CH lO
:
I
O Rerrigerats
The refrigerant is a heat carrying medium which during their cycle (i.e.
compression, condensation, expansion and evaporation)
in the refiigeration
system
absoto heat from a low temperature system and discdrd the heat so absorbed to a
higher temperature system.
The natural ice and a mixture of ice and salt were the first refrigerants. In
1834, ether, ammonia, sulphur dioxide, methyl chloride and carbon dioxide came
into use as refrigerants in compression cycle refrigeration machines. Most of the
early refrigerant materials have been discarded for safety reasons or for lack of
chemical or thermal stability. In the present days, many new refrigerants including
halo-carbon compounds, hydro-carbon compounds are used for air conditioning and
refri geration appl ications.
The suitability of a refrigerant for a certain application is determined by its
physical, thermodynamic, chemical properties and by various practical factors. There
is no one refrigerant which can be used for all types of applications i.e., there is no
ideal refrigerant.
If
one refrigerant has certain good advantages,
it will have some
disadvantages also. Hence, a refrigerant is chosen which has greater advantages and
less disadvantages.
l 0. I Desirable Properties of an Ideal Refrigerant
We have discussed above that there is no ideal refrigerant. A refrigerant is
said to be ideal
if it has all ofthe following
properties:
I. Low boiling point,
2. High critical temperature,
3. High latent heat of vaporisation.
4. Low specific heat of liquid.
5. L,ow specific volume of vapour,
6. Non-corrosive to metal,
la- l
7. Non-flammable and non-explosive,
8.
Non-toxic,
j
9. Low cost,
10. Easy
I
l.
Easy
to liqui$ at moderate pressure and temperature,
of locating leaks by odour or suitable
12. Mixes
indicator, and
well with oil.
The standard comparison of refrigerants, as used in the refrigeration
industry, is based on an evaporating temperature of - l5o C and a condensing
temperature of + 30" C.
lO,|Classification ofRefrigerants
i
The refrigerants may, broadly, be classified into the following two groups:
l. Primary refrigerants, and2. Secondary refrigerants'
The refrigerants which directly take part in the refrigeration system are
by
called primary refrigerants whereas the refrigerants which are first cooled
primary refrigerants and then used for cooling purposes are known as secondary
refrigerants
The primary refrigerants are funher classified into the following four
groups:
I. Halo-carbon refrigerants,
2. Azeotrope
refr igerants,
3. Inorganic refrigerants. and
4. Hydro-carbon refrigerants.
The American Society of Heating, Refrigeration and Air-conditioning
Engineers (ASHARAE) identifies 42 halo-carbon compounds as refrigerants, but
I0-2
only a few of them are commonly used. The following table gives some of the
commonly used halo-carbon refrigerants:
Table O
.t
Commonly used halo-carbon refrigerants
Refigerant number
Chemical name
Chemical formula
R-l I
Trichloromonofl uoromethane
ccl3F
R-12
Dich lorodifl uoromethane
ccl2F2
R-I3
Monochlorotrifl uorom ethane
ccrE
R-14
Carbontetrafluoride
CE4
R-21
Dich loromonofl uoromethane
cHCl2F
R-22
Monochlorodi fl uoromethane
cHClF2
R-30
Methylene chloride
cH2cl2
R40
Methyl chloride
cH3cl
R-I00
Ethyl chloride
c2H5cl
R-l
13
Trichlorotrifl uorothane
ccl2FccF2
R-l
14
Dichlorotetrafl uoroethane
cclECCrE
Monochloropentafl uoroethane
cc|F2cE
R-l l5
t0-?
Comparison between different refrigerants:
Synthetic refrigerants that were commonly used for refrigeration, cold storage
and air conditioning applications arq R 11 (CFC 11), R 12 (CFC 12), R 22
(HCFC 22), R 502 (CFC 12+HCFC 22) etc. However, these refrigerants have to
be phased out due to their Ozone Depletion Potential (ODP). The synthetic
replacements for the older refrigerants are: R-134a (HFC-134a) and blends of
HFCs. Generally, synthetic refrigerants are non-toxic and non-flammable.
However, compared to the natural refrigerants the synthetic refrigerants offer
lower performance and they also have higher Global Warming Potential (GWP).
As a result, the synthetic refrigerants face an uncertain future. The most
commonly used natural refrigerant is ammonia. This is also one of the oldest
known refrigerants. Ammonia has good thermodynamic, thermophysical and
environmental properties. However, it is toxic and is not compatible with some of
the common materials of construction such as copper, which somewhat restricts
its
application. Other natural refrigerants that
are being suggested
are
hydrocarbons (HCs) and carbon dLoxde (R-744). Though these refrigerants
have some specific problems owing to their eco-friendliness, they are being
studied widely and are likely to play a prominent role in future.
Prior to the environmental issues of ozone layer depletion and global
warming, the mostwidely used refrigerants were: R 11, R 12, R22, R 502 and
ammonia. Of these, R 11 was primarily used with centrifugal compressors in air
conditioning applications. R 12 was used primarily in small capacity reftigeration
and cold storage applications, while the other refrigerants were used in large
systems such as large air conditioning plants or cold storages. Among the
refrigerants used, except ammonia, all the other refrigerants are synthetic
refrigerants and are non-totic and non-flammable. Though ammonia is toxic, it
has been very widely used due to its excellent thermodynamic and
thermophysical properties. The scenario changed completely after the discovery
of ozone layer depletion in 1974. The depletion of stratospheric ozone layer was
attributed to chlorine and bromine containing chemicals such as Halons, CFCs,
HCFCs etc. Since ozone layer depletion could lead to catastrophe on a global
level, it has been agreed by the global community to phase out the ozone
depleting substances (ODS). As ?, IesUlt except ammonia, all the other
refrigerants used in cold storages had to be phased-out and a search for suitable
replacements began in earnest. At the same time, it was also observed that in
addition to ozone layer depletion, most of the conventional synthetic refrigerants
also cause significant global warming. In view of the environmental
caused by the synthetic refrigerants, opinions differed on replacements for
conventional refrigerants. The alternate refrigerants can be classified into two
problems
broad
groups:
i)
ii)
Non-ODS, synthetic refrigerants based
on
Hydro-Fluoro-Carbons
(HFCs) and their blends
Natural refrigerants including ammonia, carbon dioxide, hydrocarbons
and their blends
to-11
-
Refrigerant
R 1I(CFC)
NBP = 23.7oC
h0 at NBP=182.5 kJ/kg
To =197.98oC
Appllcation
Large air conditioning systems
Industrial heat pumps
As foam blowing agent
R 141b (N)
n-pentane (R,N)
GWP = 3500
R 12 (CFC)
NBP = -29.8oC
hrc at NBP=165.8 kJ/kg
To =112.O4oC
Domestic refrigerators
Small air conditioners
Water coolers
Smallcold storages
R 22 (R,N)
R 134a (R,N)
R 227ea (N)
R 401A,R 4018 (R,N)
R 41 1A,R 41 1B (R,N)
R 717 (N)
Air conditioning systems
Cold storages
R 4104, R 4108 (N)
R 417A (R,N)
R 407C (R,N)
Cp/Cv = 1126
ODP = 1.0
GIA/P = 7300
R 22 (HCFC)
NBP = 40.8oC
hrg at NBP=233.2kJRg
T"' =96.02oC
Cp/Cv = 1.166
ODP = 0.05
G\ /P = 1500
R 134a (HFC)
NBP = -26.15"C
h'g at NBP=222.5kJtkg
To =101.06oC
Cp/Cv = 1.102
ODP = 0.0
G\ /P = 1200
n
zrz (NHs)
I
Nep
I
= -33.35oc
hrc at NBP=1368.9 kJftg
To =133.0oC
Cp/Cv = 1.31
ODP = 0.0
G\A/P = 0.0
(GOz)
NBP = -78.4"C
h6 at 40oC=321.3 kJtkg
T"' =31.1oC
Cp/Cv = 1.3
ODP = 0.0
R 123 (R,N)
R 245fa (N)
Cp/Cv = 1.13
ODP = 1.0
R74/
Substitute suggested
RetrofiflRltNcur lNl
R 507,R 5074 (R,N)
R 404A (R,N)
R 717 (N)
Used as replacement for R 12
No replacement required
in domestic refrigerators, water
* lmmiscible
coolers, automobile A/Cs etc
in mineraloils
* Highly
hygroscopic
cold storages
lce plants
Food processing
Frozen food cabinets
Cold storages
Air conditioning systems
No replacement required
* Toxic
and flammable
* Incompatible
with copper
* Highly
efficient
* Inexpensive
and available
No replacement required
* Very
low criticaltemperature
Simultaneous cooling and " Eco-friendly
heating (Transcritical cycle)
* Inexpensive
and availabte
GWP = 1.0
Table.lO. Refrigerants, tn
lo- 5
Refrigerant
R718 (HzO)
NBP = 100.oC
hrg at NBP=2257.9 kJlkg
Application
Absorption systems
Steam jet systems
.
Replacement for R 12
Domestic refrigerators
Water coolers
No replacement required
" Flammable
* Eco-friendly
T", =374.1SoC
Cp/Cv = 1.33
ODP = 0.0
G\A/P = 1.0
R600a (iso-butane)
NBP = -11.73"C
hrs at NBP=367.7 kJ/kg
Substitute suggested
Retroflt(RllNew (N)
No replacement required
High NBP
* High freezing point
* Large specific volume
* Eco-friendly
* Inexpensive and available
T", =135.0oC
Cp/Cv = 1.086
ODP = 0.0
GWP = 3.0
TablelO-l Refrigerants, their applications and subsfifufes (contd.)
{o-6
ctlrt
Air Refrigeration Cycbs
II
Introduction
In an air refrigeration cycle, the air is used as a refrigerant. In olden days, air
was widely used in commercial applications because of its availability at free of cost.
Since air does not change its phase i.e. remains gaseous throughout the cycle,
therefore the heat carrying capacity per kg of air is very small as compared to vapour
absorbing systems. The air-cycle refrigeration systems, as originally designed and
installe4 aro now practically obsolete because
of their low coefficient of
performance and high power requirements. However, this system continues to be
for air refrigeration because of the low weight and volume of
equipment. The basic elements of an air cycle refrigeration system are
favoured
the
the
compressor, the cooler or heat exchanger, the expander and the refrigerator.
Before discussing the air refrigeration cycles, we should first know about
the unit of refrigeration, coefficient of performance of a refrigerator and
the
difference between the heat engine, a refrigerator and a heat pump.
,, ' I
Units of Refrigeration
The practical unit of refrigeration is expressed
refrigeration' (briefly written as TR).
A
in terms of 'tonne of
tonne of refrigeration is defined as the
amount of refrigeration effect produced by the uniform melting of one tonne (1000
kg) of ice from and at OoC in 24 hours.
Since the latent heat of ice is 335 H/kg, therefore one tonne of refrigeration,
ITR:
-
1000 x 335 kJ in 24 hours
looox335
=232.6kJlmin
24x60
In actual practice, one tonne of refrigeration is taken as equivalent to 210
kJlmin or 3.5 kW (i.e. 3.5 kJ/s)
U-l
Ws=Qz-Qr
tteaQt*0t
w
xf
*.Glr r lll
n*tpaor
Cef torit
Tr
tlr
Fig.tl.t. Difference
between a heat engine, refrigerator and heat pump.
The performance of a heat engine is expressed by its effrciency. We know
that the efficiency or coeffrcient of performance of an engine,
qE
or (C.O.P.)e =
Work done
=
Heat supplied
W,
= Qz-Qr
Qz
Qz
Figtl.l (b), is a reversed heat engine which either
cool or maintain the temperature of a body (T1) lower than the atmospheric
A refigerator
as shown in
temperature (T"). This is done by extracting the heat(Q1) from a cold body and
delivering it to a hot body (Qt. In doing so, work Wn is required to be done on the
system. According to First Law of Thermodynamics,
Wn=Qz-Qr
The performance of a refrigerator is expressed by the ratio of amount of heat
taken from the cold body (Q1) to the amount of work required to be done on the
system
(Wd. This ratio is called coeffrcient of performance. Mathematically,
coefficient of performance of a refrigerator,
(C.O.P.)R=
Qt =
wR
Qt
Qz
-Qr
Any refrigerating system is a heat pump as shown in
extracts heat (Qr) from a cold body and delivers
Figtt.l
(c), which
it to a hot body. Thus there is no
difference between the cycle of operations of a heat pump and a refrigerator. The
ll-3
formation of frost at the end of expansion process and clog the line. Thus in an open
cycle system, a drier should be used.
ll . tr Closed or I)ense Air Refrigeration Cycle
In a closed or
dense air refrigeration cycle, the
air is passed through the
pipes and component parts of the system at all times. The air, in this system, is used
for absorbing heat from the other fluid (say brine) and this cooled brine is circulated
into the space to be cooled. The air in the closed system does not come in contact
directly with the space to be cooled.
The closed air refrigeration cycle has the following thermodynamic
advantages
:
l.
Since
it
can work at a suction pressure higher than that of atmospheric
pressure, therefore the volume
of air handled by the compressor and expander
are
smaller as compared to an open air refrigeration cycle system.
2.
The operating pressure ratio can be reduced, which results in higher
coeffi cient of performance.
It
t6
Air Refrigerator Working on Reversed Carnot Cycle
In refrigerating systems, the Carnot cycle considered is the reversed Carmot
cycle. We know that a heat engine working on Carnot cycle has the highest possible
efficiency. Similarly, a refrigerating system working on the reversed Carnot cycle,
will
have the maximum possible coefftcient of performance. We also know that
it
is
not possible to make an engine working on the Carnot cycle. However, it is used as
the ultimate standard of comparison.
A
reversed Carnot cycle, using air a working medium (or refrigerant) is
shown on p-v and T-s diagrams in Fig.tl.2 (a) and (b) respectively. At point
v1, T1 be the pressure, volume and temperature
of air respectively.
ll-u
l,
let pr,
ftr
I
|irl
I
*vclfi
* lilist,*..*
P)f.,t$ffi
{$rvffi.
Fig.ll.2 Reversed Carnot cycle
The four processes of the cycle are as follows:
l.
Isentropic compression process. The air is compressed isentropically
shown by the curve
l-2 on p-v and T-s diagrams. During this
process, the pressure
as
of
air increases from pt to p2, specific volume decreases from vr to vz and temperafure
increases from T1
to T2. We know that during isentropic compression, no heat is
absorbed or rejected by the air.
2,Isothermal compression process. The air is now compressed isothermally
(i.e. at constant temperature, T2 = T3) as shown by the cuwe 2-3 on p-v and T-s
diagrams. During this process, the pressure of air increases from p2 to p3 and specific
volume decreases from v2 to v3. We know that the heat rejected by the air during
isothermal compression per kg of air,
q2-3
= Area 2-3-3'-2'
= Ts(sz -ss)=Tr(tr
-tr)
3. Isentropic expansion process. The air is now expanded isentropically as
shown by the curve 3-4 on p-v and T-s diagrams. The pressure of air decreases from
Pr to pa, specific volume increases from v3 to v+ and the temperafure decreases from
T3 to Ta. We know that during isentropic expansion no heat is absorbed or rejected
by the air.
ll- 6
4.Isothermal expansion process. The air is now expanded isothermally (i.e.
at constant temperature,T4= Tr)
ff
shown by the curve
4-l
on p-v and T-s diagrams.
The pressure of air decreases from pa to p1, and specific volume increases from va to
v1. We know that the heat absorbed by the air (or heat extracted from the cold body)
during isothermal expansion per kg of air,
q4-l = Area
4-l -2, -3,
= Tr(sr -sa)=To(r,
-rr)=I
(sz
-sr)
We know that work done during the cycle per kg of air
:
Heat rejected - Heat absorbed
= T2 (s2-s3)-Tr(sz-sl)
.'.
:
:
ez-r_e+-r
(Tz-Tr) (sz-sl)
coefficient of performance of the refrigeration system working
on
reversed Carnot cycle,
(C.O.'.)R
_
q4_l
:
done q2-3-q4-l
Heat absorbed
Work
Ti
(r,
(r,
-rr)
-!Xr,
T
-E)
^l
r, -rr
Though the reversed Carnot cycle is the most efficient between the fixed
temperature limits, yet no refrigerator has been made using this cycle. This is due to
the reason that the isentropic processes of the cycle. require high speed while the
isothermal processes require an extremely low speed. This variation in speed of air is
not practicable.
Note : We have already discussed that C.O.p. of a heat pump,
(C.O.P.)p:(C.O.P.)p*
1: = Ii=-+l:
Tz -I
T2
Tz
-I
and C.O.P. or efficiency of a heat engine,
(c.o.p.). = :-L
=
Tz
(c.o.P.)P
-Tr
12
ll , $f"-perature Limitations for Reversed Carnot Cycte
tf -7
We have seen in the previous article that the C.O.P. of the refrigeration
system working on reversed Carnot cycle is given by
(C.O.P.)R:#
Tl :
where
T2
:
Lower temperature, and
Higher temperature.
The C.O.P. of the reversed Carnot cycle may be improved by
1.
decreasing the higher temperature (i.e. temperature of hot body, T2), or
2.
increasing the lower temperature (i.e. temperature of cold trody, T1).
This applies to all refrigerating machines, both theoretical and practical. It
may be noted that temperatures T1 and T2 gannot be varied at will, due to certain
functional limitations. It should be kept in mind that the higher temperature (Tz) is
the temperature of cooling water or air available for rejection of heat and the lower
temperature
transfer
(Tr) is the temperature to be maintained in the refrigerator. The
heat
will take place in the right direction only when the higher temperature is
more than the temperature of cooling water or air to which heat is to be rejected,
while the lower temperature must be less than the temperature of substance to
be
cooled.
Thus
if the temperature of cooling water or air (i.e. Tz) available for heat
rejection is low, the C.O.P. of the Carnot reftigerator will be high. Since Tz in wirrter
is less than Tz in summer, therefore, C.O.P. in winter will be higher than C.O.P. in
summer. In other words, the Carnot refrigerators work more efficiently in winter than
in summer. Similarly, if the lower temperature fixed by the refrigeration application
is high, the C.O.P. of the Carnot refrigerator
used for making ice at 0'C (273
used for air-conditioned plant
is
40t.
will
be high. Thus a Carnot refrigerator
K) will have less C.O.P. than a Camot refrigerator
in summer at 20"C when the atmospheric temperature
ln other words, we can say that the Carnot C.O.P. of a domestic refrigerator
is less than the Carnot C.O.P. of a domestic air-conditioner.
tl- I
ll , I
The Carnot refrigeration cycle
cycle, hence is used as a model of
carnot refrigeration cycle is a completely reversible
temperature heat soutte
prrr"rii"" iJt u ,rrtigi*tion "y.r. op"tuiing between-a constant
10'1
wtri"ctr the real cycles are compared' Figures
and sink. It is used as reference against
and
system
comiression refrigeration
(a) and (b) show the schematic of a Camol vapour
ihe operating cycle on T-s diagram'
of
refrigeration system for puf. vapour consists
As shown in Fig.lf .l(a), the basic Carnot
(qaeffect
turbiie and evaporator' Refrigeration
four components: compressor, condenser,
as the refrigeiant undergoes the process of
r = Q") is obtained ut tt, evaporator the
latent heat from the low temperature heat
vaporization (process 4-l) and extracts
vapour is then compressed.isentropically in
source. The low temperature, low pressure
T"' The refrigerant pressure increases from P"
the compressor to the heat sink temperature
Next
(process 1-2) and ihe exit vapour is saturated'
to P" during tt e com-prfton pro."tt
iefrigerant undergoes the process of
the high pressure, high temperature saturated
(qz-r =
as it rejects the heat ofcondensation
condensation in the ."ii""r"itprocess.2-3)
through
flows
then
liquid
pt"tt*" saturated
q") to an external fr*i'rint-ut i". ft" t igS
(process 34). During this process, the
the turbine uno uni"rg;", isentropic "*fro*ion
in
P", t""'Since a saturated liquid is expanded
pressure anO temp"iotire fall frornP",T" io
the
inio nupo* and the exit condition lies in
the turbine, some amount of liquid flashes
then
*a to\' pressure liquid.vapour mixturecycle
two-phase region. This low temperature
the
Fig'l0:l(b)'
in
Thus as shown
enters the evaporator completing the- cycle.
(processes 4-l and 2-3) and two
involves two isother',,uL ir"u, iransfer-proces_ses
b2 and'34)' Heat is extracted isothermally
isentropic *ort t unri.i pro"r"r., lprocesles
rejected isothermally at
at evaporator temperature T. during proge:f- 4-l' heat,is
2-3- Work is supplied to the compressor during
condenser temperature T" during pro..r,
etaporator pressure P" to
tf-if."f refigerant uapour liom
the isentropi"
"o.p.*tioi
by the system as refrigerant liquid expands
condenser pr"rrurr'p", unJ *o.t is proiuJed
prirtut" i. to. tnuporator pressure P'' All the
isentropically in the-turbine from condenser
reversible, i.e., net entropy generation
processes are bothlnternally as well as externally
i'or the system and environment is zero'
to the carnot refrigeration cycle'
Applying first and second laws of thermodynamics
16q =
f
f
f6w
6q =qn-r -92-t =Qe -Qc
6w =w3-l
-wt-z =wT -wc =-wnet
lf- 9
00.r)
+
fig.I0.I(a):
(9" -Q") =Wner
Schematic
ola Carnot refrigeration
system
T
T"
w3-a
Fig. 1t.1(b): Carnot refrigeration clcle
It- to
on.I's diagrqm'
transfer Processes 2-3 and4-l' we can write:
now for the reversible, isothermal heat
t
g"
(1t.2)
=- j,z-t=-Jt.as=1"(sz -s:)
l
g" =go-r =JT.dS=T.(sr
(r0.3)
-sl)
9
condenser temperatures, respectively' and'
where Te and Tc are the evaporator and
st =s2 and s3
(1D'4)
=s4
(COP) is given by:
the Coeflicient of Perfornance
r"(:r
Ti)
=
coPc'.,.t =;:et
work input w*t TJt, -sr)-T"(sr
refrigeration effect
thus the
Q.
.
-sr)
=f-
t"-i
(10.5)
\T" -T" )
and condenser
coP of carnot refrigeration cycle is a function of evaporator
substance. This is the
nature of the working
#'il il*;;oittt"
t- ^ - . The Carnot COP sets an upper
operating on Carnot cycle .
temperature thermal reservoirs
refrigeration systems oierating _betwetn two constant
same heat source and sink
the
pro-t*ot's
for
theorems,
(heat source and ti"kt.
temperatures only
was obtained for air cycle refrigeration.systems
reason why exactly the same expression
limit for
ir.f.rutu."r,
rro
i."*-rriLle
cycle can have COP higher than that of Carnot COP'
bas
fig.tL.2. Carnot refrigeration
cycle represented in T's plane
/r-/2
a Camot refrigeration system
can be seen from the above expression that the CoP of
temperature decreases'
increases as the evaporator temperature increases and condenser
(Fig'11'2)' As shown
This can be explainean".y easiiy with the help of the T-sdiagram
a fixed condenser
For
l'2'34'
area
the
ir, ,fr, figure, iOp i, the iatio oi uteu a-I4-bio
lt
u, the evaporator temperature T. increases' area a-14-b (q') increases
Similarly for a fixed
area l-2-34(wna) decreases as a result, COP increases rapidly'
input
;;;;;d;." i.,
and
T" increases, the net work
evaporator temperat*e T", as the condensin-g temperature
re-mains constant, as a result the
1i1rea l-2-3-4) increases, even though cool'ing output
with evaporator temperature
UOp f.ffr. fii,.t. tD; rho*r the viriation of Carnot COP
increases sharply with
for different condenser temperatures. It can be seen that the COP
temperatures' COP reduces as
evaporator temperatures, particularly at high condensing
marginal-at low evaporator
the condenser temperatur. in.r"ur"r, but the effect becomes
refrigeration systems
t"d;;;irr;s. tt wilt be shown later that actual vapour compressionsystems
as far as the
also behave in a manner similar to that of Carnot refrigiration
performance trends are concerned.
c
o
P
T. -+
fi7.ils.
tt.
COP
Efects of evaporator and condenser temperatures on Carnot
to Practical difficulties
with carnot refrigeration system:
It is difficult to build
following
and operate a carnot refrigeration system due to the
practical difficulties:
be compressed
i. During process l-2, amixture consisting of liquid and vapour have to
due to
compressior
aswet
known
is
isentropically in the compressor. Such
" "ol-pt"ttion
difficult especially with
the presence of liquid. In practice, wet compression is very
in .T: of high speed
severe
reciprocatin; ;;d;r;orr. ffrir-pitblem is iarticularly
of liquid droplets in
presence
reciprocatini .o.pr"rrors, which get damagei due to the
the presence of liquid in
the vapour. Even though some typJs of compressors can tolerate
tl-13
dry
most widely is refrigeration' traditionally
vapour, since reciprocating compressors are
is preferred to wet compression'
compressior l"o-pr"rrio'iof vapour only)
cycle it 4d using a turbine and extracting
ii. The second practical difficulty with carnot
of liquid tl-ryg:tolt is not
work from the system durrng the isentropic expansion systems' This is due to the
o? smati capacity
economically feasible, particularly t" "-T:
of refrigerant) from the turbine is given
fact that the specific;;* J@er kilogram
by:
pc
w3-4
='iv.dr
(10'6)
volume of a
is much smaller compared to the specific
since the specific volume of liquid
liquid will be small' In
tfr" woit output iromlhe turbine in case- of the the net output will be
ineffrciencies of the turbine' then
addition, if one
"onriO..r'tte
for extracting the work from the high pressure
further reduced. e, u t .ult using a turbine
j"ttified in most of the casest'
liquid is not econo*i*ffy
vapour/gas,
onewayofachievingdrycompression.inCamotrefrigerationcycleistohavetwo
shown-in rig'f)'i'
compressors
-
one
,r.ri,tpi'and
one
i'otf'"ttui*
Condenser
r(
I
Pj>Ps>P3
o'''
Y
I
t'r-
EvaPorator
+
t
fi1.I l. l'
compre ssion
Carnot refr igerat ion system with dry
system with dry compression consists of
As shown in Fig.lf.4, the carnot refrigeration
fro,,, evaporator pressure P" to an intermediate
one isentropi"
(2-3)
"o-pr"rrionJrocess-1t--Z;
an isottrermat compression process
pressure pi and ,"-p".*i.i i", fodo*"a by
this
with
Though
lrom the
pressure P"'
intermeaiJtl';;;; p, to the condenser
be avoided' still this modified system is
can
modification the problem of wet "o.pr"rrion
tnre isothermal compression using highin'acrrieving
iitr""rrv
not practical due to |r""
speedcompressors.Inaddition,useoftwocompressorsinplaceofoneisnot
economicallY justifi ed.
ffig
made to
recoler this work of expansion in some refrigeration
systems to improve the system efficiency'
ll- tll
practical considerations' the Carnot
From the above discussion, it is clear that from
o single cornpressor is
refrigeration system need to be modified. Ory comiress.ion yith
rejection
heat rejection pt*".t is replaced by isobaric heat
;;;;il ti t# isothermal
isenthalpic
an
by
replaced
procgss.can be
;;;;;;. Sirniturty, tt, ir.ntropic-e*punsion
incorporates these two changes is
1nhith
system,
throttling process. A refrigeration
iJthe theoretical cycle on which
known as Evans-perkins oireverse Rankine cycle. This
based'
the actual vapour compression refrigeration systems are
Exp.
Device
FiS. I l. 5. Standar d Vapour compre ssion refr igerotion system
tt 4,
Vapour Compression Refrigeration Systems
ll, il
Introduction
A
vapour compression refrigeration system is an improved type
of air
refrigeration system in which a suitable working substance, termed as refrigerant, is
used.
It condenses and evaporates at temperatures
and pressures close
to
the
atmospheric conditions. The refrigerants, usually, used for this purpose are ammonia
(NH3), carbon dioxide (CO2) and sulphur dioxide (SO2). The refrigerant used, does
not leave the system, but is circulated throughout the system alternately condensing
and evaporating. In evaporating, the refrigerant absorbs its latent heat from the brine
(salt water) which is used for circulating
condensing,
it gives out its latent
it
around the cold chamber. While
heat to the circulating water of the cooler. The
vapour compression refrigeration system is, therefore a latent heat pump, as it pumps
its latent heat from the brine and delivers it to the cooler.
The vapour compression refrigeration system is now-a-days used for all
purpose refrigeration.
It is generally
used
for all industrial purposes from a small
domestic rcfrigerator to a big air conditioning plant.
It, f 2 Advantages and Disadvantages of vapour Compression Refrigeration
System over Air Refrigeration System
Following are the advantages and disadvantages of the vapour compression
refrigeration system over air refrigeration system:
Advantages
l. It has smaller size for the given capacity of refrigeration.
2. It has less running cost.
3. It can be employed over a large range of temperatures.
4. The coefficient of performance is quite high.
Disadvantages
l. The initial cost is high
u-
l(
3. Receiver. The condensed liquid refrigerant from the condenser is stored in
a vessel known as receiver from where
it is supplied to the evaporator
through the
expansion valve or refrigerant control valve.
4. Expansion valve. It is also called throttle valve or reftigerant control
valve. The function of the expansion valve is to allow the liquid refrigerant under
high pressure and temperature to pass at a controlled rate after reducing its pressure
and temperature. Some
of the liquid refrigerant evaporates as it
passes through the
expansion valve, but the greater portion is vaporised in the evaporatOr at the low
pressure and temperature.
5. Evaporator. An evaporator consists of coils
of pipe in which the liquid-
vapour refrigerant at low pressure and temperature is evaporated and changed into
vapour refrigerant at low pressure and temperature. In evaporating, the liquid vapour
refrigerant absorbs its latent heat of vaporisation from the medium (air, water or
brine) which is to be cooled.
Note
: In any compression refrigeration system, there are two different
pressure conditions. One is called the high pressure side and other is known as low
pressur€ side. The high pressure side includes the discharge line (i.e. piping from the
evaporator to the suction valve A).
f
l. f q" Pressure-Enthalpy (p-h) Chart
f
I
I
*Ht*r€
cnx nn
-rttltrilE
r.fl-ctt
Frj
tt'7
pressur€
//- /g
Fntha4 Lp-hJ cl^"rl
The most convenient chart for studying the behavior of a refrigerant is the p-
h chart, in which the vertical ordinates represent pressure and horizontal ordinates
represent enthalpy (i.e. total heat). A tlpical chart is shown in Fig.1f.| in which a
few important lines of the complete chart are drawn. The saturated liquid line and the
saturated vapour line merge into one another at the critical point.
A
saturated liquid is
one which has a temperature equal to the saturation temperature corresponding to its
pressune. The space
to the left of the saturated liquid line will, therefore, be sub-
cooled liquid region. The space between the liquid and the yapour lines is called wet
vapour region and to the right of the saturated vapour line is a superheated vapour
region.
In the following
pages, we shall drawn the p-h chart along
with the T-s
diagram of the cycle.
ll
:
t Stvpes of vapour compression cycles
consists
We have already discussed that vapour compression cycle essentially
of compression, condensation, throftling and evaporation. Many scientists
have focussed their affention to increase the coefficient of perfonnance of the cycle.
Through there are many cycles, yet the following are important from the subject
point of view
:
l. Cycle with dry saturated
vapour after compression,
2. Cycle with wet vapour after compression,
3. Cycle with superheated vapour after compression,
4. Cycle with superheated vapour before compression, and
5. Cycle with undercooling or subcooling of refrigerant.
ll-/?
tlt
(VCRS)
Compression Refrigeration System
11.,15 Standard Vapour
., I
(sSS) vapour
standard, saturated, single stage
Figure 10.5 shows the schematic of a
tn
th"-o;;ing cycte on a T s diagram' As shown
compression .rtrige.;tln ,Vrt"n, -d
system
refrigeration
saturated vapour compression
the figure the standard singte stage,
consisls of the following four processes:
Processl.2:lsentropiccompressionofsaturatedvapourincompressor
pto."t. 2-3: Isobarii heat rejection in condenser
expansion device
process 3-4: Isenthaipi" ,.p'""ri"n of saturated liquid in
piot"tt 4-1: IsobariJheat extraction in the evaporator
b"'r."n that the standard vapour compression
By comparing with Carnot cycle,- it can
ij inen"rtiUility due to non-isothermal
refrigeration cycle introdu"rr-t*o i.rrn"rriUifititt'
(process 3frren#Uiilt dt; to isenthalpic throttling smaller
heat rejection lprocess 2-3) and Z;
than
of standard cycle to be
tft""t",]t"i'COp
the
*oJA"xpect
on"
result,
a
As
4).
to these
ftl*lout"" unO sink.temperatures' Due
that of a Carnot system for the ru.n"
the
reducing
thus
uiJ *ott input increases'
s
T
ineversibilities, the cooling effect ,9Orr.-J,on
diagrams
,h; help of the cycle
svstem cop. This can be explained "";u;;,h
of
terms
in
standard vcRs
:nff].:;;;,i.dia,
shows
ro.p*i.o,i*;;;;;;oiuna
refrigeration effect'
co
fi?ilt'A@)'
VCRS
Comparison between Carnot and standard
and
is reversible for both the carnot cycle
The heat extraction (evaporation) process
is given by:
VCRS cycle. Hence the refrigeration effect
I
ge.camot
'4'
=e+,-r = jt.os=t (s, -sa,)=area
ll- 20
e-1-4'-c-e
(11.7)
For VCRS cycle (1-2-34):
I
ge,vcRS = g +-r = f T.ds = T. (s1
-
sn ) =
area
e
- I- 4- d-
e
(ll.s)
4
process of
thus there is a reduction in refrigeration effect when the isenfiopic expansion
is
reduction
this
Carnot cycle is replaced by isenthalpic throttling process of VCRS cycle,
loss
equal toihe area d44'-c-d (area Az) and is known as throttling /oss. The throttling
is equal to the enthalpy difference between state points 3 and 4" i.e,
Qe.camot
-gvcns =area
d-4-4'-c-d=(hr -ha.)=(ha -hc,)=ar€aAz
(10.9)
effect increases as the evaporator
increases. A practical consequence
temperature
temperature decreases and/or condenser
rate'
flow
mass
of this is a requirement of higher refrigerant
It is easy to show that the loss in refrigeration
The heat rejection in case of
vcRS cycle
also increases when compared to camot cycle'
Fig.I1E@. Comparative evaluation of heat reiection rate of
VCRS and Carnot cycle
given
As shown in Fig1l.$$;, the heat rejection in case of Carnot cycle (1-2"-3'4') is
by:
3
gc.camot
=-g,2,-t= - j,t.Os=T"(sz. -Sr)=area
e-2"-3-c-e
(11.10)
In case of VCRS cycle, the heat rejection rate is given by:
3
gc,vcRS
=-gz-t
=
-JT.ds=
area
e-2-3-c-e
1lf.t
t1
to
Hence the increase in heat rejection rate of VCRS compared to Carnot cycle is equal
the area 2"-2-2' (area Ar). This region is known as superheat horn, and is due to the
It-21
replacement of isothermal heat rejection process
rejection in case of VCRS.
of camot cycle by isobaric heat
cycle
when the Camot
effect
Since the heat rejection increases and refrigeratigl
ryduTs
increases compared
VCRS
the
to
input
is modified to standard VCRS cycle, the ni work
are given by:
cycles
and VCRS
to Carnot cycle. The net work iniut in case of Carnoi
w
net.VCRS
(10.12)
=(q" -9.)camot =area l-2"-3-4'-l
wnet,crnot
= (Q.
-
9r ) vsas = drg& |
- 2 -3 -
4'-c
- d - 4 -l
tl).rl
in VCRS cycle is given by:
As shown in Fig.ll.G(c), the increase in net work input
Wnet,VCRS-Wnet,Carnot=area2,,_2-2,+areac-4,_4-d_c=areaA,+areaA2(10.14)
e
cd
S
cycle
FiS.10.E(c). Figure illustrating the increase in net work input in VCRS
are given by:
To summarize the refrigeration effect and net work input of VCRS cycle
Qe,vCRs = Qe,Camot
wnet,VCRS =wnet,ca-ot
-
atea A2
+areaA, + area A'
(ll.1s)
(10.16)
The COP of VCRS cYcle is given bY:
coPvcRS
Qe'vcRS
- wna.vCRS - w
-areaA,
+ afea Al + areaAt
ge.Camot
ner,camor
ll -2,2
(ll.l7)
If we define the cycle efficiencY, rln
as
the,ratio of COP of VCRS cycle to the COP of
e
Carnot cycle, then:
r-["*"A'l
\e .,cr'"
coPvcRs
1'lo =-=
coPc-*ot
/
(10.18)
w*,cu*o,
\
of the
The cycle efficiency (also called as second law effrciency) is a good indication
cycle
the
COP,
deviation of the standard VCRS cycle from Carnot cycle. Unlike Camot
the
on
effrciency depends very much on ihe shape of T s diagram, which in turn depends
nature of the working fluid.
If we
compression
assume that the potential and kinetic energJ changes during isentropic
process
l-2 arenegligible,
then the work input wt-u is given by:
wr-z.vcns
=(h, -h1)=Gz -hr)-(hr
-hr)
(l0.le)
pressure
FiS.Il.7. Figure showing saturated liquid line 3-f coincidingwith the constant
line
line 3-f
Now as shown in Fig.llt, if we further ass'me that the saturated liquid
is a
(which
region
pressur€ line P. in the subcooled
coincides with the
"J'*ii"i
reasonably good assumption), tiren from the 2d Tds relation;
Tds
:dh - v dP = dh; when P is
f
.'.(h2
-hr
) = J.' Tds = area e
constant
-2-3 -f - g- e
lt47
(1f.20)
and, (hl
f
-hs)=JTds=area
e-l-f -g-e
(lt.2l)
I
we obtain the
substituting these expressions in the expression for net work input,
earlier
.orpr.rrot"work input to be equal to area l-2-3-f-l.Now comparing this with.the
to
equal
is
42
area
that
expression for worli input (area l-2-3-4'-c-d4-l), we conclude
area Ar.
(area 42 rv 43)
As mentioned before, the losses due to superheat (area Ar) and throttling
curves)
vapour
and
liquid
(saturation
Oepena very much on ttt" shape of the vapor dome
refrigerant'
of
o,if ; diagram. The shape of tttr saturation curves depends on the nature
types of refrigerants'
6shows T s diagrams for three different
Fig=, ll,
f
t
s
Fi7.Il.hf-s
diagramsfor three different types of refrigerants
Refrigerants such as ammonia, carbon di-oxide and water belong to Type l' These
refrigirants have symmetrigl saturation curves (vapour dome)' as a result both the
of
s,rpeitteat and throttling lossds (areas Ar and Ar) are significant. That means deviation
as
used
are
refrigerants
these
when
be
significant
VtnS cycle from Carnot cycle could
these
2,
Type
to
belong
lIFCl34a
working fluids. Refrigerants such as CFCI f, CFCI2,
refrigerints have small superheat losses (area A'1) but large throttling losses (area Al).
Higli molecular weight refrigerants such as CFC113, CFCll4, CFC115, iso-butane
inlet
bei-onging to Type 3, do not liate -y superheat losses, i.e., when the compression
condition is saturated (point l), then the exit condition will be in the 2-phase region, as a
result it is not nec"tsary to superheat the refrigerant. However, these refrigerants
il -'2+
experience significant throttling losses. Since the compressor exit condition of Type 3
refrigerants may fall in the two-phase region, there is a danger of wet compression
compressor damage. Hence for these refrigerants, the compressor inlet
condition is chosen such that the exit condition does not fall in the two-phase region. This
implies that the refrigerant at the inlet to the compressor should be superheated, the
extent of which depends on the refrigerant.
leading
to
Superheat and throffling losses:
lt can be observed from the discussions that the superheat loss is fundamentally different
from the throttling loss. The superheat loss increases only the work input to the
compressor, it does not effect the refrigeration effect. In heat pumps superheat is not a
loss, but a part of the useful heating effect. However, the process of throttling is
inherently irreversible, and it increases the work input and also reduces the refrigeration
effect.
tl , t 7 Analysis
I
of
standard vapour compression refrigeration
system
A simple analysis of standard vapour compression refrigeration system can be carried out
by asJuming a) Steady flow; b) negligible kinetic and potential energy changes across
each component, and c) no heat transfer in connecting pip€ lines. The steady flow enerry
equation is applied to each of the four components.
Evaporator: Heat transfer rate at evaporator or refrigeration capacity,
Q" =mr(hr
-hl)
where m, is the refrigerant mass flow rate in kg/s,
Q. it given by:
Q|z2)
hq and
h
are the specific enthalpies
(kJ/kg) at the exit and inlet to the evaporator, respectively. (ht -hn) is known as specific
refrigeration effect or simply refrigeration efect, which is equal to the heat transferred at
the evaporator per kilogram of refrigerant. The evaporator pressure P" is the saturation
pressure corresponding to evaporator temperature T., i.e.,
(rf.23)
P, = P,", (T")
Compressor: Power input to the compressor,
iV"
is given by:
Ws =m1(hz
-hr)
0).24)
where hz and h1 are the specific enthalpies (kJ/kg) at the exit and inlet to the compressor,
respectively. (hz-hr)is known as specific work of compression or simply work of
compression, which is equal to the work input to the compressor per kilogram of
refrigerant.
Condenser: Heat transfer rate at condenser,
Q. it given
ll-2,
by:
where h: and h2 are
respectively.
(ll.2s)
Q" =m.(hz -hr)
the specific enthalpies (kJ/kg) at the exit and inlet to the condenser,
The condenser pressure P" is the saturation pressure corresponding
to evaporator
temperature T", i.e.,
P. =Po,(T")
(1?.26)
Expansion device: For the isenthalpic expansion process, the kinetic energy change
across the expansion device could be considerable, however, if we take the control
volume, well downstream of the expansion device, then the kinetic enerry gets dissipated
due to viscous effects. and
(11.27)
hr =hl
The exit condition of the expansion device lies in the two-phase region, hence applying
the definition of quality (or dryness fraction), we can write:
ha
=(l-xl)hr,"
*x4hg.e =hr +xlhre
(10.28)
where v is the quality of refrigerant at point 4, hq", hs", hrg are the saturated liquid
enthalpy, saturated vapour enthalpy and latent heat of vaporization at evaporator
pressure, respectively.
The COP of the system is given by:
-no)
cop=[ o" l=[;,,n, -n.,1-(n,
(hz
t"J
-h,)J -hr)
I
(ll.2e)
[-,(h,
At any point in the cycle, the mass flow rate of refrigerant mr can be wriffen in terms of
volumetric flow rate and specific volume at that point i.e.,
m'=Y
(11.30)
applying this equation to the inlet condition of the compressor,
/
n,, =-Yt,/
/r,
(rr.3l)
where Vr is the volumetric flow rate at compressor inlet and v1 is the specific volume at
compressor inlet. At a given compressor speed, Vr is an indication of the size of the
compressor. We can also write, the refrigeration capacity in terms of volumetric flow rate
as:
ll -26
\
Q. =m,(h,
,
wnere
-ho)=v,rnt
-ho
Ivr
(t,-ho).
\vr )
l)
(10.32)
Generally, the type of refrigerant, required refrigeration capacity, evaporator temperature
and condenser temperature are known. Then from the evaporator and condenser
temperature one can find the evaporator and condenser pressures and enthalpies at the
exit of evaporator and condenser (safurated vapour enthalpy at evaporator pressure and
saturated liquid enthalpy at condenser pressure). Since the exit condition of the
compressor is in the superheated region, two independent properties are required to fx
the state of refrigerant at this point. One of these independent properties could be the
condenser pressure, which is already known. Since the compression process is isentropic,
the entropy at the exit to the compressor is same as the entropy at the inlet, sr which is the
saturated vapour entropy at evaporator pressure (knom). Thus from the known pressure
and entropy the exit state of the compressor could be fixed, i.e.,
h2 :h@.,sr)
sl=52
:
h@",s,)
(11.33)
The quality of refrigerant at the inlet to the evaporator (v) could be obtained from the
known values of h3, hs," and hg".
Once all the state points are known, then from the required refrigeration capacity and
various enthalpies one can obtain the required refrigerant mass flow rate, volumetric flow
rate at compressor inlet, COP, cycle efficiency €tc.
Use of Pressure-enthalpy (P-h) charts:
hl
hr:hh
hr:
Fig.ll,llStondard vapour
hz
h
compression refrigeration cycle on a P-h chart
ll-27
I I .l
g
Actual VCRS systems
The cycles considered so far are internally reversible and no change of refrigerant
state takes place in the connecting pipelines. However,
ineversibilities exist. These are due to:
l.
in
actual VCRS several
Pressure drops in evaporator, condenserand LSHX
hessure drop across suction and discharge valves of the compressor
2.
3. Heat transfer in compressor
4. Pressure drop and heat transfer in connecting pipe lines
Figures ll.l[,shows the actual VCRS cycle on P-h and T-s diagrams indicating
various ineversibilities. From performance point of view, the presslure drop in thI
evaporator, in the suction line and across the suction valve has a significant iffect on
system performance. This is due to the reason that as suction side press=ure drop increases
the specific volume at suction, compression ratio (hence volumetric efficiency) and
discharge temperature increase. All these effects lead to reduction in system capacity,
increase in power input and also affect the life of the compressor due to higher
discharge
temperature. Hence this pressure drop should be as small as posilbl. for goJd
performance. The pressurg drgq dependJ on the refrigerant velocity, length
of refrigirant
tuling and layout (bends, joints etc.). Pressur" drop can be -ieouia by red--ucing
refrigerant velocity (e.g. by increasing the inner diameter of the refrigerant tubes)-,
however, this affects the heat transfei coefficient in evaporator. More importantly
a
certain minimum velocity is required to carry the lubricating oil back to the compressor
for proper operation of the compre$6or.
Heat transfer in the suction line is detrimental as it reduces the density of refrigerant
vapour and increases the discharge temperature of the compressor. Hence,
the suction
lines are normally insulated to minimize heat transfer.
In actual
systems the compression process involves frictional effects and heat
transfer. As a result, it cannot be reversible, adiabatic (eventhough it can
be isentropic).
In many cases cooling of the compressor is provided delibeiately to maintain the
maximum compressor temperature within safe limits. This is particularly
true in case of
refrigerants such as ammonia. Pressure drops across the valves of
the compressor
increase the work of compression and reduce the volumetric efficiency
of the
compressor. Hence they should be as small as possible.
Compared to the vapour lines, the system is less sensitive to pressure
drop in the
liquid lines. However, thii also should be kept as low as possible. Heat
transfer in the condenser connecting pipes is not detrimenlrt in
oi refrigeration
systems. However, heat transfer in the subcooled liquid lines
may affect the performance.
condenser and
"*"
In addition to theabove, actual systems are also different from the
theoretical cycles
9u: !o the presence of foreign matteisuch as lubricating oil, water, air, particulate matter
inside the system. The presence of lubricating oil
avoided, however, the system
design must ensure that the lubricating oil,is carried
""nnoib,
over properly to the compressor.
This depends on the miscibility of refrigerant-lubricating oil. pr"r"rr"e
or ottr"i foreign
materials such as air (non-condensing gas), moisture, paiiculate
matter is detrimental to
system performance. Henge systems are designed and
operated such tt ut the
concentration of these materials is as low as possibli.
ll- 2g
I
I
l
fig.tlltActual
VCRS cycle on P-h
I
I
Useless superheat in suction line
Pressure drop in the deliverv line
of vapour in deli
Pressure drop in the condenser
Heat gain in liquid line
//- 2?
and
T-s diagrams
The COP of actual refrigeration systems is sometimes written in
terms of the
CoP of Carnot refrigeration system operating between the condensing and
evaporator
temperatures (coPsamol), cycle efficiency (trrr.), isentropic efficiency
o1tn.
(qi') and effrciency of the electric motor (n,-*), as givenby the equation
"on,pr"rro,
shown below:
COPac*
=
(tri}l
4cycTlisTlmoto'COPcamot
An approximate
for
l
cycle efficiency (nry") in the evaporator
^expression
te1Perature range of -50oc
to +40oc and condensing temperature range of
to
for refrigerants such as ammoni4 R 12 and n iz is zuggested by
+tot
Linge in 1966.
160oc
This expression for a refrigeration cycle operating withoJt (AT,"u
= 0f and with
subcooling (ar'"u : Ts-Tr.exit > 0 K) are given in Eqns.lt l.lf,and
,espe"tineiy:
afix
nr,
=(t-*)
n.,"
=(, *)(t.*)with
wttnout subcoorins
subcoorins
In the above equations T" and T. are condensing and evaporator
respectively.
.
ry th"
tu.fif
ft$d7
temperatures,
The isentropic effrciency of the compressor (r1;r) depends
on several factors such
compression ratio, design of the compressor, nature of
the *o*in! noia
However, in practice its.value generally lies Letvveen 0.5 to
"t".
0.g. The motoiefficiency
(tlo,o"J depends on the size and motor load.
Generally the motor efficiency is maximum
at full load. At full load its value lies around 0.7 for
srirall motors and aboui 0.95 for large
motors.
{
l/-3o
I
Vapour Absorption Refrigeration Systems
ll.
lTlntroduction
The vapour absorption refrigeration system is one of the oldest method
producing refrigerating effect. The principle
of
of
vapour absorytion was first
in 1824 while performing a set of experiments to
liquiff certain gases. The first vapour absorption refrigeration machine was
discovered by Michael Faraday
developed by a French scientist Ferdinand Canp in 1860. This system may be used in
both the domestic and large industrial refrigerating plants. The
refrigerant,
commonly used in a vapour absorption system, is ammonia.
The vapour absorption system uses heat energy, instead
of mechanical
energy as in vapour compression systems, in order to change the conditions of the
refrigerant required for the operation of the refrigeration cycle. We have discussed in
the previous chapters that the function of a compressor, in a vapour compression
system, is to withdraw the vapour refrigerant fiom the evaporator.
It then raises its
temperature and pressure higher than the cooling agent in the condenser so that the
higher pressure vapours can reject heat in the condenser. The liquid refrigerant
leaving the condenser is now ready to expand to the evaporator conditions again.
In the vapour absorption system, the compressor is replaced by att absorber,
a pump, a generator and a pressure reducing valve. These components in vapour
absorption system perform the same function as that of a compressor in vapour
compression system. In this system, the vapour refrigerant from the evaporator is
drawn into an absorber where it is absorbed by the weak solution of the refrigerant
forming a strong solution. This strong solution is pumped to the generator where it is
heated by some external source. During the heating process, the vapour refrigerant is
driven off by the solution and enters into the condenser where it is liquefied. The
liquid refrigerant then flows into the evaporator and thus the cycle is completed.
Vapour Absorption System I t , Z6J Simple
lt-7 I
The simple vapour absorption system, as shown in Fig.ff .lfconsists of an
absorber, a pump, a generator and a pressure reducing valve to replace the
compressor of vapour compression system. The other components of the system are
condenser, receiver, expansion valve and evaporator as
in the vapour compression
system.
rjrfr
ffi
llr
*flr
ll$rrmffir{F
rftr
{nr
:n
Fig f
enters
f .l
/
simple vapour absorbtion system
. In this system, the low pressure ammonia vapour leaving the evaporator
the absorber where it is absorbed by the cold water in the absorber. The warer
has the ability
to absorb very large quantities of ammonia vapour and the solution
thus formed, is known as aqua-ammonia. The absorption of ammonia vapour in
water lowers the pressure in the absorber which in turn draws more ammonia vapour
from the evaporator and thus raises the temperature of solution. Some form of
cooling arangement (usually water cooling) is employed in the absorber to remove
the heat of solution evolved there. This is necessary in order to increase the
absorption capacity
of water, because at
higher temperature water absorbs less
ammonia vapour. The strong solution thus formed in the absorber is pumped to the
generator by the liquid pump. The pump ihcreases the pressure of
the solution upto
l0
bar.
il.3L
The *strong solution of ammonia in the generator is heated by some external
source such as gas or steam. During the heating process, the ammonia vapour is
driven off the solution at high pressure leaving behind the hot weak ammonia
solution in the generator. This weak ammonia solution flows back to the absorber at
low pressure after passing through the pressure reducing valve. The high
pressure
ammonia vapour from the generator is condensed in the condenser to a high pressure
liquid ammonia. This liquid ammonia is passed to the expansion valve through the
receiver and then to the evaporator. This completes the simple vapour absorption
cycle.
It.Z|
actical Vapour Absorption System
The simple absorption system as discussed in the previous article is not very
economical. ln order to make the system more practical, it is fitted with an analyser,
a rectifier and two heat exchangers as shovm in
Figil.l$these
accessories help to
improve the performance and working of the plant, as discussed below :6{,,
Aqfe
rafr
Fig.p.tlfractical
vapour absorption system.
lt.7 3
"4,
l.
Analyser. When ammonia is vaporised in the generator, some water is
also vaporised and
will flow into the condenser along with the ammonia vapours,in
the simple system.
If
these unwanted water particles are not removed before entering
into the condenser, they will enter into the expansion valve where they freeze and
choke the pipe line.
ln order to remove
these unwanted particles flowing to the
condenser, an analyser is used. The analyser may be built as an integral part of the
generator or made as a separate piece
of equipment. It consists of a series of trays
mounted above the generator. The strong solution from the absorber and the aqua
from the rectifier are introduced at the top of the analyser and flow downward over
the trays and into the generator. In this way, considerable liquid surface area is
exposed to the vapour rising from the generator. The vapour is cooled and most
of
the water vapour condenses, so that mainly ammonia vapour leaves the top of the
analyser. Since the aqua is heated by the vapour, less external heat is required in the
generator.
2. Rectifier. ln case the water vapours are not completely removed in the
analyser, a closed type vapour cooler called rectifier (also known as dehydrator) is
used.
It is generally
water cooled and may be of the double pipe, shell and coil or
shell and tube t1pe. Its function is to cool further the ammonia vapours leaving the
analyser so that the remaining water vapours are condensed. Thus, only dry or
anhydrous ammonia vapours
flow to the condenser. The condensate fiom
the
rectifier is returned to the top of the analyser by a drip return pipe.
3. Heat exchangers. The heat exchanger provided between the pump and the
generator is used to cool the weak hot solution returning from the generator to the
absorber. The heat removed from the weak solution raises the temperature
of
the
strong solution leaving the pump and going to analyser and generator. This operation
rcduces the heat supplied to the generator and the amount of cooling required for the
absorber. Thus the economy of the plant increases.
The heat exchanger provided between the condenser and the evaporator may
also be called liquid sub-cooler. In this heat exchanger, the liquid refrigerant leaving
the condenser is sub- cooled by the low temperature ammonia vapour from the
il-7Ll
evaporator
This sub-cooled liquid is now passed to the
W
expansion valve and then to the evaporator.
In this system, the net refrigerating effect is the heat absorbed by
the
refrigerant in the evaporator. The total energy supplied to the system is the sum of
work done by the pump and the heat supplied in the generator. Therefore, the
coeffrcient of performance of the system is given by
C.O.P. =
ll
Heat absorbed in evaporator
Work done by pump * Heat supplied in generator
-T}Advantages of Vapour Absorption Refrigeration System over Vapour
Compression Refrigeration System
Following are the advantages of vapour absorption system over vapour
compression system:
l.
In the vapour absorption system, the only moving part of the
entire
system is a pump which has. a small motor. Thus, the operation of this system is
essentially quiet and is subjected to little wear.
The vapour compression system of the same capacity has more wear, tear
and noise due to moving parts of the compressor.
2.The vapour absorption system uses heat energy to change the condition of
the refrigerant from the evaporator. The vapour compression system uses mechanical
energy to change the condition of the refrigerant from the evaporator.
3. The vapour absorption systems are usually designed to use steam, either
at high pnessure or low pressure. The exhaust steam from fumaces and solar energy
may also be used. Thus this system can be used where the electric power is difficult
to obtain or is very expensive.
4. The vapour
absorption systems can operate
at reduced
evaporator
pressure and temperature by increasing the steam pressure to the generator,
decrease
with little
in capacity. But the capacity of vapour compression system drops rapidly
with lowered evaporator pressure.
u.3f
5. The load variations does not effect the performance of a
vapour
absorption system. The load variations are met by controlling the quantity of aqua
circulated and the quantity of steam supplied to the generator.
The performance of a vapour compression system at partial loads is poor.
6. In the vapour absorption system, the liquid refrigerant leaving the
evaporator has no bad effect on the system except that ofreducing the refrigerating
effect. In the vapour compression syster4,
it is essential to superheat
the vapour
refrigerant leaving the evaporator so that no liquid may enter the compressor.
7. The vapour absorption systems can be built in capacities well above 1000
tonnes of refrigeration each which is the largest size for single compressor units.
8. The space requirements and automatic control requirements favour the
absorption system more and more as the desired evaporator temperature drops.
ll
-ZJCoellicient of Performance of an Ideal Vapour Absorption
Refrigeration System
We have discussed earlier that in an ideal vapour absorption refrigeration
system,
(a) the heat (Q6) is given to the refrigerant in the generator,
(b) the heat (Q.) is discharged to the atmosphere or cooling water from the
condenser and absorber.
(c) the heat (Qs) is absorbed by the refrigerant in the evaporator, and
(d) the heat (Qp) is added to the refrigerant due to pump work.
Neglecting the heat due to pump work (Qp), we have according to First Law
of Thermodynamics,
Qc=Qc+Qe
Let
'
"'(D
T6 = Temperature at which heat (Qc) is given to the generator,
tt-36
T":
Temperature at which heat (Qc) is discharged to atmosphere or cooling
water from the condenser and absorber, and
Ts:
Temperature at which heat (Qs) is absorbed in the evaporator.
Since the vapour absorption system can be considered as
a
perfectly
reversible system, therefore the initial entropy of the system must be equal to the
entropy of the system after the change in,its condition.
... Qc
*9=k
...(iD
TGTEI
-
Qc +Qr
... I From equation (i) ]
t
o.Qo-Qc-Qe-QB
'rc
Tc Tc
TE
o"[ffi)=.'(ffi)
Qc:Qptffi]tffi]
=o,[ffi][ffi]
=o,(+)(f')
(iii)
Maximum coefficient of performance of the system is given by
(C.O.P.).*:
*.:
o'I'#)t#")
=['ft)t\")
...(iv)
It may noted that,
lt-97
'| ,,,,
3.r
(r\
I. The expression | +
\t
lis ttre c.o.P. of a carnot refrigerator working
-rr /
between the temperature limits of T s and T 6.
2.Theexpnession
f1b;
[rc
Ttlts
)
the efficiency of a carnot engine working
between the temperature limits of T6 and Ts.
Thus an ideal vapour absorption refrigeration system may be regarded as a
combination of a Carnot engine and a Carnot refrigerator. The maximum C.O.p. may
be written as
(C.O.P.)ro = (C.O.P)*"66 x
ln
rl
camor
case the heat is discharged at different temperatures
in condenser
and
absorber, then
(c.o.P.).o =
where Ta:
,, ,Zq
f
qu
rc
-t'=-lf
-r'JL
Lt
J
Temperature at which heat is discharged in the absorber.
Lithium Bromide Absorption Refrigeration System
The lithium-bromide absorption refrigeration system uses a solution of
lithium bromide in water. In this system, the *water is being used as a refrigerant
whereas lithium bromide, which is a highly hydroscopic salt, as an
absorbent. The
lithium bromide solution has a strong aflinity for water vapour because of its
very
low vapour pressur€. since lithium bromide solution is corrosive, therefore inhibitors
should be added in order to protect the metal parts of the system against
corrosion.
Lithium chromate is often uged as a corrosion inhibitor. This system is very popular
for air conditioning in which low refrigeration temperatures (not below 0" c;**
u,"
required.
ll - 3g
figlftHlhows
a lithium bromide vapour absorption system. In this system.
the absorber and the evaporator are placed in one shell which operates at the same
low pressure of the system. The generator and condenser are placed in another shell
which operates at the same high pressure of the system. The principle of operation
of
this system is discussed below:
The water for air-conditioning coils or process requirements is chilled as it
is pumped through the chilled-water tubes in.the evaporator by giving up heat to the
refrigerant water sprayed over the tubes. Since the pressure inside the evaporator is
maintained very low, therefore, the refrigerant waterevaporates. The water vapours
thus formed
will be absorbed by the strong
lithium-bromide solution which is
sprayed in the absorber. [n absorbing the water vapour, the lithium bromide solution
helps in maintaining very low pressure (high vacuum) needed in the evaporator, and
the solution becomes weak. This weak solution is pumped by a pump to the
generator where
it is heated up by using
steam or hot water in the heating coils.
A
portion of water is evaporated by the heat and the solution now becomes more
strong. This strong solution is passed through the heat exchanger and then sprayed in
the absorber as discussed above. The weak solution of lithium bromide from the
absorber to the generator is also passed through the heat exchanger. This weak
solution gets heat from the strong solution in the heat exchanger, thus reducing the
quantity of steam required to heat the weak solution in the generator.
t/-37
f ig.tt.l|I. ithium -Brom ide absorption
re
frigeration system.
The refrigerant water vapours formed in the generator due to heating of
solution are passed to the condenser where they are cooled and condensed by the
cooling water flowing through the condenser water tubes. The cooling water for
condensing is pumped from the cooling water pond or tower. This cooling water first
enters the absorber where
it takes away the heat of condensation and dilution. The
condensate from the condenser is supplied to the evaporator to compensate the water
vapour formed in the evaporator. The pressure reducing valve reduces the pressure
of
condensate from the condenser pressure to the evaporator pressure. The cooled water
from the evaporator is pumped and sprayed in the evaporator in order to cool the
water for air conditioning flowing through the chilled tubes. This completes the
cycle.
Note: The pressure difference between the generator and the absorber and
the gravity due to the height difference of the two shells is utilised to create the
pnessure for the spray.
l/- Vd
P
Ps
Pe
T@&
Fig.IS.2: H2O-L|B: system with a solution heat exchanger on Diihring plot
Dephlegmator
AJ
Heat Exchanger-l
Heat Exchanger-Il
wpv
Solution pump
Q,
Fig.171: schematic of NHs-Hzo basedvapour
absorption refrigeration system
Exp.device
Exp.dcvice
p"
I
fornp
)--Thermd .'
compression
|
",
trcRt
I
I
b)
vARs
Figs.I4.2: a) Vapour compression refrigeration
system (VC RS)
b) Vapour Absorption Refrigeration Sistem (VARS)
I
// - t//
cH /2
Refrigeration System ComponenB
I
2
Introduction
A
refrigerant compressor, as the name indicates, is a machine used to
compress the vapour refrigerant from the evaporator and to raise its pressure so that
the corresponding saturation temperature is higher than that of the cooling medium.
It also continually circulates the refrigerant through the refrigerating system.
Since
the compression of refrigerant requires some work to be done on it, therefore, a
compressor must be driven by some prime mover.
Note : Since the compressor virtually takes the heat at a low temperature
-from the evaporator and pumps it at the high temperature to the condenser. therefore
it is often referred to as a heat pump.
I 2. I
Classification of Compressors
The compressors may be classified in many ways, but the following are
important from the subject point of view:
l. According to the method of compression
(a) Reciprocating compressors.
(b) Rotary compressors, and
(c) Centrifugal compressors.
2. According to the number ofworlrinS strokes
(a) Single acting compressors, and
(b) Double acting compressors.
3. According to the number of stages
(a) Single stage (or single cylinder) compressors, and
(b) Multi-stage (or multi-cylinder)
compressors.
4. According to the method of drive employed
12-l
..-,,
,.
(a) Direct drive compressoni. and
(b) Belt drive compressors.
5. According to the location of the prime mover
(a) Semi-hermetic compressors (direct drive, motor and compressor in
separate
housings), and
(b) Hermetic compressors (direct drive, motor and compressor in
same
housings).
lL -2J^portant Terms
i
The following important terms, which
will be frequently used in
this
chapter, should be clearly understood at this stage:
l.
Suction pressure.It is the absolute pressure of refrigerant at the inlet of a
compressor.
2. Dischmge pressure.It is the absolute pressure of refrigerant at the outlet
of a compressor.
3. Compression ratio (or pressure ratio).
It
is the ratio of absolute discharge
to the absolute suction pressure. Since the absolute discharge pressure is
always more than the absolute suction pressure, therefore, the value of compression
pressure
ratio is more than unitY.
Note: The compression ratio may also be defined as the ratio of total
cylinder volume to the clearance volume.
4. Suction volume.It is the volume of refrigerant sucked by the compressor
during its suction shoke. It is usually denoted by v'.
5. Piston displacement volume or stroke volume or swept volume.It is the
volume swept by the piston when it moves from its top or inner dead position to
bottom or outer dead centre position. Mathematically, piston displacement volume or
stroke volume or swePt volume,
/2-Z
Tm.d
pro&
\_-orr*-s
- qElErg
/rFtlcdltu
Ilseqrrrd
'\-t6t
Various types of refrigeration compressors a) reciprocating b)
e) centrifugal
l2 -3
scroll
c)
dc
rotary d)
screw
Con&nsers
lL,J
ntroauction
The condenser is an important device used in the high pressune side of a
refrigeration system. Its function is to remove heat of the hot vapour refrigerant
discharged from the compressor. The hot vapour refrigerant consists
of the
heat
by the evaporator and the heat of compression added by the mechanical
energy of the compressor motor. The heat from the hot vapour refrigerant
in a
absorbed
condenser is removed first by transferring
it to the walls of the condenser tubes and
then from the tubes to the condensing or cooling medium. The cooling medium
may
be air or water or a combination of the two.
The selection of a condenser depends upon the capacity of the refrigerating
system, the type of refrigerant used and the type of cooling medium available.
12 rU \ilorking of a Condenser
The working of a condenser may be best understood by considering a simple
refrigerating system as shown in
I
tl
i
I
showing three stages
of a
Fis.ltf
(a). The corresponding p
refrigerirnt cooling
is shown
- h diagram
in Fig.fil (b) . The
compressor draws in the superheated vapour refrigerant that contains the heat it
absorbed in the evaporator. The compressor adds more bell (i. e. the heat of
compression) to the superheated vapour. This highly superheated vapour from the
compressor is pumped to the condenser through the discharge line. The condenser
cools the refrigerant in the following three stages :-
I
I
I
I
t2_4
I
w
=R"+w=l+
=l+
RE RE
RE COP
HRF= Q,
...
[...aot=*t)
W/
\.
From above, we see that the heat rejection factor depends upon
the
coefficient of performance (COP) which in turn depends upon the evaporator and
condenser temperatures.
In actual air-conditioning applications for R-12 and R-22, and operating at a
condenser temperature
of
40o
C and an evaporator temperature of 5o C, the
heat
rejection factor is about 1.25.
l?.- (-
Cussification of Condensers
According to the condensing medium used, the condensers are classified
into the following
three groups
l. Air cooled
:
condensers,
2. Water cooled condensers. and
3. Evaporative condensers.
These condensers are discusse4 in detail, in the following pages.
12
+
Air Cooled Condensers
An air-cooled condenser is one in which the removal of heat is done by air.
It consists of steel or copper tubing through which the refrigerant flows. The size of
tube usually ranges from 6 mm
of
condenser. Generally
to l8 mm outside diameter, depending upon the size
copper tube used because of its excellent heat transfer
ability. The condensers with steel tubes are- used in ammonia refrigerating systems.
The tubes are usually provided with plate type fins to increase the surface area for
heat transfer, as shown in
fig.lfl.Z The fins are usually made from
because of its light weight. The
aluminium
fin spacing is quite wide to reduce dust clogging.
The condensers with single row of tubing provides the most eflicient heat
transfer. This is because the air temperature rises at it passes through each row of
tubing. The temperature difference between the air and the vapour refrigerant
decreases
in each row of tubing and therefore each row becomes less
t2
-6
effective.
However, single row condensers require more space than multi-row condensers. The
single row condensers are usually used in small capacity refrigeration systems such
as domestic refrigerators. freezers, water coolers and room air conditioners.
f
igll.? air cooled
condenser.
The air cooled condensers may have two or more rows of tubing, but the
condensers
with up to six rows of fubing are common. Some condensers have seven
or eight rows. However more than eight rows of tubing are usually not efficient. This
is because the air temperature will be too close to the condenser temperature to
absorb any more heat after passing through eight rows of tubing.
Note: The main disadvantage of an air cooled condenser is that it operates at
a
higher condensing temperafure than
a water cooled condenser. The higher
condensing temperature causes the compressor to work more.
l2 ,8t "tno of Air-Cooled Condensers
Following are the two types of air-cooled condensers:
I. Natural convection oir-cooled condensers. In natural convection aircooled condenser, the heat transfer from the condenser coils to the air is by natural
convection. As the air comes in contact'with the warn condenser tubes,
it
absorbs
heat from the refrigerant and thus the temperature of air increases. The warm air
being lighter, rises up and the cold air from below rises to take away the heat from
the condenser. This cycle continues in natural convection air-cooled condensers.
Since the rate
of heat transfer in natural convection condenser is slower, therefore
they require a larger surface area as compared to forced convection condensers. The
/e-7
natural convection air-cooled condensers are used only in small capacity applications
such as domestic refrigerators, freezers, water coolers and room air- conditioners.
2. Forced convection air-cooled confunsers.In forced convection air-cooled
condenseN, the fan (either propeller or centrifugal) is used to force the air over the
condenser coils
to
increase
its heat transfer capacity. The forced convection
condensers may be divided into the following two groups:
(a) Base mounted air-cooled condensers, and (b) Remote air-cooled
condensers.
The base mounted air-cooled condensers, using propeller fans, are mounted
on the ,salne base of compressor, motor, receiver and other controls. The entire
ilrangement is called a condensing unit. ln small units, the compressor is belt driven
from the motor and the fan required to force the air through the condenser is mounted
on the shaft of the motor. The use of this type of compressor for indoor units is
limited up to 3 kW capacity motor only. These condensing units are used
packaged refrigeration systems
of l0 tonnes or less.
The remote air-cooled condensers are used on systems above
are available up
to
on
l0
tonnes and
125 tonnes. The systems above 125 tonnes usually have two or
more condensers. These condensers may be horizontal or vertical. They can be
located either outside or inside the building.
The remote condensers located outside the building can be mounted on a
foundation on the ground on the roof or on the side of a building away from the
walls. These condensers usually use propeller fans because they have low resistance
to air flow and free air discharge. They require 18 to 36 m3/min of air per tonne of
capacity. The propeller fans can move this volume of air as long as the resistance to
air flow is low. To prevent any resistance to air flow, the fan intake on vertical
outdoor condensers usually faces the prevailing winds. If this is not possible, the air
discharge side is usually covered with.a shield to deflect opposing winds.
The remote condensers located inside the building usually require duct work
to carry air l0 and from the unit. The duct work restricts air flow to and from the
condenser and causes high air pressure
d op. Therefore, inside condensers usually
/r4-g
use centrifugal fans which can move.the necessary volume
of air
against the
resistance to air flow.
I
l.
tQW rter Cooled Condensers
A water cooled condenser is one in which water is used
as the condensing
medium. They are always preferred where an adequate supply of clear inexpensive
water and means of water disposal are available. These condensers are commonly
used
in commercial and industrial refrigerating units. The water cooled
condensers
may use either of the following two water systems:
I. Waste water system, or 2. Recirculated water system.
vrlarr
d.far#
H5l60.L
Cart}rfi*r
rnis
rh*'*
'*ttir
fs ll{r|.
n|| lO l. rrood{ilfrtrqiltnsU*r
Figl$
l{5r#|||lfi
A|GTT'
Waler cooled condenser with recirculating water system.
In a waste water system, the water after circulating in the condenser
discharged to a sewer, as shown in
figfl.f
is
This system is used on small units and in
Iocations where large quantities of fresh inexpensive water and a sewer system large
/2 _q
enough to handle the waste water are available. The most common source
water supply is the city
of fresh
main.
ln a recirculated
,,
water system as shown in Fig.
&4
the same water
circulating in the condenser is cooled and used again and again. Thus this system
requires some type of water cooling device. The cooling water towers and spray
ponds are the most common cooling devices used in a recirculated water system. The
warm water from the condenser is led to the cooling tower where it is cooled by self
evaporation into a stream of air. The water pumps are used to circulate the water
through the system and then to the coolipg tower which is usually located on the
roof. Once a recirculated water system is filled with water, the only additional water
required is make-up water. The make Up water simply replaces the water that
evaporates from the cooling tower or spray pond.
Note: The water cooled condensers operate at a lower condensing
temperature than an air-cooled condenser. This is because the supply water
temperature is normally lower than the ambient air temperature, but the differeirce
between the condensing and cooling medium temperatures is normally the same (i.e.
14" C). Thus, the compressor for a water cooled condenser requires less power for
the same capacity.
12,.lbfypes of Water Cooled Condensers
The water cooled condensers are classified, according to their construction,
into the following three groups:
lC{tort
trtrryrt
lJsxo
{frleu.,r
tfcnrr
rigl/frube-in-tube
condenser.
,'
rig4l+6
,'12
-/a
Shell and coil condenser.
l. Tube -in-tube or double tube condensers.The
condenser, as shown
in
figt$l
consists
tube-in-tube or double tube
of a water tube inside a large refrigerant
tube. ln this type of condenser, the hot vapour refrigerant enters at the top of the
condenser. The water absorbs the heat from the refrigerant and the condensed liquid
refrigerant flows at the bottom. Since the refrigerant tubes are exposed to ambient
air, therefore some of the heat is also absorbed by ambient air by natural convection.
The cold water in the inner tubes may flow in either direction. When the
water enters at the bottom and flows in the direction opposite to the refrigerant, it is
said to be a counter-flow system. On the other hand, when the water enters at the top
and flows in the same direction as the refrigerant,
it is said to be a parallel flow
system.
The counter-flow system, as shown in
figJ&S
is preferred in all tlpes
of
water cooled condensers because it gives high rate of heat transfer. Since the coldest
water is used for final cooling ofthe liquid refrigerant and the warmest water absorbs
iti
heat from the hottest vapour refrigerant, therefore the temperature difference between
the water and refrigerant remains fairly constant throughout the condenser. In case of
parallel flow system, as the water and refrigerant flow in the same direction,
therefore the temperature difference between them increases. Thus the ability
of
water to absorb heat decreases at it passes through the condenser.
2. Shell and coil condensers. A shell and coil condenser, as shown in Fig.
4.10 consists of one or more water coils enclosed in a welded steel shell. Both the
finned and bare coil types are available.
The shell and coil condenser, may be either vertical (as shown in the figure)
or horizontal. ln this type ofcondenser, the hot vapour refrigerant enters at the top
the shell and surrounds the water coilsj As the vapour condenses,
it
of
drops to the
bottom of the shell which often serves as a receiver. Most vertical type shell and coil
condensers use countef
flow water system as it is more efficient than parallel flow
water system. In the shell and coil condensers, coiled tubing is free to expand and
conhact with temperature changes because of its spring action and can withstand any
strain caused by temperature changes. Since the water coils are enclosed in a welded
steel shell, therefore the mechanical cleaning of these coils is not possible. The coils
t2 -//
'
are cleaned
with chemicals. The shell and coil condensers are used for units upto 50
tonnes capacity.
3. Shell and tube condensers. The shell and tube condenser, as shown in Fig.
lfiJgconsists of a cylindrical
steel shell containinga number of straight water tubes.
The tubes are expanded into grooves in the tube sheet holes to form a vapour- tight
fit. The tube
sheets are welded to the shbll at both the ends. The removable water
boxes are bolted to the tube sheet at each end to facilitate cleaning of the condenser.
The intermediate supports are provided in the shell to avoid sagging of the tubes.
mt|f|rd
n
ilrttof
lic
Cffrt&L.r
th.l
r.n,j*la.r3
Fig{.6srreil and tube condenser.
The condenser tubes are made either from steel or copper, with or without
fins. The steel tubes without fins are usually used in ammonia refrigerating systems
because ammonia corrodes copper tubing.
In this type of condenser, the bot vapour refrigerant enters at the top of the
shell and condenses as it comes in contact with water fubes. The condensed liquid
refrigerant drops to the bottom of the shell which often serves as a receiver.
However, if the maximum storage capacity for the liquid refrigerant is less than the
total charge of the system, then a receiver of adequate capacity has to be added in
case the pump down
facility is to be provided as in ice plants, cold storages etc. ln
some condensers, extra rows of water fubes are provided at the lower end of the
condenser
for
sub-cooling
of the liq'uid refrigerant below the condensing
temperature.
| 2.ll
.
comparison of Air-cooled and water cooled condensers
12-{2
Following are the comparison.between air-cooled and water
cooled
condensers:
S.No.
I
2.
3.
Air-cooled condenser
Since the construction
of
Water cooled condenser
air
Since the construction of water cooled
cooled condenser is very simple,
condenser is complicated, therefore the
therefore the initial cost is less.
initial cost is high. The
The maintenance cost is also low.
cost is also high.
There
is no handling
problem
maintenance
The water cooled condensers are
with air cooled condensers.
difficult to handle.
The air cooled condensers do not
The pipes are required to take water to
require piping arrangement for
and from the condenser.
carrying the air.
4.
There is no problem in disposing
There is a problem
of used air.
used water unless a recirculatiqn system
of
disposing the
is provided.
5.
Since there
is no corrosion,
therefore fouling effect is low.
Since corrosion occurs inside the tubes
carrying the water, therefore fouling
effects are high.
6.
The air-cooled condensers
have
The water cooled condensers have high
low heat transfer capacity due to heat transfer capacity due
low thermal conductivity of air.
thermal conductivity of water.
/2_/t
to
high
7.
These condensers are used for These condensers are used
low capacity plants (less than
5
for
large
capacrty plants.
TR).
8.
Since the power required to drive
the
f-am
There is no fan noise.
is excessive, therefore, the
fan noise becomes objectionable.
9.
The distribution of air
on
condenser surface is not uniform.
10.
the condensing surface.
The air-cooled condensers have The water cooled condensers have low
high flexibility.
12.lf
There is even distribution of water on
flexibility.
Fouling Factor
The water used in water cooled condensers always contain a certain amount
of minerals and other foreign materials, depending upon its source. These materials
form deposits inside the condenser water tubes. This is called water fouling. The
deposits insulate the tubes, reduce their heat transfer rate and restricts the water flow.
The fouling factor is the reciprocal of heat transfer coefficient for the
material of scale.
The following are the recommended fouling factors:
I. For copp€r tubes used for R-12 and R-22 condensers, the fouling factor is
0.000 095 m2 s K/J.
/2-tq
17,-l7g:
These are devices used to lower the pressure from the high pressure value at the compressor,
condenser side to
the
a lower value at the evaporator side to help the refrigerant to evaporate
and absorb the required heat at the evaporator . There are two types of expansion devices that
are
:
a-Constant restriction devices : this type is used for small capacity and domestic equipments . lt
is a capillary tube with constant diameter and appropriate length .
b- Variable restriction expansion devices : These are of
two types ,the first one
is
called constant
pressure expansion valve and the second is a thermostatic expansion valve . The second type is
useful for varying load conditions
.
There exist other types such as orifice plate valves and float valves for other certain uses
Capllhry tube
Ps
Dlaphragm
I
t
lltf
ll
P"P
o
a
i
I
From
condenser
(a)
I
l
I
t-I
I
L
L
L
Thermostatic expansion valve
/2-/f
.
Q
hoAo
or Tl-T2:
where
T1 = Temperature of the refrigerant vapour condensing
= Temperature at the outside surface of the tube,
T2
tro
condensiq
film,
:
Coefficient
of
heat transfer for the refrigerant vapour
film, and
Ao:
Condensing area.
2.The heat transfer takes place from the outside surface to the inside surface
ofthe tube.
The value of this heat transfer is given by
n kA*(T2 -T3)
x
orT--Tt:*
Ox
...(ii)
kA,"(T2 -T3)
whereT3 = Temperature at the inside surface of the tube,
x:
Thickness of the tube,
k:
Thermal conductivity of the tube material, and
Ar = Meap surface area of the tubeEVAPORATORS
l}JtI
Bare Tube Coil EvaPoraters
The simplest type of evaporator is the bare tube coil evaporator, as shown in
Fis.w+
&xrfr ttr
lo+
eirc|el
.Eroeitrcn
'r {lftre
'fllornatl
Wlvh'
12-16
coil P(4f6urahr
'
The bare tube coil evaporators are also known as prime- surface
evaporators. Because of its simple construction, the bare tube coil is easy to clean
and defrost.
A little consideration will
show that this type
of evaporator
offers
relatively little surface contact area as compared to other types of coils. The amount
of surface area may be increased by simply extending the length of the tube, but
there are disadvantages of excessive tube length. The effective length of the tube is
limited by the capacity of expansion valve. If the tube is too long for the valve's
capacity, the liquid refrigerant will tend to completely vaporise early in its progress
through the tube, thus leading to excessive superheating at the outlet. The long tubes
will
also cause considerably greater pressure drop between the inlet and outlet of the
evaporator. This results in a reduced suction line pressure.
The diameter ofthe tube in relation to tube length may also be critical.
tube diameter is too large, the refrigerant velocity
refrigerant
will be too
will
Ifthe
be too low and the volume
of
great in relation to the surface area of the tube to allow
complete vaporisation. This, in turn, may allow liquid refrigerant to enter the suction
line with possible damage to the compressor (i.e., slugging). On the other hand, if the
diameter is too small, the pressure drop due to friction may be too high and will
reduce the system efficiency.
The bare tube coil evaporates may be used for any type of refrigeration
requirement. Its use is, however, limited to applications where the box temperafures
are under OoC and in liquid cooling, because the accumulation of ice or frost on these
evaporates has less effect on the heat transfer than on those equipped
bare tube coil evaporators are also extensively used
with fins. The
in house-hold refrigerators
because they are easier to keep clean.
| 2l
f
'Finned Evaporators
The finned evaporator, as shown in
[email protected],
over which the metal plates or fins are fastened.
12-t7
consists of bare tubes or coils
The metal fins are constructed of thin sheets of metal having good thermal
conductivity. The shape, size or spacing of the fins can be adapted to provide best
rate
of heat transfer for a given application. Since the fins greatly
increases the
contact surfaces for heat transfer, therefore the finned evaporators are also called
evaporators.
surface
extended
rig.Pfl
nig.nrf,
Finned evaporator.
Plate
evaporator.
The finned evaporators are primarily designed for air conditioning
applications where the refrigerator temperature is above
0t.
Because
of the rapid
it will defrost itself on the offcycle when the
temperature of the coil is near 0oC. A finned coil should never be allowed to frost
The air
because the accumulation of frost between the fins reduces the capacity.
heat transfer of the finned evaporator,
conditioning coils which operate at suction temperatures which are high enough so
that frosting never occurs, have fin spacing as small as 3 mm. The finned coils which
frost on the on cycle defrost on the offcycle have wider fin spacing.
|
2. Vt Phte EvaPorators
A common type of plate
evaporator is shown
evaporator, the coils are either welded on one side
in
fig.ltllo
In this type of
of a plate or between the two
plates which are welded together at the edges. The plate evaporators are generally
used in house-hold refrigerators, home freezers, beverage coolers, ice cream cabinets,
locker plants etc.
12,17
Shell and Tube Evaporators
/2-/g
The shell and tube evaporator, as shown in
tube condenser.
Figl9l@
is similar to a shell and
It consists of a number of horizontal tubes enclosed in a cylindrical
shell. The inlet and outlet headers with perforated metal tube sheets are connected at
each end of the tubes.
tll
lJ|illlr
Figfl.ld
Shell and tube evaporator.
These evaporators are generally used to chill water or brine solutions. When
it is operated
as a dry expansion evaporator, the refrigerant circulates through the
tubes and the liquid to be cooled fills the space around the tubes within the shell. The
dry expansion shell and tube evaporators are used for refrigerating units of 2 to 250
TR capacity. When it is operated as a flooded evaporator, the water or brine flows
through the tubes and the refrigerant circulates around the tubes. The flooded shell
and tube evaporators are used for refrigerating units
l2.f !
.'
of l0 to 5000 TR capacity.
sn"rl and Coil Evaporators
The shell and coil evaporators, as shown in
expansion evaporators to
fig.l?.lf,
are generally dry
chill water. The cooling coil is a continuous tube that
can
be in the form of a single or double spiral. The shell may be sealed or open. The
sealed shells are usually found
in shell and coil evaporators used to cool drinking
water. The evaporators having flanged shells are often used
to chill water in
secondary refr igeration systems.
Another type of shell and coil evaporator is shown in
of evaporators
ane usually used where small capacity
/2-/?
Figlt,lLBoth
types
Q to l0 TR) liquid cooling is
required. It may be noted that the shell and coil evaporator is restricted to operation
above 5"C in order to prevent the freezing problems.
qrssl
wilr
nigJtlff
Shell and coil evaporator.
figfiFt,
Shell and coil evaporator.
I 2. I q Tubs'in-Tube
or Double Tube Evaporators
The tube-in-tube evaporator (or double tube evaporator) as shown in Fig.
| lJNconsists
12- 2o
Asfrf
lsrd
f igpl,lJ.
Tube-in-tube or double tube evaporator.
of one tube inside another tube. The liquid to be cooled flows through the
'inner tube while the primary refrigerant
or secondary refrigerant (i.e. water, air or
brine) circulates in the space between the two tubes. The tube- in-tube evaporator
provides high heat fansfer rates. However, they require more space
than shell and
tube evaporators of the same capacity. These evaporators are used for wine
cooling
and in petroleum industry for chilling of oil.
ft
l6ntooded Evaporatons
ln a flooded evaporator, as shown in figft
level is always , maintained.
f{
a constant liquid refrigerant
A float
valve is used as an expansion device
",tnnot
which maintains constant liquid level in the evaporator. The liquid refrigerant
from
the receiver passes through a low side float
loorngtrnor
*
qstont*
flei",ffid
ltrf,d
ufin
|n|lhtrf
E{pmtr
ilafrril
Fooilorilr
Fhr
nfrd;
Figft I {
RooaeO evaporator.
/2-Zl
p
I
.
Cooling towers
:
This is an equipment
that be used to cool the water of the
VCRS
condensers or any other heat
exchange process . The tower cooled the water by spraying it in an air stream across several
layers of backing . These layers are used to increase the surface area of heat transfer and
to
break the water jets into small droplets for more contact .The evaporation of the droplet
surface leadstocool therestof waterandincreases the DBTandhumidityoftheoutgoingair
.There are several types of towers that are
:
;:
a- Natural cooling tower : The air is crossing the t6wer naturally by wind speed
b- Forced draft tower : The air is forced to the tower by fans
.
c-lnduced draft tower : The air is induced from the tower by fans.
Condanser
wutcr pump
/2-ZL
.