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Purdue e-Pubs
International Refrigeration and Air Conditioning
Conference
School of Mechanical Engineering
2000
Analysis of the Sub-Cooling on Refrigerating
Systems Using R-410A or R-404A
J. Benouali
Ecole des Mines de Paris
Y. S. Chang
Ecole des Mines de Paris
D. Clodic
Ecole des Mines de Paris
Follow this and additional works at: http://docs.lib.purdue.edu/iracc
Benouali, J.; Chang, Y. S.; and Clodic, D., "Analysis of the Sub-Cooling on Refrigerating Systems Using R-410A or R-404A" (2000).
International Refrigeration and Air Conditioning Conference. Paper 466.
http://docs.lib.purdue.edu/iracc/466
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ANALYSIS OF THE SUB-COOLING ON REFRIGERATING SYSTEMS
USING R-410A OR R-404A
Jugurtha Benouali, Young Soo Chang and Denis Clodic
Ecole des Mines de Paris, Centre d'energetique
60, boulevard Saint-Michel - F 75272 Paris Cedex 06
ABSTRACT
Both R-41 OA and R-404A present low critical temperatures, in the range of 72.5°C. Consequently, the
refrigerating capacity and the energy efficiency of the system decrease substantially when condensing
temperature raises. Sub-cooling, either by external group or by superfeed, improves drastically the
refrigerating capacity and the coefficient of performance.
Comparisons have been established for two evaporating temperatures (-40°C and +2°C) and
condensing temperatures from +20°C to 60°C. Simulations have been performed over a one-year period,
for two different climatic conditions on systems including floating high pressure strategy.
1. INTRODUCTION
Various strategies to sub-cool the refrigerant flowing out of the condenser were developed during the
past. They are particularly useful for refrigerants having a low critical temperature. For example R-410A
and R-404A have both a critical temperature near 72.5°C.
When condensing temperature increases, power consumption increases too, but the refrigerating
capacity decreases. This effect is penalizing, because it is not consistent with climatic conditions:
refrigerating capacity lowers when refrigerating needs increase. To limit these effects, many propositions
were made [1 , 2, 3].
Two strategies of sub-cooling are presented: the "superfeed" method, and external group. In both
cases the COP and refrigerating capacity figures will be presented for various evaporating and
condensing temperatures, and on a year basis for two French climatic conditions.
2. SUBCOOLING WITH "SUPERFEED" SYSTEM
2.1 System Layout
For low temperature applications (-40°C) and refrigerating capacity in the range of 500 kW, it is usual
to find two-stage or "superfeed" systems. The "superfeed" technology is not adapted to reciprocating
compressors but it is very much used for screw compressors. Fig. 1.a presents a typical arrangement of
"superfeed" system.
•
•
At the condenser outlet, the refrigerant flow is divided in two:
the main flow passes in a heat exchanger located in the "superfeed" capacity at the intermediate
pressure of the two-stage cycle;
the secondary flow is expanded in this capacity, the evaporated refrigerant is sucked at the
intermediate pressure of the compressor.
This arrangement is only possible with screw (or Scroll) compressors where the pressure rises along
the gas flow inside the screw. It is thus possible to inject a refrigerant flow at an intermediate pressure.
This flow will be partially compressed from this intermediate pressure to the discharge pressure whereas
the main flow leaving the evaporator is compressed from the low pressure to the high pressure of the
cycle. Fig. 1a and 1b show respectively the superfeed system layout and the h-ln p diagram of the cycle.
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28, 2000
63
lnp
h
Fig.1.b - h-ln p diagram
Fig.1.a - Superfeed system layout
2.2 Calculation Method
For modeling, the intermediate pressure is fixed such as the compression ratios Pi/Pev and PK/Pi are
~ ~ • ~v
Pi =
equal, which allows the calculation of the intermediate pressure according to:
Note: the intermediate pressure Pi of the superfeed system is fixed at a lower level than the one resulting
from the previous formula, but the results in terms of refrigerant capacity and COP are quite similar.
Parametric study is performed while varying the efficiency of the heat exchanger located in the superfeed
capacity for several series of evaporating and condensing temperatures. The heat exchanger efficiency
varies from 0 to 90%. From there, the derived refrigerant mass flow rate also varies to provide the subcooling capacity which thus depends on the heat-transfer surface.
2.3 Results
•
•
For each evaporating temperature, -40°C and +2°C, two figures are presented:
variation of the refrigerating capacity referred to the single stage system (-40°C: Fig. 2.a and +2°C:
Fig. 3.a),
variation of the relative COP also referred to the COP of the single stage system (-40°C: Fig. 2.b and
+2°C: Fig. 3.b).
1,7
z-
1, 35
"()
1,6
"'a.
"'
1,5
0
0>
/
1,4
c
~
;;-;
/
1, 3
/
0>
~.,
1,2
>
1,1
~
.,
x' /x'
£~
lp;!yrr
1,3
/
~ 1,25
_x
_....x-""
~
0
1,2
g!
-0
- o - - 30
-X·
-40
'iii
q;
a:
1,15
-so
1,1
X'
x-"x ~
h'~
...-a
--
-x--40
-so
0,2
0,4
0,6
0,8
1
0,2
0
0,4
Efficiency
0,8
0,6
Fig. 2.b
1, 3
1,14
a. 1,25
1,12
,.,...
"()
0
0>
c
~Q)
0>
~
-~
1ii
q;
a:
a.
1,2
0
1,15
1,1
0
1,08
-~
1,06
1ii
1,1
1,02
1
1
0,4
0,6
0,8
1
Efficiency
Fig. 3.a
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28, 2000
/
/
__x--
L---x·~
a:
1,()5
/
L
q; 1,04
0,2
1
Efficiency
Fig.2.a-
"'
"'
- o - 30
1
0
z-
--x
.:x-
....x-
1,05 ~~
1
a:
L
L
L
/
..x'
--x
__,..-...
0,2
---D--30
-
.68~
0
.:X
X·-4Q
__.__so
0,4
0,6
0,8
1
Efficiency
Fig.3.b
64
The higher the condensing temperature, the higher the cooling capacity. For a sub-cooling heat
exchanger the efficiency is 90% and for condensing temperatures of 30°C and 50°C, the cooling capacity
gain varies between 30% and 60%. When the condensing temperature rises to 65°C, the cooling
capacity gain is about 70% and the COP gain reaches 32%. The COP improvement is significant but
relatively less significant than the one in refrigerating capacity. This is due to the dominating role played
by the irreversibilities associated with the isenthalpic expansion, particularly when the condensing
temperature is high.
For an evaporating temperature of +2°C, the same tendencies are highlighted. The gain in terms of
refrigerating capacity or COP is reduced comparatively with the one obtained at -40°C, because the
temperature difference between source and sink is reduced (respectively 25% in refrigerating capacity
and 12% in COP with condensing temperature of 50°C}. However these gains are not negligible when
the condensing temperature approaches 50°C.
So, "superfeed" sub-cooling for systems using R-41 OA implies significant gains in terms of
refrigerating capacity and COP for high condensing temperatures. This is more significant as evaporating
temperature is low. This solution can be recommended for low temperatures but also for all evaporating
temperatures when the condensing temperature is equal or higher than 50°C. These conclusions are
also verified for systems operating with R-404A.
3. SUBCOOLING WITH AN EXTERNAL GROUP
3.1 Operation Principle
One of the methods used to reduce the energy consumption (or to improve the coefficient of
performance) of refrigerating systems working with low evaporating temperature, is to sub-cool the
refrigerant flowing out of the condenser using an auxiliary refrigerating cycle (also called mechanical subcooler) [1 ]. This method is known to yield energy savings, and to increase significantly the main system
refrigerating capacity, while necessary modifications are simple and non-expensive compared to new
system purchase [2, 3]. Figures 4.a and 4.b presents respectively layout of a system equiped with an
auxiliary group and the main loop cycle diagram.
Enthalpy.h
Fig. 4.a- Subcooling with external group
Fig. 4.b : Cycle of the principal loop
PMc
: input power for main and additional compressors
QRMc
: refrigerating capacity
T cond, T ev: refrigerant condensing and evaporating temperatures.
Calculation Assumptions
For a system sub-cooled by an external group, the mechanical power supplied to the system is the
sum of the power supplied to the compressors of both the main and auxiliary systems.
PMc = PMc_Main + PMc Auxiliary
The usual refrigerating QRMC is the one of the main system.
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28,2000
65
The higher the difference between Tcond and Tev. the lower QRMc and the energy efficiency of the
system. Using the refrigerating cycle COP allows to compare performances with different Tev and TcondCOPs are defined as follows:
COP= QRMC
PMc , COPcamot
•
Tev
and CEF =COP
Tcond-Tev
·(Teo/Ill- Tev)
T,v
COP , CEF : cycle efficiency.
COPCamot
Sub-cooling permits to achieve two goals:
reduction of the isenthalpic expansion irreversibilities (see Fig. 4.a and 4.b),
increase in the refrigerating capacity.
Notable CEF decrease is observed when Tev is low, for a given temperature Tcond , because the
difference between source and sink decreases.
The variation between the external group evaporating temperature and the sub-cooled liquid
(pinching) is fixed at 5 K.
3.2 Results For A Sub-cooling Auxiliary System Using R-134a
For each evaporating temperature, -40°C and +2°C, two figures are presented:
• variation of relative refrigerating capacity (see Fig. S.a and 6.a),
• variation of the relative coefficient of performance (see Fig. S.b and 6.b).
Evolution of the relative refrigerating
capacity Tev
4o•c
2,20
Evolution of relative COP Tev =· 4o•c
=-
,g"'
2,00
8.~
u
1,80
~
J
1,60 ,
'lii ..
a.
~
_ _ _----:.,..~71'-~-------1
.8~~ 1,30~
I .;;:.:.c~•cond
~~I
-
x-x-x-x.
1,10
1,00
l~·w·~-·· II
t-#.J5C
~~
0
I
I
I
20
40
60
Fig. S.b
Evolution of relative refrigerating capacity
.
Evolution of the relative COP
1,60
2,00 +----------------:-~-----\
+---------------,-=4'0-----1
~ ~ 1,60 --1-------=--~~~------1
; ; a..
>
:;
iii
a:
1.40 +-----,..!'~A~~----- -------1
-1----.--------,-------_;==:=::r'=..
20
40
S ubcoollng (K)
60
1,40
ll.
0
0
1,30
~
1,20
:;
a:
1,00 ...=--------------1·
0
1,50
iii
1 ,20 -t---...:>4~F"'.._________-I
o.ao
I
80
Subcooling (K)
2,20
1,80
3oll
0:
+--.K.-~
Fig.S.a
0)
\;:
1,20
1,00 F----.---~--____!;~~~~~
0
80
20
40
60
Subcoollng (K)
i
I
~<J
1,40
'i:
~ 3 1,40
n;
iii
1 ,20
a:
1.60
1,50
80
Fig. 6.a
~Tcondc30
-
)t-
-
T cond ..40
____...,.__ r cond ==50
- t - - f c o n d .. QQ
1,10
1,00
0.90
O,BO
0
20
40
60
80
Subcoollng (K)
Fig. 6.b
The COP increases up to an optimum according to the sub-cooling level. For each evaporating
temperature, the optimum is "drifting" to the left as condensing temperature decreases. Conversely, the
higher the condensing temperature, the higher the COP gain. Finally the lower the evaporating
temperature, and the higher the source/sink temperature difference, the higher the energy gain
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28,2000
66
associated with sub-cooling. This strategy is more than interesting for hot climates and it is even
advantageous for moderate ones. For an evaporating temperature of -40°C, the sub-cooling optimal
range is between 40 and 50K, whereas for +2°C, it is between 10 and 25 K.
The higher the condensing temperature, the higher the refrigerating capacity gain. Moreover, the
relative increase in the refrigerating capacity is linear according to sub-cooling.
The auxiliary group overcast can be extremely low since sub-cooling limits the over-sizing of the main
group that is always designed for high thermal loads.
Influence of the evaporating temperature level
For a single-stage system, over a large variation in temperatures (-40/+40°C) the relative additional
consumption of the auxiliary group is low and its COP is high. On the contrary, for the main system with
a small variation in temperatures between source and sink (+2/30°C), the main system COP is almost as
high as that of the auxiliary group, the global COP improvement is thus very limited.
4. R-410A AND R-404A PERFORMANCES REFERRED TO R-22
COP and refrigerating capacity of a system using R-22 were compared systematically to those of a
system using R-410A and R-404A [4].
COPR-404A , COPR_410 A for COP et PFR-404A , PFR-410A for the refrigerating capacities.
COPR-22
COPR-22
PFR-22
PFR-22
Performances of the system using R-22 are evaluated with the same sub-cooling as for R-404A and
R-41 OA cycles.
4.1 COP Evolution
Figures 7.a to B.b indicate that in all cases the use of R-410A and R-404A involves a performance
decrease of the total system since the relationship between COPs is always lower than 1.
In both cases, sub-cooling allows to reduce the performance difference between the system using
R-22 and cycles using R-410A and R-404A. Optimum of COPR-404A and COPR-4IOA exist for a given subCOPR-n
COPR_22
cooling that depends on evaporating and condensing temperatures.
•
--40°C evaporating temperature
Evolution of the ratio COP _410AICOP _221or Tev = ·40"C
..
Evolullon of the raUon COP _404A/COP _22 lor Tev
= 40"C
-:··
...,..~
~.0~~:·~~
& 0.•-jL--------------!
"8• °:: =------------l!=:::=-~=~==1
~ 0.75t-----~--------~
0,7t===========~~=~
0.55j-----___;__ _ _ _ _ _t=I·-+--~T~=""~·=..:I
30
subcoollng (K)
SuiJcoollng (K)
Fig. 7.a
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28, 2000
Fig. 7.b
67
•
+2°C evaporating temperature
Evolution of the ratio COP_ 410AICOP_22 for Tev
~
.__________ _______
Evolullon of tho ration COP _404A/COP _22 lor Tav .. +2•c
+2°C
,
~~
0,05
.::Fv____________
J!
~
---1
8 ···t--------------j
;, . 7t===========F~~
0,75+---------------j
~
8
Fig. 8.a
•
•
•
0.86
J-o-~:::::.3o[l
f-------------J-
J+o -Tcom~-50
1-t--Tcond•OO
L--------~su~~~·~Hng~(~Kl_ _ _ _ _ _ _ _ _ _J
Fig. 8.b '--------"-""_••_•'_'""-1-"1- - - - - - - - - - '
Performances variation according to evaporating and condensing temperatures:
for a given evaporating temperature, the lower the condensing temperature, the lower the subcooling necessary to reach the optimum;
the lower the evaporating temperature, the higher the COP gain;
the system using R-410A presents a better energy performance compared to the R-404A cycle;
moreover sub-cooling reduces significantly the performance variation between the R-22 and the two
blends, particularly for high condensing temperatures.
4.2 Evolution Of The Refrigerating Capacity {see Fig. 9.a to 1O.b)
Using sub-cooling with an external group makes possible to increase the refrigerating capacity of
systems using R-410A or R-404A compared to R-22 cycle. This evolution is quasi linear according to
sub-cooling. With regard to R-404A, it appears that without sub-cooling, the refrigerating capacity
supplied by the system is lower than the one available with R-22.
For a given installation, the use of R-410A allows 60% increase in-the refrigerating capacity compared
to R-22 system.
For R-404A, sub-cooling allows to increase the ratio of cooling capacity compared to R-22.
- 40°C evaporating temperature
,---------------------.
Evolution of the ration between refrigerating copoclly for R-
.--------------------~-.
Evolution of the ration between refrigerating capacity tor R-410A
and R-22 forT ev~cooc
4D4A and R-22 for Tov::::-4o•c
1,7 -·--..---
'·'+----------------------
~1,5~
.._. 1,3
r---------------------1
~·
~
1,1
t--------------------1
~
0,9
i--------------------1::-=~=::
;I
=j~~T--30~
0,7
Fig.
"',
~
_--+-Tan:bOO.
0,5 +-----~----,----~--'
0
20
40
60
8
9.a'-------------••_••_••_••_•<_~----------~
1,3+-----------------------------1
"'(!;'·'
~ --~-
~-~
__..o-
_.-
--~--~
_lf"--~-----
0.1/,.. ........-0,9 ...-
..
" ......--o--Tconol•30
l_.._rconot·•o
1. -x--Icood•SO
~
'"
Fig. 9.bL-____________s_ub_oo_ol_lng~(~K)~------------~
+2°C evaporating temperature
,---------------------------~~
r------------------------------,
EvoluUon of tho ration betwaon refrigerating capaolty for R·
404A and R-22 for Tav•+2GC
Evolution or the ration batwoon ralrlgorat/ng capacity for R410A and n~22 for Tov::::.+2°C
--------1
'·'+---------
I
i
1,:5;::=-:x-~
11,'·'~
---------------1
~ ···t-------------il
"· ·-·1-----------------------r==:r;.
It
..;:;;JTI
!:......_,:;;;;;
..
:..
+-------------------1[_
Fig~
1O.a
30 .
L __ _ _ _ _ _ _ _
M--:::::
I
__.:•c:.c••_••.....:•":c.:"":..::'".::.,_ _ _ _ _ _ _ _____,
Fig. 1O.b '------------=·=·bo=•=oi""""-"''K,_)________....J
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28, 2000
68
5. COP EVALUATION AND ANNUAL COOLING CAPACITY ACCORDING TO CLIMATIC DATAS
Two French cities were selected to perform evaluations: Trappes and Nice. For each one, the
temperature curves according to the number of hours in the year was used (see Fig. 11 ).
Nice
1200 ------------
0
-"
·------ .,
A
1000
,
Trappes
····-· ··----··-
--------
!DO
r-'
I
000
400
I
J
\
1
200
i
\_
I
"-..
'
I
L
0
0
'-,.
5
15
10
20
j
I
'\
30
25
I
~j A~~.
0
-10
30
0
10
30
20
40
ToO
Totr
F1g. 11 :Temperatures 1n N1ce and Trappes
Assumption
The condensing temperature is fixed according to the external air temperature and using the formula
TCondensing = T Air + 8 K + 2 K subcooling
This formula implies that energy gains can be added with those associated with the high floating
pressure. This assumption is used to estimate the energy savings obtained with the auxiliary system.
For condensing temperature, a 20°C threshold was selected because it corresponds to the limit used
with the high floating pressure strategy. The condensing temperature is calculated based on the air
temperatures for each site. Then the annual frequency of these temperatures is taken into account for
energy consumption calculation. Air temperatures higher than 1ooc exist 78% of the year in Nice and
53% in Trappes.
Evolution ol relative COP lor Nice
(annual averege)
~--0<-----
Evolution ol relative COP for Trappes
(annual average)
"'--
8"·"
~
lu
~
!!;"
---
'
....~
&·~1-------~---------i
"" ... t--~~-----r
____ l----:::::;;=:-~-~:::::J
•-•t-----------
~
0
L..~----~~--r==~
I ~,~-«<'C~~
-w--Tev-15'C
02+--------------------jJ
-+--Tevo-2"C
..
- - · . ,.......c
30
Subcoollng (K)
Subcoollng (K)
Fig. 12.a
Fig. 12.b
Evolution of relative cooling capacity for Nice
(annual average)
Evolution of relative cooling capacity for
Trappes (annual average)
1,6 , - - - - - - - - - - - - - - - - - - - - - - - - ,
.;::- 1,5
·c;
..
+------------~~~-----{
~ 1,4
u
~
1,3
+--------------,.,.-__..,..."-----------j
:; 1,2 -1---------..:4~=-------------l-:::::;;:::~:-;;o:c]
Oi
a:
0
10
20
30
40
50
1,1
60
+--------""""'"-----------{
0
10
20
30
40
Subcoollng (K)
Subcoollng (K)
Fig. 13.a
Fig. 13.b
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28, 2000
50
60
69
Figures 13.a and 13.b present identical relative cooling capacity gains for Trappes and Nice. These
range between 10 and 30% for the optimal sub-cooling associated with the three selected evaporating
temperatures.
For an evaporating temperature of -40°C, the annual average energy gains (see Fig. 7.1 a and b) are
about 20% for the climatic conditions of Nice and Trappes. These gains are obtained for systems
already equipped with the high floating pressure and a minimal condensing temperature of 20°C. This
COP improvement is obtained for a sub-cooling of 30K. The refrigerating capacity gain is then about
18%. Such a gain allows the modification of the initial system design, which probably makes it possible to
directly limit the auxiliary group overcast Savings in term of energy efficiency and refrigerating capacity
for an evaporating temperature of -15°C follow the same tendencies. On the other hand the gain in
terms of energy savings is equal to 8% but the refrigerating capacity gain is about 20%. These results
are obtained for a sub-cooling of 15K.
For a 2oc evaporating temperature, it is necessary to distinguish systems equiped with either
"superfeed" or auxiliary group. For "superfeed" systems, the average COP gain is about 5% and the
refrigerating capacity gain is about 10%. On the other hand, if an auxiliary group is used, it is more
judicious to make it running only for condensing temperatures higher or equal to 30°C. The major gain is
obtained for the refrigerating capacity, which is around 20% for sub-cooling of 20K.
7. CONCLUSIONS
Sub-cooling with an auxiliary group for reciprocating systems or by "superfeed" capacity for screw or
Scroll compressors presents the same tendencies. The "superfeed" system permits to obtain energy
gains slightly higher than sub-cooling using an auxiliary group, because the compressor output remains
high for various evaporating and condensing temperatures. Sub-cooling can be used systematically for
-40°C and -15°C evaporating temperatures and for all condensing temperatures. For a +2°C evaporating
temperature sub-cooling is interesting when condensing temperature is high (up to 50°C).
8. ACKNOWLEDGEMENT
The results presented in this paper are issued from a study funded by the French Utility (Eiectricite de
France, R&D Department).
9. REFERENCES
[1] Couvillion R.J., Larson M.W., Sommerville M.H., Analysis of a vapor-compression
refrigeration system with mechanical subcooling. ASHRAE Trans., 1988, Vol. 94(2), p.
[2]
[3]
[4]
641-659.
Thoronton J.W ., Klein S.A., Mitchell J.W ., Dedicated mechanical subcooling design strategies for
supermarket applications. Int. Journal of Refrigeration. 1994, Vol. 17, no. 8, p. 508-515.
Boiarski M., Podcherniaev 0., Yudin B., Giguere D., Enhancement of supermarket freezers to
reduce energy consumption and increase refrigeration capacity. 1998 International Refrigeration
Conference, Purdue University, West Lafayette, Ind. USA. p271-276.
Claux M., Clodic D.- Calcul des performances des fluides frigorigenes remplacyant du HCFC 22 pour
differentes machines frigorifiques (Calculation of performances of R-22 alternative refrigerants used
in different refrigerating systems). Center for Energy Studies, Ecole des Mines de Paris- 11/1998.
Eighth International Refrigeration Conference at
Purdue University, West Lafayette, IN, USA- July 25-28, 2000
70