CHAPTER 6 WASTE HEAT RECOVERY DEVICES
PART-I Heat Pipe
6.1 Introduction
A heat pipe is a novel device that can transfer large quantities of heat through a
small area with small temperature difference. Grover, Cotton and Erickson of Los
Alamos National laboratory are the pioneers in construction of heat pipes. The
first heat pipe was constructed by them in 1964. They constructed 3 (Three) heat
pipes, 1 with water as medium and 2 with sodium.
Initial efforts were directed towards space heat applications. Major factors are
high reliability, its capability to work under weightless condition in space as well
as workability under isothermal conditions without the need for any external
power input.
In recent years it is realized that heat pipes are equally important for application
to earth as well as space applications. One may wonder why heat pipe concept
was not given serious importance in spite of its simplicity and exceptional heat
transfer capability without external power. Reasons being:
(1) Surface tension force on which capillarity depends is weak. It was not
established that strong capillary forces are possible to develop. With
advancement of material science lead to the development of sustainable
capillary force in a fine-pored structure now.
(2) Prior to the development of space programme, there really was no
compelling need for a heat transport device based on capillary
recirculation. Advent of space programme, however, created the need for
a heat transport device for use in space power system under weightless
condition.
A heat pipe is basically a sealed container normally in the form of a tube
containing wick lining in the inside wall. It is used to transport heat from source to
sink by means of evaporation and condensation of a fluid in the sealed system.
Construction- Sealed tube, inside is lined with a weak of porous matrix
Components- (1) An evaporator section with length Le , (2) A condenser section
of length Lc and (3) a section with or without insulation, connecting the
evaporator and condenser. This section is not essential one.
Process- Evaporation and condensation. High heat transfer with relatively small
surface area
Power- No external power requirement
Environmental compatibility- Environment friendly, no noise and pollution
Transporting force- By capillary action
130
Fig. 6.1 Schematic diagram of a heat pipe
6.2 Thermal conductivity of a heat pipe
Since heat pipe transports heat by the process of evaporation and condensation,
the heat pipe can transfer heat much more effectively than any solid of same
cross section by conduction.
131
(b)
(a)
Fig 6.2 (a) Heat flow by conduction in a copper rod, (b) Heat flow
through a heat pipe of same length
The thermal conductivity of a heat pipe may be 500 times more than the best
available metallic conductor. To remove the localized heat, heat pipe is the most
effective and most widely used device. Heat pipe is also called super thermal
conductor.
6.3 Characteristics of heat pipe
1.
2.
3.
4.
5.
6.
7.
No moving parts
Requires no external energy
Reversible in operation
Completely silent
Very reliable
Rugged like any piece of pipe or tube
Can withstand a lot of abuse
Because of the reliance on capillary action to return the condensate, heat pipe is
particularly sensitive to the effect of gravity and its inclination to the horizon. Fig.
6.3a and 6.3b present two different configurations of a heat pipes. In Fig 6.3a,
return of condensate is aided by gravity whereas in Fig. 6.3b return of
condensate is against gravity. Hence arrangement in Fig.6.3b is preferred.
Gravity aids the return of condensate, the wick may be omitted. Under the
circumstances, the device is called a thermo-siphon.
132
Fig. 6.3a Along gravity
Fig. 6.3 b Against gravity
6.4 Application of heat pipe
1. Electronics- cooling of electronic circuits
2. Electrical motors, generators and transformers
3. Application in HVAC systems
4. Application in IC engines
5. Gas turbine regenerators
6.4.1 Application to production process
Transformers are large in size because they need a large surface area to
dissipate heat generated. Heat pipes can be inserted into the transformer core to
dissipate that heat and reduce transformer size substantially.
Heat pipe can be used to dissipate heat from high voltage electronic devices,
like, T.V., motor, starter and armature.
6.4.2 HVAC application
In winter, outside make up air may enter the condenser of heat pipe and room
can be heated upto 220 C from heat received at 120 C ( Fig.6.4 )
133
Fig. 6.4 Heat pipe applications to HVAC.
6.4.3 Gas Turbine Regenerator
Heat recovery from high temperature exhaust to preheat the incoming air-fuel
mixture of a gas turbine is always energy efficient. Fig. 6.5 shows a lay out of the
heat recovery from gas turbine regenerator.
Fig. 6.5 Heat recovery from gas turbine regenerator
134
6.4.4 Application to I C Engine (VAPIPE)
One of the most interesting application of heat pipe in I C Engines is known as
VAPIPE. When fitted to a car engine, it significantly reduces both the fuel
consumption and exhaust emission by using heat extracted from exhaust
emission by using the heat extracted from exhaust gas for vaporizing petrol
mixture from carburetor before it enters the engine. The vaporized mixture (petrol
+ air) makes a homogeneous mixture and improves combustion.
Fig. 6.6 VAPIPE
6.4.5 Other applications
1.
2.
3.
4.
5.
6.
7.
8.
Solar collectors, space applications, snow melting
Biscuit and bread ovens
Laundries
Pharmaceuticals
Spray drying
Welding booths
Brick kilns
Plastic lamination drying, extrusion of plastic materials. Temperature
uniformity can be maintained in texturising fibres.
9. Annealing furnaces
10. Chemical fluid bed dehumidifier
11. Epoxy coating
12. Curing oven, etc.
6.5 Limitations of heat pipe
There are 4 (four) limitations with heat pipe as shown in Fig. 6.7
(1) Sonic limitation (1-2)
135
(2) Entrainment limitation (2-3)
(3) Wicking limitation (3-4)
(4) Boiling limitation (4-5)
Fig. 6.7 Limitations of heat pipe (axial heat flux vs. temperature)
6.5.1 Sonic Limit
In a heat pipe, new vapour continuously being added to the vapour space along
the evaporator length. The vapour velocity then increases with evaporator length,
reaching a maximum at evaporator exit. As the heat transport rate and hence the
heat generation rate increases, the exit velocity becomes higher. When the exit
velocity reaches the sonic value, the vapour flow is said to be choked. The heat
transport rate at which vapour velocity becomes sonic is called sonic limit. At
that point, no further increase in vapour flow or heat transport is possible without
an increase in vapour temperature.
The sonic limit is expressed as an axial heat flux (heat transfer/vapour cross
sectional area).
Heat transfer rate is
.
.
Q = mv h fg = ρ vVAav h fg
Axial heat flux is
136
(6.1)
(6.2)
.
Q
= ρ vVh fg
Av
.
Q
Now fixing
by adjusting condenser to lower pressure, temperature and
Av
density, velocity of vapour goes to sonic limit in the evaporator exit. Once sonic
limit is achieved, any further change in parameter in condenser will not effect the
evaporation. Flow will be chocked. Thus vapour velocity should be well below the
sonic limit. The sonic limit is indicated by the curve 1-2 in Fig. 6.7.
Although heat pipes are normally not operated at sonic limit, during start-up, it
may occur when the temperature of the evaporator inlet are higher than those at
the evaporator exit.
6.5.2 Entrainment Limitation
If the vapour density is allowed to increase without an accompanying decrease in
velocity, some liquid from the wick return may be entrained, the onset of which
may be expressed by Weber number ( Wb ) given by
ρvV 2 Lc
Wb = Inertia force/Surface tension force=
=1
2πσ l
or,
2πσ l
V=
ρv Lc
(6.3)
1/ 2
where Lc is the characteristics length describing the pore size.
If, Wb > 1 , fluid circulation increases until the liquid return path can not
accommodate the increased flow. This causes draught and overheating of the
evaporator.
The entrainment limit can be estimated from
1/ 2
1/ 2
2πσ l
2πσ l ρv
Q
= ρ vVh fg = ρ v h fg
= h fg
Av
ρv Lc
Lc
which is represented by the curve between 2-3 in Fig. 6.7.
(6.4)
6.5.3 Wicking Limitation
Fluid circulation in a heat pipe is maintained by capillary forces that develop in
the wick structure at liquid-vapor interface. When a typical meniscus is
137
characterized by two principal radii of curvatures r1 and r2 , the pressure drop
across the liquid surface is
(6.5)
1 1
∆pc = σ
+
r1 r2
If the liquid wets the wick perfectly, the radii will be defined by the pore size of the
wick, which fixes the limit of heat transfer. Any further increase in heat transfer
will cause the liquid to retreat into the wick, and dry out and overheating will
occur at the evaporator end. The capillary force can be increased by decreasing
the pore size of the wick, as shown by Poiseulle equation
.
(6.6)
8π ml L
∆pl =
π r4ρ
The wicking limitation is represented by the solid line 3-4 in Fig. 6.7.
6.5.4 Boiling limitation
Formation of vapour bubble is undesirable because they could cause hot spots
and destroy the action of the wick. Therefore, heat pipes are heated isothermally
before being used to allow the liquid to wet the inner heat pipe wall and to fill all
but the smallest nucleation sites.
Boiling may occur at high input heat fluxes, and high operating temperatures.
The curve between 4-5 in Fig. 6.7 is based on the equations
pv − pl =
2σ
r
Q k (Tw − Tv )
=
A
t
where pv = vapor pressure inside the bubble
pl = liquid pressure outside the bubble
r = radius of the largest nucleation site
A = heat input area
k = thermal conductivity of the saturated wick
Tw = temperature at the inside wall
Tv = temperature at liquid vapour interface
t = thickness of first layer of the wick
138
(6.7)
(6.8)
If r is small at the nucleation site, ∆p has to be large for bubbles to grow. The
above two equations show the various factors which influence the boiling. Boiling
is, however, not a limitation with liquid metals, but when water is used as the
working fluid, boiling may be a major heat transfer limitation because k of water
is low and it does not readily fill the nucleation sites.
6.6 Pressure drop analysis for 1-D incompressible flow in a heat pipe
An incompressible fluid with density ρ flows through a circular tube segment of
diameter D and length dx .The tube is oriented at an angle β with the vertical.
The tube is also subjected to an external acceleration ng directed at an angle
α with the tube axis, where g is the acceleration due to gravity.
Fig. 6.8
.
Fluid enters the tube at an average velocity V , mass flow rate is m and pressure
p . Fluid is also added radially through the wall of the tube segment at velocity
.
.
Vr . The fluid leaves the tube segment at velocity V + dV , mass flow rate m + d m
and pressure p − dp . Within the segment, a shear stress τ s acts at the wall in a
direction opposite to that of the fluid flow.
Applying conservation of mass
ρ
π
4
D 2 + ρVr (π D dx ) = ρ (V + dV )
139
π
4
D2
(6.9)
or,
(6.10)
4Vr
dx
D
dV =
Eq. (6.10 ) relates the increment in axial velocity to the radial inlet velocity.
Conservation of momentum:
Let, Ω = Total momentum flow rate/Momentum flow rate based on
average velocity
Now,
.
.
.
Ω m + d m (V + dV ) = Ω mV +
−τ sπ D dx +
π
4
π
4
D2 p −
π
4
D 2 ( p − dp )
(6.11)
D 2 [ ρ ng cos α − ρ g cos β ] dx
.
Assuming, d m .dV = 0
We get,
(6.12)
4
τ s dx + 2ΩρVdV + ( cos β − n cos α ) ρ gdx
D
Upon integrating equation over the tube length, (x=0 to x=L) and assuming Ω to
be constant,
L
(6.13)
4
∆p =
τ s dx + Ωρ V02 − Vi 2 + ( cos β − n cos α ) ρ gL = Frictional shear +
D
0
Pressure drop due to momentum+Pressure drop due to
gravity/acceleration
dp =
(
)
From Eq. (6.13 ),
Vapour: O ( friction+momentum) >> O (gravity)
∴∆pv = Pressure drop due to friction and momentum
From Eq. (6.13 ),
Liquid: O ( momentum) << O (friction, gravity)
∴∆pv = Pressure drop due to friction and gravity
Vapour flow:
140
dp f = frictional pressure drop
dp f =
or,
dp f =
π
4
π
4
D dp f
4 dx
(6.16)
dx
D
(6.17)
ρ V2
(6.18)
Again,
dp f ∞
or,
dp f ∞
Hence,
τs =
D
4
(6.15)
D 2 dp f = π Dτ s dx
τs =
f
(6.14)
D 2 dp f = π Dτ s dx
2
(6.19)
ρV 2
2
D
=
f ρV
4
2
2
For laminar flow:
Friction factor,
f =
64 µv
64
=
ρvVav D Rea
(6.20)
Where
(6.21)
R
Vav = Va (2π r )dr / π R 2 = Vmax / 2
0
dx
∴ dpv = f
D
ρV 2
2
32
=
D2
141
µ
ρ
.
m
dx
A
(6.22)
.
µ
ρ
32
∴ ∆pv =
D2
m
Leff
A
or,
(6.24)
.
∴ ∆pv =
(6.23)
128µv mv Leff
π D 4 ρv
Some typical values of friction factor f with Reynolds number Re d shown in
Table 6.1.
Table 6.1 Typical values of Reynolds number and friction factor
Sr No
1
2
3
Reynolds’s Number
5000
10,000
100,000
Friction factor
0.039
0.037
0.022
For liquid: If the flow is through the porous medium,
Applying Darcy’s law
ml
Aw
.
∴ ∆pl =
.
µl Leff ml
ρl Aw
(6.25)
.
dpl
µ
=k l
dx
ρl
µl Leff ml
=
1
ρl Aw K w
k
where k = friction factor
K w = permeability of wick material=
µl = viscosity of liquid, Ns/m
1
k
.
ml = mass flow rate of liquid, kg/s
ρl = density of liquid, kg/m3
Aw = X-sectional area of wick, m2
K w = permeability, m2
Leff = Le + Ladb + La , effective length of heat pipe, m
142
(6.26)
6.7 Condition for flow in a heat pipe
Maximum capillary pressure drop ≥ Pressure drop required to return liquid from
evaporator to condenser + Pressure drop required to move the vapor from
evaporator to condenser+ Pressure drop due to elevation between evaporator
and condenser or external acceleration.
or,
(6.27)
∆pc |max ≥ ∆pl + ∆pv + ∆pg
Assuming no external acceleration, for the heat pipe to be operative,
.
.
2σ l cos θ µl Leff ml 128µv mv Leff
=
+
+ ρl gLeff cos β
rc
ρl Aw K w
π D 4 ρv
Generally,
128µv
µl
<<
4
π D ρv
ρl Aw K w
.
2σ l cos θ µl Leff ml
∴
=
+ ρl gLeff cos β
rc
ρl Aw K w
(6.28)
(6.29)
(6.30)
or,
.
µl Leff
2σ cos θ
ml = l
− ρc gLeff cos β
ρl K w Aw
rc
or,
.
ml =
ρlσ l h fg
µl
2 cos θ ρl gLeff
−
cos β
rc
σl
Aw K w
Leff
Heat transfer rate,
.
.
Q = m h fg
Hence, maximum heat transfer rate,
.
ρlσ l h fg Aw K w
Qmax =
µl
Leff
= (liquid property)x (wick property)x
2 cos θ ρl gLeff
−
cos β
rc
σl
2 cos θ ρl gLeff
−
cos β
rc
σl
143
(6.31)
(6.32)
(6.33)
(6.34)
The liquid property
ρlσ l h fg
µl
is know as Figure of Merit , M.
Now for heat transfer rate to be maximum, the liquid mass flow rate is to be
maximum requiring lesser and lesser values of θ . For properly wetted surfaces,
θ ≈ 00 . For, working material CH 3OH , figure of merit is low indicating low rate of
heat transfer with a limited temperature range of 200-6000C. Table 6.2 presents
some typical values of wick materials.
Table 6.2 Some data for wick materials
Sl No
Material and Mesh
Size
Capillary
height
(cm)
25.4
39.5
Pore radius
(cm)
Permeability
1
2
Glass fibre
Monel beads
( 70 − 80 µ m )
----0.019
0.061× 10−4
0.78 ×10 −10
3
Nickel Powder
( 200 µ m )
24.6
0.038
0.027 × 10−10
4
Copper Powder
( 45-56 µ m )
156.8
0.009
1.74 × 10−12
The need for a wick is to achieve the following benefits:
1. It provides axial pumping by capillary action.
2. It distributes the liquid circumferentially and ensures that the surface of the
evaporator is always wetted so that it can support all the radial heat flux.
3. The wick itself, particularly if it is integral with the wall in the form of
groove, may increase heat transfer coefficient h and promote heat
transfer rate.
The wick can inhibit entrainment of liquid in vapour restricting the heat transfer
capability of the heat pipe. Entrainment occurs when the shear force between the
counter current liquid and vapor flow is sufficient to entrain droplets of liquid to
carry them back to the condenser.
Because of provision of wick, heat pipe is more efficient than the thermo siphon.
144
6.8 Working fluid
1. Water
2. Fluorocarbons
3. Mercury
4. Methanol
5. Glycerin
6. Acetone
7. Sodium, Potassium, Lithium, etc
8. Silver
9. Molten salt
10. Ammonia
11. Organic fluids- such as liquid hydrogen, liquid oxygen, liquid nitrogen
6.9 Desirable properties of working fluid
1. It should be non-toxic
2. Non-corrosive
3. It should have low viscosity
4. It should have high surface tension ( must be wettable)
5. High latent heat
6. Good heat transfer property
7. Should be chemically compatible to the heat pipe container
6.10 Factor responsible for performance of heat pipe
1. Selection of working fluid and heat pipe materials: depends upon the
range of working temperature, corrosion and erosion of heat pipe material.
• Working fluid and material of construction are to be compatible
• Depending on the working fluid, container wall may be of aluminum,
copper or stainless steel.
2. Gas velocity- limited to 2-4 m/s to give the pressure drop across the tube
bundles to a reasonable level.
3. Length of pipe- Maximum 5 m. With increase in length wick design and
pipe length becomes complicated.
4. In case heat pipe is attached to fins, fin pitch and fin shape depends upon
the pressure drop and fouling.
6.11 Container materials
1.
2.
3.
4.
Glass
Stainless steel
Copper
Aluminum
145
5. Ceramic
6.12 Wick materials
1. Woven cloths
2. Glass fibre
3. Sintered material
4. Wire mesh
Wick and container dimensions must be such as to satisfy
∆pc >> ∆pv + ∆pl + ∆p g
(6.35)
or,
Capillary pumping >> pressure drop in vapor + pressure loss due to viscous
drag+ pressure loss due to gravity
6.13 Rating of heat pipe
The heat pipe is rated by its axial pumping rating (APR), which is the energy
moving axially along the pipe. Heat pies of different rating are available in
standard sizes (1 kW, 10 kW and so on).
Performance of heat pipe depends on the angle of operation. The tube bundle
may be horizontal or tilted with evaporator section below the condenser. Because
of this sensitivity, the angle of heat pipe may be adjusted in situ as a means of
controlling the heat transport. A number of proprietary units incorporate tilt control
mechanisms which can be adjusted either manually or automatically to cater for
changes in heat transfer requirements.
A large number of fluid and pipe material combinations have been used for heat
pipes, and some typical working fluid and material combinations as well as the
temperature ranges over which they can operate are presented in Table 6.3.
Table 6.3 Working temperature range, working fluid, heat pipe construction
material and maximum axial heat flux
Working fluid Container material
1
Temperature
Range
(K)
230-400
2
3
4
5
280-500
360-850
673-1073
773-1173
Water
Mercury
Potassium
Sodium
Sl No
Methanol
146
Copper, nickel,
stainless steel
Copper, Nickel
Stainless Steel
Nickel, Stainless Steel
Nickel, Stainless Steel
Maximum
axial heat flux
( W/m2)
0.45
0.67
25.1
5.6
9.3
PART-II Heat Pumps
6.14 Introduction
Heat pumps are used for up gradation of low grade (reject) heat. Suppose heat is
available at 300 C and heat requirement is at 700 C . Thus with the help of a heat
pump low grade heat at 300 C can be used to deliver heat at 700 C . This
increases the quality of energy. The up gradation of energy is done at the
expense of external work to the compressor.
70 deg C
C (Condenser)
CP(Compressor)
EV (Expansion valve)
E (Evaporator)
30 deg C
Fig. 6.9 Heat pump cycle
-
Up gradation of low grade energy (heat at 300C to 840C). It is done at the
expense of work to the compressor
Very much used in cold countries
It is always to be used when large demand for heat is to be met
Heat pumps are advantageous when both heating and cooling are
required. Capital cost is not much different from that of a conventional a/c
with electric resistance heaters, but energy costs are substantially lower.
6.15 Working Fluid
Mainly R11, R12, R21, R22, R114
Azeotropic mixtures R31/R114, R12/R31
6.15.1 Desirable properties of heat pump working fluid
1. High critical pressure and temperature
147
2. Large latent heat of condensation
3. low specific volume of the fluid at compressor inlet
4. Immiscible with the lubricating oil- ( NH 3 , CO2 immiscible,
R11,R12,R21, R113 completely miscible, R13, R14 miscibility is
low and R22, R114 intermittent)
5. Non toxic, chemically stable
6. Non flammable
7. Cheap
6.15.2 Applications
1. Domestic buildings
2. Commercial buildings such as apartments, big buildings
3. Industrial process heating
6.16 Heat pump size/Capacity of a heat pump
1. Cooling load in the evaporator
2. Air volume and air change requirement
3. Heating load on the condenser
4. Capital cost
6.17 Types of heat pumps
Heat pumps can be classified in 4 (four) different ways, namely,
(1) Package heat pumps with a reversible cycle
(2) Decentralized heat pumps for air conditioning moderate and large
buildings
(3) Heat pumps with a double bundled condenser
148
(4) Industrial heat pump
6.17.1 Package heat pumps with a reversible cycle
These heat pumps are used for residential and small commercial units that are
capable of heating a space in cold weather and cooling in warm weather. The
reversible heat pump operates with the help of a four way valve. During heating
operation (Fig.) the four way valve positions itself so that the high pressure
discharge gas from the compressor flows first to the heat exchanger in the
conditioned air stream. During cooling process, the refrigerant rejects heat,
warming the air, liquid refrigerant flows on to the expansion-devise section,
where the check valve in the upper line prevents flow through its branch and
instead the liquid refrigerant flows through the expansion device in the lower
branch. The cold low pressure refrigerant then extracts heat from the outdoor air
while it vaporizes. Refrigerant vapour returns to the four-way valve to be directed
to the suction side of the compressor.
To convert from heating to cooling operation the four-way valve shifts to its
opposite position so that discharge gas from the compressor first flows to the
outdoor coil, where the refrigerant rejects heat during condensation. After
passing through the expansion device in the upper branch, the low pressure low
temperature refrigerant evaporates in the heat exchanger that cools air from the
conditioned space.
Heat sources: (a) air, (b) water, (c) earth, (d) solar energy, (e) waste heat.
6.18 Different types of heat pumps
Air to Air heat pump
-Year round air conditioning
Cooling during summer
Heating during winter
149
Comp
Evaporator
Cooled air
return air
Fig. 6.10 Heat pump for summer air conditioning
Winter
Cooled air
Hot air
Q2
Comp
Q1
Hot air
Cool air
Winter: 60% energy is used for heating in cold countries
Fig. 6.11 Heat pump for winter air conditioning
150
15 deg C
Pump
Water treatment
plant
Water reservoir
or swimming pool
Evaporator
Treated water
Compressor
Condenser
R11
E.V
Water
Fig. 6.12 Swimming pool heating using water-water heat pump
Industrial process heat
-Combined heating and refrigeration
- Injection moulding plant where chilled water is required to cool the moulds
during solidification of plastics in components being moulded.
151
Underground
Storage tank
60
deg C
Inj mould m/c
Hot water
Condenser
Compr
Chilled water at
7.5 deg C
Factory building
E
Fig. 6.13 Heating requirement to factory building + chilled water to injection
moulding machine
Process Heat recovery by water – water heat pump
Ex. Wasting House “Templifier” heat pump unit.
Use of electric driven heat pumps recover heat from waste water and other
liquids for reuse in heating process of steam or heating water for space heating.
Water is delivered at 820C using the heat available at 320C . This is up gradation
of energy. If temperature difference is high—recommend 2 stage compression
with intercooling.
152
71 deg C
Hot water to
process at
82 deg C
Warm water return from
process
5
4
Condenser
6
E.V
7
2
3
H P Pump
Flash intercooler
8
L P Pump
E.V
9
1
Evaporator
Water in at
32 deg C
Water out at
27 deg C
Fig. 6.14 Multi-compressor heat pump
6
8
5
4
7
3
p
2
1 11
9 10
h
Pi = Pk P0
Fig. 6.15 P-h diagram of Multi-compressor heat pump
153
Drying of timber
100
1
W
2
Humid
air
Dehumidifier
4
3
Stack of
timber
DBT
Fig. 6.16 schematic diagram of drying of timber
Wet air
1
Evaporator
3
Condenser drain out
M
C
2
Dry air
4
Condenser
Fig. 6.17 Drying method
154
The evaporator cools the wet air below the dew point temperature, removes the
sensible and latent heat. As a result of cooling, condensation of some part of
water vapour in the air occurs and it is drained off. The dehumidified air is passed
through the condenser for up gradation of energy.
COP 3.6
Paper yarn manufacturer receive dry yarn by applying a heat pump.
Dry hot air
Condenser
dry air
EV
C
Evaporator
Wet cold air
Water
Separator
dryer
Feed (wet)
Dry material
Condensate
Fig. 6.18 Paper yarn manufacturing using heat pump
Example 1: A heat pump using R12 is to be used for winter space heating of a
residence. The building heat loss is 20 kw. Compression is reversible and
adiabatic. (Refer Fig a)
Determine the work required by compressor in kw and COP of the heat pump.
Inside temp of building is 250C, environment is at 50C.
155
Building 25 deg C
Q2=20KW
Cond
EV
Comp
E
Q1
T1=5deg C
Fig. (a)
COP =
Q2
Q2
T2
298
=
=
=
= 14.9
W Q2 − Q1 T2 − T1
20
Q2 = 20 KW
Applications:
1.
2.
3.
4.
5.
6.
7.
8.
Washing, sterilizing and clean up
Cooking
Pasteurization
Bleaching
Dye heating
Log soaking
Paint drying
Pulp heating
Chemical heat pump
Temperature Amplifier type
Propanol (C3H8O + Heat C3H6O + H2 )
156
W
C3H6O + H2
Distillation
Column
200 deg (Heat
recovery)
Reactor with Ni
Catalyser
C3H8O (Propanol)
80 deg
(Charging Head)
Fig. 6.19 Chemical heat pump
Getting heat at 200 0C from heat source = 800C
Charging process: C3 H 8 O C3H6O + H2
Endothermic process
Reactor: C3H6O + H2 Ni
C3 H 8 O + endothermic
Example 2: A heat pump is to be used to heat a house in winter and then
reversed to cool the house in summer. The interior temperature is to be
maintained at 200C. Heat transferred through the walls and roofs is estimated to
be 0.525kw per degree temperature difference between inside and outside.
(Refer Fig b)
a. If the outside temperature in winter is 50C, what is the minimum power
required to drive the heat pump.
b. If the power input is same as in part a. what is the maximum outer temperature
in summer for which the inside can be maintained at 200C.
Ans: a. W = 403.2 W). b. t1 = 35.420C
Solution :
157
h e a t lo s s
20 deg C
20 deg C
5 deg C
W
Q1
Fig b
W = Q2 – Q1
Walls and roofs = 0.522 Kw
0
C
COP =
Q2
T2
20
=
=
= 1.33
Q2 − Q1 T2 − T1 15
Temperature difference = 150C
Total heat loss = 0.525x15=7.875kw
Room
20 deg C
Q2
W
Q1 T1
Atm5 deg C
a) Q2 = 0.525KJ/SK
= 0.525 x 103 x (20-3) J/s
= 7875W
W = Q2 – Q1
158
5 deg C
COP =
Q2
Q2
T2
=
=
W Q2 − Q1 T2 − T1
273 + 20
= 19.53
20 − 5
Q2
7875
∴W =
=
= 403.16W
19.33 19.53
b) summer
∴ COP =
Assume power output to be same for both season, heat load to be
removed in summer=heat load input in winter to the room.
Room T1=20 deg C
Q1=7875w
HE
W=403.16w
Q2
T2
η=
T
W Q1 − Q2
=
= 1− 2
Q1
Q1
T1
∴
T
403.16
= 1− 2
7875
T1
or
T2
= 1 − 0.05 = 0.949
T1
∴ T1 =
T2
273 + 20
=
= 308.81K = 35.810 C
0.949
0.949
159
Wood burning
hearth
Q1
B
P
T
C
Q2
Room
Q4
Con
EV
C
E
Q3
Atmosphere
Heat pump coupled with a power plant
Heat supplied to the room = Q1+Q2
Heat multiplication factor =
Q 2 + Q 4 Q2 + Q
≅
Q1 + Q3
Q1
Q1 = 1000KJ
Then Q1+Q2 = 5000KJ
HMF = 5
Thermodynamically HMF = 6
160
Q3 = 0
PART-III Heat
Recovery from Incineration Plants
6.19 Introduction
An incinerator is a piece of equipment which assists disposal of waste products
which could not be used in a sold for re-use elsewhere.
Various countries in the world generate different types of waste every day. USA
generates 160 million tons of soild waste every year. In India, municipal waste
generation is as high as 27.4 million tons every year. (Every American generates
3-4 pounds of waste compared to an Indian who generates 100-500 gm every
day). Most of the waste materials causes environmental pollution.
There are many methods to utilize the waste materials like composting,
briquetting, anaerobic digestion, land filling, etc. Land fills gases causes’
tremendous green house gas emission.
Table 6.4 presents various types of waste materials generated/year in India.
Sl No
1
2
3
4
5
6
7
Item
Quantity of waste
generated
Municipal solid waste
27.4 MT/year
Municipal liquid waste (121 Class I and Class 12145 Ml/day
II cities)
Distillery (243 Nos)
8057 kcall/day
Food and fruit processing waste
4.5 MT/year
Dairy waste
50-60 Ml/day
Paper and pulp industry waste ( 300 mills)
1600 m3 waste water/day
Tannery (2000 Nos)
52500 m3 waste
water/day
6.20 Classification of incinerators
Depending upon the type of waste, incinerators are classified in 3 (Three)
categories.
(1) Solid incinerator- Disposal of solid material is most widely used in large
cities.
(2) Liquid incinerator- incinerators for waste liquids can be used to remove
pollutions and to recover inorganic.
161
(3) Gas incinerator- incinerators for disposal of obnoxious and poisonous gas
to prevent atmospheric pollution are called gas or fume incinerator.
6.20.1 Fume incinerator
Fig. represents a typical fume incinerator with waste heat recovery boiler. Tail
gas or fume gas enters at 430 C to a recuperative heat exchanger where it gets
heated to 3700 C . The hot fume gas now enters a combustion chamber where
natural gas is supplied at the rate of 600 Nm3/hr. After combustion, the final gas
(mainly consisting of CO2 , H 2O ) attains a temperature of 7600 C and enters the
recuperator. Once the combustion gas exchanges heat with the incoming fume
gas in the recuperator, its temperature decreases to nearly 4300 C . The flue gas
now enters in a waste heat recovery boiler to produce steam. Steam output of 11
ton/hr may be obtained by this arrangement. Exhaust gas leaving the waste heat
boiler is cooled down to 2600 C which is safely disposed through the stack. Fume
gas may be obtained from the process of producing metallic anhydrous (chemical
intermittent and monomer used in polyester synthesis).
6.20.2 Solid waste incinerator
Utility
(2) Municipal solid waste
(3) Industrial solid waste
(4) Medical solid waste
(5) Institutional solid waste
(6) Animal waste
(7) Sewage sludge
(8) Nursing home solid waste
(9) Restaurant solid waste/food waste
(10)
Tyres
Leading manufacturer
(1) BFL incinerator
(2) Crawford Equipment and Engineering Company
(3) ATI
(4) Cambi
(5) Basic International
6.20.3 Liquid waste incinerator
Various liquid wastes
(1) Organic waste- C, O, H, Hydrocarbon
(2) Halogeneted – carbon tetra chloride, venyl chloride, methyl bromide
162
(3) Metallic waste- Inorganic and organic salts
Fig. presents the working of a liquid incinerator. In this device liquid waste at the
rate of 229 kg/s is fed to the incinerator producing 10.4 kg/s steam at 19.3 bar
and 4000 C .
Most of the applications of liquid waste incineration are in petrochemical
industries.
PART-IV ORGANIC RANKINE CYCLE
6.21 Introduction
Organic substances, that can be used below a temperature of 400oC do not need
to be overheated. For many organic compounds superheating is not necessary,
resulting in a higher efficiency of the cycle. This is called an Organic Rankine
Cycle (ORC). The low temperature waste heat can be recovered/utilized with
ORC.
ORC can make use of low temperature waste heat to generate electricity. At low
temperature a steam cycle would be inefficient, due to enormous volumes of low
pressure steam, causing very voluminous and costly plants. ORCs can be
applied for low temperature waste heat recovery (industry), efficiency
improvement in power stations, and recovery of geothermal and solar heat.
Small scale ORCs have been used commercially or as pilot plant in the last two
decades. It is estimated that already about 30 commercial ORC plants werehave
been built before 1984 with an output of 100 kW.
Several organic compounds have been used in ORCs (e.g. CFCs, freon, isopentane or ammonia) to match the temperature of the available waste heat.
Waste heat temperatures can be as low as 70-80oC. The efficiency of an ORC is
estimated to be between 10 and 20%, depending on temperature levels. On
many sites no suitable use is available for low temperature waste heat, hence
upgrading by the use of a heat pump (or transformer) or an ORC are good
energy recovery candidates. However, the maximum temperature of heat pumps
is still limited, making ORC a good technology for heat recovery for base load
purposes in the range of 150 till 200oC, if no other use for the waste heat is
available on site. Some of the organic compounds used in ORC are listed in
Table-6.5 & 6.6.
163
Table-6.5: Halogenated hydro-carbons
Sl No
Nomenclature
Boiling point
R11
Chemical
formula
CFCl3
1
2
R12
CF2Cl2
−29.80 C
3
R22
CHF2Cl
−40.80 C
4
R13
CClF3
−81.50 C
Boiling point
−0.50 C
23.7 0 C
Table-6.6: Hydro carbon group
Sl No
Nomenclature
1
Propane
Chemical
formula
C3 H 8
2
n-Butane
C4 H10
3
Pentane
C5 H10
−42.120 C
For both halogenated hydrocarbon and hydrocarbons
(5) Boiling point and condensation temperatures are much lower
(6) These fluids have low critical temperature and pressure resulting in
(a) High coefficient of performance ( High critical temperature)
(b) Low condensing pressure due to low critical pressure.
These properties are utilized in ORC
To minimise costs and energy losses it is necessary to locate an ORC near the
heat source, and have a large amount of available waste heat at stationary
conditions. There is also a need to condense the working vapour. Therefore, a
cooling medium should be available on site. These site characteristics will limit
the potential application.
The theoretical potential of ORC is determined by the available waste heat, at
temperature levels between 70 and 400oC. The practical potential will, however,
be determined by the potential application of the waste heat for other purposes
(e.g. process integration) or the potential use of heat pumps or transformers.
ORC has still high capital costs, and hence other measures will generally be
more profitable.
164
6.22 DAIMLER BENZ ORC
Daimler Benz carried out a design study of bottoming cycles applied to vehicular
diesel engine. Utilizing the waste heat of the exhaust gases through an ORC
engine, a reduction of 10% in fuel consumption was achieved for long haul road
vehicles. FLUORENOL-50 was used as the working fluid.
Fig.6.20 presents a ORC bottoming cycle. Exhaust of a diesel engine is passed
through a heat exchanger (or, evaporator). Waste heat from the exhaust is
received by the organic fluid which vaporizes and enters a turbine producing
power. The organic fluid after extraction of power at the turbine is passed through
a condenser and then through a preheater to raise its temperature before entry to
the heat exchanger (evaporator).
Fig.6.20 Organic Rankin cycle
Supercritical FIAT cycle using R11 as a working fluid was used in an ORC
engine. Rating was 95 kW for recovery of waste heat from the exhaust gases of
a regenerative 260 kW gas turbine for road vehicle.
6.22.2 Other applications of ORC
The Turboden turbogenerator operates according to the Organic Rankine Cycle
(ORC) concept. The Organic Rankine Cycle (ORC) is similar to the cycle of a
conventional steam turbine, except for the fluid that drives the turbine, which is a
high molecular mass organic fluid. The selected working fluids allow to exploit
efficiently low temperature heat sources to produce electricity in a wide range of
power outputs (from few kW up to 3 MW electric power per unit).
165
The organic working fluid is vaporized by application of a heat source in the
evaporator. The organic fluid vapour expands in the turbine and is then
condensed using a flow of water in a shell-and-tube heat exchanger
(alternatively, ambient air can be used for cooling). The condensate is pumped
back to the evaporator thus closing the thermodynamic cycle. Heating and
cooling sources are not directly in contact with the working fluid nor with the
turbine. For high temperature applications (e.g. Combined Heat and Power
biomass-powered plants, high temperature thermal oil is used as a heat carrier
and a a regenerator is added, to further improve the cycle performance.
Advantages
Key technical benefits include:
a. high cycle efficiency
b. very high turbine efficiency (up to 85 percent)
c. low mechanical stress of the turbine, due to the low peripheral speed
d. low RPM of the turbine allowing the direct drive of the electric generator
without reduction gear
e. no erosion of blades, due to the absence of moisture in the vapour nozzles
f. long life
g. no operator required
The system also has practical advantages, such as simple start-stop procedures,
quiet operation, minimum maintenance requirements, and good part load
performance. Among the working fluids than can be evaluated for use in these
systems is Honeywell 245fa, a high-boiling refrigerant now available in
commercial quantities from Honeywell.
.
The favorable performance of Honeywell 245fa in Rankine cycles provides an
opportunity to realize greater electrical energy output from power generation
facilities that rely on steam-driven turbines. Likewise, large industrial enterprises
can now consider recovery of waste heat with the option to convert the energy to
Usefulelectricity.
166