130

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 eL , (2) A condenser section of length cL 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

131

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.

132

(a)

(b)

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. No moving parts 2. Requires no external energy 3. Reversible in operation 4. Completely silent 5. Very reliable 6. Rugged like any piece of pipe or tube 7. 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.

133

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 022 C from heat received at 012 C ( Fig.6.4 )

134

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

135

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. Solar collectors, space applications, snow melting 2. Biscuit and bread ovens 3. Laundries 4. Pharmaceuticals 5. Spray drying 6. Welding booths 7. Brick kilns 8. 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)

136

(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

. .

v fg v av fgQ m h VA hρ= = (6.1)

Axial heat flux is

137

.

v fgv

QVh

Aρ=

(6.2)

Now fixing .

v

QA

by adjusting condenser to lower pressure, temperature and

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

Wb = Inertia force/Surface tension force= 2

12v c

l

V Lρπσ

=

or, 1/ 2

2 l

v c

VL

πσρ

=

(6.3)

where cL is the characteristics length describing the pore size. If, 1Wb > , 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/ 22 2l l v

v fg v fg fgv v c c

QVh h h

A L Lπσ πσ ρρ ρ

ρ

= = =

(6.4)

which is represented by the curve between 2-3 in Fig. 6.7. 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

138

characterized by two principal radii of curvatures 1r and 2r , the pressure drop across the liquid surface is

1 2

1 1cp

r rσ

∆ = +

(6.5)

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

.

4

8 ll

m Lp

rππ ρ

∆ = (6.6)

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

2v lp p

rσ− = (6.7)

( )w vk T TQ

A t

−=

(6.8)

where vp = vapor pressure inside the bubble lp = liquid pressure outside the bubble r = radius of the largest nucleation site A = heat input area k = thermal conductivity of the saturated wick wT = temperature at the inside wall vT = temperature at liquid vapour interface t = thickness of first layer of the wick

139

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

rV . 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 sτ acts at the wall in a direction opposite to that of the fluid flow. Applying conservation of mass

( ) ( )2 2 4 4rD V D dx V dV Dπ πρ ρ π ρ + = +

(6.9)

140

or,

4 rVdV dx

D =

(6.10)

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,

( ) ( )

[ ]

. . .2 2

2

4 4

cos cos4s

m d m V dV mV D p D p dp

D dx D ng g dx

π π

πτ π ρ α ρ β

Ω + + = Ω + − −

− + −

(6.11)

Assuming, .

. 0d m dV = We get,

( )42 cos cossdp dx VdV n gdx

Dτ ρ β α ρ = + Ω + −

(6.12)

Upon integrating equation over the tube length, (x=0 to x=L) and assuming Ω to be constant,

( ) ( )2 20

0

4cos cos

L

s ip dx V V n gLD

τ ρ β α ρ ∆ = + Ω − + − = Frictional shear +

Pressure drop due to momentum+Pressure drop due to gravity/acceleration

(6.13)

From Eq. (6.13 ), Vapour: O ( friction+momentum) >> O (gravity)

vp∴∆ = Pressure drop due to friction and momentum From Eq. (6.13 ), Liquid: O ( momentum)<< O (friction, gravity)

vp∴∆ = Pressure drop due to friction and gravity Vapour flow:

141

fdp = frictional pressure drop

fdp = 2

4 f sD dp D dxπ π τ =

(6.14)

or,

fdp = 2

4 f sD dp D dxπ π τ =

(6.15)

4f

s

dpDdx

τ

=

(6.16)

Again,

f

dxdp

D∞ (6.17)

or, 2V

2fdp

ρ∞ (6.18)

Hence,

2

22

4 4 2s

Vf

D f VD

ρρτ

= =

(6.19)

For laminar flow: Friction factor,

64 64Re

v

v av a

fV D

µρ

= = (6.20)

Where

2max

0

(2 ) / / 2R

av aV V r dr R Vπ π= =

(6.21)

.2

2

32

2v

dx V mdp f dx

D D Aρ µ

ρ

∴ = =

(6.22)

142

.

2

32 v eff

mp L

D Aµρ

∴ ∆ =

(6.23)

or,

.

4

128 v v eff

vv

m Lp

D

µπ ρ

∴ ∆ =

(6.24)

Some typical values of friction factor f with Reynolds number Red shown in Table 6.1.

Table 6.1 Typical values of Reynolds number and friction factor

Sr No Reynolds’s Number Friction factor 1 5000 0.039 2 10,000 0.037 3 100,000 0.022

For liquid: If the flow is through the porous medium, Applying Darcy’s law

.

l l l

l w

dp mk

dx Aµρ

=

(6.25)

. .

1

l eff l l eff ll

l w wl w

L m L mp

A KAk

µ µρρ

∴ ∆ = =

(6.26)

where k = friction factor

wK = permeability of wick material=1k

lµ = viscosity of liquid, Ns/m

.

lm = mass flow rate of liquid, kg/s lρ = density of liquid, kg/m3

wA = X-sectional area of wick, m2

wK = permeability, m2

eff e adb aL L L L= + + , effective length of heat pipe, m

143

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,

max| c l v gp p p p∆ ≥ ∆ + ∆ + ∆ (6.27)

Assuming no external acceleration, for the heat pipe to be operative,

. .

4

1282 coscosl eff l v v effl

l effc l w w v

L m m LgL

r A K D

µ µσ θ ρ βρ π ρ

= + +

(6.28)

Generally,

4

128 v l

v l w wD A Kµ µ

π ρ ρ<< (6.29)

.

2 cos cosl eff ll

l effc l w w

L mgL

r A K

µσ θ ρ βρ

∴ = +

(6.30)

or,

. 2 coscosl eff l

l c effl w w c

Lm gL

K A r

µ σ θ ρ βρ

= −

(6.31)

or, . 2cos

cosl l fg l effw wl

l eff c l

h gLA Km

L r

ρ σ ρθ βµ σ

= −

(6.32)

Heat transfer rate, .

.

fgQ m h= (6.33)

Hence, maximum heat transfer rate,

.

max

2coscosl l fg l effw w

l eff c l

h gLA KQ

L r

ρ σ ρθ βµ σ

= −

(6.34)

= (liquid property)x (wick property)x 2cos

cosl eff

c l

gL

r

ρθ βσ

−

144

The liquid property l l fg

l

hρ σµ

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,

00θ ≈ . For, working material 3CH OH , 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)

Pore radius (cm)

Permeability

1 Glass fibre 25.4 ----- 40.061 10−× 2 Monel beads

( )70 80 mµ− 39.5 0.019 100.78 10−×

3 Nickel Powder ( )200 mµ

24.6 0.038 100.027 10−×

4 Copper Powder ( )45-56 mµ

156.8 0.009 121.74 10−×

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.

145

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. Glass 2. Stainless steel 3. Copper 4. Aluminum

146

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

c v l gp p p p∆ >> ∆ + ∆ + ∆ (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

Sl No Temperature Range

(K)

Working fluid Container material Maximum axial heat flux ( W/m2)

1 230-400 Methanol Copper, nickel, stainless steel

0.45

2 280-500 Water Copper, Nickel 0.67 3 360-850 Mercury Stainless Steel 25.1 4 673-1073 Potassium Nickel, Stainless Steel 5.6 5 773-1173 Sodium Nickel, Stainless Steel 9.3

147

PART-II Heat Pumps

6.14 Introduction

Heat pumps are used for up gradation of low grade (reject) heat. Suppose heat is available at 030 C and heat requirement is at 070 C . Thus with the help of a heat pump low grade heat at 030 C can be used to deliver heat at 070 C . This increases the quality of energy. The up gradation of energy is done at the expense of external work to the compressor.

C (Condenser)

EV (Expansion valve) CP(Compressor)

E (Evaporator)

70 deg C

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

148

2. Large latent heat of condensation

3. low specific volume of the fluid at compressor inlet

4. Immiscible with the lubricating oil- ( 3 2,NH CO 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

149

(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

150

Comp

Evaporator

return air Cooled air

Fig. 6.10 Heat pump for summer air conditioning

Hot air Cooled air

Comp

Q2

Cool air Hot air

Winter

Q1

Winter: 60% energy is used for heating in cold countries

Fig. 6.11 Heat pump for winter air conditioning

151

15 deg CPump

E.V

Condenser

Treated water

Water

R11

Compressor

Evaporator Water reservoiror swimming pool

Water treatmentplant

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.

152

UndergroundStorage tank

Hot water

60deg C

Inj mould m/c

Chilled water at7.5 deg C

Factory building

Condenser

E

Compr

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.

153

Condenser

Evaporator

L P Pump

H P Pump

Flash intercooler

Warm water return fromprocess

1

23

4

5

6

7

8

9

E.V

Hot water toprocess at82 deg C

E.V

Water in at32 deg C

Water out at27 deg C

71 deg C

Fig. 6.14 Multi-compressor heat pump

h

p1

234

56

78

9 10 11

0PPP ki =

Fig. 6.15 P-h diagram of Multi-compressor heat pump

154

Drying of timber

Humidair

Dehumidifier

Stack oftimber

W

DBT

1

2

3 4

100

Fig. 6.16 schematic diagram of drying of timber

Wet airEvaporator

Condenser

CM

Dry air

1

2

3

4

Condenser drain out

Fig. 6.17 Drying method

155

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.

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.

Condenser

Evaporator

Dry hot air

Wet cold air dryer

dry air EV

WaterSeparator

Condensate

Feed (wet)

Dry material

C

156

Cond

CompEV

EW

Q1

Q2=20KW

T1=5deg C

Building 25 deg C

Fig. (a)

KWQ

TTT

QQQ

WQ

COP

20

9.1420298

2

12

2

12

22

=

==−

=−

==

Applications:

1. Washing, sterilizing and clean up 2. Cooking 3. Pasteurization 4. Bleaching 5. Dye heating 6. Log soaking 7. Paint drying 8. Pulp heating

Chemical heat pump Temperature Amplifier type Propanol (C3H8O + Heat C3H6O + H2 )

157

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 :

80 deg(Charging Head)

DistillationColumn 200 deg (Heat

recovery)

C3H8O (Propanol)

Reactor with NiCatalyser

C3H6O + H2

158

20 de g C

hea t loss

5 deg C

20 deg C

W

5 deg CQ 1

Fig b

W = Q2 – Q1

Walls and roofs = C

Kw0522.0

COP = 33.11520

12

2

12

2 ==−

=− TT

TQQ

Q

Temperature difference = 150C Total heat loss = 0.525x15=7.875kw

Room20 deg C

W

Atm5 deg C

Q1

Q2

T1

a) Q2 = 0.525KJ/SK

= 0.525 x 103 x (20-3) J/s = 7875W W = Q2 – Q1

159

COP = 12

2

12

22

TTT

QQQ

WQ

−=

−=

WQ

W

COP

16.40353.19

787533.19

53.1952020273

2 ===∴

=−+=∴

b) summer Assume power output to be same for both season, heat load to be removed in summer=heat load input in winter to the room.

T2

Room T1=20 deg C

HE

Q1=7875w

W=403.16w

Q2

CKT

T

TT

or

TT

TT

QQQ

QW

021

1

2

1

2

1

2

1

21

1

81.3581.308949.0

20273949.0

949.005.01

17875

16.403

1

==+==∴

=−=

−=∴

−=−==η

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Wood burninghearth

Atmosphere

Room

Q1

Q2

Q3

Q4

E

EV CCon

C

TPB

Heat pump coupled with a power plant Heat supplied to the room = Q1+Q2

Heat multiplication factor = 1

2

31

42

QQQ

QQQQ +

≅++

Q3 = 0

Q1 = 1000KJ Then Q1+Q2 = 5000KJ HMF = 5 Thermodynamically HMF = 6

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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 Item Quantity of waste

generated 1 Municipal solid waste 27.4 MT/year 2 Municipal liquid waste (121 Class I and Class

II cities) 12145 Ml/day

3 Distillery (243 Nos) 8057 kcall/day 4 Food and fruit processing waste 4.5 MT/year 5 Dairy waste 50-60 Ml/day 6 Paper and pulp industry waste ( 300 mills) 1600 m3 waste water/day 7 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.

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(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 043 C to a recuperative heat exchanger where it gets heated to 0370 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 2 2,CO H O ) attains a temperature of 0760 C and enters the recuperator. Once the combustion gas exchanges heat with the incoming fume gas in the recuperator, its temperature decreases to nearly 0430 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 0260 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

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(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 0400 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, iso-pentane 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.

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Table-6.5: Halogenated hydro-carbons

Sl No Nomenclature Chemical formula

Boiling point

1 R11 3CFCl 023.7 C 2 R12 2 2CF Cl 029.8 C− 3 R22 2CHF Cl 040.8 C− 4 R13 3CClF 081.5 C−

Table-6.6: Hydro carbon group

Sl No Nomenclature Chemical formula

Boiling point

1 Propane 3 8C H 042.12 C− 2 n-Butane 4 10C H 00.5 C− 3 Pentane 5 10C H

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.

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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).

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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.