chapter 6 - waste heat recovery devices

7/26/2019 Chapter 6 - Waste Heat Recovery Devices

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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 asmall area with small temperature difference. Grover, Cotton and Erickson of LosAlamos National laboratory are the pioneers in construction of heat pipes. Thefirst heat pipe was constructed by them in 1964. They constructed 3 (Three) heatpipes, 1 with water as medium and 2 with sodium.

Initial efforts were directed towards space heat applications. Major factors arehigh reliability, its capability to work under weightless condition in space as wellas 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 applicationto earth as well as space applications. One may wonder why heat pipe conceptwas not given serious importance in spite of its simplicity and exceptional heattransfer capability without external power. Reasons being:

(1) Surface tension force on which capillarity depends is weak. It was notestablished that strong capillary forces are possible to develop. Withadvancement of material science lead to the development of sustainablecapillary force in a fine-pored structure now.

(2) Prior to the development of space programme, there really was nocompelling need for a heat transport device based on capillaryrecirculation. Advent of space programme, however, created the need fora heat transport device for use in space power system under weightlesscondition.

A heat pipe is basically a sealed container normally in the form of a tubecontaining wick lining in the inside wall. It is used to transport heat from source tosink 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 sectionof 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 smallsurface areaPower-No external power requirementEnvironmental compatibility- Environment friendly, no noise and pollutionTransporting force-By capillary action


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Fig. 6.1 Schematic diagram of a heat pipe

6.2 Thermal conductivity of a heat pipeSince heat pipe transports heat by the process of evaporation and condensation,the heat pipe can transfer heat much more effectively than any solid of samecross section by conduction.


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(a)(b)

Fig 6.2 (a) Heat flow by conduction in a copper rod, (b) Heat flowthrough a heat pipe of same length

The thermal conductivity of a heat pipe may be 500 times more than the bestavailable metallic conductor. To remove the localized heat, heat pipe is the mosteffective and most widely used device. Heat pipe is also called super thermalconductor.

6.3 Characteristics of heat pipe

1. No moving parts2. Requires no external energy3. Reversible in operation4. Completely silent5. Very reliable6. Rugged like any piece of pipe or tube7. Can withstand a lot of abuse

Because of the reliance on capillary action to return the condensate, heat pipe isparticularly 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 ofcondensate is against gravity. Hence arrangement in Fig.6.3b is preferred.Gravity aids the return of condensate, the wick may be omitted. Under thecircumstances, the device is called a thermo-siphon.


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Fig. 6.3a Along gravity Fig. 6.3 b Against gravity

6.4 Application of heat pipe1. Electronics- cooling of electronic circuits2. Electrical motors, generators and transformers3. Application in HVAC systems4. Application in IC engines5. Gas turbine regenerators

6.4.1 Application to production process

Transformers are large in size because they need a large surface area todissipate heat generated. Heat pipes can be inserted into the transformer core todissipate 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 roomcan be heated upto 022 Cfrom heat received at 012 C( Fig.6.4 )


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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-fuelmixture of a gas turbine is always energy efficient. Fig. 6.5 shows a lay out of theheat recovery from gas turbine regenerator.

Fig. 6.5 Heat recovery from gas turbine regenerator


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6.4.4 Application to I C Engine (VAPIPE)

One of the most interesting application of heat pipe in I C Engines is known asVAPIPE. When fitted to a car engine, it significantly reduces both the fuelconsumption and exhaust emission by using heat extracted from exhaust

emission by using the heat extracted from exhaust gas for vaporizing petrolmixture 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 melting2. Biscuit and bread ovens3. Laundries4. Pharmaceuticals5. Spray drying6. Welding booths7. Brick kilns8. Plastic lamination drying, extrusion of plastic materials. Temperature

uniformity can be maintained in texturising fibres.9. Annealing furnaces10. Chemical fluid bed dehumidifier11. Epoxy coating12. 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)


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(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 alongthe evaporator length. The vapour velocity then increases with evaporator length,reaching a maximum at evaporator exit. As the heat transport rate and hence theheat generation rate increases, the exit velocity becomes higher. When the exitvelocity reaches the sonic value, the vapour flow is said to be choked. The heattransport rate at which vapour velocity becomes sonic is called sonic limit. Atthat point, no further increase in vapour flow or heat transport is possible withoutan increase in vapour temperature.The sonic limit is expressed as an axial heat flux (heat transfer/vapour crosssectional area).

Heat transfer rate is. .

v fg v av fgQ m h VA h= = (6.1)

Axial heat flux is


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.

v fg

v

QVh

A=

(6.2)

Now fixing

.

v

Q

A by adjusting condenser to lower pressure, temperature and

density, velocity of vapour goes to sonic limit in the evaporator exit. Once soniclimit is achieved, any further change in parameter in condenser will not effect theevaporation. Flow will be chocked. Thus vapour velocity should be well below thesonic 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, itmay occur when the temperature of the evaporator inlet are higher than those atthe evaporator exit.

6.5.2 Entrainment Limitation

If the vapour density is allowed to increase without an accompanying decrease invelocity, some liquid from the wick return may be entrained, the onset of whichmay be expressed by Weber number ( Wb ) given by

Wb = Inertia force/Surface tension force=2

12

v c

l

V L

=

or,1/ 2

2l

v c

V

L

=

(6.3)

where cL is the characteristics length describing the pore size.

If, 1Wb> , fluid circulation increases until the liquid return path can notaccommodate the increased flow. This causes draught and overheating of theevaporator.

The entrainment limit can be estimated from1/ 2 1/ 2

2 2l l vv fg v fg fg

v v c c

QVh h h

A L L

= = =

(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 inthe wick structure at liquid-vapor interface. When a typical meniscus is


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characterized by two principal radii of curvatures 1r and 2r , the pressure drop

across the liquid surface is

1 2

1 1cp

r r

= +

(6.5)

If the liquid wets the wick perfectly, the radii will be defined by the pore size of thewick, which fixes the limit of heat transfer. Any further increase in heat transferwill cause the liquid to retreat into the wick, and dry out and overheating willoccur at the evaporator end. The capillary force can be increased by decreasingthe pore size of the wick, as shown by Poiseulle equation

.

4

8 ll

m Lp

r

=

(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 spotsand destroy the action of the wick. Therefore, heat pipes are heated isothermallybefore being used to allow the liquid to wet the inner heat pipe wall and to fill allbut 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

r

=

(6.7)

( )w vk T TQ

A t

=

(6.8)

wherevp = vapor pressure inside the bubble

lp = liquid pressure outside the bubble

r =

radius of the largest nucleation siteA = heat input areak = 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


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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. Boilingis, however, not a limitation with liquid metals, but when water is used as theworking fluid, boiling may be a major heat transfer limitation because kof wateris 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 flows through a circular tube segment of

diameter D and length dx .The tube is oriented at an angle with the vertical.

The tube is also subjected to an external acceleration ng directed at an angle

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 pressurep . 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 stresss

acts at the wall in a

direction opposite to that of the fluid flow.

Applying conservation of mass

( ) ( )2 24 4

rD V D dx V dV D

+ = +

(6.9)


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or,

4 rVdV dxD

=

(6.10)

Eq. (6.10 ) relates the increment in axial velocity to the radial inlet velocity.

Conservation of momentum:

Let, = Total momentum flow rate/Momentum flow rate based onaverage velocity

Now,

( ) ( )

[ ]

. . .2 2

2

4 4

cos cos

4s

m d m V dV mV D p D p dp

D dx D ng g dx

+ + = +

+

(6.11)

Assuming,.

. 0d m dV =

We get,

( )4

2 cos cossdp dx VdV n gdxD

= + +

(6.12)

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

( ) ( )2 200

4cos cos

L

s ip dx V V n gLD

= + +

= Frictional shear +

Pressure drop due to momentum+Pressure drop due togravity/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)


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fdp = frictional pressure drop

fdp =2

4 f sD dp D dx

=

(6.14)

or,

fdp = 24

f sD dp D dx =

(6.15)

4

f

s

dpD

dx

=

(6.16)

Again,

f

dxdp

D

(6.17)

or,

2V2

fdp (6.18)

Hence,

2

22

4 4 2s

Vf

D f V

D

= =

(6.19)

For laminar flow:

Friction factor,

64 64

Re

v

v av a

fV D

= =

(6.20)

Where

2

max

0

(2 ) / / 2

R

av aV V r dr R V = =

(6.21)

.2

2

32

2v

dx V mdp f dx

D D A

= =

(6.22)


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.

2

32

v eff

mp L

D A

=

(6.23)

or,.

4

128

v v eff

v

v

m Lp

D

=

(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 Reynoldss Number Friction factor

1 5000 0.0392 10,000 0.0373 100,000 0.022

For liquid: If the flow is through the porous medium,

Applying Darcys law.

l l l

l w

dp mk

dx A

=

(6.25)

. .

1

l eff l l eff l

l

l w wl w

L m L mp

A KA

k

= =

(6.26)

where k = friction factor

wK = permeability of wick material=1

k

l = viscosity of liquid, Ns/m

.

lm = mass flow rate of liquid, kg/s

l = 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


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6.7 Condition for flow in a heat pipe

Maximum capillary pressure drop Pressure drop required to return liquid fromevaporator to condenser + Pressure drop required to move the vapor fromevaporator 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 coscos

l eff l v v eff ll eff

c l w w v

L m m LgL

r A K D

= + +

(6.28)

Generally,

4

128v l

v l w wD A K


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The liquid propertyl l fg

l

h

is know as Figure of Merit , M.

Now for heat transfer rate to be maximum, the liquid mass flow rate is to bemaximum requiring lesser and lesser values of . For properly wetted surfaces,

00 . 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 presentssome typical values of wick materials.

Table 6.2 Some data for wick materials

Sl No Material and MeshSize

Capillaryheight(cm)

Pore radius(cm)

Permeability

1 Glass fibre 25.4 ----- 40.061 10

2 Monel beads( )70 80 m

39.5 0.019 100.78 10

3 Nickel Powder

( )200 m 24.6 0.038 100.027 10

4 Copper Powder

( )45-56 m 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 heattransfer rate.

The wick can inhibit entrainment of liquid in vapour restricting the heat transfercapability of the heat pipe. Entrainment occurs when the shear force between thecounter current liquid and vapor flow is sufficient to entrain droplets of liquid tocarry them back to the condenser.

Because of provision of wick, heat pipe is more efficient than the thermo siphon.


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6.8 Working fluid

1. Water2. Fluorocarbons3. Mercury

4. Methanol5. Glycerin6. Acetone7. Sodium, Potassium, Lithium, etc8. Silver9. Molten salt10. Ammonia11. Organic fluids- such as liquid hydrogen, liquid oxygen, liquid nitrogen

6.9 Desirable properties of working fluid1. It should be non-toxic

2. Non-corrosive3. It should have low viscosity4. It should have high surface tension ( must be wettable)5. High latent heat6. Good heat transfer property7. 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 therange 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 tubebundles to a reasonable level.

3. Length of pipe- Maximum 5 m. With increase in length wick design andpipe length becomes complicated.

4. In case heat pipe is attached to fins, fin pitch and fin shape depends uponthe pressure drop and fouling.

6.11 Container materials

1. Glass2. Stainless steel3. Copper4. Aluminum


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5. Ceramic

6.12 Wick materials

1. Woven cloths

2. Glass fibre3. Sintered material4. Wire meshWick 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 viscousdrag+ 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 energymoving axially along the pipe. Heat pies of different rating are available instandard sizes (1 kW, 10 kW and so on).

Performance of heat pipe depends on the angle of operation. The tube bundlemay be horizontal or tilted with evaporator section below the condenser. Becauseof this sensitivity, the angle of heat pipe may be adjusted in situ as a means ofcontrolling the heat transport. A number of proprietary units incorporate tilt controlmechanisms which can be adjusted either manually or automatically to cater forchanges in heat transfer requirements.

A large number of fluid and pipe material combinations have been used for heatpipes, and some typical working fluid and material combinations as well as thetemperature ranges over which they can operate are presented in Table 6.3.

Table 6.3 Working temperature range, working fluid, heat pipe constructionmaterial and maximum axial heat flux

Sl No TemperatureRange

(K)

Working fluid Container material Maximumaxial heat flux

( W/m2

)1 230-400 Methanol Copper, nickel,

stainless steel0.45

2 280-500 Water Copper, Nickel 0.673 360-850 Mercury Stainless Steel 25.14 673-1073 Potassium Nickel, Stainless Steel 5.65 773-1173 Sodium Nickel, Stainless Steel 9.3


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PART-II HeatPumps

6.14 Introduction

Heat pumps are used for up gradation of low grade (reject) heat. Suppose heat is

available at0

30 Cand heat requirement is at0

70 C. Thus with the help of a heatpump low grade heat at 030 C can be used to deliver heat at 070 C. Thisincreases the quality of energy. The up gradation of energy is done at theexpense 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 theexpense 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/cwith 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


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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 islow 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 largebuildings

(3) Heat pumps with a double bundled condenser


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(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 arecapable of heating a space in cold weather and cooling in warm weather. Thereversible heat pump operates with the help of a four way valve. During heatingoperation (Fig.) the four way valve positions itself so that the high pressuredischarge gas from the compressor flows first to the heat exchanger in theconditioned 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 andinstead the liquid refrigerant flows through the expansion device in the lowerbranch. The cold low pressure refrigerant then extracts heat from the outdoor airwhile it vaporizes. Refrigerant vapour returns to the four-way valve to be directedto the suction side of the compressor.

To convert from heating to cooling operation the four-way valve shifts to itsopposite position so that discharge gas from the compressor first flows to theoutdoor coil, where the refrigerant rejects heat during condensation. Afterpassing through the expansion device in the upper branch, the low pressure lowtemperature refrigerant evaporates in the heat exchanger that cools air from theconditioned 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


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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


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15 deg CPump

E.V

Condenser

Treated water

Water

R11

Compressor

EvaporatorWater 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 mouldsduring solidification of plastics in components being moulded.


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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 otherliquids 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 gradationof energy. If temperature difference is highrecommend 2 stage compressionwith intercooling.


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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

23

4

56

78

9 10 11

0PPP ki =

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


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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


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The evaporator cools the wet air below the dew point temperature, removes thesensible and latent heat. As a result of cooling, condensation of some part ofwater vapour in the air occurs and it is drained off. The dehumidified air is passedthrough 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 aresidence. The building heat loss is 20 kw. Compression is reversible andadiabatic. (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 airEV

WaterSeparator

Condensate

Feed (wet)

Dry material

C


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Cond

CompEV

E

W

Q1

Q2=20KW

T1=5deg C

Building 25 deg C

Fig. (a)

KWQ

TT

T

QQ

Q

W

QCOP

20

9.1420

298

2

12

2

12

22

=

==

=

==

Applications:

1. Washing, sterilizing and clean up

2. Cooking3. Pasteurization4. Bleaching5. Dye heating6. Log soaking7. Paint drying8. Pulp heating

Chemical heat pump

Temperature Amplifier typePropanol (C3H8O + Heat C3H6O + H2 )


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Fig. 6.19 Chemical heat pump

Getting heat at 200 0C from heat source = 800CCharging process: C3H 8OC3H6O + H2Endothermic process

Reactor: C3H6O + H2NiC3H 8O + 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 bemaintained at 200C. Heat transferred through the walls and roofs is estimated tobe 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 powerrequired to drive the heat pump.b. If the power input is same as in part a. what is the maximum outer temperaturein summer for which the inside can be maintained at 200C.Ans: a. W = 403.2 W). b. t1= 35.42

0C

Solution :

80 deg(Charging Head)

DistillationColumn

200 deg (Heatrecovery)

C3H8O (Propanol)

Reactor with NiCatalyser

C3H6O + H2


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20 deg C

heat loss

5 deg C

20 deg C

W

5 deg CQ 1

Fig b

W = Q2 Q1

Walls and roofs=C

Kw0

522.0

COP = 33.115

20

12

2

12

2==

=

TT

T

QQ

Q

Temperature difference = 150CTotal heat loss = 0.525x15=7.875kw

Room20 deg C

W

Atm5 deg C

Q1

Q2

T1

a) Q2= 0.525KJ/SK= 0.525 x 103x (20-3) J/s= 7875W

W = Q2 Q1


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COP =12

2

12

22

TT

T

QQ

Q

W

Q

=

=

WQ

W

COP

16.40353.19

7875

33.19

53.19520

20273

2===

=

+=

b) summer

Assume power output to be same for both season, heat load to beremoved 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

T

Tor

T

T

T

T

Q

QQ

Q

W

021

1

2

1

2

1

2

1

21

1

81.3581.308949.0

20273

949.0

949.005.01

17875

16.403

1

==+

==

==

=

=

==


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Wood burninghearth

Atmosphere

Room

Q1

Q2

Q3

Q4

E

EV C

Con

C

TPB

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

Heat multiplication factor =1

2

31

42

Q

QQ

QQ

QQ +

+

+ Q3= 0

Q1= 1000KJThen Q1+Q2= 5000KJ

HMF = 5Thermodynamically 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 productswhich could not be used in a sold for re-use elsewhere.

Various countries in the world generate different types of waste every day. USAgenerates 160 million tons of soild waste every year. In India, municipal wastegeneration is as high as 27.4 million tons every year. (Every American generates3-4 pounds of waste compared to an Indian who generates 100-500 gm everyday). 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 causestremendous green house gas emission.

Table 6.4 presents various types of waste materials generated/year in India.

Sl No Item Quantity of wastegenerated

1 Municipal solid waste 27.4 MT/year2 Municipal liquid waste (121 Class I and Class

II cities)12145 Ml/day

3 Distillery (243 Nos) 8057 kcall/day4 Food and fruit processing waste 4.5 MT/year5 Dairy waste 50-60 Ml/day6 Paper and pulp industry waste ( 300 mills) 1600 m3waste water/day7 Tannery (2000 Nos) 52500 m3waste

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 largecities.

(2) Liquid incinerator- incinerators for waste liquids can be used to removepollutions and to recover inorganic.


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(3) Gas incinerator- incinerators for disposal of obnoxious and poisonous gasto 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. Tailgas 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 wherenatural 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 of0760 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 gasnow enters in a waste heat recovery boiler to produce steam. Steam output of 11ton/hr may be obtained by this arrangement. Exhaust gas leaving the waste heat

boiler is cooled down to 0260 Cwhich is safely disposed through the stack. Fume

gas may be obtained from the process of producing metallic anhydrous (chemicalintermittent 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 saltsFig. presents the working of a liquid incinerator. In this device liquid waste at therate 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 petrochemicalindustries.

PART-IV ORGANIC RANKINE CYCLE

6.21 Introduction

Organic substances, that can be used below a temperature of 400o

C do not needto be overheated. For many organic compounds superheating is not necessary,resulting in a higher efficiency of the cycle. This is called an Organic RankineCycle (ORC). The low temperature waste heat can be recovered/utilized withORC.

ORC can make use of low temperature waste heat to generate electricity. At lowtemperature a steam cycle would be inefficient, due to enormous volumes of lowpressure steam, causing very voluminous and costly plants. ORCs can beapplied 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 twodecades. It is estimated that already about 30 commercial ORC plants werehavebeen 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 isestimated to be between 10 and 20%, depending on temperature levels. Onmany sites no suitable use is available for low temperature waste heat, henceupgrading by the use of a heat pump (or transformer) or an ORC are good

energy recovery candidates. However, the maximum temperature of heat pumpsis still limited, making ORC a good technology for heat recovery for base loadpurposes in the range of 150 till 200oC, if no other use for the waste heat isavailable on site. Some of the organic compounds used in ORC are listed inTable-6.5 & 6.6.


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Table-6.5: Halogenated hydro-carbons

Sl No Nomenclature Chemicalformula

Boiling point

1 R113CFCl

023.7 C

2 R12 2 2CF Cl 029.8 C 3 R22

2CHF Cl 040.8 C

4 R133

CClF 081.5 C

Table-6.6: Hydro carbon group

Sl No Nomenclature Chemicalformula

Boiling point

1 Propane3 8C H

042.12 C

2 n-Butane4 10C H

00.5 C

3 Pentane5 10

C 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 theheat source, and have a large amount of available waste heat at stationaryconditions. There is also a need to condense the working vapour. Therefore, acooling medium should be available on site. These site characteristics will limitthe potential application.

The theoretical potential of ORC is determined by the available waste heat, attemperature 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 bemore profitable.


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6.22 DAIMLER BENZ ORC

Daimler Benz carried out a design study of bottoming cycles applied to vehiculardiesel engine. Utilizing the waste heat of the exhaust gases through an ORCengine, 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 passedthrough a heat exchanger (or, evaporator). Waste heat from the exhaust isreceived by the organic fluid which vaporizes and enters a turbine producingpower. The organic fluid after extraction of power at the turbine is passed througha condenser and then through a preheater to raise its temperature before entry tothe heat exchanger (evaporator).

Fig.6.20 Organic Rankin cycle

Supercritical FIAT cycle using R11 as a working fluid was used in an ORCengine. Rating was 95 kW for recovery of waste heat from the exhaust gases ofa 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 aconventional steam turbine, except for the fluid that drives the turbine, which is ahigh molecular mass organic fluid. The selected working fluids allow to exploitefficiently low temperature heat sources to produce electricity in a wide range ofpower 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 theevaporator. The organic fluid vapour expands in the turbine and is thencondensed using a flow of water in a shell-and-tube heat exchanger

(alternatively, ambient air can be used for cooling). The condensate is pumpedback to the evaporator thus closing the thermodynamic cycle. Heating andcooling sources are not directly in contact with the working fluid nor with theturbine. For high temperature applications (e.g. Combined Heat and Powerbiomass-powered plants, high temperature thermal oil is used as a heat carrierand 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 speedd. low RPM of the turbine allowing the direct drive of the electric generator

without reduction geare. no erosion of blades, due to the absence of moisture in the vapour nozzlesf. long lifeg. no operator required

The system also has practical advantages, such as simple start-stop procedures,quiet operation, minimum maintenance requirements, and good part loadperformance. Among the working fluids than can be evaluated for use in thesesystems is Honeywell 245fa, a high-boiling refrigerant now available incommercial quantities from Honeywell..

The favorable performance of Honeywell 245fa in Rankine cycles provides anopportunity to realize greater electrical energy output from power generationfacilities that rely on steam-driven turbines. Likewise, large industrial enterprisescan now consider recovery of waste heat with the option to convert the energy toUsefulelectricity.

chapter 6 - waste heat recovery devices

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