2548 Aguk Zuhdi Mf Refrigeration Cycle

1

Chapter 4

Refrigeration cycles

DEPARTMENT OF MARINE ENGINEERINGFACULTY OF OCEAN TECHNOLOGY

INSTITUTE TECHNOLOGI SEPULUH NOPEMBERME091307 (THERMODYNAMICS)

Aguk Zuhdi Muhammad Fathallah

4.1 Refrigerators and heat pumps

Refrigerator/heat pump– Transfer heat from a low temperature region to high temperature region.

Objective of refrigerator :Maintain the refrigerated space at low temperatureby removing heatfrom it

Objective of heat pump :Maintain the heated space at a high temperatureby transfer heatto it

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eperformanc oft coefficien COP where

inputWork

effect Heating

input required

output DesiredCOP

inputWork

effect Cooling

input required

output DesiredCOP

innet

HHP

innet

LR

=

===

===

W

Q

W

Q

1COPCOP RHP +=

Performance of refrigerators and heat pumps

1COPHP >

3

4.2 The reversed Carnot cycle

High Temperature Heat Source, TH

Low Temperature Heat Sink, TL

Net Work Output, Wnet

Heat Supplied, qs

Heat Rejected, qr

Forward Heat Engine

rsnet

rnets

qqw

qwq

−=+=


High Temperature Heat Sink, TH

Low Temperature Heat Source, TL

Net Work Output, Wnet

Heat Rejected, qr

Heat Supplied, qs

Reversed Heat Engine

srnet

rnets

qqw

qwq

−==++

Carnot refrigerator

Carnot heat pump

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Operation of the Carnot refrigerator1-2 Isentropic

compression (Compressor)

2-3 Isothermal heat rejection (Condenser)

3-4 Isentropic expansion (Turbine)

4-1 Isothermal heat absorption (Evaporator)


H

LCarnot HP,

L

HCarnot R,

1

1COP

1

1COP

TT

TT

−=

−=

Performance of Carnot refrigerators and Carnot heat pumps

COPs increase as the difference between the two temperatures decrease, where

TL rises or TH falls

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Impractical approximation especially for

Process 1-2 (Compressor)

• Involve compression of liquid-vapor mixture (two phases)

Process 3-4 (Turbine)

• Expansion of high moisture content refrigerant

Refrigeration load and effect

Is the rate which heat must be removed from the refrigerated space in order to produce and maintain the desired temperature

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3.516

evaporatorat effect ingRefrigerat

load ingRefrigerat rate flow Mass

hh

kWmr −

=

=

&

Power required to drive the compressor ( )12 Power, hhmw r −= &&

Quantity of cooling water in condenser

)()( 32inout hhmTTcm rpcc −=− &&


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4.3 The ideal vapor-compression refrigeration cycle

Elimination of impracticalities with the reversed Carnot cycle by

Vaporizing the refrigerant completely before compression process

Replacing the turbine with a throttling device (expansion valve or capillary tube)


1-2 Isentropic compression (Compressor)

2-3 Isobaric heat rejection (Condenser)

3-4 Throttling (Expansion device)

4-1 Isobaric heat absorption (Evaporator)

Not an internally reversible cycle since it involves an irreversibible (throttling) process

Higher efficiency but high cost and complexity

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COP improved by 2 to 4 percent for each oC the evaporating temperature is raised or the condensing temperature is lowered


( ) ( )

case idealfor @

and @ where

COP

and

COP

equation,energy flowsteady The

33

1g1

12

32

innet

HHP

12

41

innet

LR

ieoutinoutin

Phh

Phh

hh

hh

w

q

hh

hh

w

q

hhwwqq

f=

=

−−==

−−==

−=−+−

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Refrigerant R134a enters the compressor of a refrigerator as superheated vapor at 0.14MPa and -10oC at a rate of 0.12kg/s, and it leaves at 0.7MPa and 50oC. The refrigerant is cooled in the condenser to 24oC and 0.65MPa, and it is throttled to 0.15MPa. Disregrading any heat transfer and pressure drops in the connecting lines between the components, show the cycle on a T-s diagram with respect to saturation lines, and determine

a. The rate of heat removal from the refrigerated space and the power input to the compressor

b. The isentropic efficiency of the compressor

c. The COP of the refrigerator

4.4 Actual vapor-compression refrigeration cycle

Irreversibilities in actual cycle

•Fluid friction and piping losses (cause pressure drops)

•Heat transfer to or from the surroundings

•Mechanical inefficiencies

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4.4 Actual vapor-compression refrigeration cycle

4.4 Actual vapor-compression refrigeration cycleActual compression with frictional

effects such as fluid friction and

heat transfer

(s increase 1-2 or s decrease 1-2’)

Over designed Superheated

vapor (ensure 100% vapor at

compressor inlet)

More desirable cause lower

work input requirement

Pressure drop at condenser

until throttling valve

Refrigerant is subcooled

before enteres the throttling

valve

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4.5 Selecting the right refrigerant

Choice of refrigerants

Chlorofluorocarbons (CFCs)

Ammonia

Hydrocarbons (propane, ethane, ethylene, etc)

Carbon dioxide

Air

Water

Chlorine free R-134a

Refrigerants

4.5 Selecting the right refrigerant

Selection of the refrigerant

1. Temperatures of the two media (the refrigerated space and the environment)

2. Non toxic

3. Non corrosive

4. Non flammable

5. Chemically stable

6. High enthalpy of vaporization (minimise mass flow rate)

7. Low cost

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4.6 Heat pump systems

Different between heat pump and air conditioner

1.Condenser of the heat pump (located indoors) function as the evaporator of the air conditioner.

2.Evaporator of the heat pump (located outdoors) serves as the condenser of the air conditioner

HEAT PUMP OPERATION – HEATING MODE

Fan

Fan

Compressor

IndoorCoil

Reversing valve

Expansion valve

Hot Air

Cold Air

Outdoor coil

High – Pressure liquid

Low – Pressure liquid vapor

Low – pressure vapor

High – Pressure vapor


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Fan

Fan

Compressor

Expansion valve

Out Hot Air

Cold Air

HEAT PUMP OPERATION – COOLING MODE

IndoorCoil

Reversing valveOutdoor coil


High – Pressure liquid

Low – Pressure liquid vapor

Low – pressure vapor

High – Pressure vapor

4.7 Innovative vapor-compression refrigeration systems

Ordinary vapor compression refrigeration

a. Simple

b. Inexpensive

c. Reliable

d. Practically maintenance freeNot efficient , not simplicity for large industrial application

Need modification and refinement to innovative system

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Cascade Refrigeration Systems

win

win

Closed type

Could be different working fluid for both compressor


Cascade Refrigeration Systems

Increase refrigeration capacity and decrease in compressor work

Refrigerants for both cycles not necessary to be same cause no mixing

)()(

)(

)(

)(

)()(

1256

41

innet

LCascade R

85

32

3285

hhmhhm

hhm

w

QCOP

hh

hh

m

m

hhmhhm

BA

B

B

A

BA

−+−−==

−−=

−=−

&&

&

&

&

&&

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Multistage Compression Refrigeration Systems

A

A

m=x

m=1-x

m=1

winA

winB

Direct contact heat exchanger

Two phase, closed vessel

m=x

m=1-x

winA

winB

QH

QL


Multistage Compression Refrigeration Systems

Heat exchanger replaced by a mixing chamber/flash chamber

Saturated vapor, state 3Superheated vapor from Low pressure compressor, state 2

High pressure compressor, state 9

Saturated liquid expand through second expansion process, state 7

Regeneration process

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Multipurpose Refrigeration Systems with a Single Compressor

For application with more than one temperature : Refrigerator - Freezer

Study case : Requirement for refrigerator - freezer

1.Refrigerator : maintained above the ice point due to high water content goods

2.Freezer: maintain at about -18oC for ice making or solid good storage

QL

QL

QH

Win

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Multipurpose Refrigeration Systems with a Single CompressorHigh expanding

pressure, T

above ice point

Low expanding pressure, T below ice point


Liquefaction of Gases

How to liquefy the Oxygen gas? Nitrogen gas?

What is the usage of the liquefied gases?

• Preparation of liquid propellants for Rockets

• Study of material at low temperatures

• Exciting phenomena such as superconductivity

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Liquefaction of Gases

W

4.8 Gas refrigeration cycles

Also known as reversed Brayton cycle

For the surrounding temperature, To

1-2 Gas compression process

2-3 Heat rejection of refrigeration gas to To

3-4 Gas expansion process to temperature T4

4-1 Heat absorption from the refrigerated space

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Also known as reversed Brayton cycle

[kJ/kg]

[kJ/kg]

[kJ/kg]

12in comp

43out turb

41L

out turbin comp

L

innet

LR

hhw

hhw

hhq

ww

q

w

qCOP

−=

−=

−=

−==


Use a gas refrigeration cycle with a regenerator.

This is the idealized cycle with a regenerator. This is the idealized cycle with a regenerator. Other system configurations are possibleOther system configurations are possible

QH

QL

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4.9 Absorption refrigeration systems


After the evaporator,1. NH3 dissolved in water NH3.H2O (exothermic reaction –

release heat to cooling water)

2. Pump transfer the liquid NH3.H2O to regenerator to vaporized some of the solution NH3.H2O.

3. The vapor rich in NH3 is then passes through the rectifier to separated the NH3 from water.

4. High pressure NH3 vapor is use for the rest of the refrigeration process.

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gen

Labsorption

in pumpgen

Labsorption

absorption output Required

output Desired

Q

QCOP

WQ

QCOP

COP

≅

+=

=


For maximum

COP (totally reversible

cycle)

2548 Aguk Zuhdi Mf Refrigeration Cycle

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