38658581-Chapter-11-Refrigeration-Cycles.ppt

7/29/2019 38658581-Chapter-11-Refrigeration-Cycles.ppt

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Chapter 11

Refrigeration CyclesStudy Guide in PowerPoint

to accompany

Thermodynamics: An Engineering Approach, 6th edition

by Yunus A. engel and Michael A. Boles


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The vapor compression refrigeration cycle is a common method for transferring heatfrom a low temperature to a high temperature.

The above figure shows the objectives of refrigerators and heat pumps. The purposeof a refrigerator is the removal of heat, called the cooling load, from a low-temperature medium. The purpose of a heat pump is the transfer of heat to a high-temperature medium, called the heating load. When we are interested in the heatenergy removed from a low-temperature space, the device is called a refrigerator.When we are interested in the heat energy supplied to the high-temperature space,the device is called a heat pump. In general, the term heat pump is used to describe

the cycle as heat energy is removed from the low-temperature space and rejected tothe high-temperature space.


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The performance of refrigerators and heat pumps is expressed in terms of coefficientof performance(COP), defined as

COPQ

W

COP QW

RL

net in

HPH

net in

Desired output

Required input

Cooling effect

Work input

Desired outputRequired input

Heating effectWork input

,

,

Both COPR and COPHP can be larger than 1. Under the same operating conditions,the COPs are related by

COP COPHP R 1

Can you show this to be true?

Refrigerators, air conditioners, and heat pumps are rated with a SEER number orseasonal adjusted energy efficiency ratio. The SEER is defined as the Btu/hr of heattransferred per watt of work energy input. The Btu is the British thermal unit and is

equivalent to 778 ft-lbf of work (1 W = 3.4122 Btu/hr). An EER of 10 yields a COP of2.9.

Refrigeration systems are also rated in terms of tons of refrigeration. One ton ofrefrigeration is equivalent to 12,000 Btu/hr or 211 kJ/min. How did the term ton of

cooling originate?


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Reversed Carnot Refrigerator and Heat Pump

Shown below are the cyclic refrigeration device operating between two constanttemperature reservoirs and the T-sdiagram for the working fluid when the reversedCarnot cycle is used. Recall that in the Carnot cycle heat transfers take place atconstant temperature. If our interest is the cooling load, the cycle is called the Carnotrefrigerator. If our interest is the heat load, the cycle is called the Carnot heat pump.


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The standard of comparison for refrigeration cycles is the reversed Carnot cycle. Arefrigerator or heat pump that operates on the reversed Carnot cycle is called aCarnot refrigeratoror a Carnot heat pump, and their COPs are

COPT T

T

T T

COPT T

T

T T

R Carnot

H L

L

H L

HP Carnot

L H

H

H L

,

,

/

/

1

1

1

1

Notice that a turbine is used for the expansion process between the high and low-

temperatures. While the work interactions for the cycle are not indicated on thefigure, the work produced by the turbine helps supply some of the work required bythe compressor from external sources.

Why not use the reversed Carnot refrigeration cycle?Easier to compress vapor only and not liquid-vapor mixture.

Cheaper to have irreversible expansion through an expansion valve.

What problems result from using the turbine instead of the expansion valve?


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The Vapor-Compression Refrigeration Cycle

The vapor-compression refrigeration cycle has four components: evaporator,compressor, condenser, and expansion (or throttle) valve. The most widely usedrefrigeration cycle is the vapor-compression refrigeration cycle. In an ideal vapor-compression refrigeration cycle, the refrigerant enters the compressor as a saturatedvapor and is cooled to the saturated liquid state in the condenser. It is then throttledto the evaporator pressure and vaporizes as it absorbs heat from the refrigeratedspace.

The ideal vapor-compression cycle consists of four processes.

Ideal Vapor-Compression Refrigeration CycleProcess Description1-2 Isentropic compression2-3 Constant pressure heat rejection in the condenser

3-4 Throttling in an expansion valve4-1 Constant pressure heat addition in the evaporator


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The P-hdiagram is another convenient diagram often used to illustrate therefrigeration cycle.


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The ordinary household refrigerator is a good example of the application of this cycle.

COPQ

W

h h

h h

COPQ

W

h h

h h

RL

net in

HPH

net in

,

,

1 4

2 1

2 3

2 1


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Example 11-1

Refrigerant-134a is the working fluid in an ideal compression refrigeration cycle. Therefrigerant leaves the evaporator at -20oC and has a condenser pressure of 0.9 MPa.The mass flow rate is 3 kg/min. Find COPR and COPR, Carnot for the same Tmax andTmin , and the tons of refrigeration.

Using the Refrigerant-134a Tables, we have

12

2 2

11 2

2 11

3

3

21

238.41 278.23900

200.9456 43.79

0.94561.0

3

900

0

s

so

o

s

s

StateState kJ

kJh Compressor exit hCompressor inlet kgkgP P kPa

kJT Cs T CkJ

kg K s sxkg K

State

Condenser exit

P kPa

x

3 4

44 13

4 3

4101.61 0.358

0.4053200.3738

.0

o

StatekJh xThrottle exitkg

kJskJ T T C

s kg Kkg K h h


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1 4 1 4

, 2 1 2 1

( )

( )

(238.41 101.61)

(278.23 238.41)

3.44

LR

net in

Q m h h h hCOP

W m h h h h

kJ

kg

kJ

kg

The tons of refrigeration, often called the cooling load or refrigeration effect, are

1 4( )

13 (238.41 101.61)

min211

min

1.94

LQ m h h

kg kJ Ton

kJkg

Ton

,

( 20 273)

(43.79 ( 20))

3.97

LR Carnot

H L

TCOP

T T

K

K


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Another measure of the effectiveness of the refrigeration cycle is how much inputpower to the compressor, in horsepower, is required for each ton of cooling.

The unit conversion is 4.715 hp per ton of cooling.

, 4.715

4.715

3.44

1.37

net in

L R

WQ COP

hp

Ton

hp

Ton


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Actual Vapor-Compression Refrigeration Cycle


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Heat Pump Systems


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Other Refrigeration Cycles

Cascade refrigeration systems

Very low temperatures can be achieved by operating two or more vapor-compression

systems in series, called cascading. The COP of a refrigeration system alsoincreases as a result of cascading.


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Multistage compression refrigeration systems


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Multipurpose refrigeration systems

A refrigerator with a single compressor can provide refrigeration at severaltemperatures by throttling the refrigerant in stages.


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Liquefaction of gases

Another way of improving the performance of a vapor-compression refrigerationsystem is by using multistage compression with regenerative cooling. The vapor-compression refrigeration cycle can also be used to liquefy gases after some

modifications.


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Gas Refrigeration Systems

The power cycles can be used as refrigeration cycles by simply reversing them. Ofthese, the reversed Brayton cycle, which is also known as the gas refrigeration cycle,is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is

modified with regeneration. The work output of the turbine can be used to reduce thework input requirements to the compressor. Thus, the COP of a gas refrigerationcycle is

COPq

w

q

w wR

L

net in

L

comp in turb out

, , ,


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Absorption Refrigeration Systems

Another form of refrigeration that becomes economically attractive when there is asource of inexpensive heat energy at a temperature of 100 to 200oC is absorptionrefrigeration, where the refrigerant is absorbed by a transport medium and

compressed in liquid form. The most widely used absorption refrigeration system isthe ammonia-water system, where ammonia serves as the refrigerant and water asthe transport medium. The work input to the pump is usually very small, and the COPof absorption refrigeration systems is defined as

COPQ

Q W

Q

QR

L

gen pump in

L

gen

Desired output

Required input

Cooling effect

Work input,


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Thermoelectric Refrigeration Systems

A refrigeration effect can also be achieved without using any moving parts by simplypassing a small current through a closed circuit made up of two dissimilar materials.This effect is called the Peltier effect, and a refrigerator that works on this principle is

called a thermoelectric refrigerator.

38658581-Chapter-11-Refrigeration-Cycles.ppt

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