Week 9. Refrigeration Cycles I

GENESYS Laboratory

Objectives

1. Introduce the concepts of refrigerators and heat pumps and the measure of their performance.

2. Analyze the ideal vapor-compression refrigeration cycle. 3. Analyze the actual vapor-compression refrigeration cycle. 4. Review the factors involved in selecting the right refrigerant for an

application. 5. Discuss the operation of refrigeration and heat pump systems. 6. Evaluate the performance of innovative vapor-compression

refrigeration system . 7. Analyze gas refrigeration systems. 8. Introduce the concepts of absorption-refrigeration systems. 9. Review the concepts of thermoelectric power generation and

refrigeration

GENESYS Laboratory

Refrigerators And Heat Pumps

• The transfer of heat from a low-temperature region to a high-temperature one

• The performance of refrigerators and heat pumps is expressed in terms of the

coefficient of performance (COP)

LR

net,in

HHP

net,in

HP R

Desired output Cooling effect

Required input Work input

Desired output Heating effect=

Required input Work input

1

QCOP

W

QCOP

W

COP COP

GENESYS Laboratory

The Reversed Carnot Cycle

Carnot refrigerator and T-s diagram of the reversed Carnot cycle

• Carnot Refrigerator

• Carnot Heat Pump

1

1COP CarnotR,

L

H

TT

H

L

TT

1

1COP CarnotHP,

The reversed Carnot cycle is not a suitable model for refrigeration cycles Process 1→2, 3→4 : achievable Process 2→3 the compression of a liquid-vapor mixture Process 4→1 the expansion of high- Moisture-content refrigerant in a turbine

GENESYS Laboratory

The Ideal Vapor-Compression Refrigeration Cycle

• Many of the impracticalities associated with the reversed Carnot cycle can be

eliminated by vaporizing the refrigerant completely before it is compressed and by

replacing the turbine with a throttling device, such as an expansion valve or

capillary

T-s diagram for the ideal vapor-compression refrigeration cycle

• Four processes: 1-2 Isentropic compression in a compressor 2-3 Constant-pressure heat rejection in a condenser 3-4 Throttling in an expansion device 4-1 Constant-pressure heat absorption in an evaporator

GENESYS Laboratory

The Ideal Vapor-Compression Refrigeration Cycle

• P-h diagram

-Process 3-4 is isenthalpic process (expansion valve)

-Processes 2-3 and 4-1: Q is determined as deviation between “h”s

-Process 1-2: W is determined as h2-h1

The P-h diagram of an ideal vapor-compression refrigeration cycle

12

32

innet,

HHP

12

41

innet,

LR

hh

hh

w

qCOP

hh

hh

w

qCOP

• The COPs of refrigerators and heat pumps operating on the vapor-compression refrigeration cycle can be expressed as

GENESYS Laboratory

Ex 1) The Ideal Vapor-Compression Refrigeration Cycle

GENESYS Laboratory

Ex 1-1) The Ideal Vapor-Compression Refrigeration Cycle

Consider an ideal refrigeration cycle that uses R-134a as the working fluid. The temperature of the refrigerant in the evaporator is -20oC, and in the condenser exit, it is 40oC. The refrigerant is circulated at the rate of 0.03 kg/s. Determine the COP and the capacity of the plant in rate of refrigeration.

GENESYS Laboratory

Actual Vapor-Compression Refrigeration Cycle

• An actual vapor-compression refrigeration cycle differs from the ideal one owing

to the irreversibilities (e.g. fluid friction causing pressure drops and heat transfer to

or from the surroundings) that occur in various components.

GENESYS Laboratory

• The compression process in the actual cycle is not isentropic • The entropy of the refrigerant may increase (process 1-2) or decrease (process 1-2’) due to cooling effect

• The refrigerant is subcooled somewhat before it enters the throttling valve and superheated before it enters the compressor

Ex 2) The Actual Vapor-Compression Refrigeration Cycle

GENESYS Laboratory

Ex 2-1) The Actual Vapor-Compression Refrigeration Cycle

A refrigeration cycle utilizes R-134a as the working fluid. The following are the properties at various points of the cycle designated in Figure. P1=125 kPa, T1=-10

oC P2=1200 kPa, T2=100

oC P3=1190 kPa, T3=80

oC P4=1160 kPa, T4=45

oC P5=1150 kPa, T5=40

oC P6=P7=140 kPa, x6=x7 P8=130 kPa, T8=-20

oC The heat transfer from R-134a during the compression process is 4 kJ/kg. Determine the COP of this cycle.

GENESYS Laboratory

Innovative Vapor-Compression Refrigeration Systems

• The ordinary vapor-compression refrigeration systems are simple, inexpensive,

reliable, and practically maintenance-free

• However, for large industrial applications efficiency, the major concern is not

simplicity.

• Modifications and refinements are necessary

- Cascade Refrigeration Systems

- Multistage Compression Refrigeration Systems

- Multipurpose Refrigeration Systems with a Single Compressor

- Liquefaction of Gases

GENESYS Laboratory

Cascade Refrigeration Systems

• Need to operate a large temperature range

-Way of dealing with a large pressure range in the cycle and a poor performance for

reciprocating compressor

• Solution

- Two or more refrigeration cycles that operate in series

-Refrigerants in both cycles can be the same or different, but a certain refrigerants

with more desirable characteristics can be used on each cycle

• Result

-The compressor work decreases and the

amount of heat absorbed from the refrige-

rated space increases

- The ratio of mass flow rates is

1256

41

innet,

LcascadeR,

85

323285

hhmhhm

hhm

W

QCOP

hh

hh

m

mhhmhhm

BA

B

B

ABA

GENESYS Laboratory