# Chapter 9. Refrigeration and Liquefaction

(1)

(2)

## Introduction

###  Maintain the temperature below that of the surrounding

- Absorption of heat at a low temperature level

- Accomplished by evaporation of a liquid in a steady-state flow process - Then, recycled via compression and condensation

###  Liquefaction

- A gas is liquefied

- Very similar to refrigeration process

(3)

(4)

## 9.1 The Carnot Refrigeration

###  Reversed heat-engine cycle

Heat is absorbed at low TC

Heat is rejected at high TH

Requires external source of energy : W

Coefficient of Performance (COP), w

- A measure of the effectiveness of a refrigerator

C H

C C

C H

### 

Coefficient of Performance

(COP)

Hot reservoir at TH

Cold reservoir at TC

C

### Q

H

Carnot refrigeration

(5)

## 9.2 Vapor-Compression Cycle

Liquid returns to its original P by

expansion (Throttling process)

Vapor is cooled and condensed

(Rejection of heat at high TH)

Vapor is compressed to high P

Liquid evaporates at const T & P (Heat absorption

at low TC)

(6)

## 9.2 Vapor-Compression Cycle

2 3

1 C 2

1 2

C

### 

Rate of circulation of refrigerant Coefficient of Performance

Throttling Process (Const. H)

1 2

C

2

3

4 3

H

(7)

### Vapor-compression refrigeration cycle on P-H diagram

P-H diagrams are more commonly used in refrigeration cycle than TS diagrams.

(8)

o

o

o

(a)

(b)

(9)

C H

C C

o

C

o

H

o

(10)

## Solution (b)

Const. S

1 2

4 3’ 3

We should find the enthalpy at 1, 2, 3, and 4!

State 2

0oF, saturation condition (V) P = 21.162 psia

H2= 103.015 Btu/lbm S2= 0.22525 Btu/lbm.F State 4

80oF, saturation condition (L) P = 101.37 psia

H4= 37.987 Btu/lbm State 3’

Read H from Fig.G.2.

P = 101.37 psia, S=0.22525

 H3’ = 117 Btu /lbm H1 = H4

ln P

H

(11)
(12)
(13)

2 3

S

S 3 2

S '

(14)

2 3

1

2

m

1 2

C

(15)

## 9.3 The Choice of Refrigerant

###  Two requirements for refrigerant

Air cannot leak into the refrigeration system  vapor pressure at the evaporator

temperature should be greater than atmospheric pressure (1 atm)

The vapor pressure at the condenser temperature should not be too high  cost for high pressure

(16)

## Choice of Refrigerant

###  Fully halogenated chlorofluorocarbons are most common

- CCl3F (CFC-11), CCl2F2 (CFC-12)

- Cause severe ozone depletion  no more in use

###  Replacement

- CHCl2CF3 (HCFC-123), CF3CH2F (HCF-134a), CHF2CF3 (HCF-125)

(17)

H

C

H

C

(18)

## 9.4 Absorption Refrigeration

Condenser

Evaporator

Compressor Throttling

Valve

This part can be replaced by heat engine – a work

producing device

###  Vapor-compression refrigerator

Work of compression is supplied by an electric motor  mostly from heat engine

###  Absorption refrigeration

Direct use of heat as the energy source for refrigeration

(19)

Throttling Valve

(20)

## Analysis

C C

C

S

S H

H H

H S

H

### 

Work required for the refrigeration cycle

Heat required for the production of the work.

C C S

S H

H C

H

(21)

## 9.5 The Heat Pump

Winter : Heating

Summer : Cooling

###  Economic advantage depends on the cost of electricity comparing to the cost of fuels such as oil and natural gases.

1. Compressor: This increases the pressure of the refrigerant so that it will accept the maximum amount of heat from the air.

2. Condenser: Coils that move heat to or from the outside air.

3. Evaporator: Coils that move heat to or from the air inside the home.

4. Air handler: Fan that blows the air into the ducts of the home.

Components 1, 2, 3 and 4 are found in all standard air conditioners.

5. Reversing valve: Changes the heat pump from air conditioning to heating, and vice versa.

(22)

## Example 9.2

A house has a winter heating requirement of 30 kJ/s and a summer cooling requirement of 60 kJ/s. Consider a heat-pump installation to maintain the house temperature at 20 oC in winter and 25 oC in summer. This requires circulation of the refrigerant through interior exchanger coils at 30 oC in winter and 5 oC in summer. Underground coils provide the heat source in winter and the heat sink in summer. For a year-round ground temperature of 15 oC, the heat-transfer characteristics of the coils necessitate refrigerant temperatures of 10 oC in winter and 25 oC in summer. What are the minimum power requirements for winter heating and summer cooling?

(23)

## Example 9.2

Hot reservoir (House coil) TH= 30 oC

Cold reservoir (underground coil)

TC = 10 oC

C

H

### 

In winter,

a house coil at 30 oC is a hot reservoir (heat sink) and

a underground coil at 10 oC is a cold reservoir (heat source)

Carnot heat pump

C H

H H C C

C H C

H

(24)

## Example 9.2

Hot reservoir (underground coil)

TH= 25 oC

Cold reservoir (House coil) TC = 5 oC

C

### Q 

H

In summer,

a house coil at 5 oC is a cold reservoir (heat source) and a underground coil at 25 oC is a heat reservoir (heat sink)

Carnot heat pump

C H

C C H H

C H C

H

(25)

(26)

## Liquefaction Processes

###  Sufficiently low T and high P desired.

compression Constant P

cooling

(27)

(28)

(29)

(30)

Updating...

관련 주제 :