Refrigeration System.pdf

MEC653 HVAC SYSTEM CHAPTER 9 REFRIGERATION SYSTEM

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Vapor Compression Refrigeration Cycle The processes in the vapor compression refrigeration cycle are as per following: Process 1-2

Process Description Isentropic compression

2-3 3-4

Constant pressure heat rejection Throttling process

4-1

Constant pressure heat absorption

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Refrigeration cycle is based on the modification of reversed Carnot cycle

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The P-h diagram is another convenient diagram often used to illustrate the vapor-compression refrigeration cycle.

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Components Description COMPRESSOR: Provide the driving force for the entire system by drawing low pressure refrigerant in and adding pressure such that it exits at a higher temperature. CONDENSER: Exhaust heat from the system by virtue of heat transfer across a temperature gradient. The refrigerant in the condenser is at a higher temperature than the ambient temperature.

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EXPANSION VALVE: Allow the refrigerant to expand dramatically in a controlled process such that it exits the valve at a low quality liquid-vapor mixture. EVAPORATOR: Absorb heat from the cold space by virtue of a temperature gradient, similar to the condenser.

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Performance of Refrigeration System Results of First Law Component

Compressor

First Law Result .

.

W in  m(h2  h1 ) .

.

Condenser

Q H  m(h2  h3 )

Expansion valve

h3  h4

Evaporator

.

.

Q L  m(h1  h4 )

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Results of Second Law

Cycle

Second Law Result .

Refrigerator

QL

COPR 

.

W in

h1  h4  h2  h1

.

Heat pump

COPHP 

QH .

W in



h2  h3 h2  h1

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Chiller COP vs Chiller Efficiency

.

COPR 

QL



.

W in

h1  h4 h2  h1

.

Efficiency 

W in in kW .

Q L in RT

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Screw compressor

Centrifugal compressor

Reciprocating compressor Global Aspiration …… A World Class University

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Integrated Part Load Value Typical figures of chiller efficiency (kW/RT) at part load: % Load

Centrifugal

Screw

Reciprocating

A

100

0.68

0.81

0.85

B

75

0.70

0.72

0.95

C

50

0.73

0.67

1.10

D

25

0.98

0.69

1.42

*100 RT chiller operating 12 hours per day, 300 days per year??? Global Aspiration …… A World Class University

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Chiller Plant COP vs Chiller Plant Efficiency

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Refrigerants Type of refrigerants:

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Refrigerant Selection Two important parameters: the temperatures of the refrigerated space and the environment with which the refrigerant exchanges heat. R-134a (replaced R-12, which damages ozone layer) is used in domestic refrigerators and freezers, as well as automotive air conditioners. R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets. Global Aspiration …… A World Class University

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COP comparison between different refrigerants under the same operating conditions.

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Absorption Refrigeration Systems The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small, and the COP of absorption refrigeration systems is defined as: Desired output Cooling effect QL QL COPR     Required input Work input Qgen  Wpump ,in Qgen

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Cooling water flows

Cooling tower

Make-up water tank 3

Condenser

2

Expansion valve

Compressor

Refrigerant R-22 flows Evaporator 4

Make-up water flows

1 From building

To building Chilled water flows

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Water-cooled chiller Split unit

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Air-cooled packaged

Water-cooled packaged

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