lecture 2. refrigeration cycles

7/29/2019 Lecture 2. Refrigeration Cycles

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Lecture 2

Refrigeration Cycles

By: Addisu Dagne1

Engineering and Technology college

Mechanical Engineering Department

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The Reversed

Carnot Cycle

Schematic of aCarnot refrigerator

and T-sdiagram of

the reversed Carnot

cycle.

Both COPs increase

as the difference

between the two

temperatures

decreases, i.e. as TL

rises orTH

falls.

The most efficient refrigeration cycle

operating between TL and TH. But not a

suitable model for refrigeration cycles

because: (i) process 2-3 involvescompression of a liquidvapor mixture -

requires a compressor that will handle

two phases, (ii) process 4-1 involves

expansion of high-moisture-content

refrigerant in a turbine.

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Ideal Vapor-compression Refrigeration

CycleIs the ideal model for refrigeration systems. The refrigerant is vaporizedcompletely before it is compressed and the turbine is replaced with a

throttling device.

Schematic and T-sdiagram for the ideal vapor-

compression refrigeration cycle.

The mostwidely usedcycle for

refrigerators,A-C systems,and heatpumps.

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An

ordinary

household

refrigerator

.The P-hdiagram of an ideal

vapor-compression

refrigeration cycle.

The ideal vapor-compression refrigeration cycle involves an

irreversible (throttling) process to make it a more realistic model for

the actual systems.Steady-flow

energybalance

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1112

A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal

vapor-compression refrigeration cycle between 0.12 and 0.7 MPa. The mass flow

rate of the refrigerant is 0.05 kg/s. Show the cycle on a T-s diagram with respect to

saturation lines. Determine:

a) the rate of heat removal from the refrigerated space,

b) the power input to the compressor,

c) the rate of heat rejection to the environment, and

d) the coefficient of performance.

Answers: (a) 7.41 kW, 1.83 kW, (b) 9.23 kW, (c) 4.06

ProblemIdeal and Actual Vapor-Compression Refrigeration Cycles

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Actual Vapor-Compression Refrigeration Cycle

Schematic and T-sdiagram for the actual vapor-

compression refrigeration cycle.

An actual vapor-compression refrigeration cycle involves

irreversibilities in various components - mainly due to fluid friction

(causes pressure drops) and heat transfer to or from the surroundings.As a result, the COP decreases.

Differences

Non-isentropiccompression;

Superheatedvapor atevaporator exit;

Sub-cooled liquidat condenser exit;

Pressure drops incondenser andevaporator.

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Consider a 300 kJ/min refrigeration system that operates on an ideal vapor-

compression refrigeration cycle with refrigerant-134a as the working fluid. The

refrigerant enters the compressor as saturated vapor at 140 kPa and is

compressed to 800 kPa. Show the cycle on a T-s diagram with respect to

saturation lines, and determine the:a)quality of the refrigerant at evaporator inlet,

b)coefficient of performance, and

c)power input to the compressor.

Problem Class

ExerciseIdeal and Actual Vapor-Compression Refrigeration Cycles

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1118

Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at

0.14 MPa and 10C at a rate of 0.12 kg/s, and it leaves at 0.7 MPa and 50C. The

refrigerant is cooled in the condenser to 24C and 0.65 MPa, and it is throttled to

0.15 MPa. Disregarding any heat transfer and pressure drops in the connecting

lines between the components, show the cycle on a T-s diagram with respect tosaturation lines, and determine:

a)the rate of heat removal from the refrigerated space,

b)the power input to the compressor,

c)the isentropic efficiency of the compressor, and

d)the COP of the refrigerator.

Answers: (a) 19.4 kW, 5.06 kW, (b) 82.5 percent, (c) 3.83

ProblemIdeal and Actual Vapor-Compression Refrigeration Cycles