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CHEMICAL ENGINEERING THERMODYNAMICS-II (CH.E – 307)

ByMuhammad Asif Akhtar

[email protected] Asif Akhtar CH.E 307

REFRIGERATION CYCLES

Muhammad Asif Akhtar CH.E 307

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

Analyze the ideal vapor-compression refrigeration cycle.

Analyze the actual vapor-compression refrigeration cycle.

Review the factors involved in selecting the right refrigerant for an application. Muhammad Asif Akhtar CH.E 307


INTRODUCTION

A major application area of thermodynamics is refrigeration, which is the transfer of heat from a lower temperature region to a higher temperature one.

Devices that produce refrigeration are called refrigerators, and the cycles on which they operate are called refrigeration cycles.

The most frequently used refrigeration cycle is the vapor-compression refrigeration cycle in which the refrigerant is vaporized and condensed alternately and is compressed in the vapor phase.


INTRODUCTION

The purpose of a refrigeration system is to maintain a cold region at a temperature below the temperature of its surroundings.


REFRIGERATORS AND HEAT PUMPS

The objective of a refrigerator is toremove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium.


Coefficient of Performance

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


The Cooling Capacity of a Refrigeration System

The rate of heat removal from the refrigerated space—is often expressed in terms of tons of refrigeration. The capacity of a refrigeration system that can freeze 1 ton (2000 lbm) of liquid water at 0°C (32°F) into ice at 0°C in 24 h is said to be 1 ton. One ton of refrigeration is equivalent to 211 kJ/min or 200 Btu/min.


THE REVERSED CARNOT CYCLE

Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.


THE IDEAL VAPOR-COMPRESSIONREFRIGERATION CYCLE


An Ordinary Household Refrigerator


THE IDEAL VAPOR-COMPRESSIONREFRIGERATION CYCLE…

The vapor-compression refrigeration cycle is the most widely used cycle for refrigerators, air-conditioning systems, and heat pumps. It consists of four processes:

1-2 Isentropic compression in a compressor2-3 Constant-pressure heat rejection in a condenser3-4 Throttling in an expansion device4-1 Constant-pressure heat absorption in an evaporator


P-h Diagram

Another diagram frequently used in the analysis of vapor-compression refrigeration cycles is the P-h diagram. On this diagram, three of the four processes appear as straight lines, and the heat transfer in the condenser and the evaporator is proportional to the lengths of then corresponding process curves.


Analyzing Vapor-Compression Refrigeration Systems

Evaporator

Compressor

Condenser

Expansion valve


Analyzing an Ideal Vapor-Compression Refrigeration Cycle

A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa. If the mass flow rate of the refrigerant is 0.05 kg/s, determine

(a) the rate of heat removal from the refrigerated space and the power input to the compressor,(b) the rate of heat rejection to the environment, (c) the COP of the refrigerator.






Analyzing an Ideal Vapor-Compression Refrigeration Cycle (Home Assignment)

Refrigerant 134a is the working fluid in an ideal vapor-compression refrigeration cycle that communicates thermally with a cold region at 08C and a warm region at 26C. Saturated vapor enters the compressor at 08C and saturated liquid leaves the condenser at 26C. The mass flow rate of the refrigerant is 0.08 kg/s. Determine

(a) the compressor power, in kW, (b) the refrigeration capacity, in tons, (c) the coefficient of performance, and (d) the coefficient of performance of a Carnot refrigeration cycle operating between warm and cold regions at 26 and 0C, respectively.




ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE


The Actual Vapor-Compression Refrigeration Cycle

Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.14 MPa and 10°C at a rate of 0.05 kg/s and leaves at 0.8 MPa and 50°C. The refrigerant is cooled in the condenser to 26°C and 0.72 MPa and is throttled to 0.15 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, determine

(a) the rate of heat removal from the refrigerated space and the power input to the compressor,(b) the isentropic efficiency of the compressor, and (c) the coefficient of performance of the refrigerator.




