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Refrigeration and Air Conditioning

NME:604Unit2

Presented byMr Naphis AhmadAssistant professorJETGI Barabanki

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The processes of this cycle are

Process 4-1: two-phase liquid-vapor mixture of refrigerant is evaporated through heat transfer from the refrigerated space.

Process 1-2: vapor refrigerant is compressed to a relatively high temperature and pressure requiring work input.

Process 2-3: vapor refrigerant condenses to liquid through heat transfer to the cooler surroundings.

Process 3-4: liquid refrigerant expands to the evaporator pressure.

Two-phaseliquid-vapor mixture

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►Engineering model:

►Each component is analyzed as a control volume at steady state.

►Dry compression is presumed: the refrigerant is a vapor.

►The compressor operates adiabatically.

►The refrigerant expanding through the valve undergoes a throttling

process.

►Kinetic and potential energy changes are ignored.►Applying mass and energy rate balances

Evaporator 41in hhmQ

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►The term is referred to as the refrigeration capacity, expressed in kW in the SI unit system

►A common alternate unit is the ton of refrigeration which equals about 211 kJ/min.

inQ

►Applying mass and energy rate balances

CompressorAssuming adiabatic compression

12c hhmW

Condenser

32out hhmQ

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Expansion valveAssuming a throttling process

34 hh

Coefficient of Performance (COP)

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p-h Diagram The pressure-enthalpy (p-h) diagram is a thermodynamic property diagram commonly used in the refrigeration field

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Multistage compression refrigeration systems

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Use of flash chamber for flash gas removal

In this system, instead of passing refrigeration vapour from the low stage

compressor through the flash tank, vapour from the flash chamber blended

with the vapour coming from the low stage compressor .Thus the inlet

condition to the high stage condition will be superheated.

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This is P-H diagram for flash removal

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Cascade refrigeration systems

Very low temperatures can be achieved by operating two or more vapor

compression systems in series, called cascading. The COP of a refrigeration

system also increases as a result of cascading.

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Psychometric Processes Sensible Heating Sensible Cooling Dehumidification Humidifying Adiabatic Cooling Adiabatic Mixing (Moist Air & Water Vapor)

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