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REFRIGERATION

&

HEAT PUMPS

Anjana Tharanga

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Evaporative Cooling

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Is used to reduce the temperature ofcooling water.

Water to be cooled is sprayed into the topof the tower.

And then it is allowed to flow down a fill ofmaterial

Ambient air stream is flown in counter

flow manner

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Vapour compression refrigeration

Highly compressed fluids at a certain temperaturetend to get colder when they are allowed toexpand.

fluid is used to cool a low temperatureenvironment and reject the heat to a hightemperature environment

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Thermo-dynamics aspect of

refrigeration The 1st law of thermodynamics says heat flow

occurs from a hot source to a cooler sink.

So energy in the form of work must be added tothe process to get heat to flow from a lowtemperature region to a hot temperature region.

The Carnot cycle can serve as the initial model ofthe ideal refrigeration cycle.

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1

2

4

5

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The choice of refrigerant and the requiredcooling temperature and load determine thechoice of compressor, as well as the design of

the condenser, evaporator, and otherauxiliaries.

Additional factors such as ease ofmaintenance, physical space requirementsand availability of utilities for auxiliaries(water, power, etc.) also influence componentselection.

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Vapour absorption refrigeration

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Absorption refrigeration systems that use Li-Br-water as a refrigerant have a CoefficientofPerformance (COP) in the range of 0.65 -0.70

and can provide chilled water at 6.7 oC with acooling water temperature of 30 oC. Systems

capable of providing chilled water at 3 oC arealso available.

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How cooling efficiency is

expressed

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Energy flows through a cooling

machine

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EFFICIENCY IS DEFINED AS

USEFUL ENERGY OUTPUT DIVIDED

BY TOTAL ENERGY INPUT.

But for chillers we cant apply it.

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COEFFICIENT OF PERFORMANCE

(COP)

COP is dimensionless.

The higher the COP, the more efficient

the heat pump.

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COOLING MACHINE

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COEFFICIENT OF PERFORMANCE

(COP)

The cooling process with the maximum possible efficiency is theCarnot cycle.

The COP of a cooling machine using the Carnot cycle is;

Where THot

and Tcold

are the absolute temperatures of the hot andcold heat reservoirs respectively.

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The COP of any machine

depends strongly on the

temperature difference againstwhich it operates.

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Heat Pump

Similar to the refrigeration cycle but theenergy released in the condenser isconsidered instead of the energy absorbed inthe evaporater.