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Types of evaporators

How a direct expansion evaporator works

By: Grant Laidlaw – regular contributor

In this module we look at evaporators – from their purpose to the various types available and even factors affecting their performance

On completion, you should:

  • Understand the purpose and application of evaporators in refrigeration systems;
  • Be able to describe the various types and construction of evaporators; and
  • Be able to determine the various factors affecting the performance of evaporators.

Purpose of evaporators
The purpose of an evaporator is to transfer heat from one medium to another.

Any device in which refrigerant is boiled (or evaporated) for the purpose of extracting heat from another medium is called an evaporator.

Thus chillers, unit coolers or the ice-cube makers in a domestic refrigerator are all classified as evaporators. It is because of the very many and varied applications of refrigeration and there are so many different types of evaporators.

Direct expansion evaporators
In a direct expansion evaporator the amount of liquid refrigerant supplied is the amount that is actually demanded by the load. It is completely vaporised by the time it reaches the end of the evaporator.

Liquid refrigerant is fed to the direct expansion evaporator through an expansion valve or throttling device in just the right amount so that all liquid will be converted to vapour before the refrigerant reaches the suction line to the compressor. In order to ensure that only refrigerant vapour enters the suction line, the vapour is allowed to be superheated by approximately 5K. For this, approximately 20% of extra heat exchange surface area is required.

Heat transfer to the liquid refrigerant from the evaporator tubes is better than to refrigerant vapour to the tubes. The surface temperature (apparatus dew point) of the first part of the DX (dry expansion or direct expansion) evaporator is lower than the outlet part because of the superheat.

DX coils are popular because of simplicity, low initial cost and low refrigerant charge required in spite of being somewhat less efficient than the flooded type. An additional advantage is that direct expansion evaporators will not give oil return problems if they are correctly installed.

Read the full feature in Cold Link Africa January 2016 page 34.

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