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Talking Technically: Compressors

By: Grant Laidlaw (F. SAIRAC) – ACRA CEO

How should you go about choosing the correct compressor for a particular refrigeration system?

Type of compressor 

There are various types of compressors on the market today. All have somewhat different characteristics and as such will be used in applications where advantages outweigh the disadvantages. A reasonably accurate assessment about when the various compressors are used is based on their refrigerating capacity.

Reciprocating compressors dominate from very small refrigerating capacities up to about 300kW.
Rotary compressors are used mostly in domestic refrigerators and small air-conditioning units.
Screw compressors are used in the 300-500kW capacity range, mainly in industrial refrigeration plants but also on water chillers.
Centrifugal compressors are used in plants with capacities in excess of 500kW, particularly water-chilling plants used for large air-conditioning installations.

This section mainly concerns itself with reciprocating compressors and as such only a selection of these will be discussed.

Information required
The information required to select a compressor is as follows:

a. The capacity of the compressor in kW;
b. The saturated suction temperature in °C (i.e. evaporating temperature);
c. The saturated discharge temperature in °C (i.e. condensing temperature);
d. The amount of superheating of the vapour in K;
e. The amount of sub-cooling of the liquid refrigerant in the compressor in K;
f. The type of refrigerant; and
g. Supply voltage.

The above will make the selection of a compressor possible. However, in order to select the best compressor for the particular job, the following additional information is required:

h. Method of starting and/or current restriction (if any);
i. Whether unloading is required;
j. Type of compressor, i.e. hermetic, semi-hermetic or open;
k. Type of drive required; and
l. Accessories required.

Selection considerations
So what are our selection considerations?

Reciprocating compressors are made in three different types:

1. Hermetic compressors;
2. Semi-hermetic or accessible hermetic compressors; or
3. Open compressors.

Each type of compressor has its advantages and disadvantages and selection of the type of compressor depends on a number of factors as follows:

Capacity requirement: The capacity requirement of the compressor is the primary consideration in the selection of the type of compressor.

Hermetic compressors are made in the lower range, up to a refrigeration capacity of approximately 35kW. The main advantages of the hermetic compressor are its cost and the ease of installation. Because of this, the use of this type of compressor is increasing continuously.

Semi-hermetic compressors are made in capacities up to approximately 100kW and are very popular, especially in packaged equipment.

Hermetic and semi-hermetic compressors are made for a standard range of suction temperatures and a maximum condensing temperature. These operating conditions have to be strictly adhered to. Operations at suction temperatures above this range will cause motor cooling problems and excessive power requirements. Both of these can result in motor burn-outs. At a suction temperature lower than what the compressor is designed for, the mass of gas passing over the motor is insufficient. This results in excessive temperature rise of the motor windings, high superheat and high discharge temperatures. Condensing pressures higher than design will result in excessive power requirements.

Read more about this technical in Cold Link Africa September/October 2015, page 35.

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