Cold Link Africa VOL. 01 - No.01 | September / October 2015 | Page 35

TALKING TECHNICALLY
INCORPORATING COLD CHAIN

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business administration and an associate degree in educational
administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for 17 years
and has been a Johannesburg committee member for the past eight (chairman in 2011 and 2012. Currently he is the vice-chairman (Johannesburg council) as well as vice-president (national council).

Compressors

#TalkingTechnically

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

T

here 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 airconditioning 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, semihermetic 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.
Open compressors are manufactured
in capacities up to approximately 500kW.
In the above, it has been assumed that
one compressor is used to take care of
the entire load. This is not always the best
solution as on some installations ‘standby’
capacity is required. If the compressor
in a ‘one compressor’ system becomes
inoperative, it means that no cooling at
all is available. For this reason, two or three
or more compressors can be used. In this
case, the system becomes a great deal less
vulnerable. This also extends the capacity
ranges of the v