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Types of Compressors and compressor selection

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

In the last issue we looked at the first section involving types of compressors and now move on to power supply

In areas where only limited or no electrical power is available, compressors can be driven by internal combustion engines. In this case only open type compressors are suitable. It is possible to generate power with a generating set, though, because of the high currents required to start compressors, it is often not a proposition to provide a big generator to start the compressor which then runs at reduced capacity.

Municipal regulations in most areas forbid the starting of larger motors direct on line (which requires high starting currents). This often makes it impossible to use hermetic compressors which do not have provisions for reduced current starting. Semi-hermetic compressors do have provision for starting on the star-delta system which reduces starting current. On very big compressors, the torque on standard motors is often insufficient and special motors are required. In this case, only open compressors can be used.

Packaged systems
The tendency is more and more towards packaged air-conditioning and refrigeration units, i.e. units that include the entire refrigeration system (compressor, condenser and evaporator). The advantage of packaged equipment is the fact that matching and assembling of the components is done by the manufacturer, often after extensive field tests. Installation is greatly simplified, giving advantages of savings in labour and time. It is also cheaper for a manufacturer to design and produce in quantity. Another advantage lies in the fact that assemblies of piped-up components can be tested for leaks in pipe joints in the factory.

Packaged units greatly simplify selection as instead of selecting and matching the various components, only one piece of equipment needs to be selected.

Because of the strictly controlled manufacturing procedure and the predictable operating conditions of packaged equipment, hermetic and semi-hermetic compressors are suitable and hence often used on this type of equipment.

Field-assembled systems
Field-assembled systems often include long refrigeration lines. The longer the refrigerant pipes, the greater is the possibility of problems with oil return, moisture and other impurities in the system as well as loss of refrigeration through leaks. Hermetic compressors have very small sumps and hence little oil charge. If this oil is lost in the system without extra oil being added, the compressor will fail. Semi-hermetic and open compressors are usually fitted with an oil sight glass and oil failure protection which enables the operator to prevent oil failure problems.

Moisture and/or impurities in a system can cause a breakdown in the windings of hermetic and semi-hermetic compressors which causes ‘burn-outs’. This leaves deposits throughout the refrigeration system which must be cleaned out before repair or replacement of the compressor. For this reason, in systems with long refrigerant pipes, open compressors are used in preference to the other types.

Compressor drives
Open compressors which are driven by electric motors can be either direct or belt driven. Direct drives are simple, needless maintenance and have reduced power loss. The disadvantage is that the selection of compressor speed is limited to the available motor speeds which are approximately 48 revolutions per second (rps) for two-pole speed, 24 rps (four-pole speed), 16 rps (6-pole speed) or 12 rps (8-pole speed). These speeds correspond to the more popular 2 900, 1 450, 960 and 735 rpms.

If the compressor capacity must match the load of the capacity of other equipment exactly, it is at times necessary to provide a belt drive which can provide virtually any compressor speed. It must be remembered, however, that there is approximately 10% power loss in belt drives. In addition, more maintenance is required. The advantages of a belt drive are:

(a) unlimited selection of speeds, and
(b) reduced lining up problems and the possibility of changing the speed if and when required.

Note: Compressors have certain minimum and maximum design speeds which must be considered.

Read the full technical in Cold Link Africa November/December 2015 Vol 1 No 2 page 37.

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