Compressor Protection Devices- Part 24

Tag: Air Sensing Switch

In the last article we completed our study of low ambient control methods. This month we will concentrate on compressor protection devices with the emphasis on lubrication and its associated control devices for system protection.

It is ironic that refrigeration system designers have to spend considerable time and money keeping the majority of the system free of oil and yet are obliged to introduce oil into the system in order to protect one component, the compressor.

Oil must be introduced into the compressor in order to provide lubrication for the surfaces experiencing friction, primarily the bearings, these being on the crankshaft, connecting rod bearings, both the big and small end bearings. The other rubbing surfaces are the cylinder walls where in addition to providing lubrication the oil film assists in providing a seal between the piston and cylinder wall to reduce blow-back.

Lubrication of these components can be provided by splash-feed or pressure-feed.

Splash-feed is caused by the action of the crank shaft rotating and dipping the cranks and the big end of the connecting rods into the oil sump and splashing oil onto the internal surfaces of the compressor. This type of lubrication is normally restricted to reciprocating compressors (open and semi-hermetic) of less than 2.5kW and hermetic compressors (reciprocating, rotary sliding vane, and scroll).

Compressors using this form of Air Sensing Switch do not normally incorporate any form of oil failure protection, although some manufacturers can supply an oil float switch to give warning if the oil level falls below a safe level. A float switch would be recommended if long pipe lines are involved or on applications at risk from liquid refrigerant returning, such as with reverse cycle defrosting.

Some compressors, normally open type, feature an extension of the connection rod big-end bearing in the form of a dipstick which splashes into the oil bath in the sump.

A splash rotor can also be fitted at the rear end of the crankshaft in the form of two arms which splash lubricant from the sump to the roof of the compressor body where it then falls into a gallery at the end of the crankshaft for subsequent distribution along drillings in the crankshaft and channels in the journals. The oil is then allowed to flow freely through drillings in the connecting rod big-ends back to the sump. Such compressors can be run for prolonged periods at very low speed (200 rpm) making them especially suited to transport refrigeration applications.

Larger compressors use pressurised oil which is pumped to the various bearings and surfaces from a mechanical oil pump via small channels drilled into the crankshaft, connecting rods and the compressor body. These oil pumps are usually fitted as an integral part of the compressor and are directly driven by the crankshaft.

Larger compressors such as screw and centrifugal machines use an oil pump in the form of a totally separate item driven by its own motor. The oil management systems of certain large water chillers and other systems can be very complex.

Failure of the oil pump in any type of compressor (other than oil-less type) will usually result in catastrophic damage to the compressor if such failure is not detected quickly enough. There are a number of components to protect against oil failure but they all must be able to measure the crankcase pressure and the oil net pressure. Possibly the most common is the oil differential pressure switch.