Page 168 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 153

a compressor is a piece of mechanical equipment it re- the right conditions liquid refrigerant can be carried into
quires lubrication. And, since the moving parts are in the cylinders as well. So, always check to see if the oil
direct contact with the refrigerant, lubricating oils for heater is working before starting a compressor and check
compressors are specific to the refrigerant used. With that sight glass immediately after starting the compres-
the exception of centrifugal compressors some of the lu- sor for foaming. A small amount of foaming that doesn’t
bricating oil is broken up into small droplets forming raise the oil level out of the glass would be acceptable.
an aerosol that travels with the refrigerant through the Compressor controls are typically fitted with a timer to
system. The oil is cooled and occasionally heated by the prevent the motor starting after power is interrupted for
refrigerant itself so oil coolers are not always required. sufficient time to allow the heater to boil off any liquid
There is no dipstick in a refrigeration compressor refrigerant that’s migrated to the crankcase. That’s why
so other means are required to determine the level of the your home air conditioner compressor won’t start right
oil. Some compressors, like the one in a window air-con- away after you over-adjusted the thermostat to stop it.
ditioner and your car, don’t require a lot of oil and be-
cause the systems are sealed, an adequate amount of oil
is assumed. The wise operator always looks around and
under compressors, evaporators, condensers and asso-
ciated piping to detect any accumulation of leaking oil.
First of all it’s an indication of a possible refrigerant leak
and secondly it can indicate a potential failure of the
compressor. Larger compressors are usually fitted with
a sight glass in the compressor crankcase that permits
observation of the oil level. An idle compressor can have
a high oil level due to accumulation of oil drained from
connecting piping as well as portions of the compressor
itself. On my one trip as a ship’s refrigeration engineer
I noticed entries in the log of my predecessor that men-
tioned frequent topping off and draining of lubricating
oil from the compressors. I did note a higher oil level
on compressors that were shut down but the level al-
ways returned to normal shortly after the compressor
was placed in operation. That’s because the oil filled the
cavities in the compressor that had drained back to the
bottom and some of the oil was circulated into the sys-
tem. Simply allowing what happened naturally to occur
saved me all the work of adding and removing oil.
While it seems strange every refrigerant compres- Figure 5-5. Suction accumulator
sor has a heater in it. The purpose of the heater is to
boil off refrigerant that condenses in the crankcase or oil
sump. When the compressor is operating the heater is The description for evaporators mentioned the
normally turned off because the operation of the com- traps for accumulating and re-injecting oil into the flow
pressor heats the oil. When the compressor is idle, or out of refrigerant gas. That has the potential of sending a
of service, refrigerant tends to migrate to the compres- slug of oil into the compressor that could be carried di-
sor crankcase, condense, and mix with the oil. This can rectly into one of the cylinders resulting in damage to
happen if the heater doesn’t work and the result will be the compressor. The device shown in Figure 5-5 is called
higher crankcase oil level. This situation is problemat- a suction accumulator and is installed in the vapor pip-
ic because as soon as a compressor starts operating the ing near the compressor inlet to protect the compressor
lower crankcase pressure drops below the saturation from slugs of refrigerant or oil. A slug of oil, refrigerant,
pressure of the liquid refrigerant in the oil and it boils. or a combination thereof is trapped in the accumulator.
The boiling refrigerant mixes with the lubricating oil to The small hole in the riser tubing allows a gradual re-
create foam which results in slugs of lubricating oil en- injection of oil into the flow of refrigerant to return it
tering the valves and cylinders of the compressor. Under to the compressor and the superheat in the gas should