Page 172 - Boiler_Operators_Handbook,_Second_Edition
P. 172
Refrigeration & AC 157
unacceptable and a means is required to keep the com-
pressor in operation constantly. That’s because the delay
time between starting and stopping is typically around
15 minutes for those designs of compressors to ensure
the oil in the crankcase is free of liquid refrigerant. The
solution to the problem is hot gas bypass.
I’ve only encountered one situation where hot gas
bypass to keep the compressor running was essential.
An entryway for the plant laboratory at a plastic fibers
plant was to be used to temporarily store pallet loads of
production samples shipped to the lab for testing. The
spools of fiber were not light and the laboratory tech-
nicians were not happy with a 15 minute wait time be-
fore cooling restarted. A pressure reducing valve and
connecting tubing that delivered compressor discharge
Figure 5-11.
vapor to the inlet of the evaporator maintains the evap-
Scroll compressor
orator pressure to keep the compressor running.
Hot gas bypass can be confused with hot gas de-
RIGHT: Figure 5-12. frosting because they’re piped the same. The difference
Scroll progression in control operation will let you tell which system it is.
is no way to unload a scroll Screw Compressors
compressor. Scroll compressors Screw compressors consist of two screw shaped ro-
have an advantage of lower tors that are interlocked inside a casing. The two screws
sound power levels because form cavities between the screws and the casing (Fig-
they are smoother operating. ure 5-13) that are sealed where the two screws meet and
To a degree scroll compressors compression is achieved by capturing a gulp of refriger-
can be turned down using vari- ant vapor in a cavity and that cavity progressing from
able speed drives or two speed the inlet where the pressure is low to the outlet. Theoret-
motors for a better match to ically the vapor would progress along the screws at suc-
cooling loads. My home’s heat tion pressure until the cavity is opened at the discharge
pump contains a two speed where high pressure vapor would flow into the cav-
compressor and in real cold ity and, like a piston, compressing what was trapped.
periods is switching between However, leakage between and along the screws serves
high and low speed rather than to raise the pressure in the cavity as it travels along so
starting and stopping. the sudden compression when the pocket of gas reaches
the discharge is not as violent as it could be. A certain
Hot Gas Bypass and degree of unloading is provided with screw compres-
Defrosting sors through the movement of a plug laying along the
Many reciprocating and screws. That same plug also provides a means of partial-
scroll compressors, especial- ly unloading the compressor for start up.
ly the smaller ones, are on/ Lubrication is very important for screw compres-
off operation only. The typical sors because the oil helps seal the clearances between
refrigerator, freezer, icemaker, the screws and the casing to reduce refrigerant leakage
wine cooler, and window & at the same time as it creates a wedge of oil between the
auto air-conditioners can be op- moving parts. A typical screw compressor will have pro-
erated in this manner because visions for removing oil from the compressor discharge
swings in temperature can be vapor and injecting it back in at the inlet to the screws.
tolerated. On occasion the load Large screw compressors will also have gears mounted
can swing considerably and/ on the two screw shafts that maintain screw alignment
or temperature deviations are without exposure to gas leakage and they must be lubri-
