7/28/2019 AE1235
1/1
1 2010 Emerson Climate Technologies
Printed in the U.S.A.
AEx
Application Engineering
B U L L E T I N
In order to provide greater operating flexibility on
largesystems, the use of multiple compressors connectedin parallel
is an attractive approach. By choosing theproper combinations of
compressors to operate, thesystem capacity can be closely matched
to the load.Full power savings for the compressors not in
operationis achieved, while at the same time providing a
built-insafety factor in the event of system malfunction.
Parallel systems inherently have potential operatingproblems.
Adequate oil levels must be maintained in allcompressors, whether
operating or not, and the suddenincrease or decrease in system
capacity caused by oneor more compressors cycling on or off can
cause liquidrefrigerant floodback to the operating compressors.
Some type of suction accumulator should be providedto protect
the compressors from liquid floodback on allmultiple compressor
systems. One widely used solutionis to provide a horizontal 4 inch
suction header severalfeet long, with individual suction standpipe
connectionsfor each compressor. The header becomes, in effect,
anaccumulator, with oil being returned to the compressorsby means
of a small diameter port in the individualstandpipe or in a
separate oil return fitting. Returning hotoil from an oil separator
to the suction header provides
a means of cooling the oil returned to the compressorand
protects against malfunction of the separator.
In the past, oil equalization was achieved on
parallelcompressors by providing a connection for
pressureequalization between compressor crankcasesabove the oil
level and another connection betweencrankcases at or below the oil
level for oil equalization.With large compressors having large
crankcases andconnections, this approach was feasible. However,the
majority of compressors used today on parallelsystems are not
manufactured with special equalizingfitting connections.
The tandem compressor, in which two compressors areassembled at
the factory into a common center housingprovides a successful
solution to parallel operationwithout the necessity for external
oil equalizing fittings.
In the case of individual compressors connected inparallel, a
simple connection between compressorcrankcase sight glass fittings
has been used. An oilseparator is essential, and careful attention
mustbe paid to system design. However, as compressor
displacement increases, this method of equalizationbecomes more
difficult due to limitations on the diameterof the equalizing line.
Since crankcase pressure ina running compressor is usually slightly
lower thansuction pressure, there is a normal flow of
refrigerantvapor through the crankcase of an idle compressorinto
the equalization line to the crankcase of theoperating compressor.
This flow can cause oil to betransferred from the crankcase of the
idle compressorto the crankcase of the running compressor,
resultingin occasional trips of the oil pressure safety control
on
compressor startup.To eliminate or substantially reduce the
problemsdescribed above, proprietary oil equalization systemshave
been developed which provide continuous oilreturn to each
compressor. These systems utilizeexisting system pressure
differentials to provide oilequalization through a relatively small
diameter line atthe desired oil level. This type of system has
provenvery successful.
Because of the extensive testing necessary to provethe
acceptability of a parallel system design and thecritical nature of
oil equalization to system reliability,Emerson Climate Technologies
does not recommend
field construction of parallel systems unless anEmerson approved
oil equalization system is used. Inthe absence of such a system,
Emerson specificationsrequire a factory designed and tested
refrigeration andoil equalization system.
The oil equalization systems which Emersonrecommends forfield
erected parallel systems utilizeindividual oil float chambers on
each compressor,fed from an oil separator and a common
reservoir.This approach has been thoroughly field proven.
Oilequalization systems utilizing this technology aremanufactured
by AC&R Products (Chicago, Illinois,USA) and Sporlan Valve
Company (St. Louis, Missouri,
USA) and are commonly available at refrigerationwholesalers.
They are available with standard fittings tomount in place of the
crankcase sight glass and providea reliable pretested means of oil
equalization.
Field experience has proven that major operatingproblems can be
created by improper parallel systemdesign, and in the best interest
of all concerned, suchapplications should be restricted to properly
designedand tested systems.
Parallel Compressor Operation
AE17-1235 R1 August 1988Reformatted October 2010
Application Engineering
B U L L E T I N
