7/28/2019 AE1238
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1 2010 Emerson Climate Technologies
Printed in the U.S.A.
AEx
Application Engineering
B U L L E T I N
Many larger horsepower Copelametic compressorsare designed for
immersion type crankcase heaterswhich mount in the compressor
crankcase with theheater element in direct contact with the oil.
Dueto the limited space available and the need for asufficient
amount of heat to maintain the crankcase oiltemperature a
reasonable differential above outsidetemperature under low ambient
conditions, high surfacetemperatures on the heater surface are
unavoidable.
As long as the surface temperature remains below the
point at which oil carbonizes, no operating problemarises.
However, if the surface temperature reaches350F or higher, then oil
breakdown can occur, carboncan form, and chemical reactions can
occur whichmay cause compressor failure. Since the design
safetymargin is relatively small, extreme operating conditionscan
create dangerous temperatures, and system designprecautions may be
necessary.
Heater elements are simple resistance heaters, so theheat
developed is proportional to the square of thevoltage. A 10%
increase in voltage results in an increasein the heat generated in
excess of 20%. Obviously theapplication of crankcase heaters on
systems where high
voltage is encountered is far more critical.
The rate at which heat can be dissipated from the heaterdepends
on the surface heat transfer coefficient, whichis much higher when
in contact with liquid as opposedto vapor. Therefore, if a high oil
level is maintained inthe crankcase at all times, heat transfer
will be efficient,but if there are recurring periods when oil is
low in thecrankcase, as might occur on commercial
applications,carbonizing temperatures are much more likely to
occur.
Another factor affecting heat dissipation is thetemperature of
the oil in which the heater is immersed.The higher the oil
temperature, the greater the heatersurface temperature required to
give off the sameamount of heat.
Dangerous temperatures on air conditioning applicationswill
seldom if ever be encountered unless thecompressor is required to
operate in very high ambienttemperatures at extremely high voltage.
However, lowtemperature applications at high voltage conditions
are
almost certain to be at the danger point.
To provide adequate compressor protection, immersiontype
crankcase heaters on low temperature compressorsmust be
de-energized when the compressor isoperating, if above nominal
voltage is to be expected.It is strongly recommended that this type
of heater bede-energized during operation on low
temperaturecompressors regardless of the voltage, and even onair
conditioning compressors, it is good design practiceto de-energize
the heater when the compressoris operating. Disconnecting the
heater may beaccomplished by a reverse acting auxiliary contact
onthe contactor, or by means of a normally closed relay
operated from the pilot circuit.
Heaters mounted in external wells, or externallymounted heaters
dissipate their heat over a muchgreater area and are not in direct
contact with theoil, so the above restrictions would not apply in
theirapplication.
Application of Immersion Type Crankcase Heaters
AE17- 1238 September 1973Reformatted October 2010
Application Engineering
B U L L E T I N
