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1 2010 Emerson Climate Technologies
Printed in the U.S.A.
AE17-1252 R2
Application Engineering
B U L L E T I N
Heat reclaim systems have proven to be a majorsource of energy
savings, particularly in connectionwith supermarket refrigeration.
During periods whenstore areas require heating, heat normally
dischargedoutdoors through the condenser can be diverted to aheat
transfer coil in the heating duct, providing heatto the store with
substantial savings in both cost andenergy. Throughout the year,
hot water requirementsmay be satisfied with discharge line heat
exchangers.
With proper design, large savings can be obtained
with no threat to the compressor. However systemdesigners must
remain alert to the fact that compressorsin low temperature systems
must operate very close todangerous temperature levels, and the
operating safetyof the compressor cannot be ignored when
consideringsystem modifications for heat reclaim.
Condenser Heat Reclaim
Various system designs have been utilized, and bothparallel and
series connections have been used for theheat reclaim coil and the
condenser. Due to systembalancing problems, the series connection
has provento be more satisfactory, and a schematic diagram of
atypical heat reclaim system is shown in Figure 1.
Since the heat reclaim coil and the condenser areboth in effect
serving as condensers in the circuit,some means of condensing
pressure control must beprovided to prevent the condensing
temperature fromfalling to a level which could not efficiently be
used forstore heating. With return air temperature typically in
the70F to 75F range, condensing temperatures of 100For higher are
usually required to provide a reasonabletemperature difference for
effective heat transfer. In thesystem as shown, this is
accomplished by means ofthe head pressure control valves and some
additionalrefrigerant charge.
A number of compressor failures have been reportedon heat
reclaim systems over the years and fieldinvestigation reveals that
these failures have been onlow evaporating temperature condensing
units wherethe condensing pressure was maintained by cycling
thecondenser fan. Any single stage Copeland compressoroperating at
evaporating temperatures below 0F musthave adequate air over the
compressor for propercooling. It appears the source of the
compressor failureshas been operation during the heat reclaim cycle
of
compressors on condensing units where compressorcooling is
provided by the condenser fan or fansresulting in inadequate air
over the compressor. It isconceivable that the heat reclaim coil
could perform allor most of the condensing function, and the
condenserfan could be cycled off for hours at a time.
The requirement for adequate air over any Copelandcompressor
operating at 0F and below is a standardEmerson Climate Technologies
specification. Operation
of compressors at low evaporating temperatures withoutair over
the compressor is considered misuse, andcompressors failing due to
such abuse are not subjectto warranty replacement.
Fan cycling for condensing pressure control wherea single
condenser fan is used is not recommendedon any air-cooled
compressor or low temperaturecompressor unless an auxiliary
compressor coolingfan is used.
Design Considerations for Heat Reclaim Systems
AE17-1252 R2 February 1988Reformatted November 2010
Application Engineering
B U L L E T I N
Figure 1Schematic Diagram of
Typical Heat Reclaim System
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2 2010 Emerson Climate Technologies
Printed in the U.S.A.
AE17-1252 R2
Application Engineering
B U L L E T I N
On condensing units with multiple condenser fansapplied to heat
reclaim systems, an auxiliary compressorcooling fan must be used
unless specifically approvedotherwise by the Emerson Climate
Technologies
Application Engineering Department.
Discharge Line Hot Water Heaters
Discharge line heat exchangers such as shown inFigure 2 can
provide an economical source of hot water.The heat transfer takes
place with the superheateddischarge gas, so water temperatures much
higher thancondensing temperatures can be obtained.
Instances have been reported of field personnelintentionally
setting head pressure controls at ahigher than normal level in
order to get hotter waterfrom a heat reclaim water heater. This not
onlyendangers compressor life, it increases the overallenergy
consumption of the compressor, and may be
counter-productive from both a cost and energy savingstandpoint.
Obviously the available heat is limited sinceremoving the superheat
from discharge gas is only afraction of the heat given off during
the condensingprocess.
One other potential danger with this type of heatexchanger is
excessive pressure drop. Any pressuredrop introduced into the
discharge line increases thedischarge pressure at which the
compressor mustoperate. 3 to 4 psi pressure drop will not introduce
amajor penalty, but on commercial applications, greaterpressure
drop may lead to compressor overheating andearly failure. Pressure
drops on this type of device inexcess of 5 psi should not be
considered acceptable,particularly on medium and low temperature
systems.
Figure 2Schematic Diagram of Discharge Line Water Heater
