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  • 7/28/2019 AE1252

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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

  • 7/28/2019 AE1252

    2/2

    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