Heat ExchangersDOE-HDBK-1018/1-93TYPES OF HEAT EXCHANGERS(2-1)D Tlm D T2 D T1lnD T2D T1Heat transfer in a heat exchanger is by conduction and convection. The rate of heattransfer, "Q", in a heat exchanger is calculated using the following equation.(2-2)QUoAoD TlmWhere:=Heat transfer rate (BTU/hr)QUo = Overall heat transfer coefficient (BTU/hr-ft2-F)Ao = Cross sectional heat transfer area (ft2)DTlm =Log mean temperature difference (F)Consider the following example of a heat exchanger operated under identical conditions as acounter flow and then a parallel flow heat exchanger.T1= represents the hot fluid temperatureT1in= 200FT1out= 145FUo = 70 BTU/hr-ft2-FAo = 75ft2T2=represents the cold fluid temperatureT2in=80FT2out=120FCounter flow DTlm =(200120oF) (14580oF)ln(200120oF)(14580oF)72oFRev. 0ME-02Page 7
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