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Heat Transfer

Heat Transfer . Flow Inside Heat Exchangers. Flow inside Heat Exchangers. In Heat Exchangers, one fluid is cooled while the other fluid is heated without direct contact between fluids. There are two types of flow inside Heat Exchangers. 1- Parallel Flow (Co current Flow).

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Heat Transfer

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  1. Heat Transfer Flow Inside Heat Exchangers

  2. Flow inside Heat Exchangers • In Heat Exchangers, one fluid is cooled while the other fluid is heated without direct contact between fluids. • There are two types of flow inside Heat Exchangers.

  3. 1- Parallel Flow (Co current Flow) 2- Countercurrent Flow W,Cp,T2 W,Cp,T1 W,Cp,T1 W,Cp,T2 w,cp,t1 w,cp,t1 w,cp,t2 w,cp,t2 T1 T1 T2 T2 temperature temperature t2 t2 t1 t1 x x

  4. Typically in heat exchangers, the heat lost by the hot fluid is gained by the cold fluid through indirect contact (neglecting any heat losses), an overall Energy Balance Equation can be written as: Q=w cp(t2-t1)=W Cp (T1-T2)=UoAotm • Uo:Overall heat transfer coefficient • Ao:Heat transfer area • tm:Average temperature difference • Rwallcan be neglected compared with convection resistances. hi ho • Where is the heat transfer coefficient of the inner fluid based on the outside area

  5. For a differential element inside the exchanger, with a thickness dx, T & t are the mean values of the hot and cold fluids respectively, t=T-t T t dQ dx • The differential rate of heat transfer dQ is • dQ=Uo Do dx t dQ= T1 T1 T2 T2 t2 t2 t1 t1 temperature temperature Q=0 Q Qtotal Parallel Flow Countercurrent Flow Q=0 Q Qtotal

  6. Parallel Flow Countercurrent Flow =Constant T1 T1 T2 T2 t2 t2 temperature t1 t1 temperature Q=0 Q Qtotal Q=0 Q Qtotal

  7. Where is the LMTD: Logarithmic Mean Temperature Difference

  8. Example (1) A hot fluid enters the exchanger at T1=300oF and leaves at T2oF, A cooler fluid enters the exchanger at t1=100oF and leaves at t2oF a) If T2=200oF and t2=170oF b) If T2=t2=182oF c) If T2=110oF and t2=233oF

  9. In case of Isothermal Fluids If a cold fluid is heated from 100oF to 275oF by condensing steam at 300oF, Calculate LMTD for both cases 300 300 300 300 275 275 100 100 temperature temperature Countercurrent Flow Parallel Flow 200 25 200 25

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