Heat Exchangers | Gate

covered topics

  1. Logarithmic Mean Temperature Difference
  2. Number of transfer units

1. Heat exchanger: heat exchanger is a device which is designed for effective heat transfer between two fluids. the flow of heat transfer is from high temperature to low-temperature fluid. 

Ex. Boilers, condenser, etc.

2. Logarithmic mean temperature difference (LMTD): the temperature difference which, if constant would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference.  

heat transfer by the heat exchanger 

Q = UAdTm

# LMTD in parallel flow heat exchanger

ΔT1 = Th1-Tc1

ΔT2 = Th2-Tc2 

# LMTD in counter flow heat exchanger

ΔT1 = Th1-Tc2

ΔT2 = Th2– Tc1


  • ΔTm = ΔT1-ΔT2 / ln(ΔT1/ΔT2)                                                                                                                                
  • Q (heat transfer) = UAΔTm

3. Heat capacity ratio: it is denoted by C, and defined by the ratio of minimum heat capacity rate to maximum heat capacity rate of two fluids. 

C= Cmin/ Cmax

4. Effectiveness: it is defined as the ratio of actual heat transfer rate to the maximum possible heat transfer rate of any heat exchanger.

ε = actual heat transfer/ maximum possible heat transfer 

# if (mCp)hot > (mCp)cold, 

   ε = (Tc1-Tc1)/ (Th1-Tc1)

# if (mCp)hot < (mCp)cold, 

 ε (Th1-Th2)/(Th1-Tc1)

5. Number of transfer units(NTU): NTU is the measure of heat transfer size of a heat exchanger. 

       NTU = UA/Cmin

# effectiveness from NTU method for parallel flow

 ε = 1 – e-NTU(1+C) / (1 + C)

# effectiveness from NTU method for counter flow

ε = 1 – e-(1-C)NTU /  1 + Ce-(1-C)NTU




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