Log Mean Temperature Difference (LMTD)

The LMTD is a logarithmic average of the temperature difference between the hot and cold fluid streams at each end of the heat exchanger. The larger the value of LMTD, the higher heat is transferred.
The rate of heat transfer can be expressed as q = UAΔTm

Where ∆Tm is the log mean temperature difference given as
log mean temperature difference
In Parallel Flow Heat Exchanger
Parallel Flow Heat Exchanger

∆T1 = Thot,in – Tcold,in
∆T2 = Thot,out – Tcold,out

In Counter Flow Heat Exchanger
Counter Flow Heat Exchanger LMTD

∆T1 = Thot,in – Tcold,out
∆T2 = Thot,out – Tcold,in

Assumptions made in LMTD method:

  1. The overall heat transfer coefficient (U) is constant throughout the heat energy
  2. The specific heats (Cp) and mass flow rates of both fluids is constant.
  3. The flow conditions are steady.
  4. There is no change of phase either of the fluid during the heat transfer.
  5. There is no loss of heat to the surroundings, due to heat exchanger being perfectly insulated.
  6. Axial conduction along the tubes of heat exchanger is negligible.
  7. The changes in kinetic energy and potential energy are negligible.
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All Comments

  • Hello there. May I ask, why are we using LMTD instead of the normal temperature difference(the one that I concern is in the heat exchanger experiment which is the Wilson’s plot). Thank you

    Nur Fitrah Nov 29, 2015 11:02 pm Reply
  • LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is?

    yogesh kumar Apr 12, 2016 10:45 am Reply

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