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 = U*A*ΔT_m

_{Where ∆Tm is the log mean temperature difference given as}

Parallel Flow Heat Exchanger

Counter Flow Heat Exchanger

Assumptions made in LMTD method are

• The Internal Energy (U) is constant throughout the Heat Energy
• Cp and m are constant.
• The Heat Exchanger is perfectly insulated
• The Cross Section of stream is uniform
• No conduction of heat along the tubes of Heat Exchanger
• The Kinetic Energy and Potential Energy changes are negligible
• The flow conditions are steady