LMTD METHOD

This is a method used to decide the temperature variation due to heat transfer in heat exchanging

devices. The LMTD gives the difference of the temperatures with respect to two temperatures

and logarithmic value of those. When the value of LMTD is higher in a case the heat transfer is

high.

=

ln

Where, the temperature difference between the two streams at the end A, and is the

temperature difference between the two streams at end B

=

Where, Q is the heat exchanged per unit time, U is the overall heat transfer coefficient and is

the exchange area

For the calculation,

Hot Side Temperature in - 110

Hot Side Temperature out - 75

Cold Side Temperature in - 35

Cold Side Temperature out - 70

= 110 35

= 75

= 75 70

= 5

=

ln

= 75 5

ln755

= 25.85

=

= 0.32 25.85 = 8.272 /2

Limitations of the LMTD method

The approach is merely depending on the assumptions of the specific heat capacity of the

fluids in constant. However in real scenario the specific heat changes with the

temperature variation and the assumption is only valid for small temperature differences.

Because of this assumption in some cases calculated values may vary according to

LMTD method or the values may not be accurate.

With this method the overall heat transfer coefficient is also considered as constant in

practice the value varies with temperature therefore the LMTD approach no longer

accurate in this case.

The LMTD method assumes the state of the condition as steady state so this can only be

used for steady state conditions and for transient cases cannot be analysed.