### LMTD Heat Exchanger (Logarithmic Mean Temperature Difference)

The main factor to calculate and design heat exchanger is heat transfer problem. If amount of heat which is released is equal to Q with regulations of time, then cold fluid will receive heat as Q with equation:

Q = U . A . ΔTm

Where:
Q = released / received heat
U = overall heat transfer coefficient
A = area of ​​heat transfer
ΔTm  = Difference in average temperature

Before determining surface area of ​​heat (A), then the value of LMTD Heat Exchanger (Logarithmic Mean Temperature Difference) should be first determined. This is based on the difference in inlet temperature of fluid and outlet temperature of heat.

For different parallel flow direction of fluid, in which:

ΔTmax = ( T1 – t1 ) : ΔTmin = ( T2 – t2 )

For opposite luid flow, then:

ΔTmax = ( T1 – t2 ) : ΔTmin = ( T2 – t1 )

Where:
LMTD = Logarithmic Mean Temperature Difference
T1 = temperature of fluid into shell
T2 = temperature of fluid out of shell
t1 = temperature of fluid into tube
t2 = temperature fluid out of tube

In design heat exchanger, actual difference in average temperature should be calculated by using correction factor (Ft). Amount of actual difference in average temperature (ΔTm) is:

ΔTm = Ft × LMTD