How is LMTD calculated for heat exchangers?

Formula for LMTD calculation – For a Parallel Flow Heat Exchanger, Hot fluid entering at temperature 100 degree Celsius and exiting at 90 degree Celsius . Cold fluid Entering at 30 degree Celsius and exiting at 50 degree Celsius. Find the LMTD . LMTD = ((100 – 30)-(90-50)) / ln (100-30/90-50) = 53.6 degree Celsius .

Why LMTD is introduced in a heat exchanger?

The log mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The use of the LMTD arises, straightforwardly, from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.

Why do we use LMTD in shell and tube heat exchanger?

The LMTD allows to represent the driving force of heat exchange along the exchanger and accounts for the fact that the difference in between the cold side and the hot side is changing all along the equipment. The use of the LMTD is valid for co-axial heat exchangers.

What do you mean by LMTD and Amtd?

LMTD (logarithmic mean temperature difference) = (dt1 – dt2)/ln (dt1/dt2) AMTD (Arithmetic mean temperature difference) is to calculate this by taking Arithmetic mean of the two fluids. This is less accurate than LMTD so it is not used for calculation. It is calculated as. AMTD = (t1i + t1o/2) – (t2i + t2o/2)

What is lmtd for co current flow when?

(At the other end cold fluid enters and hot fluid exits.) by definition given above, LMTD for counter current flow = (60-50) / ln(60/50) = 10 / 0.1823 = 54.850C. For co-current heat exchanger, ΔT1 = TH1 – TC1 = 100 – 30 = 700C (At first end hot and cold fluids enter the heat exchanger.)

What is the value of LMTD?

Explanation: The value of corrected LMTD should be less than the theoretical LMTD considering countercurrent flow. Thus the LMTD correction factor is less than 1. Also for steady state operation of the HE, its value should be more than 0.75.

How do you calculate LMTD?

In this example, a simple heat exchanger with single shell pass and single tube pass is considered. For multiple shell and tube passes, the LMTD calculated has to be multiplied by a correction factor to account for geometric changes.