How to Calculate the Log Mean Temperature Difference (LMTD)

This short article aims to show you how to calculate the LMTD for Counter current flow and Co-current flow also called (Parallel flow). The calculation has been referenced by many text books. You can also download the free calculator attached with this article.

Why do we need to calculate it?
The log mean temperature difference represents the driving force for heat exchanger operation. It will allow us to make a decision on the flow arrangement type and making the right choice here will also improve your final design and mainly the heat transfer.

Flow arrangements
There is mainly two different flow arrangement in heat exchangers,
Counter Current flow:
See the image below

Counter current flow arrangement

Where:

T1 - Inlet temperature of hot stream

T2 - Outlet temperature of hot stream

t1 - Inlet temperature of cold stream

t2 - Outlet temperature of cold stream

Co-current flow (Parallel flow):

See the image below

Parallel flow arrangement

How do we calculate it?
The log mean temperature difference is normally calculated from the terminal temperature differences.

For Counter current flow:

 For Co-current flow

Reference:
D. Q. Kern, Process Heat Transfer, McGraw-Hill, International Edition, 1950

Free LMTD Calculator

Log mean temperature difference calculator

Download from webbusterz

How to determine stream allocation in heat exchangers

Stream allocation is an important decision in heat exchanger design.  This decision can impact the heat exchanger life, can also make the difference between higher and lower maintenance costs.  Can also have a high impact on the thermal performance the heat exchanger.  This short article aims to discuss the process of determining the fluid allocations in shell and tube heat exchangers. The decision is made based on the criteria below:
1- Fouling:
Check fouling factors for each fluid.
The fluid/stream with the high fouling factor should be in tubes – this makes it easy to clean and prolongs the exchanger life. Placing the fouling fuild in the tubes also allows better velocity control as increased velocities tend to reduce fouling.
2- Pressure:
Higher pressure fluid/stream is placed in Tube side. This hasan effect on the shell thickess as placing the high pressure fluid in the tubes would mean that the shell thickness can be reduced.
3- Corrosion rate:
Higher rate fluid/stream is placed in Tube side. In general fewer corrosion resistant alloys are needed if the corrosive fluid is placed on the tube side
4- Viscosity
Fluid/Stream with higher viscosity is placed in the Shell Side as higher heat transfer rates are generally obtained using this practice.
5- Phase change
The stream with phase change is assigned to Shell Side
in steam heated vaporizers/reboilers the condensing steam is placed in Tube Side
I hope the above provide a simple guidline for anyone not sure about this issue.
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