# 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

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

How do we calculate it?

The **log mean temperature difference** is normally calculated from the terminal temperature differences.

For Co-current flow

Reference:

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

Free LMTD Calculator