Basic Heat Exchanger Duty Calculation

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In this article

– We will look into the two types of flow in a heat exchanger.
– We will also check the formula of LMTD. We will then compute for the heat exchanger duty.
– Lastly, we will provide the assumptions in which the LMTD may be valid.

 

 

Parallel Flow (or Con-current)
There are two types of flow in a heat exchanger – parallel flow and counter flow. In parallel flow, the hot and cold fluid steams enter from the same end and exit at the other end of the heat exchanger.

In physics, we learned that heat transfers from hot to cold. The hot fluid stream transfers its heat to the cold fluid stream as the streams flow in the heat exchanger. Thus during the heat transfer process, the temperature of the hot fluid is decreased while the temperature of the cold fluid is increased.

con-current con-current

 

Counter Flow (or Counter-current)
Conversely, in counter flow, the hot and cold fluid streams enter from different ends of the heat exchanger. Same concept as above, the temperature of the hot fluid is decreased while the temperature of the cold fluid is increased.

 

Counter-currentCounter-current

 

Logarithmic Mean Temperature Difference (LMTD)
Before we compute for the heat transfer, we first calculate the LMTD.

LMTD


Heat Transfer Rate (or Heat Exchanger Duty)

The equation for heat transfer process is as follows:

Q = U*A*LMTD

where: Q = heat transfer rate
U = heat transfer coefficient
A = surface area where the heat transfer takes place

Note that an LMTD correction factor can be found in Tubular Exchanger Manufacturers Association (TEMA) for different types of heat exchangers.

 

Assumptions:
When using LMTD, we should consider the following assumptions:

a. The specific heat of the fluids are constant.
b. There is no phase change. Thus, only sensible heat (no latent heat) is transferred.
c. The heat transfer coefficient is constant.
d. The flow is a steady-state concept.

 

Photo credit to Supakitmod3 of freedigitalphotos.net

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