Fouling factor in heat exchanger: Definition, Formula, Units [with Pdf]

Before discussing the fouling factor let’s know, what is fouling?

What is fouling?

Fouling is the phenomenon of deposition of fluid impurities on the heat transferring surface of the body and that layer offers resistance for the transfer of heat.

The deposited impurities may be,
1) Impurities in raw water
2) Salt present in water
3) Formation of algae
4) Formation of the layer due to the reactive nature of passing fluid

The layer of fouling resists the flow of heat transfer and also reduces the flow rate of fluid through the utilities.

The fouling is a slower process and it is identified when the performance of the equipment decreases.

The rate of fouling increases with an increase in working time and temperature while it decreases with an increase in the velocity of the fluid.

What is fouling factor:

The fouling factor is the reciprocal of the heat transfer coefficient of the layer formed due to the fouling.

The fouling factor is denoted by the symbol `R_{f}`.

Therefore as per the definition, it is given by,

`R_{f}= \frac{1}{h}`

Fouling factor formula:

As discussed before it is given by,

`R_{f}= \frac{1}{h}`

In actual practice, the fouling factor is determined by testing the equipment in clean and scaling conditions.

Then by evaluating the heat transfer coefficient in both conditions the fouling factor is calculated as,

`R_{f}= \frac{1}{U_{SCALING}}-\frac{1}{U_{CLEAN}}`

Fouling factor units:

The SI and FPS units of the Fouling factor are as follows:-

In SI system:

In the SI system, the unit of convective heat transfer coefficient is Wm ².°C

Therefore the unit of fouling factor is given by,

Fouling factor `=\frac{1}{h}=\frac{1}{\frac{W}{m^{2}.°C}}=\frac{m^{2}.°C}{W}`

Hence the SI unit of fouling factor is m².°C/W

In FPS system:

In the FPS system, the unit of the convective heat transfer coefficient is Btu/hr.ft².°F

Therefore unit of fouling factor is given by,

Fouling factor`= \frac{1}{h}-\frac{1}{\frac{Btu}{hr.ft^{2}.°F}}`= hr.ft².°F/Btu

Hence the FPS unit of fouling factor is hr.ft².°F/Btu.

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