Lesson 6 - Radiation and Combined

In practice, heat transfer surfaces do not remain clean. Even with a proper cleaning procedure in place, scale, dirt, and other solids form on heat transfer surfaces as they are continually used. The effects of this add additional resistance to heat transfer known as fouling. Fouling factors can be very variable in magnitude and difficult to estimate because they are often dependent on time. Taking account of fouling factors in wort boiling or cooling is very important in order to maintain acceptable heat transfer rates during production.

The amount of ‘thermal resistance’ (r) that fouling adds is dependent on the thickness of the layer (x) and the thermal conductivity of the scale layer (k). Thermal resistance is defined as:

It is common to account for fouling when calculating the overall heat transfer coefficient by adding an addition term ‘’ for each film of resistance. In the case of our previous example, let’s say we have fouling on the hot side of the wall. The equation for overall heat transfer coefficient would then become:

Resistance resulting from fouling can be as high as 0.005 m² KW ^-1.

Fouling factors provide a safety factor for designing heat exchanging systems. You will learn more about this application later in the unit.