and rearranging gives an expression for UA in a circular cross section where heat is being transferred:

In this example, the effect of the scale deposit, hivl, is on the inside wall of the pipe. There are commonly five resistances to heat transfer caused by the (1) heat transfer wall and (2) inside (i) and (3) outside (o) of the heat transfer wall, (4) 2 fluids (h0 and hj, and (5) 2 fouling scales (hls and hos). Often there is a fouling scale on the surface of a transfer surface caused by oxidation or burn-on during operation. Protein foods are particularly bad in that the heat causes denaturation and subsequent adherence to the surface. When gas comes into contact with heat transfer surfaces (eg, a fire tube boiler) heavy scales can be formed. In fact, the maintenance and cleaning of heat exchanger surfaces is a continuous chore, usually accomplished by chemical cleaners. However, in extreme cases (especially in boiler tubes), a periodic rodding out is necessary to maintain the efficiency of the heat transfer.

Boilers are particularly important to the food processing industry since they are the source of steam for many heat transfer systems. The boiler is a heat exchanger that converts a liquid to a vapor. The vapor, after being condensed in a food processing heat exchanger, is normally collected and recycled. Often there are fouling scales or film deposits on both sides of a pipe or plate heat exchanger. This must be recognized in the calculation of the overall heat transfer coefficient.

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