‘Fouling’ is a general term that includes any kind of deposit of extraneous material that material that appears upon the heat exchanger surface during the lifetime of the heat exchanger.
Whatever the cause or exact nature of the deposit, additional resistances to heat transfer is introduced and operational capability of the heat exchanger is correspondingly reduced. In many cases, the deposit is heavy enough to significantly interfere with fluid flow and increase the pressure drop required to maintain the flow rate through the exchanger.
The effect of fouling is to form an essentially solid deposit upon the surface, through which heat must be transferred by conduction. If we knew both the thickness and the thermal conductivity of the fouling, we could treat the heat transfer problem simply as another conduction resistance in series with the wall.
In general, we know neither of these quantities and the only possible technique is to introduce the additional resistance as fouling factors in computing the overall heat transfer coefficient.
Fouling effects inside the tube usually cause the reduction in heat transfer and the small increase in flow resistance.