There are several possible causes of poor thermal performance of a condenser, as discussed below. Fouling, common to all beat exchangers, is not included, as this is not likely to be a problem except when the coolant is dirty water (see Section 367E). The problems discussed may lead to a serious reduction in the rate of beat transfer. Further information is given by K. J. Bell in Taborek (1993) and in Section 264 of HEDH.

A. Temperature difference

The saturation temperature of a condensing stream depends on its pressure and composition. Consequently if the pressure drop exceeds the predicted amount, the temperature difference, and hence the rate of heat transfer, will be less than that predicted. Excessive pressure drop may occur in the vapour inlet nozzle or where the vapour first enters the condensing region. The problem is especially difficult with shell-side condensation of steam under vacuum.

Similarly if the condensing stream contains more incondensable gases than was anticipated, the saturation temperature will be less than was anticipated and again the rate of beat transfer will be reduced. A stream of vapours specified as being free of any gases may in reality contain small traces of gas that will increase to quite a high condensation at outlet, where the temperature difference is at its lowest; this leads to a considerable reduction in the local heat flux, due to reductions both in the temperature difference and in the beat transfer coefficient. In particular, vacuum steam condensers may fail in their performance as a result of an excessive amount of air leaking into the steam.

In a multi-component condensing stream, the temperature profile must be carefully specified, especially if there is a "temperature pinch" in the condenser, i.e. a point where the temperature difference is considerably less than elsewhere. A change in process conditions may lead to a reduction in the saturation temperature, with consequent loss in beat flux.