As the name implies, a condenser is a heat exchanger used to convert vapour into liquid. The vapour may be a single component or a mixture. In either case, if it is possible to condense all the vapour, the exchanger is referred to as a Total Condenser. If, however, non-condensable gas is mixed with the vapour, it is not possible to condense all the vapour because the non-condensable leaving the condenser will be saturated with vapour. Hence, such an exchangers is called a Partial Condenser.

So called total condensers will almost always have trace non-condensable present but these can often be ignored in the thermal design except for ensuring that provision is made to prevent accumulation of non-condensable during long-term operation Such removal is normally by venting although it may rarely be possible to rely on the non-condensable being desolved in the condensate.

This section of HEDH is concerned with exchangers whose primary purpose is condensation. There are many cases, however, where the condensation is only there to supply heat needed for another stream which may be single-phase or vaporising. The vapour undergoing condensation in such cases is normally service steam. While the general principles described here will still apply to such exchangers, the design will be mainly dictated by the stream being heated or vaporised. Other sections of this Handbook should therefore be consulted in these cases. These are Section 3.3 for single-phase heating, 3.5 for evaporators and 3.6 for reboilers.

In most cases, the condensing stream is separated from the coolant by a solid wall (usually metal) which may be a plate or a tube wall. The term surface condenser can be used to describe this arrangement although this name tends to be confined to tubular condensers used in the power industry The alternative, is a direct contact condenser where the coolant is sprayed or poured directly into the vapour. Clearly, this arrangement is only possible if the condensate and coolant may remain mixed or if they are immiscible to enable separation. Direct contact condensers are discussed in Section 3.19.

This section of HEDH is concerned mainly with shell-and-tube condensers. It should be noted, however, that other exchanger types may be used as condensers. These are listed below together with the section of this Handbook where more information may be found (where no section number is given, there is nothing yet in the Handbook).