Heat exchangers on board ship are mainly coolers where a hot liquid is cooled by sea water. There are some instances where liquid heating is required, such as heavy fuel oil heaters and sea water heaters fortank cleaning. Although being heat exchangers, main condensers for a steam ship and the evaporator/distiller are dealt with separately.
The heat exchange process is aceomplished by having the two liquids pass on either side of a conducting surface. The heat from the hot liquid passes to the cold liquid and the conducting surface, i.e. the tube wall, is at a temperature between the two. It is usual for marine heat exchangers to have the two liquids flowing in opposite directions, i.e. counter or contra flow. This arrangement provides a fairly constant temperature difference between the two liquids and therefore the maximum heat transfer for the available surface area.