●Heat exchangers are used in condensers and boilers in steam plants, radiators, intercoolers and preheaters and condensers and evaporators in refrigerators and air conditioners

●A heat exchanger is a device which used to transfer heat between a solid object and a fluid or betweentwo or more fluids.

●The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact.

Hence, the correct option is (D).