Heat exchanger is a heat exchanger that transfers heat between two or more liquids. Each liquid is separated by a wall to prevent them from mixing. Sometimes, in an HVAC system, the heat exchange will occur between a gas and a liquid. They can be used for both heating and cooling. Heat exchanger is a heat exchanger which employs a two-phase heat transfer mechanism. Heat exchanger is when one or more fluids undergo a phase change during the heat transfer process, either changing from a liquid to a gas or a gas to a liquid.