Editing Heat exchanger (section)

===Waste heat recovery units===
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A [[waste heat recovery unit]] (WHRU) is a heat exchanger that recovers heat from a hot gas stream while transferring it to a working medium, typically water or oils. The hot gas stream can be the exhaust gas from a gas turbine or a diesel engine or a waste gas from industry or refinery.

Large systems with high volume and temperature gas streams, typical in industry, can benefit from steam [[Rankine cycle]] (SRC) in a waste heat recovery unit, but these cycles are too expensive for small systems. The recovery of heat from low temperature systems requires different working fluids than steam.

An organic Rankine cycle (ORC) waste heat recovery unit can be more efficient at low temperature range using [[refrigerant]]s that boil at lower temperatures than water. Typical organic refrigerants are [[ammonia]], [[pentafluoropropane]] (R-245fa and R-245ca), and [[toluene]].

The refrigerant is boiled by the heat source in the [[evaporator]] to produce super-heated vapor. This fluid is expanded in the turbine to convert thermal energy to kinetic energy, that is converted to electricity in the electrical generator. This energy transfer process decreases the temperature of the refrigerant that, in turn, condenses. The cycle is closed and completed using a pump to send the fluid back to the evaporator.