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Desuperheaters

In refrigerators and air conditioners that use vapor-compression cycles, the gas-phase refrigerant leaving the compressor is much hotter than its condensing temperature. A desuperheater is a heat exchanger that extracts heat from the gas-phase refrigerant and heats another fluid, in this case water.

Desuperheaters are sold as options for new air conditioners and refrigeration equipment, or as add-ons for existing equipment. Retrofit installation of a desuperheater usually requires disconnecting or cutting into the refrigerant plumbing of an existing refrigerator or air conditioner.

How It Works
When operating, a desuperheater and the associated refrigeration or air conditioning system deliver heat to the water in the same manner as a HPWH, in other words as heat rejected from a vapor cycle condenser. The key distinction is that the primary function of a dedicated HPWH is heating water (with byproduct cooling that may or may not be useful), while a desuperheater is appended to a cooling system that operates to meet a cooling load (with waste heat available only when cooling is required). Desuperheater heat exchangers are plate-type or shell-and-tube type; they are of double-wall vented designs to avoid contamination of the potable water if a refrigerant passage develops a leak.

Desuperheater Schematic

Efficiency
The amount of heat that a desuperheater can recover from an air conditioner of given capacity depends on the air conditioner's efficiency; more efficient air conditioners have less heat available for recovery. Air conditioner efficiency is expressed as Seasonal Energy Efficiency Rating (SEER), which is the cooling capacity in Btu/hr divided by the electric energy consumption in Watts. For residential size 2 to 5 ton air conditioners (where 1 ton = 12,000 Btu/hr cooling capacity), a desuperheater on an SEER 14 air conditioner can heat about 5 gallons of water per hour per ton from 70 to 130°F; a desuperheater on a SEER 10 air conditioner can heat 7 gallons per hour per ton.

Capacity/Energy Input
When attached to a continuously operating refrigeration system, such as a central refrigeration system in a supermarket, ample waste heat to meet water heating requirements will be available throughout the year. However, when attached to a central air conditioning system, where cooling loads vary seasonally, daily, and hourly, a desuperheater will provide a limited portion of annual water heating needs (at no incremental energy), while requiring a significant amount of back-up heat input.