A refrigeration system does not destroy heat, it moves it. The sum of the heat removed from the room and the electricity drawn by the compressor is rejected at the condenser: a plant doing 100 kW of cooling and drawing 40 kW of electricity continuously gives 140 kW of heat to the air. The same plant mostly burns fuel separately for wash water and heating. Heat recovery transfers this heat to water through a heat exchanger added to the discharge line. The heat has already been produced; its price has been paid.
There are two levels. A desuperheater takes only the superheat of the hot gas leaving the compressor; this is 10–25% of the total heat rejected but its temperature is high, and water at 55–65 °C is obtained. In full condensing heat recovery all the heat is taken; the water temperature stays a few degrees below the condensing temperature, at the 35–45 °C level. In transcritical CO₂ systems water at 60–70 °C can be produced in large proportion. Which is chosen is determined by the temperature and daily quantity of hot water needed in the plant.
In food plants the most suitable uses are these: preheating of cleaning and wash water in slaughterhouses and dairies, staff showers, underfloor heating of frozen stores (glycol at 10–20 °C is enough against frost heave), heating of offices and processing areas, and preventing icing around doors and docks. Because heat production and consumption do not coincide in time, an insulated storage tank is needed. The tank volume is chosen according to the daily hot water consumption.
The trap is raising the condensing pressure to get hotter water: every 1 K rise in condensing temperature increases compressor consumption by 2–3% and the gain turns into a loss. The superheat should be taken first, and the pressure raised only if the calculation shows it is worthwhile. In a drinking water circuit a double-wall heat exchanger is used so that refrigerant and oil cannot mix with the water. In hard water the plates block quickly; water softening and regular cleaning are planned. When recovery is out of service the condenser must be able to reject all the heat by itself.