In general, the practical use of waste energy is called heat recovery. Figure 10.5 shows an example of such a system. Refrigeration plants with air-cooled condensers produce a lot of waste energy by dumping the condensation energy to the ambient air. By installing a brazed plate heat exchanger desuperheater, a large proportion of this waste energy can be turned into hot water that may be used for many purposes such as:
- sanitary hot water
- room heating
- hot water for processes
- cleaning water
Desuperheater units are located between the compressor and condensers to utilize the high-temperature energy of the superheated refrigerant gas. By using a separate heat exchanger to utilize the high temperature of the discharge gas, it is possible to heat water to a higher temperature than would be possible in a condenser (see Figure 10.6).
The total amount of available superheating depends on the difference between the discharge temperature from the compressor (point a in Figure 10.7) and the condensing temperature of the refrigerant gas (point b). The system can be arranged to bypass the desuperheater brazed plate heat exchanger if no hot water is required. The condenser should then be designed to be able to handle the full condensing capacity.
The desuperheater is normally designed not to condense any refrigerant. However, some liquid refrigerant may form, depending on operating conditions. This liquid must be carried to the condenser, which ideally should be situated below the desuperheater. However, for practical reasons it is often placed above the desuperheater. In a brazed heat exchanger, the droplets of refrigerant leaving will be dispersed and easily carried over by the dominant vapor phase. Designing the connection pipe from desuperheater to condenser for a gas velocity of 5-10 m/s will provide sufficient turbulence to avoid the accumulation of liquid condensate.
Scaling is a potential problem in desuperheaters when the secondary fluid is water, because the solubility of limestone (CaCO3) decreases with increasing temperature. The maximum water temperature should ideally not exceed 65-70°C to avoid scaling problems. If the risk of scaling is increased by the use of hard water, etc., the use of co-current flow should be considered to reduce the risk of excessively high water temperatures.