A desuperheater in the context you're referring to is an add-on (secondary) heat exchanger that transfers otherwise wasted heat from an air conditioner's hot gas line to a storage water heater. It requires expert installation skills and additional plumbing between outdoor unit and storage tank. It provides "free" hot water. Whether or not it may save more than it costs depends on climate, building cooling load and local energy prices.
In order to get full benefit of a desuperheater, a buffer tank is necessary. Otherwise, the primary water heater will maintain its temperature independently, which reduces or eliminates heat transfer from the desuperheater (capacity is proportional to the temperature difference between the water and the refrigerant, and the water flow rate). Also, a circulator pump is necessary. Otherwise, heat transfer would only happen when hot water draws coincide with A/C cycles. The cost of the desuperheater, pump and buffer tank, plus installation makes it difficult to justify.
Desuperheaters fell out of favor after the conversion to R410a refrigerant but are still offered as factory installed options by many ground source heat pump manufacturers.
Thanks for your response.
The students I taught in my HVAC classes always seemed to have a hard time understanding what Superheat is and a greater difficulty applying the processes involved. I always returned to the basics to explain this concept. The following may clear up any questions regarding Superheat.
Taking water, at atmospheric pressures, we need to raise the temperature of the water to 100 deg. C (212 deg. F) to achieve its boiling point. Heating liquid water to its boiling point uses “Sensible Heat” or that heat applied that can be measured with a normal thermometer. Notice that when we add Sensible Heat, the water stays in its (liquid) state or phase, but its actual temperature rises.
When this water’s boiling point is reached, the water goes through a change of state with the addition of heat energy…converting liquid water to steam. During this water phase change, liquid water is converted into water vapor - steam (water in a gaseous form). The added heat to accomplish this is called Latent Heat. The addition of Latent Heat involves a change in the state of matter. In this case liquid water is changing its state to a gas and does so keeping the temperature of the process constant.
Going back to our example, liquid water at its boiling point is at 100 deg. C (212 deg. F) at atmospheric pressures. While the state is changing from liquid to gas, the temperature remains the same. When the change of the state of the water is completed, the temperature of the water vapor (steam) is 100 deg. C. (212 deg. F). If processes require this steam temperature to increase to 110 deg. C (230 deg. F.) Sensible Heat needs to be added.
Increasing the temperatures of matter beyond their boiling point is called SuperHeat.