What is a desuperheater?
Superheated steam is steam that is at a temperature higher than the saturation temperature for the steam pressure. For example, steam at a pressure of 3 bar g has a saturation temperature of 143.762°C. If further heat were to be added to this steam and the pressure remained at 3 bar g, it would become superheated.
So, desuperheating is the process by which superheated steam is restored to its saturated state, or the superheat temperature is reduced.
The idea behind desuperheating is that saturated steam has a better energy exchange capacity (U coefficient) than superheated steam.
Superheated steam must cool down before condensing, therefore it is less efficient than saturated steam in appliances such as heat exchangers.
Also, superheated steam is a thermal insulator, just like air.