In case the boiler is required to generate superheated steam, the temperature of steam should be brought down to saturation temperatures before utilization in the process.
At many occasions plants have a boiler that generates superheated steam which is used in co-generation systems to generate electricity. When this superheated steam is to be used to meet the process steam requirements, its temperature needs to be brought down to saturation temperatures.
The most practical way of reducing the superheat value of steam is by the direct addition of water. To achieve temperature stability of the conditioned steam and prevent thermal shock in downstream pipes, cooling water must be atomized. It is also necessary to have a correct mix of superheated steam and cooling water. This is achieved via equipment called the de-superheater.
In a de-superheater water is injected in to the superheated steam, whereby the water absorbs the excess heat in steam and itself converts to steam. Thus there is an increase in the quantity of steam after the de-superheater. It is absolutely necessary that the outlet steam temperature should be as close as possible to its saturation temperature. The sole reason for this is the saturated steam is the best conductor of heat and as the degree of Superheat increases, the heat transfers become inefficient.
If the outlet steam temperature is, say,15 deg.C higher than saturation temperatures of180 deg.C, then the steam consumption shall be 10900 Kgs/Hr to achieve the same enthalpy transfer. This means that you will end up spending 900 kgs/Hr steam extra due to the system inefficiency. Considering a typical coal fired boiler, the steam costs approx Rs. 1/- per Kg, the losses each hour shall be in the region of Rs. 900/- per hour!