Q. We use steam in several processes, including the kilns. The new boiler supervisor mentioned that we needed a desuperheater as the steam was too hot after the steam goes through a pressure reducer. What is a desuperheater and why is the steam too hot?

A. First, your new boiler person is indeed alert to a fact that many people overlook. When steam, let’s say 50 psi, is first made in the boiler, it has a temperature of about 300F. Now, when the steam goes through a pressure reducer valve, the pressure does indeed drop, let’s say to 10 psi. If you had a boiler operating at 10 psi, the steam would be at 240F. However, when you drop the pressure of 50 psi, 400F steam to 10 psi, all the heat is still there.

This extra heat, called superheat, means that you have 10 psi steam at 400F. This is dry steam, rather than saturated. So, what is sometimes done is that this hot steam is bubbled up through water. The heat will actually boil the water and make steam until 240F is reached. Now you have 10 psi steam at 240F; now the steam is wet steam. Wet steam heats more uniformly, faster, and more efficiently. The vessel (often only 10 inches in diameter and 3 feet long) with fresh water continually being added to make sure that it does not run dry is called a desuperheater.