Desuperheating of Steam
Rapid Design
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A desuperheater is a device that cools superheated steam to a temperature close to its saturation temperature, ususally by spraying atomised droplets of water into the flow of supeheated steam. Superheated steam is steam that is at a temperature above its saturation temperature.
Desuperheating of steam is an almost universal feature of a sugar factory. This is for two reasons
- Steam turbines are generally designed to leave some superheat in their exhaust to prevent erosion of turbine blades by water droplets
- Juice and syrup should be boiled at less than about 125°C to reduce colour formation and sucrose destruction
There are numerous methods of desuperhing steam each with their own advantages and disadvantages: a good discussion on the various approaches to desuperheating is given by Spirax Sarco
Theory
A heat and mass balance over the desupeheater yield two equations
ms2hs2 = ms1hs1 + mwhw
ms2 = ms1 + mw
Combining these yields
mw = ms1 · (hs1 - hs2) / (hs2 - hw)
Symbols
- mmass flow rates [kg/s]
- henthalpy [kJ/kg]
Subscripts
- wcooling water
- s1steam upstream of the desuperheater
- s2steam downstream of the desuperheater