Control systems Desuperheaters
4
Types of desuperheater From the preceding paragraphs, it is easy to understand why there has to be a period of good contact between the droplets of cooling water the superheated steam. If good contact is lost, the water can no longer absorb heat effectively from the steam, evaporation stops and the desuperheating process comes to a halt.
When the steam velocity is too low, 'water droplet fall-out' occurs and a pool of water is formed which runs along the bottom of pipe. At this point good contact between cooling water and the steam is lost and effective desuperheating will not occur. By following the guidelines presented in this document or using the Spirax Sarco online sizing software, problems due to droplet fall-out can be avoided.
There are three basic types of Spirax Sarco desuperheater (shown below) which all use a different method to atomize the cooling water droplets. Each one has its own merits and the desuperheater selection chart shown on the following page determines which type should be selected.
Co mplete spray type desuperheater
Cooling water
Spray nozzle assembly
Thermal sleeve
Superheated steam
Desuperheated steam
4.8 13
Venturi type desuperheater
Cooling water
Internal diffuser
Main diffuser
Internal nozzle
Steam flow
Seal
Seal
Steam atomizing desuperheater
Atomizing steam Cooling water
Internal nozzle
Internal diffuser
Steam flow
Finer droplets, therefore smaller absorbtion length
Steam flow
and/or better turndown and approach to saturation
Seal
Seal
TI-P475-06 CTLS Issue 4
Page 3 of 12
Desuperheater Online Program Sizing Guidance
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