Control valves

Desuperheaters

Types of desuperheater

From the foregoing 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 atomise 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.

Complete spray type

desuperheater (STD)

Thermal sleeve

Desuperheated steam

Superheated

steam

Venturi type

desuperheater (VTD)

Steam

flow

Steam atomising

desuperheater (SAD)

Steam

flow

Steam

flow

Internal

nozzle

Internal

diffuser

Seal

Internal

nozzle

Main

diffuser

Spray nozzle assembly

Cooling water

Seal

Cooling water

Seal

Seal

Internal

diffuser

TI-P475-06 CH Issue 3

Page 3 of 12

Atomising

steam

Cooling

water

Finer droplets, therefore

smaller absorbtion length

and / or better turndown and

approach to saturation

Desuperheater Online Program Sizing Guidance

4.8.14