DE SUPERHEATER - SPRAY & SURFACE TYPE

DE SUPERHEATER

The purpose of a de-superheater is to reduce the temperature of a quantity of superheated steam and make it suitable for auxiliary services requiring low temperature or saturated steam conditions.

The reason for doing it in this way, rather than by taking the saturated steam direct from the steam drum, is to protect the superheaters; these must be protected against overheating under all-conditions of load by circulating steam through them.

Although attemperators and de-sup erheaters are both used to protect the

superhe ater from overheating, they do this in different ways. 

Attemperators are fitted to control the temperature of the steam passing through, while de-superheaters help to provide the necessary circulation of steam

Two other important differences between these two items of equipment are

# firstly, that de-superheaters are fitted after the final superheater stage and the de-superheated steam is not then raised again temperature.

# Secondly, that while attemperators give control over a range of superheat temperature, a de-superheater usually produces steam at a constant temperature.

TYPE

SPRAY TYPE DE-SUPERHEATER

1. These are fitted independent of the boiler, and are suitable for producing large amounts of de-superheated steam.  . It consists basically of a mixing tube, water spray nozzle, and a thermostat to provide control.

Superheated steam enters and passes down through the inner mixing tube   

2. Water is sprayed into this steam through an a to mizing nozzle, and an exchange of heat by direct contact takes place .

3. The superheat temperature of the steam is reduced , while the water receiving th is heat as latent heat changes to steam. 

4. Any water droplets remaining are separated out by the baffle, and are drained off through the steam trap

5. while the de-superheated steam  passes up the outer mixing tube to an orifice plate fitted in the outlet pipe to ensure a thorough mixing and final evaporation of any remaining particles of water before the steam leaves.

SURFACE TYPE DE-SUPERHEATER

1.These consist of a tube or tubes placed below the water level in the steam or water drums.

2. The steam passing through is cooled to about 30°C above the  saturation temperature corresponding to the pressure in the drum.

3. In some cases fins are fitted on the tubes to extend the heat exchange surface , but usually bare tubes formed into a number of loops, or in the form of a series of U-tubes, are used.

4. If larger quantities of steam are required, the de-superheater may be placed in an external chamber connected to the steam and water drums by balance connections.

5. The sensible heat given up by the superheated steam passing through the de-superheater is used to generate steam which is then returned via the top balance connection to the steam drum.