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Methods of De-superheating : Week 8/21/2012

In our last set of article, we discussed about what is De-superheating, why it is required & how it is done. In this article, let’s see various methods of desuperheating.

Methods of Desuperheating

Primarily, there are two methods, one is direct & the one is indirect.

In case of indirect method, the cooling medium doesn’t get mixed with superheated steam. The cooling medium can be liquid or gas, even air. The shell & tube heat exchanger is a good example of this type. The steam temperature control is taken care of by controlling the pressure or temperature of the cooling medium.

In case of direct method, the cooling medium comes into direct contact with steam. Usually, it is cool water that is mixed. The water, when gets mixed with the superheated steam, evaporates, thus resulting in reduction in steam temperature. The water mixing arrangement is what defines various design methods of desuperheaters.

1) Fixed Nozzle Desuperheater (FND)

This design is usually for large pipes, of size 6” and above. This is one of the simplest forms of desuperheaters. The performance of this desuperheater depends on spray nozzle design. This can usually handle Kv as low as 0.0162 mÑ / hr and as high as 1.6 mÑ / hr. The number of nozzles is decided according to the pipeline diameter. These desuperheaters can typically handle turndown ratio upto 4:1 (depending on water quality). Maximum recommended pressure difference between steam and water is typically 45 bar (653 psig). If it exceeds this limit, then a pressure control valve is installed in waterline to reduce water pressure within limits. This is one of the most economic method of desuperheating.

2) Minicooler Desuperheater (MCD)

This is a unique device for specific applications like the Ejector/Gland Sealing Steam, where the cooling water injection quantity is very low. This design involves fixed nozzle. The Steam is cooled by injecting a little water in the zone where steam has highest turbulence. This is good for use with pipe sizes less than 4”. The turndown ratio is upto 1:4. This type is ideal for use with water control valve and control system. The main purpose of using this type is to avoid erosion of pipe surface, as the ejection happens inside the valve body only. The advantages include instantaneous water mixing, less straight length requirement and absence of moving parts avoiding possibility of choking.

In our next article, we will discuss about Variable Nozzle Desuperheater & how desuperheating can be done effectively.