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About Variable Nozzle De-superheating : Week 9/7/2012

In our last set of article, we discussed about some methods of De-superheating and their significance. In this article, let us discuss about Variable Nozzle Desuperheater & how desuperheating can be done effectively.

Variable Nozzle Desuperheater

When there is a need for higher Kv, or when the process conditions are changing or changes in output conditions are required, the earlier two types (i.e. Fixed Nozzle or Mini cooler type design) may not offer the appropriate solution. A Variable Nozzle Desuperheater (VND) would be the most appropriate solution in such cases. This design can ensure a fine conical spray that evaporates almost instantaneously, avoiding pipe impingement. It also eliminates need of a separate coolant valve, while offering more turndown ratios. A large desuperheating range can be covered because of more number of available nozzles. Let’s see how this is done.

The nozzle assembly consists of three components: A) Spray head B) Nozzle C) Nozzle cap

The high quality surface finish of the spray nozzles minimises frictional losses, thereby ensuring that the total pressure difference between water and steam is available for atomisation of water. Each nozzle assembly is served by individual feed holes in the spray cylinder. Water enters the chamber through these openings. The relatively large volume of this chamber ensures that water is proportioned evenly through each orifice. The pressure drop across this spray cylinder head results in an increase in the fluid velocity. Water is subsequently rotated in the nozzle chamber before being emitted through the central hole. The combination of splitting the feed flow, increasing velocity and rotating effect ensures that water is injected into the system in a symmetrical hollow cone spray. The nozzles are finally assembled with the spray cylinder and are sealed.

The Variable Nozzle Desuperheater may be equipped with a variety of spray heads. The uniform threading accepts spray cylinder heads with a wide range of Kv values. Typical configurations are with either 6 or 9 equally sized spray nozzles. Special combinations are possible to suit process requirements. This feature enables the VND to be customised for specific system requirement.

For effective desuperheating:

There are few guidelines to ensure effective desuperheating in your process. While selecting or designing a desuperheater care must be taken for the following aspects:

  • Check the maximum allowable steam pipe size (usually upto 1200mm)
  • Check minimum pipe size required for installation (usually 150mm)
  • Water pressure should be minimum 5 bar more than the steam pressure
  • Maximum recommended pressure difference between steam and water is 45 bar. If it exceeds 45 bar then you need to install pressure control valve in waterline to reduce water pressure within limit
  • Spray water should be very clean (equivalent to boiler feedwater)

In our next article, we will discuss some more aspects of the Variable Nozzle Desuperheater (VND).