The Sugar Engineers

Desuperheating of Steam

Rapid Design

A desuperheater is a device that cools superheated steam to a temperature close to its saturation temperature, ususally by spraying atomised droplets of water into the flow of supeheated steam. Superheated steam is steam that is at a temperature above its saturation temperature.

Desuperheating of steam is an almost universal feature of a sugar factory. This is for two reasons

• Steam turbines are generally designed to leave some superheat in their exhaust to prevent erosion of turbine blades by water droplets
• Juice and syrup should be boiled at less than about 125°C to reduce colour formation and sucrose destruction

There are numerous methods of desuperhing steam each with their own advantages and disadvantages: a good discussion on the various approaches to desuperheating is given by Spirax Sarco

Theory

A heat and mass balance over the desupeheater yield two equations

m_s2h_s2 = m_s1h_s1 + m_wh_w

m_s2 = m_s1 + m_w

Combining these yields

m_w = m_s1 · (h_s1 - h_s2) / (h_s2 - h_w)

Symbols

• m
  mass flow rates [kg/s]
• h
  enthalpy [kJ/kg]

Subscripts

• w
  cooling water
• s1
  steam upstream of the desuperheater
• s2
  steam downstream of the desuperheater