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Control systems Desuperheaters
Detailed example 2 Steam is required at 75 psi g (5 bar g) (saturated) for use in a Shell & Tube Heat Exchanger. The Client has steam available at 75 psi g (5 bar g), 660 °F (349 °C). Minimum steam flow is 18,750 lbs/hr (8 505 kg / h), maximum is 55,000 lbs/hr (24,948 kg/h). Boiler feedwater at 70 °F (21 °C) is available as the cooling medium.
B
Desuperheater
Steam supply
2
3
Check valve
Cooling water
Temperature controller
4.8 20
4
Temperature regulating control valve
Method As the turndown is approximately 3:1 we can select any type of Desuperheater. There are 3 options: SAD - Steam atomising desuperheater This would require atomizing steam at 110 psi g (7.5 bar g) min. VTD - Venturi type desuperheater This could be selected, however, this desuperheater is not part of a pressure reducing station, consequently if we select a VTD venturi type desuperheater, the discharge steam pressure will be reduced by the pressure drop across the unit. The steam could be desuperheated to Tsat +5 °F (+3 °C). STD - Spray type desuperheater The unit is capable of handling the turndown with no steam-side pressure drop and can desuperheat to Tsat +9 °F (+5 °C). In this case the client states that Tsat +9 °F (+5 °C) is acceptable so we will select an STD. The Spirax Sarco Online Spray Type Desuperheater Sizing Program would be completed as shown opposite:
This section does not need to be completed
TI-P475-06 CTLS Issue 4
Page 10 of 12
Desuperheater Online Program Sizing Guidance
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