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Control systems Desuperheaters
Detailed example Steam is required to heat a vessel jacket at 30 psi g (2 bar g) and 275 °F (133.7 °C). A low pressure steam supply is available at
145 psi g (10 bar g) and 390 °F (200 °C) for this purpose. Condensate at 200 °F (95 °C) and 75 psi g (5 bar g) is available as the cooling medium. Minimum steam flow required is 2200 lbs/hr (1000 kg / h), maximum flow 11000 lbs/hr (5000 kg / h).
Pressure controller
Steam supply
Desuperheater
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Pressure regulating control valve
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Temperature regulating control valve
Check valve
Cooling water
Temperature controller
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Method As the turndown required equals 5:1 and the application involves simultaneous pressure reduction, this would seem to be a good application for a venturi type desuperheater (VTD). Please note that a steam atomizing desuperheater (SAD) could also be used for this duty, but the overall installation cost would be higher due to the greater complexity of the installation. As already stated, it is not possible to control at the saturated temperature of the steam so any associated temperature controller should have a set point of 5 °F (3 °C) above the saturation temperature (which is 275 °F (136.7 °C) in this example). We can use the Spirax Sarco Online Sizing Program to calculate the unit size (available at www.spiraxsarco.com/uk with password). The form would be completed as follows: The steam temperature at the inlet to the desuperheater is calculated by the program automatically and is shown on the datasheet. Once the calculation button is pressed the software will calculate the required cooling water pressure and flowrate. It will also calculate the pressure drop across the unit. The user will see a summary screen, at which point the desired flange rating can be selected and then the 'Save & Email' button can be pressed. You will then receive an e-mail with a drawing and datasheet to your chosen e-mail address.
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TI-P475-06 CTLS Issue 4
Page 8 of 12
Desuperheater Online Program Sizing Guidance
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