Flash Steam Calculator

How much flash steam forms when hot condensate is released to a lower pressure.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Condensate at a high pressure is hot water at the saturation temperature for that pressure. When the pressure drops, the water is suddenly hotter than the new saturation temperature, and the excess heat turns part of it into steam. That is flash steam.

The fraction that flashes is the drop in the liquid enthalpy divided by the latent heat at the lower pressure. The flash steam is low-pressure steam that can be used, for example in a heater, instead of being lost to atmosphere.

Formulas used

  • x = \frac{h_{f1} - h_{f2}}{h_{fg2}}
  • \dot m_{flash} = x\,\dot m

Worked example

With the default numbers in the calculator above:

  1. Liquid enthalpy before and after: 742.8 and 468.2 kJ/kg
  2. Latent heat at the lower pressure: 2225 kJ/kg
  3. Flash steam fraction: 12.34 %
  4. Flash steam produced: 123.4 kg/h

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Why does a condensate return line sometimes "steam"?

Because the condensate flashes as its pressure drops. It is flash steam, not live steam leaking through a trap.

Where do the values come from?

The IAPWS-95 steam table, interpolated linearly between rows.

What is the pressure after?

The pressure in the vessel or line the condensate flows into, in bar gauge.

Related calculators

Further reading: Cengel and Boles, Thermodynamics: An Engineering Approach; Moran and Shapiro, Fundamentals of Engineering Thermodynamics.

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