Calculate the state of moist air from any pair of properties. Enter the dry bulb temperature with relative humidity, wet bulb, dew point or humidity ratio, and the pressure, and the calculator returns humidity ratio, RH, dew point, wet bulb, enthalpy, specific volume, density and vapour pressures, draws the point on a psychrometric chart, and mixes two air streams by volume flow.
What it gives you
- Full state point: W, RH, dew point, wet bulb, enthalpy, specific volume, density
- Any humidity input: RH, wet bulb, dew point or humidity ratio
- Pressure input for altitude or pressurised systems
- Adiabatic mixing of two streams on a dry-air mass basis, with fog detection
- Live chart with saturation curve, RH and enthalpy lines
Method and standards
The relations are those of ASHRAE Fundamentals: the Hyland-Wexler saturation pressure over water and ice, ideal-gas humidity ratio, and the standard enthalpy and specific volume expressions with dry air at 0 °C as the datum. Dew point and wet bulb are solved iteratively. Mixing combines humidity ratio and enthalpy in proportion to dry-air mass flow.
How to use it
- Enter dry bulb and one humidity property.
- Read the state point and check it on the chart.
- Enter two streams with flows to see the mixed condition.
Questions engineers ask
- Which pressure should I use?
- Local atmospheric pressure. 101.325 kPa at sea level, about 95.5 kPa at 500 m and 89.9 kPa at 1000 m. Use the system pressure for pressurised air.
- What does the fog warning mean?
- The mixed point lies above the saturation line, so moisture condenses and the air leaves saturated at a lower humidity ratio.
- How accurate is it?
- The relations reproduce the ASHRAE tables to the last published digit over their validated range of −60 to 100 °C.
Need this calculation done properly, with a signed-off calculation pack or FEA behind it? Forgepoint provides mechanical design, analysis and pressure equipment engineering from Manchester, UK-wide.
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