The partial pressure is the pressure exerted by each component as if it was occupying the same volume of the mixture. The effective partial pressure of the steam can be expressed as:
ps_effective = vs / V p (2)
where
ps_ effective = effective steam pressure, absolute (Pa, bar, psi)
vs = volume of steam (m3, in3)
V = volume of mixture (m3, in3)
p = absolute pressure (Pa, bar, psi)
Reducing the part of steam reduces the effective steam pressure. Increasing the part of steam (until 100%) increases the effective pressure.
The effective pressure in a steam/air mixture made up by 3 parts steam and 1 part air, with total pressure 5 bar absolute, can be expressed as:
ps_effective = (3 parts) / (3 parts + 1 part) (5 bar abs)
= 3.75 bar absolute
Important! Since the steam has an effective pressure of 3.75 bar instead of the pressure of 5 bar a, the mixture would have a temperature of approximately 139 oC rather than the expected saturation temperature of 152 oC. This has a major effect on the heat transfer capability of an heat exchanger.
Mixture Pressure | 0% Air (Pure Steam) | 5% Air | 10% Air | 15% Air | |||||
---|---|---|---|---|---|---|---|---|---|
(psig) | (bar) | (oF) | (oC) | (oF) | (oC) | (oF) | (oC) | (oF) | (oC) |
2 | 0.15 | 219 | 104 | 216 | 102 | 213 | 101 | 210 | 99 |
5 | 0.35 | 227 | 108 | 225 | 107 | 222 | 106 | 219 | 104 |
10 | 0.7 | 239 | 115 | 237 | 114 | 233 | 112 | 230 | 110 |
20 | 1.4 | 259 | 126 | 256 | 124 | 252 | 122 | 249 | 121 |
Steam & condensate systems- properties, capacities, pipe sizing, systems configuration and more.
Thermodynamics of steam and condensate systems.
Basic air changing state processes - heating, cooling, mixing, humidifying and dehumidifying by adding steam or water - psychometric diagrams and the Mollier charts.
Air can be humidified by adding water or steam.
Estimate the amount of steam required (lb/h in 100 cfm) in humid air.
Using steam to humidify air.
The moisture holding capacity of air increases with temperature.
Gibbs' Dalton's law of the total pressure of a mixture of gases.
Sprayed coils, spinning discs and steam humidifiers.
The change in state wwhen mixing moist air - enthalpy, heat, temperature and specific humidity.
The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.
Density of the mix of dry air and water vapor - moist humid air.
Calculate steam heated air systems.
Calculating the amount of steam in non-flow batch and continuous flow heating processes.
An introduction to vapor and steam.
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