Vacuum Technology Book, Volume II

4.1.7 Water vapor tolerance / water vapor capacity

Water vapor tolerance $p_w$ is the maximum water vapor pressure with which a vacuum pump can continuously intake and displace pure water vapor under normal ambient conditions (20 °C, $p_0$ = 1,013 hPa). It can be calculated from the pumping speed, gas ballast flow, relative humidity and saturation vapor pressure at a given pump temperature.

\[p_W=\frac{q_{pV,\,Ballast} \cdot (p_S-p_a)}{S \cdot (\alpha \cdot p_0-p_S)}\]

Formula 4-6: Water vapor tolerance

$p_W$ Water vapor tolerance
$q_{pV,\,Ballast}$ Gas ballast flow
$S$ Pumping speed of pump
$p_S$ Saturation vapor pressure of water vapor at exhaust gas temperature
$p_a$ Partial pressure of water vapor in the air
$p_0$ Atmospheric pressure
$\alpha$ Correction factor, dimensionless

The correction factor takes into account the fact that a higher pressure than atmospheric pressure is required to open the outlet valve. In our example $\alpha$ can be assumed as 1.1.

The water vapor tolerance has the dimension of a pressure and is expressed in hPa.

DIN 28426 describes the use of an indirect process to determine water vapor tolerance. The water vapor tolerance increases as the exhaust temperature of the pump rises and gas ballast flow qpV,Ballast increases. It decreases at higher ambient pressure.

Without gas ballast, a vacuum pump having an outlet temperature of less than 100°C would not be capable of displacing even small amounts of pure water vapor. If water vapor is nevertheless pumped without gas ballast, the condensate will dissolve in the pump oil. As a result, the base pressure will rise and the condensate could cause corrosion damage.

The water vapor capacity is the maximum volume of water that a vacuum pump can continuously intake and displace in the form of water vapor under the ambient conditions of 20 °C and 1,013 hPa.

\[q_{m,\,water}=p_W \cdot S \cdot M \cdot (RT)^{-1}\]

Formula 4-7: Water vapor capacity

$q_{m,\,water}$ Water vapor capacity
$M$ Molar mass of water
$R$ General gas constant
$T$ Absolute temperature

The water vapor capacity is expressed in g · h-1. It is therefore a water vapor mass flow rate. The symbol $c_W$ (water vapor capacity) is commonly used to express this in formulas.

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