Page 101 - Industrial Ventilation Design Guidebook

66 CHAPTER 4 PHYSICAL FUNDAMENTALS

Using partial pressures, this definition can be written as

Humidity x is thus a dimensioniess number, but sometimes a "dimension" is
added to it, as a reminder of its definition. We can, for instance, wrire
x = 0.05 or x = 0.05 kg H 2 O/kg d.a., where d.a. stands for dry air.
According to Eqs. (4.76), (4.78), and (4.80), humidity can be written as

By solving the partial pressure of water vapor from the equation above, we receive

We denote again the mass of dry air in a volume V as m { and the mass of
water vapor as m h. When humid air is treated as an ideal mixture of two com-
ponents, dry air and water vapor, the enthalpy of this mixture is

where hi is the specific enthalpy of dry air (J/kg) and h h is the specific enthalpy
of water vapor.
Technical calculations dealing with humid air are reasonable to solve with
dry air mass flow rates, because these remain constant in spite of changes in
the amount of water vapor in the air. For that reason a definition for enthalpy,

which is the enthalpy of humid air divided by the dry air mass, is made. The dimen-
sion of enthalpy h k is J/kg, but it is often written as J/kg d.a. as a reminder that the
total enthalpy of the mixture is calculated in terms of a kilogram of dry air.
Combining Eqs. (4.85) and (4.86), we get

and using Eq. (4.81) we have

In calculations with humid air, when the pressure is not high (usually the
atmospheric pressure of 1 bar), water vapor and dry air can be handled as an
ideal gas, as we have already done in Eqs. (4.76) and (4.78). For ideal gases
the specific enthalpy is just a function of temperature:

and

When 0 °C dry air is chosen as the zero point of dry air enthalpy, and 0 °C
water as the zero point of water vapor enthalpy, the enthalpies of dry air and
water vapor can be calculated from the equations