Percentage saturation

This is not the same as relative humidity but is sometimes confused with it. However, for saturated air and for dry air the two are identical and within the range of states used for comfort conditioning they are virtually indistinguishable.

Percentage saturation is defined as the ratio of the moisture content of moist air at a given temperature, t, to the moisture content of saturated air at the same temperature t.

It is also known as the degree of saturation.

Applying the general gas law to the superheated steam present in moist air which is not saturated, we may write

ms P$ Vs ^a ^a

8 ~ ma “ RSTS /?aVa

Since the steam and dry air occupy the same volume and are at the same temperature, being in intimate contact with one another.

We may then write

A = Ps.

(Pat — Ps) Rs

For the unsaturated moist air.

Similarly, for saturated air the moisture content is given by

From the definition of percentage saturation we can write

G at a given temperature

(2.14)

= 6 ^ _ x 100 gss at the same temperature

(2.15)

= Pss.) • 100 Pss (Pat Ps)

From what has been said earlier it is clear that Rss is the same as Rs and hence the ratio RJRS is absent from equation (2.15).

EXAMPLE 2.5

Calculate the percentage saturation of air at 20°C dry-bulb and a moisture content of 0.007 34 kg per kg dry air for (a) a barometric pressure of 101.325 kPa and (b) a barometric pressure of 95 kPa.

Answer

From Example 2.4 the moisture content of saturated air at 20°C is 0.014 68 kg per kg dry air at 101.325 kPa and 0.015 69 kg per kg dry air at 95 kPa. Hence, by equation (2.14)

0.00734 0.014 68