Dryness Fraction of Steam:

Dryness fraction of steam is factor which used to specify the quality of steam. It can be defined as the ratio of weight of dry steam Wds present in known quantity of wet steam to total weight of Wet steam Wws. It is unit less quantity and is denoted by x. Thus

x = W_ds/W_ds + W_ws

it is evident from above equation that x = 0 in pure water state as WdS = 0. It can also be seen, in figure 1 that W_ds  = 0 in water state. But for presence of very small amount of dry steam that is W_ds = 0, x will be greater than zero as shown in the Figure 2. On the other hand for no water particles at all in sample of steam, W_ws = 0. Thus x can acquire maximum value of 1. It cannot be above 1. The values of dryness fraction for different states of steam are shown in figure given below, and are as follows.

(i) Wet steam 1 > x > 0    

(ii) Dry saturated steam   x = 1

(iii) Superheated steam   x =1

The dryness fraction of sample of steam can be found experimentally by means of calorimeters.