The quality (i.e. the dryness) of wet steam can be found by using a separating and throttling calorimeter. Figure 2.6.4 shows the general arrangement of the device.
The separator, as its name suggests, physically separates the water droplets from the steam sample. This alone would give us a good idea of the dryness of the steam, despite that the separation is not complete, because, as we have seen the dryness fraction is the ratio of the mass of pure steam to the total mass of the steam.
Having separated out the water droplets we can find their mass to give us the mass of water in the sample, m1. The ‘pure steam’ is then condensed to allow its mass to be found, m2. Then,
Dryness fraction from separator, x = m2/ (m1 + m2)
A more accurate answer is obtained by connecting the outlet from the separator directly to a throttle and finding the dryness fraction of the partly dried steam.
In the throttling calorimeter, the steam issuing through the orifice must be superheated, or we have two dryness fractions, neither of which we can find. Throttling improves the quality of the steam, which is already high after passing through the separator, therefore superheated steam at this point is not difficult to create.
To find the enthalpy of the superheated steam, we need its temperature and its pressure.
For the throttling calorimeter,
Enthalpy before = enthalpy after throttling
hf + x.hfg = enthalpy from superheat tables
If we call the dryness from the separator, x1, and the dryness from the throttling calorimeter x2, the dryness fraction of the steam sample is x, given by, x = x1 × x2