Dryness Fraction 1
DETERMINING DRYNESS FRACTION
Figure 1 Temperature/Enthalpy
Dryness Fraction
This handout assumes the student is familiar with Steam Tables and the terms found therein. If required,
please review Steam Tables before conti nuing.
The heavy solid line in the above chart represents the transformati on of 1 kg of water at 0˚C and atmospheric
pressure into steam, with the additi on of heat.
From 0˚C to 100˚C, energy is added to the water in the form of sensible heat. This causes an increase in
temperature while its state remains the same. This energy is referred to in steam tables as “hf”. The change
from 1 kg of water at 0˚C into 1 kg of water at 100˚C requires the additi on of 419.04 kJ of energy. Therefore,
at atmospheric pressure:
hf = 419.04 kJ/kg
The saturati on temperature of water at atmospheric pressure is 100˚C. The additi on of more heat will not
cause a temperature change but will, instead, cause a change of state. In this case, evaporati on into steam
at 100˚C. The change from 1 kg of water at 100˚C into dry saturated steam at 100˚C requires the additi on of
2257.0 kJ of energy. Therefore, at atmospheric pressure:
hfg = 2257.0 kJ/kg.
Referring to the diagram, you will noti ce that hg = hf + hfg
If not all of hfg is added to the water, then not all of the water can change into steam. If, say, 50% of hfg is
added then only 50% of the water will be changed into steam and the steam will be referred to as 50% dry.
This is known as the “dryness fracti on” of the steam.
Expanding on this, then, it can be seen that the total enthalpy content of steam of a certain dryness fracti on,
and made from water at 0˚C, can be found out by totalling hf and that porti on of hfg which has been added
(hfg multi plied by the dryness fracti on). If the steam is 100% dry, then the enti re amount of hfg has been
added and the dryness fracti on of the steam would be 100% or 1.
Putti ng this into a formula we get:
total enthalpy = hf + (dryness fracti on) x (hfg)
Using this formula and the informati on found in Steam Tables, one can determine the dryness fracti on of
steam at any pressure or temperature.
