Sensible Heat of Water or Heat of the Liquid or Enthalpy of Liquid (hf )
It is the quantity of heat required to raise unit mass of water from 0°C to the saturation temperature (or boiling point temperature) corresponding to the given pressure of steam generation. In Fig 5.5, ‘hf ’ indicates enthalpy of liquid in kJ/kg. It is different at different surrounding pressures.
Laten Heat of Vapourisation of Steam (hfg) : Or, Latent Heat of Evaporation
It is the quantity of heat required to transform unit mass of water at saturation temperature to unit mass of steam (dry saturated steam) at the same temperature. It is different at different surrounding pressures.
Saturated Steam It is that steam which cannot be compressed at constant temperature without partially condensing it. In Fig. 5.5, condition of steam in the line AB is saturated excepting the point A which indicates water at boiling point temperature. This water is called saturated water or saturated liquid. The steam as it is being generated from water can exist in any of the three different states given below.
(1) Wet steam
(2) Dry (or dry saturated) steam
(3) Superheated steam.
Amongst these, the superheated state of steam is most useful as it contains maximum enthalpy (heat) for doing useful work. Dry steam is also widely utilized, but the wet steam is of least utility. Different states of steam and sequential stages of their evolution are shown in Fig. 5.4 a-e. Their corresponding volumes are also shown therein.
WET SATURATED STEAM Wet steam is a two-phase mixture comprising of boiling water particles and dry steam in equilibrium state. Its formation starts when water is heated beyond its boiling point, thereby causing start of evaporation.A wet steam may exist in different proportions of water particles and dry steam. Accordingly, its qualities are also different. Quality of wet steam is expressed in terms of dryness fraction which is explained below.
Dryness Fraction of Steam Dryness fraction of steam is a factor used to specify the quality of steam. It is defined as the ratio of weight of dry steam Wds present in a known quantity of wet steam to the total weight of Wet steam Wws. It is a unit less quantity and is generally denoted by x. Thus
it is evident from the above equation that x = 0 in pure water state because WdS = 0. It can also be seen, in Fig. 5.4a that Wds = 0 in water state. But for presence of even a very small amount of dry steam i.e. Wds = 0, x will be greater than zero as shown in Fig. 5.4b. On the other hand for no water particles at all in a sample of steam, Wws = 0. Therefore x can acquire a maximum value of 1. It cannot be more than 1. The values of dryness fraction for different states of steam are shown in Fig. 5.4, 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 a sample of steam can be found experimentally by means of calorimeters
Dry (Or Dry Saturated) Steam A dry saturated steam is a single-phase medium. It does not contain any water particle. It is obtained on complete evaporation of water at a certain saturation temperature. The saturation temperature differs for different pressures. It means that if water to be evaporated is at higher pressure, it will evaporate at higher temperature. As an illustration, the saturation temperatures at different pressures are given below for a ready reference.
| p (bar) | 0.025 | 0.30 | 2.0 | 9.0 | 25.0 | 80.0 | 150.0 | 200.0 |
| tsat (C) | 21.094 | 69.12 | 120.23 | 175.35 | 223.93 | 294.98 | 342.11 | 365.71 |
Superheated Steam
When the dry saturated steam is heated further at constant pressure, its temperature rises-up above the saturation temperature. This rise in temperature depends upon the quantity of heat supplied to the dry steam. The steam so formed is called superheated steam and its temperature is known as superheated temperature t sup °C or T sup K. A superheated steam behaves more and more like a perfect gas as its temperature is raised. Its use has several advantages. These are
(i) It can be expanded considerably (to obtain work) before getting cooled to a lower temperature.
(ii) It offers a higher thermal efficiency for prime movers since its initial temperature is higher.
(iii) Due to high heat content, it has an increased capacity to do work. Therefore, it results in economy of steam consumption. In actual practice, the process of superheating is accomplished in a super heater, which is installed near boiler in a steam (thermal) power plant.
Degree of Super Heat
It is the difference between the temperature of superheated steam and saturation temperature corresponding to the given pressure.
So, degree of superheated = t sup – ts
Where; tsup = Temperature of superheated steam ts = Saturation temperature corresponding to the given pressure of steam generation.
Super Heat
It is the quantity of heat required to transform unit mass of dry saturated steam to unit mass of superheated steam at constant pressure so,
Super heat = 1 × Cp × (tsup – ts ) KJ/Kg
Saturated Water
It is that water whose temperature is equal to the saturation temperature corresponding to the given pressure.
