As in the case of gases, the vapours may also be expanded or compressed by different methods as given below:
Method # 1. Constant Volume Process (fig. 3-14):
An example of constant volume process is the heating of steam in a closed rigid vessel. As there is no change in volume, work done in this process is zero. By the first law of thermodynamics
Q = W + (U2 – U1) = 0 + [(H2 – P2 V2) – (H1 – P1 V1)].
Method # 2. Constant Pressure Process (fig. 3-15):
An example of constant pressure heating is the generation of steam in a boiler.
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Q = W + (U2 – U1)
= [P2V2 – P1V1] [(H2 – P2V2) – (H1 – P1V1)] = H2 – H1
In this process, the heat added or rejected is same as change in enthalpy.
Method # 3. Constant Temperature (Isothermal) Process (fig. 3-16):
During evaporation or condensation (wet region) constant temperature process is the same as constant pressure process. Once the steam is in superheated region it behaves like a perfect gas obeying gas Maws and hence, the constant temperature expansion in this region is hyperbolic (PV = constant).
Method # 4. Hyperbolic (PV = constant) Process (fig. 3-17):
The hyperbolic expansion is not isothermal in wet region but may be regarded isothermal in superheated region.
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= P1V1 loge r + [(H2 – P2V2) – (H1 – P1V1)]
= P1V1 loge r + (H2 -H1) (as P2v2 = P1V1).
Method # 5. Polytropic Process (fig. 3-18):
In this process there is a change in entropy, it is also known as polytropic process.
Method # 6. Reversible Adiabatic or Isentropic Process (fig. 3-19):
In reversible adiabatic process the heat supplied is zero. In this process entropy remains constant, so this process is known as isentropic process.
Method # 7. Throttling Process (fig. 3-20):
In this process, the throttling of steam occurs in throttle value.
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The enthalpy before throttling is same as enthalpy after throttling.
In throttling process, the steam is made to flow from a higher pressure to lower pressure under conditions such as the heat interchange and work done are both zero. Thus, in this process the enthalpy at end points is constant. This process is essentially adiabatic (isentropic) but irreversible, causing an increase in entropy.
