Isoentropic expansion

Step 2. When steam passes through a turbine, it undergoes an isoentropic expansion. The work that the steam does in transferring its momentum to the turbine wheel exactly equals the shaft horsepower developed by the turbine. The entropy of the system is therefore constant. On this basis, extend a line through point A straight down the Mollier diagram. This line represents a constant entropy expansion. 

Step 3. The exhaust-steam pressure is 17 psia. The intersection of the isoentropic expansion line and 17-psia constant-pressure line together determine point B in Fig. 17.2. 

Supersonic isoentropic expansion of a gas stream leads to its cooling, as follows from the well-known thermodynamic relations... 

When steam enters a turbine through the nozzle block, its pressure energy is converted to velocity energy. This is an isoentropic expansion, in that the potential of the steam to do work is preserved. When a turbine runs too fast, it may be slowed down in one of two ways. First, allow the governor steam inlet valve to reduce the flow of steam. Not only is the steam flow reduced, but the pressure of the steam flowing into the nozzle block, hence its ability to do work, is diminished. The other way is to reduce the nozzle block cross-sectional area by closing one or more hand valves. This simply reduces the flow of steam but does not impair the ability of the steam to do work. 

Isoentropic expansion, 289 Isostripper turndown (hydrofluoric acid alkylation), 153-154... 

The ejector steam undergoes an isoentropic expansion, which converts much of its pressure into velocity. It is this high velocity (up to 600 mph) that pulls a vacuum. A reduction in steam temperature and pressure or an increase in the steam s moisture content reduces the motive steam s ability to pull a vacuum. 

I like to call the isoenthalpic expansion a parasitic expansion because it wastes the potential ability of the steam to do work. On the other hand, we have the good isoentropic expansion that maximizes the ability of the expanded steam to do work. Figure 20.1(b) - Case 2, also illustrates this sort of good expansion. 

Figure 20.2 An isoentropic expansion. Heat decreases and velocity increases.

The net effect of this exercise will be to save not 20 percent of the motive steam, but 10 percent. The 20 percent reduction in nozzle area is partially offset by the opening of the governor valve. The inefficient, irreversible, isoenthalpic expansion and pressure drop across the governor speed control valve is reduced. The efficient, reversible, isoentropic expansion and pressure drop across the nozzles is increased. 

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