Thermodynamics

558 | ThermodynamicsTIDEAL CYCLETIrreversibilityin the pump21ACTUAL CYCLEPressure dropin the boilerPressure dropin the condenser3Irreversibilityin the turbine42a2s14s34a(a)s(b)sFIGURE 10–4(a) Deviation of actual vapor power cycle from the ideal Rankine cycle. (b) The effect of pump andturbine irreversibilities on the ideal Rankine cycle.the steam in the boiler to compensate for these undesired heat losses. As aresult, cycle efficiency decreases.Of particular importance are the irreversibilities occurring within thepump and the turbine. A pump requires a greater work input, and a turbineproduces a smaller work output as a result of irreversibilities. Under idealconditions, the flow through these devices is isentropic. The deviation ofactual pumps and turbines from the isentropic ones can be accounted for byutilizing isentropic efficiencies, defined asandh P w sw a h 2s h 1h 2a h 1(10–10)h T w aw s h 3 h 4ah 3 h 4s(10–11)where states 2a and 4a are the actual exit states of the pump and the turbine,respectively, and 2s and 4s are the corresponding states for the isentropiccase (Fig. 10–4b).Other factors also need to be considered in the analysis of actual vaporpower cycles. In actual condensers, for example, the liquid is usually subcooledto prevent the onset of cavitation, the rapid vaporization and condensationof the fluid at the low-pressure side of the pump impeller, which maydamage it. Additional losses occur at the bearings between the moving partsas a result of friction. Steam that leaks out during the cycle and air thatleaks into the condenser represent two other sources of loss. Finally, thepower consumed by the auxiliary equipment such as fans that supply air tothe furnace should also be considered in evaluating the overall performanceof power plants.The effect of irreversibilities on the thermal efficiency of a steam powercycle is illustrated below with an example.