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only working substance used, because of its good thermal properties and availability. A caution note is that the right-hand-side end of the two-phase region, the saturated vapour line, that for water in the T-s diagram has the shape shown in Fig. 17.9, may be more vertical and even have negative slope at low temperatures for heavier molecular substances, naturally avoiding the problem of wet-vapour at the turbine. Since, at ambient temperature (the heat sink), water change phase at lower-than-atmospheric pressure (e.g. 5 kPa at 33 oC) the condenser must operate under vacuum and a so-called 'deaerator' is needed to remove non-condensable gasses from the feed water or infiltration; moreover, removal of oxygen and carbon dioxide in feed water is always desirable to avoid corrosions in the circuit. Gas solubility in a liquid decreases near the pure-liquid vapour saturation curve, thus deaeration may be achieved by heating the liquid at constant pressure (e.g. adding some vapour) or by making vacuum at constant temperature (e.g. with a small jet of vapour by venturi suction). Maximum temperature in a steam power plant is limited by metallurgical constraints to less than 900 K (some 600 oC), and the maximum pressure depends on the variations to the simple Rankine cycle used, with typical values of 10 MPa (supercritical Rankine cycles surpass 22 MPa). For a simple Rankine cycle with turbine exhaust above the vapour saturation line, the energy efficiency, with the perfect substance model, is: W W W h h nosat e,Rankine net T B 3 4 cp(T3 T4) hlv1 cp(T3 T2) (17.7) Qpos Qcaldera h3 h2 where the work for pumping the liquid is neglected, cp=2 kJ/(kg·K) is an average isobaric thermal capacity for steam, and hlv1=2400 kJ/kg the enthalpy of phase change at T1 (a few degrees above the maximum cooling temperature); in most cases, however, the turbine exhaust is in the two-phase region, as sketched in Fig. 17.9, state 4,with steam mass-fractions down to x=0.9, and h4 in (17.7) must be found using the lever rule for liquid/vapour mixtures). The main variants of the simple Rankine cycle are reheating (a multistage expansion as explained in Chapter 5) and regeneration (bleeding some vapour from the middle of the turbine, and before reheating if used, to heat the feed water). These feed-water heaters may be of the open or closed type (Fig. 17.10). In an open feed-water heater, steam extracted at some turbine stage is added to the main feed water stream (that must be previously pressurised to avoid boiling). In a closed feed-water heater, the extracted steam goes through the shell of a shell-and-tubes heat-exchanger and discharges in a lower-pressure heater or the condenser. The mass fraction of vapour to be extracted is designed to be able to heat the main feed-water stream until the saturation temperature of the extracted steam. Besides those water heaters, fuel-fired steam plants always incorporate a heat-recovery exchanger to preheat water from the condenser with flue gases, known as economiser. The ‘boiler’ itself (or steam generator) is usually a set of vertical tubes surrounding the hearth and connecting a lower liquid drum and a higher vapour drum, from which the steam goes through a superheater before entering the turbine. Fig. 17.10. Open and closed feed-water heaters.PDF Image | THERMAL MACHINES AND HEAT ENGINES
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