Single pressure steam raising

Fig. 7.7. (a) Overall efficiency of CCGT plant with feed water heating by bled steam and single pressure steam raising (after Rufli (9)). (b) Overall efficiency of CCGT plant with feed water heating with bled steam and dual... 

The single pressurizer serves all four coolant loops. It serves to hold the primary circuit pressure constant and to allow for expansion and contraction of the coolant volume caused by changes of load. An electric immersion heater of 1800-kW capacity is used to raise the pressure to, and maintain it at, the required value during negative pressure surges caused by an increase in load demand. In the event of a positive surge, caused by a reduction in load demand, a spray system, fed from the cold leg of one of the reactor coolant loops, condenses steam in the pressurizer to avoid tripping the pressure relief valves with which it is fitted. 

The transformation between water and steam can be depicted in the manner shown in Figure 2.3.2, which represents a plot of P vs the molar volume V at various temperatures close to the critical point. The shaded region indicates an excluded domain for example, at F = 15.3 bar and T = 473.2 K water with molar volume Va coexists with steam whose molar volume is V, . No intermediate molar volumes characteristic of a single phase can exist under these conditions. As the pressure is raised beyond 15.3 bar at 473.2 K steam gas at molar volume Vb condenses to form liquid at molar volume Va- The diagram also shows that as the temperature is raised the two volumes approach each other and ultimately coalesce at 647.2 K to a common value Vc- Above that temperature the two phases merge into one fluid phase. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

Thermal Compression—Ot the two vapor-compression methods, thermal compression requires less capital but yields lower heat recovery than does mechanical compression. A steam-jet booster is used to compress a fraction of the vapor leaving the evaporetor so that the pressure and temperature are raised. Thermal compression is normally applied to the first effect on existing evaporators or where the conditions are right for the application in single-effect units. 

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