It is instructive to consider some recent and current CEGB design cycles by comparing their Mollier diagrams (Fig 1.46), the selected cycles are:
- (1) 660 MW oil-fired with BFP turbine.
- (2) 660 MW AGR with BFP turbine.
- (3) New subcritical coal-fired with motor-driven feed pump.
- (4) New supercritical coal-fired with motor-driven feed pump.
The steam conditions for these cycles are summarised below:
- (1) 660 MW oil-fired 160 bar/538°C reheating to 538°C at 39 bar (24.4% TSV) with 61 mbar condenser pressure.
- (2) 660 MW AGR 160 bar/530°C reheating to 538°C at 37.4 bar (23.3% TSV) with 41 mbar condenser pressure.
- (3) New subcritical coal-fired 176 bar/565°C reheating to 565°C at 50 bar (20.8% TSV) with 57 mbar condenser pressure.
- (4) New supercritical coal-fired 240 bar/565°C reheating to 565°C at 60 bar (25.0% TSV) with 57 mbar condenser pressure.
In this example, single-reheat has been selected for the supercritical cycle. The advantages and disadvantages of double-reheat require careful consideration. All CEGB coal or oil-fired plant is designed to be capable of 'two-shifting' for a significant part of its life. It is clearly more difficult to achieve this kind of flexibility with double-reheat and the type of plant eventually built will need to take these factors into account.
The two 660 MW cycles are typical of the latest units in service, although the coal-fired 660 MW units use 565°C TSV and reheat temperatures.
Both the AGR and the oil-fired 660 MW units use a back pressure boiler feed pump turbine, supplied with steam from the HP exhaust and returning it to the IP/LP crossover point.
The oil-fired unit has bled-steam extractions at HP exhaust, at BFPT midpoint, at BFPT/IP turbine exhaust and from three positions in the LP turbine. The AGR requires cooler feedwater and takes bled-steam from the BFPT/IP exhaust and from four positions in the LP turbine.
The new large coal-fired units will use motor-driven feed pumps. They can take advantage of higher final feed temperature and the economics justify bled-steam extraction from the HP turbines at approximately 70-76 bar. They also have bled-steam extraction from HP exhaust, IP exhaust and four stages from the LP turbine.
The diagram shows how much larger are the IP and LP heat drops than those of the HP turbines. It is also notable how close together are the IP and LP expansion lines for all four examples. The HP expansion line for the subcritical coal-fired machine is not very different from that for the two 660 MW units, although its higher pressures will mean reduced specific volumes throughout. The new supercritical unit has larger HP and IP heat drop than the subcritical machine. This implies additional available work but this is bought at the price of increased feed pump power.
The feed pump power is an important part of the cycle selection.
The low back pressure of the AGR is clearly apparent. This pressure is largely site-dependent and is a function of the mean year-round cooling water temperature. The oil-fired 660 MW unit is located on the Thames where the water temperature is quite high. The two new coal units assume tower cooling, while the AGR is at a coastal site with cold water.