3.3.5 Effect of exhaust pressure
The efficiency of the cycle is strongly influenced by the LP turbine exhaust pressure. The back pressure of the condenser sets the saturation temperature at which the expanded steam rejects its latent heat of vaporisation to the cooling water. Consequently, changes in back pressure affect the temperature of cycle heat rejection.
The effect of a change in condenser saturation temperature on the cycle work done is shown on a T-S diagram in Fig 1.42. Generally, a low exhaust pressure is sought as it improves cycle efficiency.
However, there are several plant constraints affecting the selection design exhaust pressure. Consider the effect of lowering the exhaust pressure on the turbine condition line on the Mollier diagram (Fig 1.43). Expansion to a lower pressure results in increased wetness, and increased specific volume at the exhaust.
The increase in exhaust wetness tends to increase the erosion of the last-stage blades. However, more significant is the effect of increased specific volume, implying the need for a higher volume flow rate. The volume flow rate through the condenser is the product of the mean steam axial velocity and the annular exhaust area. The annular exhaust area is limited by the maximum length of LP blade. At 3000 r/min, the centrifugal forces on the long blades become very high and there is a limit to the mechanical stress which blade roots can sustain. Hence, having established a maximum annular area, the increase in volume flow rate must be accommodated by an increase in exit velocity. Steam issuing from the last stage of the turbine with high residual velocity represents a loss of kinetic energy. This kinetic energy performs no useful work on the turbine blades and therefore is a loss of available energy, known as the 'leaving loss', and varies with the square of velocity. The other loss associated with the turbine exhaust is the 'hood loss', which defines the hydraulic pressure loss between the last row of moving blades and the condenser; this also varies with the square of the same velocity.
The sizing of the LP turbine exhaust area affects the overall design of machine, in particular the thermal design of the condenser (see Chapter 3). The requirement to provide a certain volume flow rate affects the configuration of the LP cylinders since 1, 2, 3 or even 4 double-flow LP cylinders may be operated in parallel to give the desired flow rate. The number and size of the chosen arrangement of LP turbines affects many other areas of design — the rotor dynamics; the plant arrangement; size of civil structures.
Having introduced all factors against decreasing exhaust pressure, the design compromise is once more between improvements in cycle efficiency versus the increase in plant complexity and therefore reliability and cost. Finally, a major factor affecting the choice of exhaust pressure is the ambient temperature at which the CW can reject heat. This factor seasonally effects the thermal efficiency of the plant.