2.2 System parameters
The final feed temperature (FFT) out of the ultimate HP heater is fixed within a few degrees by the bleed pressure and steam temperature available to the heater. On all current 660 and 500 MW units, the bleed point is the HP exhaust or 'cold reheat' pressure.
The feed temperature out of a heater is conditional on several factors. First, a bled-steam pipework temperature loss, usually 1.1°C, which is subtracted from the saturation temperature equivalent of the bleed point pressure. The resultant temperature is the saturation temperature equivalent to the pressure of the steam entering the heater shell. The heater thermal performance and hence its heat transfer surface is determined by the values of the temperature terminal differences (TTDs) on the steam and drain sides.
The steam TTD is defined as the temperature difference between the saturated steam temperature at entry to the heater shell and the feed water leaving the heater. The drain TTD is the temperature difference between the feedwater entering the heater and the drains leaving the heater. To achieve optimum HP heater performance, it is usually necessary to partition each HP heater into three zones; namely, the de-superheating, condensing and drain cooling zones. The effect which TTDs have on the size of these zones is fully explained in Section 6 of this chapter.
By applying the line temperature drops to all HP heater bled-steam extraction pressures and the steam TTDs, the feed temperature out of each of the HP heaters is determined. Once the temperatures out of the heaters (and hence into the next heater) are known, the drain temperatures are found by applying the drain TTDs. However, it should be remembered that the smaller the TTDs, the greater will be the heat transfer surface to achieve the intended performance.
HP heaters which are supplied with steam with a high degree of superheat can have negative TTDs, the higher feed temperatures out of the heaters being made possible by the total steam temperature as seen by the desuperheating zone. Figure 3.17 illustrates the principles explained above and shows typical TTDs for a 660 MW unit and the resultant heater inlet, outlet and drain temperatures. It should be noted that the second HP heater draws its steam from the BFP turbine which is relatively low in superheat, hence the steam TTD is larger than that of the ultimate heater which has more superheat in the bled-steam.
The TTDs are determined by the economics of the cycle, the increase in cycle efficiency by use of smaller TTDs being weighed against the increased cost of heater surface.