POSSIBLE SCHEMES FOR PREHEATING HEATING-NETWORK MAKEUP WATER IN THE CONDENSERS OF ONE CHP PLANT

V. A. Bragin, V. I. Bespalov, L. A. Belyaev, N. N. Galashov

(Presented by the scientific seminar of the Department of Thermal Power Plants)

Under the load conditions of one of the CHP plants, its equipment in the heating season operates predominantly according to the heat schedule. Therefore, electricity generation by the condensing cycle is determined only by the availability of the necessary ventilation steam flows in the low-pressure sections of district-heating turbines. To increase the economy of the station, it was proposed to use the ventilation flows for preliminary heating of the heating-network makeup water in the condensers of two K-50-90 turbines (station Nos. 3 and 5), modernized for operation with controlled steam extraction, and of a PT-25-90 turbine (station No. 6). Before this, the heating-network makeup water was preheated in a raw-water heater (RWH), then sent to the chemical water-treatment plant, from which, for heating, it went to TG No. 1 condenser, transferred to a degraded-vacuum mode, and then to the chemically treated water heaters (CTWH). From the CTWH the water was discharged into vacuum deaerators, from which it was supplied to the heating network. Heating in the RWH and CTWH was carried out by steam from district-heating extractions. The original basic scheme for heating the makeup water is shown in Fig. 1.

Fig. 1

Analysis of the proposed scheme for preliminary heating of the makeup water showed that, taking into account its discontinuity by the chemical water-treatment plant, the mutual arrangement of the equipment, and the existing circulating-pipeline lines, the following heating schemes can be selected from it (Fig. 2):

scheme No. 1—with sequential heating of the water in the condensers of TG Nos. 3 and 1 (Fig. 2, a);

scheme No. 2—with heating in the condensers of TG Nos. 5 and 1 (Fig. 2, б);
scheme No. 3—with heating in the condensers of TG Nos. 6 and 1 (Fig. 2, в);
scheme No. 4—with heating in the condensers of TG Nos. 5, 3, and 1 (Fig. 2, г);
scheme No. 5—with heating in the condensers of TG Nos. 6, 3, and 1 (Fig. 2, д);
scheme No. 6—with heating in the condensers of TG Nos. 6, 5, and 1 (Fig. 2, е);
scheme No. 7—with heating in the condensers of TG Nos. 6, 5, 3, and 1 (Fig. 2, ж).

The temperature of the heating-network makeup water at the inlet to the scheme may vary from 0 to 25°C, and its quantity, allowing for replenishment of the station’s internal losses, may vary from 111 to 389 kg/sec. The temperature of the water at the inlet to the chemical water-treatment plant, according to the condition for its rational operation, must not exceed 20–35°C. The ventilation steam flow rates are: for TG No. 3—12.5 kg/sec, for TG No. 5—8.35 kg/sec, and for TG No. 6—

Fig. 2. Open gate valves with the following numbers:
а) scheme No. 1—2, 5, 7, 9; б) scheme No. 2—2, 4, 6, 8; в) scheme No. 3—1, 3, 5, 8; г) scheme No. 4—2, 4, 6, 7, 9; д) scheme No. 5—1, 3, 5, 7, 9; е) scheme No. 6—1, 3, 4, 6, 8; ж) scheme No. 7—1, 3, 4, 6, 7, 9

5.56 kg/sec. The maximum temperature of the exhaust nozzle for all these turbines, according to the reliability condition, must not exceed 50°C. The turbine condensers are converted to four-pass operation. It is assumed here that if, as a result of heating the water in the condensers before the chemical water-treatment plant, its temperature does not reach 20°C, then the water is additionally heated in the PSV by steam from district-heating extractions.

The choice of possible schemes for preliminary heating of makeup water in the condensers of TG Nos. 3, 5, and 6 was determined primarily on the basis of the permissible operating conditions of the turbines and condensers. It was assumed that, in the presence of condensate injection, the temperature of the exhaust steam must not exceed 50°C. The latter was determined from the known relation [1]

\[ t_k = t_1 + \Delta t + \delta t^\circ\mathrm{C}. \]

The water temperature rises \(\Delta t\) were found from the heat-balance equations of the corresponding condensers. Calculations of the terminal temperature difference \(\delta t\) showed that, within the ranges of changes in temperature and steam and water flow rates indicated above, its value is 5–2°C, which corresponds to station data. Therefore, in the calculations \(\delta t\) was taken equal to 3°C. To determine the enthalpy of the exhaust steam of the low-pressure part of the tur-

turbines Nos. 3, 5, and 6 were calculated for variable operation. For TG No. 1, an empirical dependence of power on steam flow rate and pressure in the condenser was used.

The admissibility of the corresponding operating conditions for the condensers was checked from the water pressures in the water chambers, the thermal stresses, and the vibration of the condenser tubes. In all possible operating conditions there were no restrictions imposed by the condensers.

Table 1

The calculations performed showed that, according to reliability conditions, the possibility of operation of the individual schemes is determined by the water temperature at the inlet to the scheme and by its quantity. The regions of admissible operating conditions of the individual schemes with respect to these indicators are characterized by the data of Table 1.

Fig. 3

More clearly, the regions of admissible operating conditions for the example of schemes Nos. 1 and 5 are shown in Fig. 3. Here, for scheme No. 1 (Fig. 3, a) and No. 5 (Fig. 3, b), \(t_2\) is understood as the temperature of the water at the outlet from the condensa-

...of TG No. 3, and by \(G\) the amount of makeup water. Similar graphs can also be constructed for other schemes.

The results obtained make it possible to draw the following conclusions.

Not all schemes can be used under given operating conditions. Thus, scheme No. 7 can be applied only at low temperatures of the initial water and at substantial quantities of it. At small makeup-water flow rates, on the order of 139–259 kg/sec, only the intermediate schemes Nos. 2, 3, and 6, etc., can be used.

The fact that, for certain values of the quantity and initial temperature of the water, different schemes can be used indicates that the most optimal of them can be established only on the basis of economic indicators.

The selection from the complete scheme No. 7 of intermediate schemes (Nos. 1–6) and the establishment of the limits of their permissible application make it possible to analyze the possibility of the station’s participation, when condensers are used to heat water, in covering peak loads of the power system. Thus, for example, transition from scheme No. 6 to scheme No. 3 makes it possible to increase the station output by loading part of the low-pressure section of TG No. 5.

The rational choice of the number of possible intermediate schemes is determined by the conditions of the particular station.

REFERENCES

1. I. N. Kirsanov. Condensing Installations. Moscow, “Energiya,” 1965.