ANALYSIS OF THE THERMAL EFFICIENCY OF DISTRICT HEATING NETWORK MAKEUP WATER HEATING SCHEMES IN THE CONDENSERS OF A COMBINED HEAT AND POWER 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)

As was noted in [1], when using the condensers of three district-heating turbines of one CHP plant for preliminary heating of the district-heating network makeup water, for reasons of reliability it is fundamentally possible to distinguish seven heating schemes. They differ in the combination of successively connected condensers. The range of permissible operating modes of each scheme depends on definite values of the amount of makeup water and its inlet temperature. At the same time, it turns out that, for some values of the raw-water flow rate and its temperature, the use of different schemes is permissible under the reliability conditions. The regions of their permissible operating modes overlap. It is therefore of interest to determine the thermal efficiency of the schemes for preliminary heating of the district-heating network makeup water. This will make it possible in each specific case to establish the most optimal scheme for connecting the condensers and to choose, in a substantiated way, the final preliminary-heating scheme for the plant.

The thermal efficiency was investigated as applied to the schemes and the conditions of their operation noted in [1]. The economic effect of applying each scheme was determined from the change in fuel consumption at the CHP plant caused by replacing the original scheme for heating the makeup network water, shown in Fig. 1 in [1], with a preliminary-heating scheme. For significant makeup-water flow rates not provided for by the existing original plant scheme, additional booster pumping units were included in the original scheme. As a result of using one or another preliminary-heating scheme, extraction steam of TG Nos. 3, 5, and 6 is displaced; it was assumed that the amount of displaced steam among the machines is distributed in proportion to the load of their regulated district-heating extractions. The displacement of extraction steam leads to a reduction in the output of the district-heating turbines by \(\sum \Delta N_t\). In addition, the change in output \(\sum \Delta N_p\) due to a change in steam flow to the regenerative extractions and \(\sum \Delta N_d\) due to deterioration of the vacuum in the condensers when the latter are switched to cooling by makeup water was taken into account. The operating mode of the remaining turbines of the CHP plant was assumed unchanged.

The total change in plant output was determined from the expression

\[ \Delta N_c = \sum \Delta N_t + \sum \Delta N_p + \sum \Delta N_d \pm \Delta N_{cн}. \tag{1} \]

Here \(\Delta N_{cн}\) is the change in the power of the station’s own needs when ...

transition to schemes with preliminary heating of the district-heating water. It takes into account the change in the capacity of the auxiliary-service pumps remaining in operation and the capacity of the added booster pumping units.

To determine the change in the capacity \(\Delta N_{\mathrm{t}} + \Delta N_{\mathrm{p}}\) of the turbine generators, station diagrams of their operating conditions were used. The change in capacity due to deterioration of the vacuum was determined from correction curves. In those cases where the vacuum deterioration went beyond the limits of the correction curves, the change in turbine capacity was determined by calculating the low-pressure section for off-design operation at the required new exhaust pressure.

Fig. 1

The heat output from the station was assumed unchanged. The shortfall in electric power was compensated for by a replacement K-300-240 unit with a specific fuel consumption of \(320\ \text{g}/\text{kWh}\).

The results of the calculations are presented in the form of graphs of the dependence of fuel savings at the station on the water temperature at the inlet to the first of the series-connected condensers along its path (\(t_1\)) and on its quantity (\(G\)) in Figs. 1 and 2. Figure 1 shows these dependences for preliminary-heating schemes No. 1—(a), No. 2—(b), No. 3—(c), No. 4—(d), and Fig. 2—for schemes No. 5—(a), No. 6—(b). The limiting curves on each graph define the range of possible operating conditions for the particular scheme, based on the maximum amount of makeup water and on the conditions for reliable turbine operation.

It is evident from the graphs that, for all schemes, the economic effect of preliminary heating increases with an increase in the amount of makeup water and with a decrease in its initial temperature, and also when the condensers of those turbines are included in the scheme for which the absolute values of the exhaust-steam flows through the low-pressure sections are larger. The graphs make it possible, by simple analysis in each specific case, to establish the most economical scheme for connecting the condensers. However, the preliminary-heating scheme selected in this way will be optimal only for quite specific values of \(t_1\) and \(G\). But the latter, under operating conditions, do not remain in

remain constant over the year, but may vary within fairly wide limits. Therefore, for the final selection of the preliminary-heating scheme it is necessary to analyze, on the basis of the graphs in Figs. 1 and 2, the annual economic effect for the plant. For example, it is known that, at a makeup-water flow rate of \(G\) kg/sec, the initial water temperature \(t_1^\circ\mathrm{C}\) will be maintained for \(n_1\) hours during the year, temperature \(t_1'\)—for \(n_1'\) hours, temperature \(t_1''\)—for \(n_1''\) hours, etc. Then, using the graphs in Figs. 1 and 2,

Fig. 2

one can determine for which scheme the annual fuel saving, equal to

\[ \Delta B_c=\Delta B_1\cdot n_1+\Delta B_1'\cdot n_1'+\Delta B_1''\cdot n_1''+\ldots \tag{2} \]

will be greatest. Here \(\Delta B_1\), \(\Delta B_1'\), \(\Delta B_1''\), etc., are the hourly economic effects for the particular scheme, found from the graphs respectively at water temperatures \(t_1\), \(t_1'\), \(t_1''\), etc. A similar analysis can be carried out for different makeup-water flow rates.

Similarly, it is possible to establish the most economical schemes for individual periods of the year (winter, autumn–spring, summer) and then finally select the required arrangement of connecting pipelines, which will make it possible, by switching, to obtain the necessary sequence in the connection of the condensers. When selecting the scheme, it is also possible to take into account the requirements imposed on the plant by the power system with respect to the participation of the CHP turbogenerators in cutting peaks of electrical load. By changing from one scheme to another, it is possible to free the low-pressure sections of individual turbines for their full loading.

REFERENCES

1. V. A. Bragin, V. I. Bespalov, L. A. Belyaev, N. N. Galashov. Possible schemes for preliminary heating of district-heating network makeup water in the condensers of one CHP plant. Izvestiya TPI. Present volume.