On the Influence of Replacement Capacity on the Economic Efficiency of Utilizing Heat Loss in the Condenser of a District Heating Unit for Heating Make-Up Water of the Heat Network
Abstract Generated abstract
This paper examines how the assumed replacement capacity of a condensing power plant affects the thermal economic efficiency of using condenser heat losses from a district-heating turbine to preheat make-up water for a heat network. It develops criteria for determining the limiting specific heat consumption of the substituting turbine at which the scheme ceases to be thermally advantageous, accounting for changes in turbine output, regenerative steam flows, condenser heat return, and auxiliary power consumption. A numerical example for a reconstructed K-50-90 turbine shows that the threshold value can change substantially when auxiliary needs or regenerative-flow effects are included, indicating that these factors may materially alter conclusions about the economic rationality of the heat-recovery scheme.
Full Text
Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, holder of the Order of the October Revolution and the Order of the Red Banner of Labor
Volume 283 1974
On the Influence of Substitution Capacity on the Economic Efficiency of Using Heat Loss in the Condenser of a District-Heating Unit for Heating Make-Up Water of the Heat Network
V. A. Bragin, V. I. Bespalov, L. A. Belyaev, N. N. Galashov
(Presented by the scientific seminar of the Department of Thermal Power Plants)
In accordance with [1], the effectiveness of using the heat of the ventilation steam flow in the condensers of district-heating turbines is determined by the amount of fuel savings under the condition that the combined heat-and-power plant supplies a constant quantity of electric and heat energy. As a result of preliminary heating in the condenser of the make-up water of the heat network (with constant heat supply), the capacity of the turbine unit decreases because of a reduction in the available heat drop of the turbine and a reduction in the steam flow to the controlled extraction and to regeneration. In addition, the electric power consumption for the plant’s own needs changes. Depending on the specific conditions of the station, the power expenditures for circulating pumps, pumps supplying make-up water to the heat network, feed pumps, etc., may vary. All this leads to a change in the electric power of the combined heat-and-power plant delivered to external consumers. To satisfy the necessary conditions according to [1], the capacity of the substituting state district power plant (condensing turbine), equal to
\[ \Delta N_z = \Delta N_{\mathrm{tu}} \pm \Delta N_{\mathrm{sn}} . \]
is brought in.
Here \(\Delta N_{\mathrm{tu}}\) is the change in the capacity of the district-heating turbine unit when circulating water is replaced by make-up water of the heat network;
\(\Delta N_{\mathrm{sn}}\) is the change in the power expenditure for the drive of the plant’s own needs of the combined heat-and-power plant.
Thus, when determining the thermal efficiency of using heat loss in the condenser for heating the make-up water of the heat network, it is necessary to add to the heat consumption for the turbine unit in the new operating mode the additional heat consumption at the substituting state district power plant:
\[ \Delta Q_z = q_z \cdot \Delta N_z, \]
where
\(q_z\) is the specific heat consumption for generating power on the substituting turbine.
The efficiency of heating make-up water in the condenser of the turbine unit, according to [2], will be determined by the value
\[ \Delta Q_{\mathrm{e}} = Q_{\mathrm{k}}\left[1-\varepsilon(q_z-860)(1+a_{\mathrm{r}})\right] -\Delta N_1\left[1+860\,\varepsilon(1+a_{\mathrm{r}})\right](q_z-860) + \]
\[ +\Delta Q_{\mathrm{k}}\varepsilon(1+a_{\mathrm{r}})(q_z-860) \pm \Delta N_{\mathrm{sn}}\cdot q_z . \tag{1} \]
Here \(\Delta N_1\) is the decrease in turbine output due to a reduction in the heat drop of part of the low-pressure section;
\(Q_k\) is the heat loss in the condenser under the initial scheme, when it is cooled by circulating water;
\(\Delta Q_k\) is the change in the amount of heat returned from the condenser with the condensate;
\(\mathcal{E}\) is the specific generation of electric power on the basis of heat supplied to consumers from the controlled extraction;
\(\alpha_p\) is the share of the change in the output produced by regenerative steam flows relative to the change in the output produced by the steam of the controlled extraction.
In calculations, the most economical condensing turbine of the given power system is usually taken as the substituting turbine. Depending on the specific conditions, this may be, for example, a turbine of type K-200-130, K-300-240, or K-100-90, etc. Therefore the effectiveness of using heat in the condenser of a district-heating turbine unit in different power systems, depending on the type of substituting capacity (the value of \(q_z\)), will be different. It is of interest to establish, in each specific case, that value \(q_z^*\) at which the use of the heat of the ventilation steam flow in the condenser for heating the make-up water of the heat network becomes uneconomical from the thermal standpoint.
The value \(q_z^*\) for individual operating periods can be found from (1) under the condition that \(\Delta Q_e=0\). If heating of the make-up water of the heat network is carried out successively in the condensers of several machines of the TPP, then the most accurate annual-average value \(q_z^*\) is determined from the condition \(\Delta B_{\text{tap}}=0\) according to equation (13) in [2].
Assuming approximately that \(\Delta N_{\text{sn}}=0\), and taking \(\Delta Q_e=0\), from (1) we obtain
\[ q_z^* = 860 + C, \tag{2} \]
where
\[ C=\frac{1}{A} = \frac{1}{ \mathcal{E}(1+\alpha_p) \left\{ 1-\frac{\Delta Q_k}{Q_k} + \frac{\Delta N_1}{Q_k} \left[ 860+\frac{1}{\mathcal{E}(1+\alpha_p)} \right] \right\} }. \tag{3} \]
It follows from this that if \(q_z-860 \ge C\), then the use of the heat of the ventilation flow in the condenser for heating the make-up water of the heat network is disadvantageous. If \(q_z-860<C\), then the indicated use will be rational from the thermal standpoint. The economy of the scheme will be greater, the larger the difference between \(q_z-860\) and \(C\). If \(q_z=0\), then in this case the saving from applying the scheme will be the greatest.
Analysis of (3) shows that the value of the coefficient \(C\) is determined by the properties of the specific turbine unit and by its operating regime. Thus, for example, the higher the pressure in the chamber of the controlled extraction (the smaller the value of \(\mathcal{E}\)), the larger \(C\) and \(q_z^*\), and, consequently, the higher the economy of the scheme.
If the change in auxiliary-power output is taken into account, then equation (2) takes the form:
\[ q_z^* = K(860+C), \tag{4} \]
where
\[ K=\frac{AQ_k}{AQ_k \pm \Delta N_{\text{sn}}}. \tag{5} \]
Allowance for auxiliary needs changes the value of \(q_3^*\). If, when using heat loss in the condenser, the power for auxiliary needs increases (positive \(\Delta N_{\rm sn}\)), then the coefficient \(K<1.0\) and the value of \(q_3^*\) decreases; the economic efficiency of the scheme also decreases, all other conditions being equal. If the power consumption for auxiliary needs decreases (negative \(\Delta N_{\rm sn}\)), then \(K>1.0\) and the value of \(q_3^*\) increases, and consequently the economic efficiency of the scheme also increases.
The value of the coefficient \(K\), besides the properties of the turbine unit and the mode of its operation, is also determined by the particular scheme for connecting the auxiliary-needs pumps, their characteristics, etc.
Let us determine the value of \(q_3^*\) for the case of heating the make-up water of the heat network in the condenser of a K-50-90 turbine unit reconstructed for operation with a regulated steam extraction \(p_t = 1.2\ \text{kgf}/\text{cm}^2\). The condenser, on the water side, has been converted to a four-pass arrangement. The make-up-water flow rate is \(1200\ \text{t}/\text{h}\), and the ventilation steam flow through the low-pressure section is \(30\ \text{t}/\text{h}\). The temperature of the make-up water at the condenser inlet is \(15^\circ\text{C}\). As a result of replacing the circulating water with make-up water of the heat network, the pressure in the condenser changes from \(0.036\) to \(0.051\ \text{kgf}/\text{cm}^2\). For the operating mode under consideration, for the turbine unit \(Q_k = 17.45 \cdot 10^6\ \text{kcal}/\text{h}\); for simplicity, the discharge of drains into the condenser is not taken into account; \(\Delta Q_k = 0.195 \cdot 10^6\ \text{kcal}/\text{h}\); \(\Delta N_1 = 780\ \text{kW}\); \(e = 0.355 \cdot 10^{-3}\ \text{kW}\cdot\text{h}/\text{kcal}\); \(\alpha_p \approx 0.125\).
We take \(\Delta N_{\rm sn} = 0\).
Then, according to (3), \(C \approx 2190\ \text{kcal}/\text{kW}\cdot\text{h}\), and \(q_3^* = 3050\ \text{kcal}/\text{kW}\cdot\text{h}\).
If, under the new operating mode of the turbine unit, the auxiliary-needs power has increased by \(1000\ \text{kW}\), then the value of \(q_3^*\) decreases to \(2710\ \text{kcal}/\text{kW}\cdot\text{h}\) (\(K = 0.889\)). It follows from this that if power from a state district power plant with K-100-90 turbines is used as the substitute power, then the scheme for heating make-up water will be uneconomical.
If, in the calculations, the change in turbine-unit power due to regenerative steam flows is not taken into account (take \(\alpha_p = 0\)), then \(q_3^*\) increases (at \(\Delta N_{\rm sn}=0\)) to \(3310\ \text{kcal}/\text{kW}\cdot\text{h}\). This may lead to a significant error in determining the efficiency of the scheme.
When the thermal economic efficiency of the scheme for preliminary heating of make-up water in the condenser of a district-heating turbine unit is established according to (2) or (4), that is, under the condition \(q_3 - 860 < C\) or \(q_3 < q_3^* = K \cdot (860 + C)\), the final question of the effectiveness of the scheme can be resolved only after comparing the calculated costs. Over time, as increasingly economical condensing units appear in the power system, the efficiency of the scheme will increase.
REFERENCES
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Basic methodological principles of techno-economic calculations in power engineering. Metallurgizdat, 1959.
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V. A. Bragin, V. I. Bespalov, L. A. Belyaev, N. N. Galashov. Fuel savings when using heat loss in the condenser of a turbine unit for heating make-up water of a heat network. Proceedings of TPI. Present collection.