On Increasing the Efficiency of Heat Utilization in Continuous Blowdown
Abstract Generated abstract
This paper examines how to improve heat and working medium recovery from continuous blowdown in steam generator thermal schemes. It compares conventional continuous-blowdown expanders with an adiabatic flashing turbine, using a PT-50-130/13 turbine scheme and calculating power increments from regenerative steam admission, reduced make-up water heating, condensate heat use, and turbine output. The calculations indicate that replacing an expander with an adiabatic flashing turbine yields an energy gain whenever its internal relative efficiency is above zero, with the gain proportional to turbine efficiency and blowdown flow rate. The study also finds that the additional gain generally increases as separator pressure decreases, while its detailed dependence on pressure is influenced by the regeneration scheme and the point of heat introduction.
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PROCEEDINGS
OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR POLYTECHNIC INSTITUTE named after S. M. KIROV
Volume 249 1973
ON INCREASING THE EFFICIENCY OF UTILIZATION OF HEAT FROM CONTINUOUS BLOWDOWN
V. I. BESPALOV, L. A. BELYAEV
(Presented by the scientific seminar of the Department of TEE)
A known method for reducing in-plant losses of heat and working medium with the blowdown water of steam generators is the installation of one or more stages of continuous-blowdown expanders. Analysis shows that, for a constant amount of heat supplied to the working medium in the steam generator, introducing continuous-blowdown expanders into the scheme leads to a gain in plant output, which is the sum of the following components:
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An increase in the output of the turbine installation due to the admission of steam \((G'_c)\) from the continuous-blowdown expanders into the corresponding regenerative heaters \((\Delta N')\).
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Additional output obtained through a reduction in heat expenditure in the thermal scheme of the turbine installation for heating a smaller amount of make-up water \((\Delta N'')\).
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Additional output obtained through the use of the heat of the blowdown condensate \((G''_c)\) for heating the make-up water \((\Delta N''')\).
However, the use of continuous-blowdown expanders has its own characteristic drawback, namely that the throttling process occurring in the expander contains an energy loss associated with a decrease in the availability of the high-pressure water. If the process in the expander is approximated by an isentropic process, using the kinetic energy of the working-medium flow to obtain mechanical work, then the loss of availability of the high-pressure water will decrease. In this case the expander must be constructed in the form of an adiabatic phase-transformation turbine (APT) [1], and the degree of utilization of the kinetic energy of the working medium will depend on its relative internal efficiency \((\eta_{oi})\).
If the APT expander-turbine operates with \(\eta_{oi}=0\), then the effect of using such an expander in the thermal scheme will consist of the components mentioned above. At \(\eta_{oi}>0\), the gain in output must be determined taking into account the output of the APT turbine \((N_{\text{APT}})\). It is obvious that, as \(\eta_{oi}\) of the APT expander-turbine increases, the percentage content of flashed steam will decrease, which will entail a reduction of the output-gain components \(\Delta N'\), \(\Delta N''\), while \(\Delta N'''\) will increase owing to the increase in the amount of blowdown condensate.
The purpose of the present work was to identify the qualitative effect of replacing the expander of an adiabatic flashing turbine (AF) at different values of its \(\eta_{oi}\) and pressures in the separator \((P_c)\). To reveal this effect, the thermal scheme of a PT-50-130/13 turbine, shown in Fig. 1, was selected, with a blowdown rate equal to \(3.6\%\) of the nominal steam flow rate to the turbine at a nominal output \(N^{и}=50\) MW. The components and the total gain
\[ \delta N=\Delta N' + \Delta N'' + \Delta N''' + N_{\mathrm{AF}} \]
were determined by the method of calculating changes in the economy of thermal power plants when their thermal scheme is varied, based on the application
Fig. 1. Thermal scheme of a thermal power installation with a PT-50-130/13 turbine and a drum steam generator. AF—adiabatic flashing turbine
of power-change coefficients [2]. The validity of applying this method in the present case may be justified by the fact that a relatively small amount of heat and working fluid is introduced into the scheme, which does not lead to a change in the power-change coefficients.
Figures 2 and 3 present the results of calculations of the relative magnitude of the power increment \(\delta N/N^и\) with complete utilization of the heat of continuous blowdown for various \(\eta_{oi}\) and \(\bar h_i=f(P_c^i)\). The relative utilized heat drop is defined as
\[ \bar h_i=\frac{i_0-i_i}{i_0-i_k}, \]
where
\(i_0\)—enthalpy of the steam before the turbine;
\(i_i\)—enthalpy of the steam in the extraction with pressure \(P_c^i\);
\(i_k\)—enthalpy of the steam at the condenser inlet.
It is seen from Fig. 2 that, at constant pressure in the separator, the magnitude of the additional gain from installing the AF turbine is directly proportional to its \(\eta_{oi}\). The character of the change in the magnitude of the gain with separator pressure (Fig. 3) at unchanged \(\eta_{oi}\) is more complex and depends on the point at which the heat of the continuous-blowdown concentrate is introduced into the scheme and on the structure of the regeneration scheme.
The work carried out makes it possible to draw the following conclusions:
- Replacing the continuous-blowdown expander with an AF turbine always leads to an energy gain if its internal relative efficiency is greater than zero.
Fig. 2. Dependences of the relative increases in power, in percent, on the relative internal efficiency of the AF turbine: 1 — \(P_c = 30.5\) bar; 2 — \(P_c = 19.4\) bar; 3 — \(P_c = 13\) bar; 4 — \(P_c = 5.87\) bar; 5 — \(P_c = 2.7\) bar; 6 — \(P_c = 1.2\) bar; 7 — \(P_c = 0.18\) bar; d — \(P_c = 5.9\) bar (steam discharge from the AF turbine to the deaerator).
Fig. 3. Dependence of the relative increases in power, in percent, on the magnitude of the relative utilization of the heat drop of the displaced steam extraction at various \(\eta_{oi}\) of the AF turbine: \(+\) — \(\eta_{oi}=0\); \(\square\) — \(\eta_{oi}=0.3\); \(\Delta\) — \(\eta_{oi}=0.6\); \(\circ\) — \(\eta_{oi}=0.9\).
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The magnitude of the additional gain is directly proportional to the internal relative efficiency of turbine AF and to the flow rate of purge water.
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The additional gain from replacing the expander with turbine AF increases as the pressure in the separator decreases.
REFERENCES
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S. V. Polozhii. Steam-power cycles with adiabatic steam extraction. Izvestiya TPI, Vol. 150, Tomsk, 1968.
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Ya. M. Rubinshtein, M. I. Shchepetilnikov. Calculation of the effect of changes in the thermal scheme on the economy of power plants. Moscow, 1969.
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D. P. Elizarov. Thermal-power installations of power plants. Moscow, 1967.