EVAPORATOR-GENERATORS OF STEAM POWER PLANTS WITH ADIABATIC VAPORIZATION

S. V. Polozhii

(Presented by Prof. I. N. Butakov, Doctor of Technical Sciences)

Evaporator-generators are the principal distinguishing element of steam power plants with adiabatic vaporization (SPP AV), in which saturated steam is produced and the energy of the vaporization process is utilized [1]. In such plants, high thermal efficiency is achieved at low pressure of the superheated steam, which leads to favorable technical-and-economic performance indicators for SPP AV of both large and small capacity.

In SPP AV, instead of the costly and complex steam boilers made of alloy steel used in modern district power stations, water-heating boilers made of inexpensive carbon steel are installed. Low pressure and low specific steam consumption make it possible to have a small heating surface for the steam superheater, made of thin-walled tubes, instead of the expensive, metal-intensive, and bulky superheaters for superhigh and supercritical pressure in modern unit-type district power stations. The expensive thick-walled steam lines of modern district power stations are replaced in plants with adiabatic vaporization by thin-walled, less expensive steam lines. The complex and expensive steam turbines with superhigh- and high-pressure cylinders of unit-type district power stations give way to simple and inexpensive evaporator-generators with medium- and low-pressure steam turbines. In SPP AV there are no intermediate superheaters and steam lines connecting the boiler and turbine for reheated steam, without which the construction of modern—

Fig. 1. Approximate cost of steam turbines with an evaporator-generator and of modern steam turbines per kW of capacity.

1 — steam turbine with an evaporator-generator at a mean steam pressure of 35 bar.
2–4 — modern steam turbines:
2 — 98 bar, 500°C; 3 — 180 bar, 550–580°C; 4 — 250–300 bar, 600–650°C.

temporary supercritical-pressure steam power plants. The complex system of regenerative feedwater heating is replaced in installations with EG by one that is several times cheaper and less metal-intensive. The low cost of the main and auxiliary equipment, and the considerable thermal economy at low steam pressure, make it possible to employ a deep vacuum in steam turbines, allow a low temperature of the exhaust gases in boiler units, and raise the initial steam temperature to the level attained in gas-turbine installations, and make it economically expedient to achieve a further increase in the thermal economy of the AP steam power plant.

The initial theoretical and experimental investigations of adiabatic steam generation made it possible to establish and analyze the dependence of steam generation on the magnitude, interval, and character of the pressure drop as the principal driving forces of this process. These same investigations established the general tendency of the adiabatic steam-generation process: an increase in the kinetic energy and in the degree of dispersion of the flow, and in the rate of steam generation, with increasing magnitude of the pressure drop, initial temperature, and temperature head. On the basis of these investigations it proved possible to reveal the erroneousness of the methods used in theoretical and experimental studies of phenomena associated with adiabatic phase transformations; to establish that experimental data obtained for some conditions and forms of evaporating elements cannot be transferred to others [2—7]. These same investigations revealed the basic law of adiabatic steam generation: the simultaneous occurrence, in equal proportions, of mutually opposite processes of adiabatic steam generation and condensation as the pressure of a liquid in the critical state is reduced. This leads to a molecular-dispersed structure of the flow during adiabatic steam generation. In such a continuously interacting flow, in the form of associations and groups of molecules of liquid and steam, any substantial difference in the velocities and trajectories of motion of the liquid and steam particles, and the existence of those phenomena that cause erosion and moisture losses in steam turbines operating in the wet-steam region, are excluded.

Experimental investigation of adiabatic steam generation under vacuum [2—4], of atomization during pressure reduction of highly compressed solutions and water [8], and also study of the energy of adiabatic steam generation in expanding evaporating elements (nozzles) at excess pressure [9, 10] confirmed a number of theoretical propositions: the high degree of perfection and the kinetic energy of the steam-generation process in expanding evaporating elements, the dispersed structure of the flow, and the intensification of these processes as the magnitude of the pressure drop and the temperature head increase. The experiments of the Moscow Power Engineering Institute [8] fully confirmed the microdispersed structure of the flow at a high initial pressure of heated water and the increase in the dispersion of the flow as the temperature head and the magnitude of the pressure drop increase. The sufficiently high energy efficiency of the process of adiabatic steam generation in expanding evaporating elements, even with a small pressure drop of the order of one atmosphere into a vacuum, small temperature heads, and unfavorable physical properties of water under these conditions [3], was an encouraging factor and, with the corresponding theoretical analysis, made it possible to conclude that the efficiency of the evaporating elements of IG turbines is high. In subsequent years these theoretical conclusions received experimental confirmation from other investigators [9, 10] (Table 1). Thus

Table 1

Energy efficiency of expanding evaporative elements

Thus, the first part of the problem of creating efficient EG turbines has been experimentally substantiated: the high efficiency of evaporative elements, whose velocity coefficient practically does not differ from the velocity coefficient of steam-turbine nozzles. The assertions of critics concerning the impossibility of obtaining high kinetic energy in the nozzles of EG turbines proved untenable. The physical and thermodynamic properties of the flow and of the process of adiabatic steam generation, as studies have shown, proved very favorable for the operation of EG turbines of steam-power plants with adiabatic steam generation. The high energy efficiency of the nozzles of EG turbines, the molecularly dispersed structure of the flow, the single-stage principle of operation of evaporator-generator turbines, due to the low heat drops \(kz\) of water,* together with the low pressure and temperature of the working fluid in the evaporator-generator casing, ensure reliable and efficient operation of such turbines, eliminate erosion and losses due to the two-phase state of the flow (moisture). The calculated value of the internal relative efficiency of an EG turbine, depending on capacity, ranges from 0.62 for low-power turbines to 0.80 for powerful EG turbines. Table 2 gives the design data of steam turbines with evaporator-generators. Figure 2 shows a longitudinal section of a medium-pressure saturated-steam evaporator-generator turbine with a capacity of 11.6 thousand kW, located on the same shaft as a steam turbine of 20 thousand kW capacity. Table 3 gives some characteristics and flow parameters of EG turbines for various pressures in the EG casing. Figure 3 presents—

Fig. 2. Longitudinal section of an evaporator-generator turbine with a capacity of 11.6 thousand kW for a district-heating steam turbine of medium pressure rated at 20 thousand kW.

1 — high-pressure water inlet,
2 — outlet of unevaporated water,
3 — separation device in the evaporator-generator casing,
4 — saturated-steam outlet,
5 — end seal,
6 — seal between the EG casing and the casing of the steam turbine,
7 — steam-turbine shaft.

Water parameters at the inlet: pressure 220 bar, temperature 373°C. Flow parameters at the outlet from the EG: pressure 37 bar, temperature 246°C of water and saturated steam. Water flow rate 152 kg/sec (546 t/h). Steam production of the EG — 61 kg/sec (220 t/h). Relative internal efficiency, 0.74.

Table 2

Design characteristics of steam turbines with evaporator-generators

the results of theoretical studies of the motion of a two-phase flow in the flow path of EG turbines, carried out according to a known method [12]. On the basis of these studies it was established that the angle of attack of liquid droplets smaller than 0.2 micron approaches zero, and such liquid droplets have a velocity of motion close to the velocity of the steam flow and cannot have an impact entry onto the working blades of the turbine wheel. Liquid particles of one micron and less do not change their trajectory, as compared with the trajectory of motion of the steam flow, when passing through the curvilinear channel of the runner. These studies made it possible to establish those critical sizes of liquid droplets in the flow for the operating conditions of EG turbines,

Table 3

Some characteristics of turbines of evaporator-generators

the exceeding of which will lead to disruption of the velocity and trajectory characteristics of the two-phase flow and to the occurrence of losses from the two-phase state (moisture) and erosion of the working blades. This critical size of liquid droplets proved to be hundreds of times greater than the sizes of liquid droplets in the flow at the outlet from the evaporating elements of EG turbines. The absence of erosion of the working blades and of losses from moisture in EG turbines, substantiated theoretically, is to a certain extent confirmed by experimental studies and by the operating practice of solid-fuel gas turbines, in which ash particles of 10 microns and less do not lead to erosion of the working blades, while ash particles of several microns, as having the same velocities and trajectories of motion as the gas flow in curvilinear working channels, are generally not taken into account in the calculation of gas turbines.

As a result of theoretical and experimental studies of adiabatic vaporization and the operating conditions of evaporator-generator turbines, carried out at the Tomsk Polytechnic Institute, as well as experimental studies of the energy and dispersion of the flow during adiabatic vaporization available in the literature, together with the practice of using a two-phase flow in gas turbines, it proved possible to substantiate the reliable and effective operation of EG turbines even before special experimental studies were undertaken for the design development of EG turbines needed for the broad introduction of steam-power installations with adiabatic vaporization into the power engineering of our country.

In the course of carrying out research on the problem under consideration, the work was criticized by a number of prominent scientists engaged in the development and introduction of modern steam-power installations of superhigh and supercritical steam pressure.

The main objections and critical comments on the work focused on denying the effective and reliable operation of EG turbines. As evidence for such negative conclusions about the effective and reliable operation of EG turbines, facts of erosion and losses due to moisture in steam turbines were cited, as well as experimental data on the outflow of heated water from nozzles and diaphragms and other types of non-power evaporating elements. Denial, on this basis, of the effective and reliable operation of EG turbines is unscientific. Extending the throttling process in diaphragms and cylindrical nozzles to the ordered process in expanding evaporating elements, and on this basis denying the high energy efficiency of the evaporating elements of EG turbines—as is done by Doctor of Technical Sciences A. M. Zysina, Candidate of Technical Sciences A. S. Zykov, and others—is a gross error. These processes are incompatible in all respects, and especially in terms of the energy effect of converting heat drop into the kinetic energy of the flow, not only for adiabatic vaporization but also for the outflow of steam. It is well known that, in order to obtain the kinetic energy of a flow at the expense of heat drop, it is necessary to have a special nozzle profile, a definite angle of divergence, and other design characteristics coordinated for the specific conditions, parameters, etc. The extent to which such negative re—

![Figure 3 diagram]

Fig. 3. Angles of attack and trajectories of characteristics of liquid droplets of various sizes for the conditions of the flow section of a medium-pressure EG in a casing.

conclusions for assessing the energy efficiency and degree of perfection of the process in the nozzles of IG turbines, is evident from experimental studies [2–4, 8, 9, 10] (Table 1).

At a similar level, denial has been made of the effective and reliable operation of IG turbines. The uncritical transfer of phenomena of losses from moisture and erosion of blading in steam turbines to IG turbines, without taking into account and analyzing an entirely different set of physical, thermodynamic, design, and other factors, is an equally gross error and is difficult to explain. Here, a non-objective approach to the question under consideration, poor awareness of the phenomena of adiabatic steam generation, and the unsatisfactory state of scientific research on losses from moisture in steam turbines may have played a role [5–7].

REFERENCES

1. S. V. Polozhii. Steam-power installations with adiabatic steam generation. Izv. vuzov SSSR — Energetika, No. 1, 1965.

2. S. V. Polozhii. On the process of steam generation during the outflow of heated water. Izv. vuzov SSSR — Energetika, No. 2, 1960.

3. S. V. Polozhii. Experimental study of adiabatic steam generation during flow in nozzles. Izv. vuzov SSSR — Energetika, No. 9, 1963.

4. S. V. Polozhii. On the calculation of flow parameters in adiabatic steam generation. Izv. TPI, 125, 1964.

5. S. V. Polozhii. On the influence of steam moisture on the efficiency of a turbine stage. Izv. vuzov SSSR — Energetika, No. 7, 1962.

6. S. V. Polozhii. Once again on the question of the influence of steam moisture on the efficiency of a turbine stage. Izv. vuzov SSSR — Energetika, No. 3, 1963.

7. S. V. Polozhii. On the kinetics of the process of steam condensation in a turbine stage. Izv. TPI, vol. 137, 1965.

8. P. D. Lebedev, M. I. Verba, B. I. Leonchik. Certain regularities in the spraying of superheated liquid. Izv. vuzov SSSR — Energetika, No. 10, 1959.

9. O. Frenzl. Strömung Verdampfen den Wassers in Dusen. “Maschinenbau und Wärmewirtschaft,” No. 2, 1956.

10. A. V. Zysin. Combined steam-gas installations. GEI, 1962.

11. K. V. Olesevich. Wear of gas-turbine elements during operation on solid fuel. Mashgiz, 1959.

12. K. V. Olesevich. Determination of the trajectories of ash particles in the interblade channels of gas turbines operating on solid fuel. Nauchnye zapiski OPI, vol. 44, 1962.