In steam power plants, vapors of various liquids (water, mercury, etc.), but most often water vapor, are used as the working fluid.

In the steam boiler of a steam power plant (1) due to the heat supply Q 1 produced by combustion of fuel in the furnace, steam is formed when constant pressure p 1(Fig. 33). In the superheater (2), it is additionally heated and passes into the state of superheated steam. From the superheater, steam enters the steam engine (3) (for example, in steam turbine), where it fully or partially expands to pressure p 1 with receipt useful work L 1. The exhaust steam is sent to the refrigerator-condenser (4), where it is completely or partially condensed at constant pressure p 2. Steam condensation occurs as a result of heat exchange between the exhaust steam and the coolant flowing through the refrigerator-condenser (4).

After the refrigerator, the condensed steam enters the inlet of the pump (5), in which the liquid pressure increases from p 2 to the original value p 1 after which the liquid enters the steam boiler (1). The installation cycle is completed. If partial condensation of exhaust steam occurs in the refrigerator (4), then in the steam power plant instead of a pump (5) a compressor is used, where the pressure of the steam-water mixture also increases with p 2 to p 1. However, in order to reduce the work of compression, it is advisable to completely condense the steam in the condenser and then compress not the steam-water mixture, but the water leaving the condenser. The described cycle of a steam power plant is called the Rankine cycle (Fig. 34).

The Rankine cycle consists of an isobar ( 4–1 ), where heat is supplied to the heater, adiabats ( 1–2 ) expansion of steam in a steam turbine, isobars ( 2–3 ) heat removal in the refrigerator-condenser and isochores ( 3–4 ) increasing the water pressure in the pump. Line ( 4–a) on the isobar corresponds to the process of increasing the temperature of the liquid after the pump to the boiling point at pressure p 1. Plot ( a–b) corresponds to the transformation of a boiling liquid into dry saturated steam, and the section ( b–1) – the process of adding heat to a superheater to convert dry saturated steam into superheated steam.

Rice. 34. Rankine cycle in coordinates p-v (A) And T-s (b)

The work done by steam in a turbine is equal to the difference in enthalpies of steam before and after the turbine

The work spent on compressing water in the pump is also determined by the difference in the enthalpy of the working fluid at points (4) and (3).

In coordinates р-v this work is determined by the area e-3-4-f(Fig. 34a). This work is very small compared to the work of the turbine.

The useful work of the cycle is equal to the work of the turbine minus the work expended on driving the pump w N

Specific quantity warmth q 1 supplied in the boiler and superheater is determined from the first law of thermodynamics (no work is done) as the difference in enthalpies of the working fluid in the process of heat supply

Where h 4– enthalpy hot water at the inlet to the steam boiler at pressure p 2 practically equal in value to the enthalpy of boiling water at point (3),
those. h 4 @ h 3.

By comparing the ratios, we can determine the thermal efficiency of the Rankine cycle as the ratio of the useful work received in the cycle to the amount of heat supplied

. (309)

Other important characteristic steam power installations– specific steam consumption d, which characterizes the amount of steam required to produce 1 kWh energy ( 3600 J), and is measured in .

Specific consumption a pair in the Rankine cycle is equal to

. (310)

The specific steam consumption determines the size of the units: the larger it is, the more steam must be generated to obtain the same power.

Ways to increase the efficiency of steam power plants

The thermal efficiency of the Rankine cycle, even in installations with high steam parameters, does not exceed 50 % . In real installations due to the presence internal losses in the engine the efficiency value is even lower.

There are two ways to increase the efficiency of steam power plants: increasing the parameters of the steam in front of the turbine and complicating the circuits of steam power plants.

1 – steam generator; 2 – steam superheater; 3 – steam turbine;
4 – capacitor; 5 – feed pump; 6 – heat consumer

The first direction leads to an increase in heat drop during the expansion of steam in the turbine ( h 1 - h 2) and, as a consequence, to an increase in specific work and cycle efficiency. In this case, the heat drop across the turbine h 1 -h 2 can be further increased by reducing the back pressure in the installation condenser, i.e. reducing pressure r 2. Increasing the efficiency of steam power plants in this way is associated with solving a number of difficult technical problems, in particular, the use of high-alloy, heat-resistant materials for the manufacture of turbines.

The efficiency of using a steam power plant can be significantly increased by using the heat of exhaust steam for heating, hot water supply, drying materials, etc. For this purpose, the cooling water heated in the condenser (4) (Fig. 35) is not thrown into the reservoir, but pumped through heating installations heat consumer (6). In such installations, the station generates mechanical energy in the form of useful work L 1 on the turbine shaft (3) and heat Q etc. for heating. Such stations are called thermal power plants ( CHP). Combined generation of thermal and electrical energy is one of the main methods for increasing the efficiency of thermal installations.

The efficiency of a steam power plant can be increased compared to the Rankine cycle by using the so-called regenerative cycle
(Fig. 36). In this scheme feed water, entering the boiler (1), is heated by steam, partially taken from the turbine (3) . According to this scheme, steam produced in the boiler (1) and superheated in the superheater (2) is sent to the turbine (3), where it expands to the pressure in the condenser (4). However, part of the steam, after it has performed work, leaves the turbine and is sent to the regenerative heater (6) , where, as a result of condensation, it heats the feed water supplied by the pump (5) to the boiler (1) .

The condensate itself, after the regenerative heater, enters the pump inlet (5) or condenser 4, where it mixes with the steam condensate that has passed through all stages of the turbine. Thus, the same amount of feed water enters the boiler as exits it in the form of steam. From the diagrams (Fig. 37) it is clear that every kilogram of steam entering the turbine expands due to pressure p 1 up to pressure p 2, doing work w 1 =h 1 -h 2. Pairs in quantity ( 1 - g) fractions of a kilogram expands to final pressure p 3, doing work w 2 =h 2 -h 3. The total work of 1 kg of steam in the regenerative cycle will be

where is the fraction of steam taken from the turbine and supplied to the regenerator.

Rice. 37. Graph of adiabatic expansion of steam in a turbine with intermediate extraction ( A) and changes in the amount of steam ( b)

The equation shows that the use of heat recovery leads to a decrease in the specific expansion work compared to the Rankine cycle with the same steam parameters. However, calculations show that the work in the regenerative cycle decreases more slowly than the heat consumption to produce steam in the presence of regeneration, therefore the efficiency of a steam power plant with regenerative heating is ultimately higher than the efficiency of the conventional cycle.

The use of high and ultra-high pressure steam in order to increase the efficiency of installations encounters a serious difficulty: its humidity in the last stages of the turbine is so high that it significantly reduces the efficiency of the turbine, causes erosion of the blades, and can cause their failure. Therefore, in installations with high steam parameters it is necessary to use the so-called intermediate superheating of steam, which also leads to an increase in the efficiency of the installation (Fig. 38).

Rice. 38. Scheme of a steam power plant with intermediate steam superheating:

1 – steam generator; 2 – steam superheater; 3 – high pressure turbine (HPT); 4 – low pressure turbine (LPT); 5 – capacitor; 6 – feed pump; 7 – intermediate superheater; 8 – consumer

In a steam power plant with intermediate superheating of steam, after expansion in the high-pressure turbine (3), the steam is discharged into a special superheater (7) , where it is reheated at pressure r rp to a temperature that is usually slightly lower than the temperature t 1.Superheated steam enters the low pressure turbine (4) and expands in it to the final pressure p 2 and goes into the capacitor (5) (Fig. 39).

The humidity of steam after the turbine in the presence of steam superheating is significantly less than it would be without it ( x 1 >x 2) (Fig. 39). The use of intermediate superheating in real conditions gives increased efficiency approximately 4 % . This gain is obtained not only due to an increase in the relative efficiency of the low-pressure turbine, but also due to an increase in the total work of steam expansion through the low-pressure turbine and high pressure. The fact is that the sum of the segments and , characterizing the operation of high and low pressure turbines, respectively, is greater than the segment 1 –e, which characterizes the expansion work in the turbine of an installation in which intermediate superheating of steam is not used (Fig. 39 b).

Rice. 39. Steam expansion process in an installation with intermediate superheating

Refrigeration cycles

Refrigeration units are designed to cool bodies to a temperature below environment. To carry out such a process, it is necessary to remove heat from the body and transfer it to the environment due to work supplied from the outside.

Refrigeration units are widely used in the gas industry in the preparation of gas for transport in integrated gas treatment units (CGTUs), for cooling gas at compressor stations main gas pipelines, laid in areas of permafrost, during processing natural gas, when receiving and storing liquefied natural gas, etc.

Theoretically, the most profitable refrigeration cycle is the reverse Carnot cycle. However, the Carnot cycle in refrigeration units is not used because of the design difficulties that arise when implementing this cycle, and, in addition, the effect of irreversible work losses in real refrigeration machines is so great that it negates the advantages of the Carnot cycle.

STEAM POWER INSTALLATION

Steam power plants (SPU) are designed to produce electrical energy and water steam used for the production needs of industrial enterprises. Currently, all large chemical plants and industrial complexes have their own control systems.

Figure 20 shows a schematic diagram of a steam power plant. The PSU consists of a steam boiler (1.1"), a steam turbine (2), a condenser (3) and a feed pump (4). The steam boiler is complex engineering structure. The diagram conventionally shows only two of its elements - the boiler drum (1) and the superheater (1").

Rice. 20. Schematic diagram steam power plant

The installation works as follows. Feedwater (condensate and water returning from the plant) is pumped into the steam boiler drum (1) by a pump (4). In the drum, due to the chemical heat of the fuel, which is burned in the boiler furnace (the furnace is not shown in Fig. 3), and in some cases due to the energy potential of combustible or high-temperature secondary energy resources, water at constant pressure turns into moist saturated steam (X = 0, 9 – 0.95). Then the wet saturated steam enters the boiler superheater (1"), where it is superheated to a predetermined temperature. The superheated steam is sent to the steam turbine (2). Here it expands adiabatically to produce useful work, which is transformed into electrical energy using a generator. Modern turbines have a series of extractions through which steam is sent to the technological needs of the workshops of an industrial enterprise. After the turbine, the exhaust steam is sent to the condenser (3). The condenser is a regular one. shell and tube heat exchanger, the main purpose of which is to create a vacuum behind the turbine. This leads to an increase in heat loss in the turbine, which increases the efficiency of the PSU cycle. In the condenser, due to the removal of heat from the exhaust steam to the cooling water, it condenses. The resulting condensate is again supplied to the boiler drum by the pump (4).

Rice. 21. Cycle P.S.U. in P – v and T – S diagrams

In Fig. Figure 21 shows the PSU cycle in the P – υ and T – S diagrams. In these diagrams, line 1–2–3–4 corresponds to the isobaric process of producing superheated steam in a steam boiler. Section 1-2 characterizes the process of heating feed water to boiling point, section 2-3 corresponds to the process of vaporization, i.e. the transformation of water into steam, section 3-4 characterizes the process of steam overheating. Line 4-5 reflects the adiabatic process of steam expansion in the turbine. Section 5-6 – isobaric process of steam condensation in a condenser. Line 6-1 characterizes the process of increasing the pressure of feed water in the pump. The process of increasing water pressure in the pump practically occurs at constant temperature and without heat exchange with the environment. In addition, given that liquids are practically not compressible, this can be considered isochoric. Under these conditions, process 6-1 occurs at q = 0, T = const, υ = P – υ and T – S and S = P – υ and T – S. Therefore, line 6-1 in the T - S diagram is transformed into a point .

When analyzing the cycles of steam power plants, the following concepts are introduced:

1. Turbine technical operation. The technical work of a turbine refers to the work of all thermodynamic processes in the cycle.

For isobaric process 1-4 we have:

(7.12)

During the process of adiabatic expansion of steam in a turbine:

During an isobaric condensation process in a capacitor:

(7.14)

For process 6-1, characterizing the technical operation of the pump at q = 0,

T = const, υ = const and S = const, we get

Hence:

2. Cycle work. The cycle work is defined as the difference between the technical work of the Pipe and the work expended by the pump.

The efficiency of the PSU cycle is assessed using coefficients useful action cycle. There are thermal and internal relative efficiency of the cycle. The thermal efficiency of the cycle is understood as the ratio of the cycle work to the heat supplied from the upper source. The work of the cycle is determined by formula (7.17). The top source of heat in in this case are flue gases produced during the combustion process of fuel, or high-temperature H.E.R.

Heat from the upper source to the working fluid ( q 1) is supplied to the steam boiler in a 1-2-3-4 process. This heat is numerically equal to:

In this case, the thermal efficiency of the PSU cycle can be written as follows:

(7.19)

In practice, when analyzing the operation of a PSU, a formula is often used that does not take into account the work of the pump, due to its smallness compared to the technical work of the cycle:

(7.20)

where Δh is the heat loss in the turbine.

In an actual PSU cycle, the adiabatic expansion process in the steam turbine nozzles is irreversible. Irreversibility is associated with an increase in entropy, so the actual heat loss Δh d less than theoretical Δh. In Fig. 22 shows the theoretical and actual heat loss in a steam turbine in the h - S diagram.

Rice. 22. Graphical representation of heat loss in a turbine on an h – S diagram.

The thermal efficiency of the real PSU cycle is determined by the expression.

The efficiency of the Rankine cycle, even in installations with high steam parameters, does not exceed 50%. In real installations, due to the presence of internal losses in the turbine, the efficiency value is even lower.

The values ​​of enthalpies included in expression (9) are influenced by three parameters of the working fluid—the initial pressure r 1 and initial temperature T 1 superheated steam at the turbine inlet and final pressure r 2 at the turbine outlet. This leads to an increase in heat transfer and, as a consequence, to an increase in specific work and cycle efficiency.

In addition to changing steam parameters, the efficiency of steam power plants can be increased by complicating the circuitry of the installation itself.

Based on the above, the following ways to increase thermal efficiency are identified.

1. Increasing initial pressure p 1 with unchanged parameters T 1 and r 2 (Fig. 15, A). The diagram shows Rankine cycles at maximum pressures r 1 and r 1a > r 1. A comparison of these cycles shows that with increasing pressure to r 1A heat change has higher value, than , and the amount of heat input decreases. Such a change in the energy components of the cycle with increasing pressure r 1 increases thermal efficiency. This method gives a significant increase in cycle efficiency, but as a result of increased r 1 (the pressure in steam power plants can reach up to 30 ata), the humidity of the steam leaving the turbine increases, which causes premature corrosion of the turbine blades.

2. Increase in initial temperature T 1 with unchanged parameters r 1 and r 2 (Fig. 15, b). Comparing cycles in a diagram at temperatures T 1 and T 1a > T 1 you can see that the enthalpy difference increases to a greater extent than the difference, since the isobar flows more steeply than the isobar. With such a change in the enthalpy difference with increasing maximum temperature cycle thermal efficiency increases. The disadvantage of this method is that the superheater requires a heat-resistant metal; the temperature of the superheated steam can reach up to 650 °C.

3. Simultaneous increase in pressure p 1 and temperature T 1 at constant pressure r 2. Promotion as r 1 and T 1 increases thermal efficiency. Their effect on the moisture content of steam at the end of expansion is opposite, with an increase r 1 it increases, and with increasing T 1 – decreases. Ultimately, the state of the steam will be determined by the degree of change in the quantities r 1 and T 1 .

4. Decrease in pressure p 2 at constant parameters T 1 and r 1 (Fig. 15, V). With a decrease r 2 the degree of expansion of steam in the turbine increases and the technical work increases ∆ l = l a – l. In this case, the amount of heat removed less than (the isobar at lower pressure is flatter), and the amount of heat input increases by the amount . As a result, the thermal efficiency of the cycle increases. Lowering the pressure r 2 can be achieved at the condenser outlet temperature equal temperature environment, but at the same time condensing device you will have to create a vacuum, since temperature corresponds to pressure r 2 = 0.04 ata.

5. Use of secondary (intermediate) steam superheating(Fig. 15, G). The diagram shows a straight line 1 –2 shows the expansion of steam to a certain pressure r 1A in the first cylinder of the engine, line 2–1 a–– secondary superheating of steam at pressure r 1A and straight 1 a–2 a–– adiabatic expansion of steam in the second cylinder to final pressure r 2 .

The thermal efficiency of such a cycle is determined by the expression

The use of secondary superheating of steam leads to a decrease in the humidity of steam at the outlet of the turbine and to a slight increase technical work. Increased efficiency in this cycle is insignificant, only 2–3%, and such a scheme requires a more complex design of the steam turbine.

6. Application of the regenerative cycle. In the regenerative cycle, feedwater after the pump flows through one or more regenerators, where it is heated by steam, partially taken after its expansion in some stages of the turbine (Fig. 16).

Rice. 15. Ways to increase thermal efficiency Rankine cycle

Rice. 16. Diagram of a steam power plant operating

according to the regenerative cycle:

1 –– boiler; 2 –– steam superheater; 3 –– steam turbine; 4 –– electric generator; 5 –– cooler-condenser; 6 –– pump; 7 –– regenerator; α is the share of steam extraction

The amount of steam taken will be determined from the equation heat balance for regenerator

where is the enthalpy of condensate at finite vapor pressure r 2 ; –– enthalpy of steam taken from the turbine; –– condensate enthalpy at steam extraction pressure.

The useful work of 1 kg of steam in a turbine will be determined by the formula:

The amount of heat expended per 1 kg of steam is

Then the thermal efficiency in the regenerative cycle will be found

.

A detailed study of the regenerative cycle shows that its thermal efficiency is always greater than thermal efficiency. Rankine cycle with the same initial and final parameters. Increased efficiency when using regeneration it is 10–15% and increases with increasing amounts of steam extraction.

7. Application of the heating cycle. The heating cycle utilizes the heat given off by steam to cooling water, which is usually used in heating systems, in hot water supply systems and for other purposes. In this case, the heat q 1 supplied to the working fluid can be redistributed to varying degrees to obtain technical work and heat supply. In the heating cycle (Fig. 17), part of the electricity is not processed, since part of the heat of the steam taken from the turbine is consumed by the consumer.

Rice. 17. Diagram of a steam power plant operating on

heating cycle:

1 –– boiler; 2 –– steam superheater; 3 –– steam turbine; 4 –– electric generator; 5 –– cooler-condenser; 6 –– pump; 7 –– heat consumer

The amount of heat received by the working fluid is partially converted into useful work of turbine blades, and partially spent for the purpose of heat supply to consumers. Since both jobs are useful, thermal efficiency loses its meaning.

Efficiency heating cycle will be determined

.

Since two types of products are produced in the heating cycle (electricity and heat), it is necessary to distinguish between the internal efficiency for heat production and the weighted average efficiency for the production of electricity and heat. Each of them equal to one, since there are no losses within the cycle.

In reality, efficiency heating cycle cannot be equal to unity, since there are always mechanical losses in the turbine and hydraulic losses in heat supply systems.

Steam power plant(PSU) is a complex energy equipment, in which water vapor is used as the working fluid. Various PSU cycles are known, including the Carnot cycle, which, as shown in Chap. 4, the highest thermal efficiency of all possible cycles in a given temperature range. The advantage of water vapor is precisely that during the process of vaporization, heat can be supplied to it along an isotherm and heat can also be removed along an isotherm during condensation. If the processes of heat supply are not associated with phase transformations, it is technically very difficult to carry them out strictly at constant temperatures. It can be argued that technically the Carnot cycle is only possible in the wet steam region.

To do this, the liquid, which is in a state of saturation (vol. 7, Fig. 8.1), should be sent to a steam generator, in which heat is supplied to it, for example, from the combustion products of organic fuel or released during nuclear reaction. In the region of wet steam, the isotherm and isobar coincide, so the essentially isobaric boiling process in the steam generator also occurs at a constant temperature. From the steam generator, dry saturated steam (i.e. 2) sent for adiabatic expansion to condenser pressure

Rice. 8.1.

(T. 3 ) in a steam engine - a piston steam engine or a steam turbine. In a condenser, heat is removed from exhaust steam at constant pressure and constant temperature and the steam condenses, but not completely (i.e. 4). Capacitor - This heat exchanger, in which the so-called circulating water, which removes the heat generated by steam during condensation on the outer surface of the pipes. Wet steam after the condenser enters a steam piston or blade compressor and is adiabatically compressed to the state of saturated water, including. 1.

Thermal efficiency of the Carnot cycle in the wet steam region

This efficiency is the highest possible value for any cycles carried out in the temperature range T (_2 and G 3_4.

Unfortunately, the ratio cannot be reduced arbitrarily

way in order to increase efficiency. For water vapor the natural limit for T (_2 is T cr = 647 K, and for the condensation temperature the lower limit is the ambient temperature into which the heat must be removed - G 3 _ 4 > 300 K. Thus,

The actual effective efficiency of the cycle under consideration will be significantly less, since the expansion and, especially, compression of wet steam are accompanied by large energy losses. Moreover, a machine for adiabatic compression of wet steam, which must first work as a compressor, compressing steam with a relatively high degree dryness and then like a pump, must have too complex design and cannot be reliable and cheap.

It should be noted that the use of temperatures 7\_ 2 close to T kr, leads to a decrease in the useful work produced by 1 kg of steam in a cycle. To verify this, it is enough to compare the areas 1-2-3-4i G-2"-3"-4" in Fig. 8.1.

The noted disadvantages of the Carnot cycle are organically inherent in it and hinder it practical use. At the same time, minor improvements to the considered cycle, proposed by William John McQuarne Rankine (1820-1872), turn it into a cycle through which more than 80% of all electricity produced on Earth is generated at thermal and nuclear power plants.

The energy balance of a steam power station with a turbine is shown in Fig. 519. He is exemplary; The efficiency of a steam power plant can be even higher (up to 27%). The energy losses that occur during the operation of a steam power plant can be divided into two parts. Part of the losses is due to imperfect design and can be reduced without changing the temperature in the boiler and condenser. For example, by arranging more advanced thermal insulation of the boiler, it is possible to reduce heat loss in the boiler room. The second, much larger part - the loss of heat transferred to the water cooling the condenser, turns out to be completely inevitable at given temperatures in the boiler and condenser. We have already indicated (§ 314) that the condition for the operation of a heat engine is not only to receive a certain amount of heat from the heater, but also to transfer part of this heat to the refrigerator.

Extensive scientific and technical experience in the design of heat engines and in-depth theoretical studies concerning the operating conditions of heat engines have established that the efficiency of a heat engine depends on the temperature difference between the heater and the refrigerator. The greater this difference, the greater the efficiency of a steam power plant (of course, provided that all the technical imperfections in the design mentioned above are eliminated). But if this difference is small, then even the most technically advanced machine cannot provide significant efficiency. Theoretical calculation shows that if the thermodynamic temperature of the heater is equal to , and the refrigerator is equal to , then the efficiency cannot be greater than

Rice. 519. Approximate energy balance of a steam power station with a turbine

So, for example, in a steam engine, the steam which has a temperature of 100 (or 373) in the boiler and 25 (or 298) in the refrigerator, the efficiency cannot be greater , i.e. 20% (practically, due to the imperfection of the device, the efficiency of such an installation will be significantly lower). Thus, to improve the efficiency of heat engines, it is necessary to move to higher temperatures in the boiler, and therefore to higher steam pressures. Unlike previous stations, which operated at a pressure of 12-15 atm (which corresponds to a steam temperature of 200), modern steam power stations began to install boilers of 130 atm or more (temperature about 500).

Instead of increasing the temperature in the boiler, it would be possible to lower the temperature in the condenser. However, this turned out to be practically impossible. At very low pressures the vapor density is very low and at large quantities steam passed through in one second by a powerful turbine, the volume of the turbine and condenser would have to be prohibitively large.

In addition to increasing the efficiency of a heat engine, you can take the path of using “waste heat,” i.e., heat removed by water cooling the condenser.

Rice. 520. Approximate energy balance of thermal power plant

Instead of releasing condenser-heated water into a river or lake, it can be directed through hot water heating pipes or used for industrial purposes in the chemical or textile industries. It is also possible to expand steam in turbines only to a pressure of 5-6 atm. At the same time, very hot steam comes out of the turbine, which can be used for a number of industrial purposes.

A station using waste heat supplies consumers not only electrical energy obtained through mechanical work, but also by heat. It is called a combined heat and power plant (CHP). An approximate energy balance of a thermal power plant is shown in Fig. 520.