Gas turbine generator. Gas turbines and low power gas turbine units in the Russian market. Gas turbine design

"Turbo", "turbojet", "turboprop" - these terms have firmly entered the lexicon of 20th century engineers involved in the design and maintenance of vehicles and stationary electrical installations. They are used even in related areas and advertising, when they want to give the name of the product some hint of special power and efficiency. In aviation, rockets, ships and power plants, the gas turbine is most often used. How is it organized? Does it run on natural gas (as the name might suggest), and what are they like? How is a turbine different from other types of internal combustion engine? What are its advantages and disadvantages? An attempt to answer these questions as fully as possible is made in this article.

Russian machine-building leader UEC

Russia, unlike many other independent states formed after the collapse of the USSR, managed to largely preserve the machine-building industry. In particular, the Saturn company is engaged in the production of special-purpose power plants. The gas turbines of this company are used in shipbuilding, the raw materials industry and energy. The products are high-tech, they require a special approach during installation, debugging and operation, as well as special knowledge and expensive equipment for scheduled maintenance. All these services are available to customers of UEC - Gas Turbines, as it is called today. There are not so many such enterprises in the world, although the principle of arranging the main product is at first glance simple. The accumulated experience is of great importance, which makes it possible to take into account many technological subtleties, without which it is impossible to achieve a durable and reliable operation of the unit. Here is just a part of the UEC product range: gas turbines, power plants, gas pumping units. Among the customers are "Rosatom", "Gazprom" and other "whales" of the chemical industry and energy.

The manufacture of such complex machines requires an individual approach in each case. The calculation of a gas turbine is currently fully automated, but the materials and features of the wiring diagrams matter in each individual case.

And it all started so easy...

Searches and couples

The first experiments of converting the translational energy of the flow into rotational force were carried out by mankind in ancient times, using an ordinary water wheel. Everything is extremely simple, liquid flows from top to bottom, blades are placed in its flow. The wheel, equipped with them around the perimeter, is spinning. The windmill works the same way. Then came the age of steam, and the wheel turned faster. By the way, the so-called "eolipil", invented by the ancient Greek Heron about 130 years before the birth of Christ, was a steam engine that works exactly on this principle. In essence, this was the first gas turbine known to historical science (after all, steam is a gaseous state of aggregation of water). Today, however, it is customary to separate these two concepts. Heron's invention was then treated in Alexandria without much enthusiasm, although with curiosity. Turbine-type industrial equipment appeared only at the end of the 19th century, after the Swedish Gustaf Laval created the world's first active power unit equipped with a nozzle. Approximately in the same direction, engineer Parsons worked, supplying his machine with several functionally connected steps.

The birth of gas turbines

A century earlier, a certain John Barber had a brilliant idea. Why do you need to heat the steam first, is it not easier to use directly the exhaust gas generated during the combustion of fuel, and thereby eliminate unnecessary mediation in the energy conversion process? This is how the first real gas turbine came about. The 1791 patent lays out the basic idea of ​​being used in a horseless carriage, but elements of it are used today in modern rocket, aircraft, tank, and automobile engines. The beginning of the process of jet engine building was given in 1930 by Frank Whittle. He came up with the idea of ​​using a turbine to propel an airplane. Later, she found development in numerous turboprop and turbojet projects.

Nikola Tesla gas turbine

The famous scientist-inventor has always approached the issues under study in a non-standard way. It seemed obvious to everyone that wheels with paddles or blades "catch" the movement of the medium better than flat objects. Tesla, in his characteristic manner, proved that if you assemble a rotor system from discs arranged in series on the axis, then by picking up the boundary layers with a gas flow, it will rotate no worse, and in some cases even better, than a multi-bladed propeller. True, the direction of the moving medium should be tangential, which is not always possible or desirable in modern units, but the design is greatly simplified - it does not need blades at all. A gas turbine according to the Tesla scheme is not being built yet, but perhaps the idea is just waiting for its time.

circuit diagram

Now about the fundamental device of the machine. It is a combination of a rotating system mounted on an axis (rotor) and a fixed part (stator). On the shaft there is a disk with working blades forming a concentric lattice, they are affected by gas supplied under pressure through special nozzles. Then the expanded gas enters the impeller, also equipped with blades, called workers. For the inlet of the air-fuel mixture and the outlet (exhaust), special pipes are used. The compressor is also involved in the overall scheme. It can be made according to a different principle, depending on the required working pressure. For its operation, a part of the energy is taken from the axis, which is used to compress the air. The gas turbine works by means of the process of combustion of the air-fuel mixture, accompanied by a significant increase in volume. The shaft rotates, its energy can be used usefully. Such a scheme is called single-circuit, but if it is repeated, then it is considered multi-stage.

Advantages of aircraft turbines

Since about the mid-fifties, a new generation of aircraft has appeared, including passenger ones (in the USSR these are Il-18, An-24, An-10, Tu-104, Tu-114, Tu-124, etc.), in designs of which aircraft piston engines were finally and irrevocably supplanted by turbine ones. This indicates a greater efficiency of this type of power plant. The characteristics of a gas turbine are superior to those of carburetor engines in many ways, in particular, in terms of power / weight, which is of paramount importance for aviation, as well as in equally important indicators of reliability. Lower fuel consumption, fewer moving parts, better environmental performance, reduced noise and vibration. Turbines are less critical to fuel quality (which cannot be said about fuel systems), they are easier to maintain, they require less lubricating oil. In general, at first glance it seems that they do not consist of metal, but of solid virtues. Alas, it is not.

There are disadvantages of gas turbine engines

The gas turbine heats up during operation and transfers heat to the surrounding structural elements. This is especially critical, again in aviation, when using a redan layout scheme that involves washing the lower part of the tail unit with a jet stream. And the engine housing itself requires special thermal insulation and the use of special refractory materials that can withstand high temperatures.

Cooling gas turbines is a complex technical challenge. It's no joke, they work in the mode of a virtually permanent explosion occurring in the body. The efficiency in some modes is lower than that of carburetor engines, however, when using a dual-circuit scheme, this drawback is eliminated, although the design becomes more complicated, as in the case of including "booster" compressors in the scheme. Acceleration of turbines and reaching the operating mode requires some time. The more often the unit starts and stops, the faster it wears out.

Correct Application

Well, no system is without flaws. It is important to find such an application of each of them, in which its advantages will be more clearly manifested. For example, tanks such as the American Abrams, which is powered by a gas turbine. It can be filled with anything that burns, from high-octane gasoline to whiskey, and it puts out a lot of power. This may not be a very good example, as experience in Iraq and Afghanistan has shown the vulnerability of compressor blades to sand. Repair of gas turbines has to be done in the USA, at the manufacturing plant. Take the tank there, then back, and the cost of the maintenance itself, plus accessories ...

Helicopters, Russian, American and other countries, as well as powerful speedboats, are less affected by clogging. In liquid rockets, they are indispensable.

Modern warships and civilian ships also have gas turbine engines. And also energy.

Trigenerator power plants

The problems faced by aircraft manufacturers are not as worrying for those who make industrial equipment for generating electricity. Weight in this case is no longer so important, and you can focus on parameters such as efficiency and overall efficiency. Gas turbine generator units have a massive frame, a reliable frame and thicker blades. It is quite possible to utilize the generated heat, using it for a variety of needs, from secondary recycling in the system itself, to heating domestic premises and thermal supply of absorption-type refrigeration units. This approach is called trigenerator, and the efficiency in this mode approaches 90%.

Nuclear power plants

For a gas turbine, it makes no fundamental difference what is the source of the heated medium that gives its energy to its blades. It can be a burnt air-fuel mixture, or simply superheated steam (not necessarily water), the main thing is that it ensures its uninterrupted power supply. At its core, the power plants of all nuclear power plants, submarines, aircraft carriers, icebreakers and some military surface ships (the Peter the Great missile cruiser, for example) are based on a gas turbine (GTU) rotated by steam. Safety and environmental issues dictate a closed primary loop. This means that the primary heat agent (in the first samples this role was played by lead, now it has been replaced by paraffin) does not leave the near-reactor zone, flowing around the fuel elements in a circle. The heating of the working substance is carried out in subsequent circuits, and the evaporated carbon dioxide, helium or nitrogen rotates the turbine wheel.

Wide application

Complex and large installations are almost always unique, their production is carried out in small batches or in general single copies are made. Most often, units produced in large quantities are used in peaceful sectors of the economy, for example, for pumping hydrocarbon raw materials through pipelines. It is these that are produced by the UEC company under the Saturn brand. Gas turbines of pumping stations are fully consistent with their name. They really pump natural gas, using its own energy for their work.

A gas turbine is commonly referred to as a continuously operating engine. Next, we will talk about how a gas turbine is arranged, what is the principle of operation of the unit. A feature of such an engine is that inside it, energy is produced by compressed or heated gas, the result of which is the mechanical work on the shaft.

History of the gas turbine

Interestingly, turbine mechanisms have been developed by engineers for a very long time. The first primitive steam turbine was created in the 1st century BC. e.! Of course, its essential
This mechanism has reached its heyday only now. Turbines began to be actively developed at the end of the 19th century, simultaneously with the development and improvement of thermodynamics, mechanical engineering and metallurgy.

The principles of mechanisms, materials, alloys have changed, everything has been improved, and now, today, mankind knows the most perfect of all previously existing forms of a gas turbine, which is divided into various types. There is an aviation gas turbine, and there is an industrial one.

It is customary to call a gas turbine a kind of heat engine, its working parts are predetermined with only one task - to rotate due to the action of a gas jet.

It is arranged in such a way that the main part of the turbine is represented by a wheel on which sets of blades are attached. , acting on the blades of a gas turbine, makes them move and rotate the wheel. The wheel, in turn, is rigidly fastened to the shaft. This tandem has a special name - the turbine rotor. As a result of this movement, which takes place inside the gas turbine engine, mechanical energy is obtained, which is transmitted to an electric generator, to a ship's propeller, to an aircraft propeller and other operating mechanisms of a similar principle of operation.

Active and jet turbines

The impact of the gas jet on the turbine blades can be twofold. Therefore, turbines are divided into classes: the class of active and reactive turbines. Reactive and active gas turbines differ in the principle of the device.

Impulse turbine

An active turbine is characterized by the fact that there is a high rate of gas flow to the rotor blades. With the help of a curved blade, the gas jet deviates from its trajectory. As a result of the deflection, a large centrifugal force develops. With the help of this force, the blades are set in motion. During the entire described path of the gas, a part of its energy is lost. Such energy is directed to the movement of the impeller and shaft.

jet turbine

In a jet turbine, things are somewhat different. Here, the flow of gas to the rotor blades is carried out at low speed and under the influence of a high level of pressure. The shape of the blades is also excellent, due to which the gas velocity is significantly increased. Thus, the jet of gas creates a kind of reactive force.

From the mechanism described above, it follows that the device of a gas turbine is rather complicated. In order for such a unit to work smoothly and bring profit and benefit to its owner, you should entrust its maintenance to professionals. Service profile companies provide service maintenance of installations using gas turbines, supplies of components, all kinds of parts and parts. DMEnergy is one such company () that provides its customer with peace of mind and confidence that he will not be left alone with the problems that arise during the operation of a gas turbine.

Power plants of relatively small capacity can include both gas turbine engines (GTE) and reciprocating engines (RP). As a result, customers often ask which drive is better. And, although it is definitely impossible to answer it, the purpose of this article is an attempt to understand this issue.

Introduction

The choice of the type of engine, as well as their number for driving electric generators at a power plant of any capacity, is a complex technical and economic task. Attempts to compare piston and gas turbine engines as a drive are most often made using natural gas as a fuel. Their fundamental advantages and disadvantages have been analyzed in the technical literature, in the brochures of manufacturers of power plants with piston engines, and even on the Internet.

As a rule, generalized information is given about the difference in fuel consumption, in the cost of engines, without taking into account their power and operating conditions. It is often noted that it is preferable to form the composition of power plants with a capacity of 10-12 MW on the basis of reciprocating engines, and higher power - on the basis of gas turbines. These recommendations should not be taken as an axiom. One thing is obvious: each type of engine has its advantages and disadvantages, and when choosing a drive, some, at least indicative, quantitative criteria for their evaluation are needed.

Currently, the Russian energy market offers a fairly wide range of both reciprocating and gas turbine engines. Among piston engines, imported engines prevail, and among gas turbine engines, domestic ones.

Information about the technical characteristics of gas turbine engines and power plants based on them, proposed for operation in Russia, has been regularly published in the "Catalogue of gas turbine equipment" in recent years.

Similar information about reciprocating engines and power plants, which they are part of, can only be obtained from advertising brochures of Russian and foreign companies that supply this equipment. Information about the cost of engines and power plants is most often not published, and published information is often not true.

Head-to-Head Comparison of Reciprocating and Gas Turbine Engines

The processing of the available information makes it possible to form the table below, which contains both a quantitative and a qualitative assessment of the advantages and disadvantages of reciprocating and gas turbine engines. Unfortunately, some of the characteristics are taken from promotional materials, the complete accuracy of which is extremely difficult or almost impossible to verify. The data required for verification on the results of the operation of individual engines and power plants, with rare exceptions, are not published.

Naturally, the figures given are generalized; for specific engines, they will be strictly individual. In addition, some of them are given in accordance with ISO standards, and the actual operating conditions of the engines differ significantly from the standard.

The presented information gives only a qualitative characteristic of the engines and cannot be used in the selection of equipment for a particular power plant. Some comments can be given for each position of the table.

The energy market has a very large selection of engines with significant differences in technical characteristics. Competition between the engines of the considered types is possible only in the range of unit electric power up to 16 MW. At higher powers, gas turbine engines replace piston engines almost completely.

It must be taken into account that each motor has individual characteristics, and only these should be used when choosing a drive type. This makes it possible to form the composition of the main equipment of a power plant of a given capacity in several versions, varying, first of all, the electric power and the number of required engines. The versatility makes it difficult to choose the preferred type of engine.

On the efficiency of piston and gas turbine engines

The most important characteristic of any engine in power plants is the power generation efficiency (KPIe), which determines the main, but not the full volume of gas consumption. The processing of statistical data on the values ​​of efficiency makes it possible to clearly show the areas of application in which, according to this indicator, one type of engine has advantages over another.

The mutual arrangement and configuration of the three selected in Fig. 1 zones, within which there are dot images of the values ​​of the electrical efficiency of various engines, allows us to draw some conclusions:

• even within the same type of engines of the same power, there is a significant scatter in the values ​​​​of efficiency for generating electricity;
• with a unit power of more than 16 MW, gas turbine engines in the combined cycle provide an efficiency value of more than 48% and monopolize the market;
• electrical efficiency of gas turbine engines up to 16 MW, operating in both simple and combined cycles, is lower (sometimes very significantly) than that of piston engines;
• gas turbine engines with a unit capacity of up to 1 MW, which have recently appeared on the market, are superior in terms of efficiency to engines with a capacity of 2-8 MW, which are most often used today in power plants;
• the nature of the change in the efficiency of gas turbine engines has three zones: two with a relatively constant value - 27 and 36%, respectively, and one with a variable - from 27 to 36%; within two zones, the efficiency coefficient weakly depends on the electric power;
• the value of the efficiency for the generation of electricity of reciprocating engines is in constant dependence on their electrical power.

However, these factors are not a reason to give priority to piston engines. Even if the power plant will produce only electrical energy, when comparing equipment options with different types of engines, it will be necessary to perform economic calculations. It is necessary to prove that the cost of saved gas will pay for the difference in the cost of reciprocating and gas turbine engines, as well as additional equipment for them. The amount of saved gas cannot be determined if the operating mode of the station for the supply of electricity in winter and summer is unknown. Ideally, if the necessary electrical loads are known - maximum (winter working day) and minimum (summer day off).

Use of both electrical and thermal energy

If the power plant must produce not only electrical, but also thermal energy, then it will be necessary to determine from which sources it is possible to cover heat consumption. As a rule, there are two such sources - the utilized heat of engines and/or the boiler house.

For piston engines, the heat of the cooling oil, compressed air and exhaust gases is utilized, for gas turbine engines only the heat of the exhaust gases is utilized. The main amount of heat is recovered from the exhaust gases with the help of waste heat exchangers (UHE).

The amount of recovered heat largely depends on the mode of operation of the engine to generate electricity and on climatic conditions. Incorrect assessment of the engine operation modes in winter will lead to errors in determining the amount of utilized heat and an incorrect choice of the installed capacity of the boiler house.

Graphs in Fig. 2 show the possibility of reclaimed heat supply from gas turbine and piston engines for heat supply purposes. The points on the curves correspond to the manufacturer's data on the capabilities of the available equipment for heat recovery. On the engine of the same electric power, manufacturers install various UTOs - based on specific tasks.

The advantages of gas turbine engines in terms of heat generation are undeniable. This is especially true for motors with an electric power of 2-10 MW, which is explained by the relatively low value of their electrical efficiency. As the efficiency of gas turbine engines increases, the amount of utilized heat must inevitably decrease.

When choosing a piston engine for power and heat supply of a particular facility, the need to use a boiler house as part of a power plant is almost beyond doubt. The operation of the boiler house requires an increase in gas consumption in excess of what is necessary to generate electricity. The question arises of how the gas costs for the energy supply of the facility differ if in one case only gas turbine engines with exhaust heat recovery are used, and in the other case piston engines with heat recovery and a boiler house are used. Only after a thorough study of the features of the object's consumption of electricity and heat can this question be answered.

If we assume that the estimated heat consumption of an object can be completely covered by the utilized heat of the gas turbine engine, and the lack of heat when using a piston engine is compensated by the boiler house, then it is possible to identify the nature of the change in the total gas consumption for the energy supply of the object.

Using the data in Fig. 1 and 2, it is possible for the characteristic points of the zones marked in Figs. 1, get information about gas savings or overruns when using various types of actuators. They are presented in the table:

The absolute values ​​of gas savings are valid only for a specific object, the characteristics of which were included in the calculation, but the general nature of the dependence is reflected correctly, namely:
with relatively close values ​​of electrical efficiency (difference up to 10%), the use of piston engines and a boiler room leads to excessive fuel consumption;

• with relatively close values ​​of electrical efficiency (difference up to 10%), the use of piston engines and a boiler room leads to excessive fuel consumption;
• with a difference in efficiency values ​​of more than 10%, the operation of reciprocating engines and the boiler house will require less gas than for gas turbine engines;
• there is a certain point with maximum gas savings when using reciprocating engines and a boiler room, where the difference between the efficiency values ​​of engines is 13-14%;
• the higher the efficiency of a piston engine and the lower the efficiency of a gas turbine, the greater the gas savings.

As a supplement

As a rule, the task is not limited to the choice of the type of drive, it is required to determine the composition of the main equipment of the power plant - the type of units, their number, auxiliary equipment.

The choice of engines to produce the right amount of electricity determines the possibilities for generating recovered heat. In this case, it is necessary to take into account all the features of changes in the technical characteristics of the engine associated with climatic conditions, with the nature of the electrical load, and determine the effect of these changes on the supply of utilized heat.

It must also be remembered that the power plant includes not only engines. On its site, there are usually more than a dozen auxiliary structures, the operation of which also affects the technical and economic performance of the power plant.

As already mentioned, from a technical point of view, the composition of the power plant equipment can be formed in several ways, so its final choice can only be justified from an economic standpoint.

At the same time, knowledge of the characteristics of specific engines and their impact on the economic performance of a future power plant is extremely important. When performing economic calculations, it is inevitable to take into account the motor resource, maintainability, timing and cost of major repairs. These indicators are also individual for each specific engine, regardless of its type.

The influence of environmental factors on the choice of the type of engines for the power plant cannot be ruled out. The state of the atmosphere in the area where the power plant is to be operated can be a major factor in determining the type of engine (regardless of any economic considerations).

As already noted, data on the cost of engines and power plants based on them are not published. Manufacturers or suppliers of equipment refer to the possible difference in configuration, delivery conditions and other reasons. Prices will be presented only after filling in the corporate questionnaire. Therefore, the information in the first table that the cost of reciprocating engines with a power of up to 3.5 MW is lower than the cost of gas turbine engines of the same power may turn out to be incorrect.

Conclusion

Thus, in the unit power class up to 16 MW, neither gas turbine nor piston engines can be given unequivocal preference. Only a thorough analysis of the expected modes of operation of a particular power plant for the generation of electricity and heat (taking into account the characteristics of specific engines and numerous economic factors) will fully justify the choice of engine type. A specialized company can determine the composition of the equipment at a professional level.

References

1. Gabich A. Application of gas turbine engines of low power in the energy sector // Gas turbine technologies. 2003, No. 6. S. 30-31.
2. Burov VD Gas-turbine and gas-piston power plants of low power // Mining magazine. 2004, special issue. pp. 87-89,133.
3. Catalog of gas turbine equipment // Gas turbine technologies. 2005. S. 208.
4. Salikhov A. A., Fatkulin R. M., Abrakhmanov R. R., Shchaulov V. Yu. Development of mini-CHP using gas piston engines in the Republic of Bashkortostan. 2003, No. 11. S. 24-30.

This article, with minor changes, is taken from the journal "Turbines and Diesels", No. 1 (2) for 2006.
Author - V.P. Vershinsky, OOO "Gazpromenergoservis".

A gas turbine is an engine in which, in the process of continuous operation, the main organ of the device (the rotor) converts (in other cases, steam or water) into mechanical work. In this case, the jet of the working substance acts on the blades fixed around the circumference of the rotor, setting them in motion. In the direction of the gas flow, turbines are divided into axial (gas moves parallel to the axis of the turbine) or radial (perpendicular movement relative to the same axis). There are both single and multi-stage mechanisms.

A gas turbine can act on the blades in two ways. Firstly, it is an active process, when gas is supplied to the working area at high speeds. In this case, the gas flow tends to move in a straight line, and the curved blade part standing in its way deflects it, turning itself. Secondly, it is a reactive type process, when the gas supply rate is low, but high pressures are used. type in its pure form is almost never found, because in their turbines it is present which acts on the blades along with the reaction force.

Where is the gas turbine used today? The principle of operation of the device allows it to be used for drives of electric current generators, compressors, etc. Turbines of this type are widely used in transport (ship gas turbine installations). Compared to steam analogues, they have a relatively small weight and dimensions, they do not require the arrangement of a boiler room, a condensing unit.

The gas turbine is ready for operation quite quickly after start-up, develops full power in about 10 minutes, is easy to maintain, requires a small amount of water for cooling. Unlike internal combustion engines, it does not have inertial effects from the crank mechanism. one and a half times shorter than diesel engines and more than twice as light. The devices have the ability to run on low quality fuel. The above qualities make it possible to consider engines of this kind of particular interest for ships on and hydrofoils.

The gas turbine as the main component of the engine has a number of significant disadvantages. Among them, they note high noise, less than diesel engines, efficiency, short life at high temperatures (if the gas medium used has a temperature of about 1100 ° C, then the turbine can be used on average up to 750 hours).

The efficiency of a gas turbine depends on the system in which it is used. For example, devices used in the power industry with an initial temperature of gases above 1300 degrees Celsius, from air in the compressor no more than 23 and no less than 17, have a coefficient of about 38.5% during autonomous operations. Such turbines are not very widespread and are mainly used to cover load peaks in electrical systems. Today, about 15 gas turbines with a capacity of up to 30 MW operate at a number of thermal power plants in Russia. On multi-stage plants, a much higher efficiency index (about 0.93) is achieved due to the high efficiency of structural elements.

The principle of operation of gas turbine plants

Fig.1. Scheme of a gas turbine unit with a single-shaft gas turbine engine of a simple cycle

Clean air is supplied to the compressor (1) of the gas turbine power unit. Under high pressure, air from the compressor is sent to the combustion chamber (2), where the main fuel, gas, is also supplied. The mixture ignites. When a gas-air mixture is burned, energy is generated in the form of a stream of hot gases. This flow rushes at high speed to the turbine wheel (3) and rotates it. Rotational kinetic energy through the turbine shaft drives the compressor and electric generator (4). From the terminals of the power generator, the generated electricity, usually through a transformer, is sent to the power grid, to energy consumers.

Gas turbines are described by the Brayton thermodynamic cycle. The Brayton/Joule cycle is a thermodynamic cycle that describes the working processes of gas turbine, turbojet and ramjet internal combustion engines, as well as gas turbine external combustion engines with a closed loop of a gaseous (single-phase) working fluid.

The cycle is named after American engineer George Brighton, who invented the reciprocating internal combustion engine that operated on this cycle.

Sometimes this cycle is also called the Joule cycle - in honor of the English physicist James Joule, who established the mechanical equivalent of heat.

Fig.2. P,V Brayton cycle diagram

The ideal Brayton cycle consists of the processes:

• 1-2 Isentropic compression.
• 2-3 Isobaric heat input.
• 3-4 Isentropic expansion.
• 4-1 Isobaric heat removal.

Taking into account the differences between real adiabatic processes of expansion and contraction from isentropic ones, a real Brayton cycle is constructed (1-2p-3-4p-1 on the T-S diagram) (Fig. 3)

Fig.3. T-S Brayton cycle diagram
Ideal (1-2-3-4-1)
Real (1-2p-3-4p-1)

The thermal efficiency of an ideal Brayton cycle is usually expressed by the formula:

• where P = p2 / p1 - the degree of pressure increase in the process of isentropic compression (1-2);
• k - adiabatic index (for air equal to 1.4)

It should be especially noted that this generally accepted way of calculating the cycle efficiency obscures the essence of the ongoing process. The limiting efficiency of the thermodynamic cycle is calculated through the temperature ratio using the Carnot formula:

• where T1 is the refrigerator temperature;
• T2 - heater temperature.

Exactly the same temperature ratio can be expressed in terms of the pressure ratios used in the cycle and the adiabatic index:

Thus, the efficiency of the Brayton cycle depends on the initial and final temperatures of the cycle in exactly the same way as the efficiency of the Carnot cycle. With an infinitesimal heating of the working fluid along the line (2-3), the process can be considered isothermal and completely equivalent to the Carnot cycle. The amount of heating of the working fluid T3 in the isobaric process determines the amount of work related to the amount of the working fluid used in the cycle, but in no way affects the thermal efficiency of the cycle. However, in the practical implementation of the cycle, heating is usually carried out to the highest possible values ​​limited by the heat resistance of the materials used in order to minimize the size of the mechanisms that compress and expand the working fluid.

In practice, friction and turbulence cause:

• Non-adiabatic compression: for a given total pressure ratio, the compressor discharge temperature is higher than ideal.
• Non-adiabatic expansion: although the turbine temperature drops to the level necessary for operation, the compressor is not affected, the pressure ratio is higher, as a result, the expansion is not enough to provide useful work.
• Pressure losses in the air intake, combustion chamber and outlet: as a result, the expansion is not sufficient to provide useful work.

As with all cyclic heat engines, the higher the combustion temperature, the higher the efficiency. The limiting factor is the ability of the steel, nickel, ceramic or other materials that make up the engine to withstand heat and pressure. Much of the engineering work is focused on removing heat from parts of the turbine. Most turbines also try to recover heat from exhaust gases that are otherwise wasted.

Recuperators are heat exchangers that transfer heat from exhaust gases to compressed air before combustion. In a combined cycle, heat is transferred to the steam turbine systems. And in combined heat and power (CHP), waste heat is used to produce hot water.

Mechanically, gas turbines can be considerably simpler than reciprocating internal combustion engines. Simple turbines may have one moving part: shaft/compressor/turbine/alternate rotor assembly (see image below), not including the fuel system.

Fig.4. This machine has a single stage radial compressor,
turbine, recuperator, and air bearings.

More complex turbines (those used in modern jet engines) may have multiple shafts (coils), hundreds of turbine blades, moving stator blades, and an extensive system of complex piping, combustion chambers, and heat exchangers.

As a general rule, the smaller the motor, the higher the speed of the shaft(s) required to maintain the maximum linear velocity of the blades.

The maximum speed of the turbine blades determines the maximum pressure that can be reached, resulting in maximum power, regardless of engine size. The jet engine rotates at about 10,000 rpm and the micro-turbine at about 100,000 rpm.