Compilation of exam questions on thermal engineering for engineering students.
Exam Question # 1. What is the difference between enthalpy and entropy?
A working substance in any state has a certain amount of internal energy. Besides, it also has some potential energy which is proportional to the absolute pressure and the specific volume i.e., to the product PV. This product, PV, is the work that has to be done to introduce a working substance with a specific volume V into a medium with a pressure P. The sum of the internal energy U and the product PV is known as the enthalpy or heat content and is denoted by H.
Hence H = U + PV J/kg.
We know that for an ideal gas the internal energy depends only on temperature. It follows from the equation of state that the product PV for such a gas also depends only on the temperature. Therefore, the enthalpy for an ideal gas depends only on the temperature.
It is possible to represent work involved in a reversible process by an area on pressure volume diagram. Both pressure and volume are the properties of a system. The pressure serves as the potential which causes work to cross a boundary. If we can plot a diagram which represents heat flow during a reversible processes it will be of much use to us.
We know that the temperature is a potential which causes the heat flow i.e., the heat flows due to difference in temperature. Thus the absolute temperature will be used as the first dimension, like the pressure, on the diagram for which we are searching. Now we must fix second dimension which must also be the property of the system.
The second dimension should be so chosen such that the area under the curve bounded by two ordinates must give us the heat flow during the reversible process. The second dimension is called by the name entropy which is also the property of the system.
If we write in differential form, we get-
dQ = dɸ x T (see fig. 2-4).
A temperature entropy diagram has the same use in representing the heat flow as the pressure volume diagram has in connection with work done during a process.
Exam Question # 2. What is a thermodynamic system?
Ans. All physical things in nature have some form of boundary, whose shape in general identifies it as the object that it is. Inside its boundary there are certain things with particular functions to carry out. This inside arrangement is called system. Outside the boundary of the object are the surroundings and the reaction between the system and surroundings in general control, the behaviour pattern of the object.
The heat engines are the systems. It is not necessary that at any time a complete object must be under study. Only part may be under study and then this part may be considered as the system. In other words, system can be defined as a particular region which is under study. It is identified by their boundaries around which are surroundings. Fig. 2-5 shows the thermodynamic system.
The boundary need not be fixed. For example, a mass of a gas (the system) may expand and hence, the boundary in this system will modify and interactions will occur with the surroundings at the boundary.
There are three types of systems, namely closed system, open system and isolated system.
The closed system is a system of fixed mass and identity. The boundaries of a closed system are determined by the space the matter occupies. Heat and work may cross the boundary of the system, but the mass that comprises the system is fixed.
The example of a closed system is a mass of gas or vapour contained in an engine cylinder, the boundary of which is drawn by the cylinder walls, the cylinder head and the piston crown. The boundary is continuous and no matter may enter or leave.
In an open system mass crosses the boundary of the system. Heat and work may also cross the boundary. An example of an open system is the flow of working fluid in turbines and compressors, as well as in piston type of engines during the injection and discharge processes. This is called a two flow boundary system.
Another example of an open system is air leaving a compressed air tank. This would be a one-flow boundary system, since air is only leaving the tank and none is entering.
In the isolated system, the mass, heat and work do not cross the boundary of the system.
If the volume of a system under study remains constant, then this volume is called the control volume. The control volume is bounded by the control surface.
The thermodynamic system is called uniform if all its components have identical properties. If different parts of the system have different properties and there are interfaces, the system is called heterogeneous. In the absence of interfaces, the system is called homogeneous.
A reservoir containing a boiling liquid and saturated vapour above it is an example of a heterogeneous system. A mixture of ice and water also constitutes a heterogeneous system. The individual components of a heterogeneous system, which are separated by interfaces are called phases. A heterogeneous system can also contain three phases when solid, liquid and gaseous phases coexist in the system.
The state of a system where the values of the parameters characterizing it do not vary with time is called the steady state. If the values of the parameters characterizing the state of a system vary, with time, the state is called a transient state.
Exam Question # 3. Describe the working of modern boilers.
Ans. Boiler is a container into which water is fed and is heated which evaporates into steam. In the earlier designs the boiler a simple shell with a feed pipe and steam outlet, mounted on a brick shell was used. The fuel was burnt over a grate and the heat so released was directed over the lower shell surface. This resulted in a loss of heat from surfaces.
The fire-tube boiler increases the heating surface but also distributes area of steam formation more uniformly.
The water-tube boiler consists of one or more relatively small drums with number of tubes in which water-steam mixture circulates. Heat flows from outside tubes to water. This makes it possible to have construction of large capacity and high pressure boilers.
The fire-tube boilers were limited to a maximum design working pressure of 250 N/cm2 and steam generating capacity of 25 tonnes per hour. The conventional water tube boilers work upto steam pressures of about 700 N/cm2 and 300°C superheat with a steam generating capacity of 40 tonnes per hour.
The shell or fire-tube boilers are cheaper than water-tube boilers. The water- tube boiler can be constructed very easily at site. They are flexible from constructional point of view. They are capable of quick steam generation and their constructional design can be varied to suit a wide range of situations. The furnaces of a water- tube boiler are of different shapes. Therefore, water tube boilers are generally preferred for high pressure high duty performance.
In recent days the demand for higher power outputs from the thermal power plants requires high pressure, high duty boilers. For increasing the steam pressure and the rate of steam generation of a boiler, forced circulation of water and / or steam and radiant heat transfer from the furnace to the water were considered essential.
As the demand for large capacity and high pressure boilers grew, the demand for more active furnace wall were developed because of increasing rate of heat transfer in furnace proper. The water from drum is supplied to lower header. Steam is actively generated in walls, to rise to upper drum where it separates from boiler water.
In a simple water tube circuit, steam bubbles are formed n the heated side. The resulting steam-water mixture weighs less than cooler water on the unheated side and thus free convection current are established.
The free circulation is affected by two factors:
(i) Difference in density between water and steam-water mixture, and
(ii) The frictional losses opposing circulation.
At a higher pressure, the effect of first factor reduces and thus forced circulation is inevitable. Also the forced circulation increases the rate of heat transfer thus permitting higher rate of steam generation and reduction in size. Thus large capacity boilers are possible.
The high water velocities rather than high gas velocities are suitable, as a smaller quantity of fluid is dealt with and increase in pressure can be more easily attained than gas. Hence the tubes of smaller diameter may be used for a boiler of a given output.
If the flow takes place through one continuous tube, large pressure drop takes place due to friction. This can be reduced by arranging the flow to pass through parallel system of tubing.
Exam Question # 4. What is a nozzle? What are the types of it?
Ans. A nozzle is a passage of varying cross-sectional area in which the pressure energy of the steam is converted into kinetic energy. This result in increase of velocity of the steam jet at the exit of the nozzle is obtained due to decrease in total heat content of the steam. The nozzles are used in steam turbine to obtain high velocity jets.
There are three types of nozzle:
(1) Convergent Nozzle:
This type of nozzle has convergent shape and size. In this case the velocity of steam increases at the throat. This nozzle has got the convergent section and end is throat at which the velocity is high.
(2) Convegent-Divergent Nozzle:
This type of nozzle has convergent and then divergent portion. The divergent portion is long and convergent portion is small. The two portions meet at the throat.
(3) Divergent Nozzle:
This type of has divergent section and the velocity is reduced by use of the divergent section.
Exam Question # 5. Explain the supersaturated or metastable expansion of steam in the nozzle.
Ans. The steam flow through the nozzle and discharge of steam is less than the theoretical discharge because of the losses in the nozzle. But sometimes it is found that the discharge of the wet steam through the nozzle is more than the theoretical discharge.
This occurs because of the converging part of the nozzle is very short and steam velocity is very high.
Then the molecules of steam do not get sufficient time to collect and form the droplets for condensation. This rapid expansion is called metastable expansion and results into supersaturated state. This results in undercooling of the steam to a temperature corresponding to its pressure. This increases the density of steam and mass flow rate.
The steam at point 1 expands isentropically and reaches to the point 2 as a normal process. But in the case of super saturation, the wet steam expands to a point 2a isentropically and at this point super saturation occurs which results into increase of entropy with constant enthalpy due to super saturation upto point 2b. Then steam expands to the point 2c with isentropic expansion back to the pressure for point 2. This results into drop of enthalpy.
H2c – H2 = drop of enthalpy.
Exam Question # 6. What is erosion of turbine blades?
Ans. In the multi-stage steam turbines, the pressures are quite high for first and intermediate stages and naturally the temperatures are high. The blade material must be high to withstand the temperature. In the intermediate stages the steam is wet at the leaving point from the turbine. Therefore the blade material must be able to withstand both corrosion and erosion due to presence of the water particles.
The centrifugal stresses are very high because of the high speed of operation of the turbine. If the speeds are very high and the moisture content is more than 10 % then the erosion of blades occur. The most effected portion is the back of the inlet edge of the blades where either the groove formed or even some portion breaks off.
The water particles in the moisture tend to concentrate in the outer annulus because of the centrifugal force and their speed are greater than the root speeds. This causes erosion in the tip of the blades. Fig. 11-58 shows the erosion points.
The erosion can be prevented by:
(1) By using the reheat cycle so that the wetness of steam can be reduced and maintained within the accepted values.
(2) Raising the temperature of the inlet steam, so that the steam leaves with high dryness fraction
(3) The turbines can be designed with drainage belt to drain the water droplet on the outer periphery.
(4) The leading edge of the turbine is provided with a shield of the hard material.
The most suitable method adopted is providing the tungsten shield. This improves the blade life.
Exam Question # 7. What are the advantages of steam turbine over gas turbine?
Ans. (1) The governing steam turbine is simple and easy. The throttle and cut-off governing can be used.
(2) The gas turbines the air-fuel ratio becomes too high causing problems of flame propagation.
(3) Steam turbine work on Rankine cycle and gas turbine uses Braytron cycle which is less efficient.
(4) In gas turbine, part load efficiency is very less because maximum cycle temperature is reduced.
(5) The material used for steam turbine blades are cheap, whereas gas turbine blades are costly.
(6) The steam turbine can recycle the steam to water, where as in open cycle turbines, the air-fuel are exhausted.
(7) Erosion of turbine blades is less in steam turbine.
(8) The rotor speed can be reduced by compounding of steam turbine.
Exam Question # 8. How does air pumps maintain vacuum in condenser?
Ans. The vacuum in the condenser is affected by the leakage of air into the vacuum space. Some of the air is carried away with the steam which was dissolved in the feed water. In jet condensers, air enters with the cooling water and this air is released when the cooling water is sprayed into the evacuated condensing when the cooling water is sprayed into the evacuated condensing chamber.
This air expands to a very large volume and the pressure in the condenser increases. The air thus accumulated in the condenser is removed continuously by means of an air pump. The vacuum is formed by the condensation of the steam whereby a large volume of steam is converted into a relatively small volume of water.
The air pump removes the air that continually enters the condenser. As certain power is required to operate the pump, it should be seen that the air entering the condenser should be reduced to a minimum.
The capacity of an air pump depends upon the temperature of the air or the mixture of air and water which it has to withdraw from the condenser. It also depends upon the quantity of air to be removed from the condenser.
Thus, it is desirable that air should be removed, at a point where the air has the lowest temperature and consequently the smallest volume. If the leakage of air is prevented the capacity of the pump required is reduced considerably.
In surface condensers, the condensate is to be used as a feed water and therefore the temperature of the condensate should be as high as possible while the temperature of air should be as low as possible and therefore compromise should be affected when wet pumps are used. When a dry pump is used, the air is cooled first by being exposed to the cooling water before the cooling water gets heated by the steam.
The main function of air pump is to maintain vacuum in the condenser such that the pressure in the condenser is as equal to the exhaust steam pressure. This can be done by removing the air from the condenser. The pump may remove condensate also.
There are two classification of air pump:
(1) Dry air pump- These are the pump which removes the dry air.
(2) Wet air pump- These are the pumps which removes the dry air and condensate.
Exam Question # 9. Describe the working of semi-closed cycle gas turbine.
Ans. Some gas turbine plants work on a combination of two cycles the closed cycle and the open cycle. Such a combination is called the semi-closed cycle.
Fig. 15-40 shows a scheme for such a plant without heat exchanger. A sufficient quantity of air is fed to the low pressure compressor which after compression passes through the precooler where its temperature is lowered.
Then it is further compressed in the high pressure compressor heated in the combustion chamber and expanded in the turbine through a common shaft, the remaining power being available for driving the generator or generation of electricity or taking up other external load.
Some of the exhaust gases of the turbine are bled away and further expanded in the turbine which drives the compressor through a common shaft, thus completing the open cycle. The remaining exhaust gases are cooled by the precooler as shown and recycled thus forming a closed cycle.
It may be noted that the oxygen in the working fluid is gradually replaced by carbon dioxide and a large percentage of water in the products of combustion gets condensed in the precooler; thus we get a working fluid which has a higher molecular weight and hence higher density than that of air. Thus for the same output a small machine is required.
But since the products of combustion mix up with the working fluid there is a serious problem of carbon deposits in the precooler tubes and the blades of compressor and turbines. Moreover, there is a possibility of a severe corrosion in the precooler because of the possible presence of sulphur in the fuel and the water vapour from the combustion.
Exam Question # 10. What are the different co-generation systems?
Ans. In the conventional power plants like steam turbine or gas turbine, the efficiency is only 35% and remaining 65% energy is lost. The co-generation is used to conserve energy which is likely to be lost. The regeneration of energy is possible by co-generation plant.
It is known as Combined Heat and Power (CHP) or total energy system. It offers energy savings ranging between 15% to 40% when compared against the supply of electricity and heat from the conventional systems.
There are two types of classifications of co-generation systems:
(1) Topping Cycle:
The topping cycle plant generates electricity or mechanical power first and the lost heat is used for regeneration.
The gas turbine topping cycle is used for co-generation as shown in fig. 15-45. A natural gas turbine drives a generator and the exhaust from the gas turbine goes to heat recovery boiler that makes process steam and process heat.
In the bottoming cycle the heat is generated first and then the exhaust from the boiler or furnace is send to gas turbine. The bottoming cycle is used in heavy industries where glass or metal manufacturing is done with very high temperature furnaces are used.
The fig. 15-46 below shows the bottoming cycle.
Exam Question # 11. How are boilers classified?
The boilers may be classified as below:
(i) Water-tube and Fire-tube
(ii) Forced circulation
(iii) Natural circulation
(iv) Externally fired and Internally fired
(v) Stationary, Portable, Locomotive and Marine, and
(vi) Horizontal, Inclined and Vertical.
The classification is described as below:
(i) Water-tube and fire-tube boilers are classified according to relative positions of water and hot gases.
In water-tube boilers there is a large number of comparatively small diameter tubes through which water circulates and hot gases surround these tubes. The principal boilers belonging to this type are Babcock and Wilcox, Stirling etc.
In fire-tube boilers water surrounds the tubes through which the hot gases from the furnace pass. There may be one or two large tubes at the front end of which the grates are arranged, as in Cornish or Lancashire boilers. The tubes may be smaller but more in number as in a Locomotive boiler which is known as a multi-tubular fire-tube type boiler.
(ii) In the forced circulation type of boilers the circulation of water in the boiler is carried out by definite mechanical means. They are used to increase circulation as in a La-Mont boiler, Velcox boiler, etc.
A good circulation of water is essential where rapid steam generation is required, because the heat energy of fuel is liberated at high temperature and is being transferred by direct conduction through, and radiation on to, the metal walls of the water spaces of the boiler.
This causes bubbles of steam to form on and cling to these walls. The bubbles are removed by the circulation of water which also distributes the heat throughout the bulk of the water. This type of boilers is of a very recent origin and they are used for high pressure steam with larger quantity of steam supply.
(iii) In natural circulation type of boilers circulation of water in the boiler takes place due to natural convection currents produced by the application of heat as in Lancashire, Locomotive, Babcock and Wilcox boilers, etc.
(iv) According to the position of the furnace, the boilers are classified as externally fired or internally fired. In externally fired boilers the furnace is placed outside the shell of the boiler as in Babcock and Wilcox boiler.
In internally fired boilers the position of the furnace, is inside the shell of the boiler as in a Lancashire boiler.
(v) Boilers are classified according to use as Stationary, Portable, Locomotive and Marine.
(vi) They are also classified as horizontal, inclined and vertical according to the direction of the principal axis of the boiler.
Boilers used for supplying steam to machines, which generate power, are called steam power boilers. They generate steam under pressure.
Most common types of natural circulation boilers used as steam power boilers are as follows:
Lancashire, Cochran, Locomotive, Scotch Marine, Babcock and Wilcox, Stirling, etc. The last two are water-tube type of boilers.
Exam Question # 12. What are the units of work and power?
Ans. The unit of work is the joule (J) and this is defined as the work done when a force of 1 newton is exerted through a distance of 1 metre in the direction of the force. The joule is, therefore, considered to be 1 newton metre (N-m).
It should be appreciated, however, that work is a form of energy and the joule, although defined as above, is the unit not only of work but also of all other forms of energy, including heat and electrical energy.
1 kilojoule (kj) = 1000 J = 103 j
1 megajoule (MJ) = 10,00,000 J = 106 J.
Power is defined as the rate of doing work. Thus, if W is the amount of work done in time t, then
Power = work done/time taken = W/t
The unit of power is the watt (W), which is defined as a rate of working 1 joule per second i.e., 1 = newton metre per second.
Thus 1 N-m/s = 1 J/s = 1 W.
Like the joule, the watt can also refer to other forms of energy. Since the joule is the unit not only of work but of energy in general, an alternative definition of the watt is a rate of transfer of energy of 1 joule per second. Thus the watt is also the unit of electrical power and of rate of heat transfer.
In practice the watt is found to be inconveniently small. Consequently the kilowatt (kW) is frequently used, the kilowatt being 1000 watts. For still larger powers the megawatt (MW) is used.
= 1000000 W = 106 W.
Similarly when we are dealing with large amounts of work (or energy), it is usually more convenient to express the work in kilowatt hours (kWh), where
1 kWh = 1000 watt hours
= 1000 x 3600 watt seconds or joules
= 3.6 x 106 J = 3.6 MJ.
The SI unit of torque is the newton metre (Nm). To prevent confusion with the millinewtons (mN), we quote for torque the unit of force first. Hence with SI units, a unit such as the “Nm” represents both work and torque. In order to distinguish between the units of these quantities it is preferable to express work (or energy) as J and torque as N-m.
Exam Question # 13. Explain thermodynamic state and thermodynamic process.
Ans. The properties of working substances depend on their state. The quantities describing the state of working substances are known as their parameters. The simplest parameters of state that can be measured directly are pressure P, absolute temperature T, temperature t, specific volume v and volume V. The parameters of state also include internal energy U, specific internal energy u, specific enthalpy h, enthalpy H, entropy ɸ and specific entropy s.
The thermodynamic state of a system or a working substance is defined by particular values of independent properties. Experiment shows that only two properties are necessary to completely define the state of a working substance. This is known as two property rule.
The defining properties will also indicate the physical form of a working substance i.e., phase. The various phases are solid, liquid or vapour/gas. When only one phase is present, the description is single-phased.
When two or more phases are present together, the description is mixed-phased. When a working substance exists in the mixed-phase form, the various phases are assumed to be in equilibrium i.e., there is no tendency to spontaneously change into one phase or another.
The sequence in which the states of a working substance change, resulting from its interaction (exchange of energy) with the surrounding system, is called the thermodynamic process.
There are equilibrium and non-equilibrium processes. Any process is performed under mechanical (in expansion or compression of the working body) or thermal (in heating or cooling of the working body) action on the body of the surroundings, whose state also changes.
The slower the change in the state of the surroundings, the slower will the process go on, the more will the temperatures and pressures of the surroundings and the body be equalized and the nearer will they be to the state of equilibrium at each moment of time.
At an infinitely small velocity, the process will be equilibrium because the surrounding medium and the working body will pass slowly from one state of equilibrium into another. In this way, the entire process may be represented as consecutive series of the states of equilibrium.
Actual (real) processes are non-equilibrium, since they occur at a finite velocity with a considerable difference in temperatures and pressures between the surroundings and the working body.
When an equilibrium process is completed in one direction, it can be made to proceed in the reverse direction through the same sequence of states of equilibrium. As a result, there will be no changes in the surroundings, and the working body will return to its initial state. For this reason, equilibrium processes are also known as reversible ones.
All actual processes are irreversible, for example – the combustion of fuel, the transfer of heat from hot bodies to cooler ones, the throttling of steam or gas, etc.
Thermodynamics mainly studies reversible processes. The study of irreversible processes occurring in heat engines is extremely complicated. Besides, the study of reversible processes makes it possible to find more advantageous conditions for carrying out actual processes in heat engines.
The results of studying reversible processes are applied to irreversible ones with the aid of experimental factors which take into account the influence of the differences between these two kinds of processes.
If processes are carried out on a substance such that, at the end, the substance is returned to its original state, then the substance is said to have been taken through a cycle. This is commonly required in many engines. A sequence of events takes place which must be repeated. In this way the engine continues to operate. Each repeated sequence of events is called a cycle.
The reversible cyclic processes form the foundation of the theoretical cycles of heat engines.
A comparison between the efficiency of real cycles of heat engines and the efficiency of theoretical cycles can serve as a measure of perfectness of the processes developing under real conditions.
Exam Question # 14. Write notes on impulse turbine, simple de-laval turbine, Parson’s reaction turbine.
The important components of an impulse turbine are the nozzles and blades. In nozzle the expansive property of the steam is utilized to produce a jet of steam moving with very high velocity. The function of the blades is to change the direction and hence momentum of the jet or jets of steam and so to produce a force which will rotate the blades.
Since the force is due to a change of momentum caused mainly by the change in the direction of flow, it becomes essential to draw velocity diagram showing how the velocity of the steam varies during its passage through the blades. The velocity diagram indicates velocities of steam at inlet and exit.
2. Simple De-Laval Turbine:
The De-Laval turbine was the first impulse turbine successfully built in 1889. This is the simplest turbine in form. It has single impulse wheel on which steam jets impinge from several nozzles arranged around the circumference.
The steam is expanded in nozzles which are inclined to the wheel tangent at an angle of about 20°. The smallest diameter and a speed of 30000 r.p.m. It is most suitable for low pressure steam supply. The blades are made symmetrical with angles of about 20° at inlet and outlet.
The horse power developed is about 500 kW and the blade speed is 200 m/sec. It has spherical bearings. Helical gearing is used to reduce the high rotational speed of the wheel to a practical value, without undue noise or friction losses.
In this turbine the steam enters the turbine through a double seated throttle valve, which is controlled by a governor driven from a worm gear on the main shaft, and passes in succession through the rings of fixed and moving blades until it reaches the end of the turbine cylinder and passes to the exhaust.
In passing through each ring of blades the steam drops in pressure and increases in volume. To allow for this increased volume and keep the velocity of steam uniform, the blade ring areas are increased in steps. The blade rings between one step and next form an expansion group, and all the blade rings of a particular group have the same external and internal radius.
In impulse turbines, the steam pressure on the back and front of a set of moving blades is the same and any thrust exerted by the steam in the direction of the rotor axis is negligible. In the reaction turbine this thrust is considerable owing to the fall of pressure within the blades and difference between the blade sizes in the various steps.
Dummy pistons and thrust bearings are used to balance this axial thrust. The face of dummy piston on the right is exposed to entering high pressure steam, while the face of dummy piston on the left is under steam pressure conveyed by pipe between the third and fourth expansions.
The back of the dummy piston on the left is under pressure conveyed by pipe between sixth and seventh expansions. The rotor is a steel forging, and the dummy pistons are solid with it.
It indicates roughly how the blade height increases a the specific volume of the steam increases with reduction in pressure; also how the pressure falls gradually as the steam passes through the groups of blades. It will be observed from the diagram that there is a pressure drop across each row of blades, fixed and moving.
This is of considerable practical importance specially at the high pressure end of the turbine where the pressure drop is greatest, because this difference of pressure tends to force some steam through the clearance space between the moving blades and casing, and between the fixed blades and the rotor. The available energy possessed by this leaking steam is partly lost.
Exam Question # 15. What are the types of pulverized fuel burners?
Ans. The efficient utilization of pulverized coal depends to a large extent upon the ability of the burners to produce uniform mixing of coal and air and turbulence within the furnace. The air which carries the pulverized fuel in the furnace through the burner with primary air and remaining secondary air is required for complete combustion is admitted separately around the burner or elsewhere in the furnace.
The pulverized coal burners should satisfy the following requirements:
(1) It should mix the coal and primary air thoroughly and project the same in the furnace properly with secondary air which is generally added around the burner.
(2) It should create proper turbulence and maintain stable combustion of coal and air throughout the operating range of the plant.
(3) It should control the flame shape and its travel in the furnace. This is generally done by the secondary air vanes and other control adjustments provided in the burner.
(4) The mixture of coal and air should move away from the burner at a rate more than or equal to the speed of flame travel to avoid the flash back with the burner.
(5) The burner should also be provided with adequate protection against overheating, internal fires and excessive abrasive wear.
The factors which affect the performance of the pulverized fuel burner are, the characteristics of the fuel used, fineness of the powdered coal, volatile matter, the geometry of the burner, place of mixing the fuel and air, proportions of primary and secondary airs, furnace design and patterns of load changes.
The classification of burners is made on the rapidity of burning the coal and air in the furnace.
Tertiary air is supplied around the burner to form an envelope around the primary air and fuel to provide better mixing. The burner discharges air and fuel mixture vertically in thin streams with practically no turbulence and produces a long flame. Heated secondary air is introduced at right angles to the flame which provides necessary mixing for better and rapid combustion.
The furnace for low volatile coal is equipped with such burners to give a long flame path for slower burning coal particles. The longer path provides more time to burn and it is necessary to control the velocity in this zone. Less heat of ignition is available due to low volatile contents and it is necessary to reduce the cooling effect from the wall tubes in the ignition zone by using the refractory belt round the furnace or by a refractory front wall.
Generally low volatile coals have higher fusion temperature than bituminous coal and therefore higher fusion temperature than bituminous coal and therefore higher furnace rating are permissible.
The turbulent burners are usually set into furnace walls and project the flame horizontally into the furnace. The fuel air mixture and secondary hot air are arranged to pass through the burner in such a way that there is good mixing and the mixture is projected in highly turbulent form in the furnace.
The mixture burns intensively and combustion is completed in the short distance. This burner gives high rate of combustion compared with other types. The velocity at the burner tip is as high as 50 m/Sec. The bituminous coal is successfully used with this burner. By proper adjustments a long penetrating flame or short intensely hot flame can be produced. All modern plants use this type of burner. This is generally referred for high volatile coals.
The tangential burners set in furnace discharge the fuel air mixture tangentially to an imaginary circle in the center of furnace. The swirling action produces sufficient turbulence in the furnace to complete the combustion in short period and avoids the necessity of producing high turbulence at the burner itself. High heat release rates are possible with this method of firing.
This type of burner is sometimes constructed with tips that can be angled through a small vertical arc so as to raise or lower the position of the turbulent combustion region in the furnace. This arrangement controls the temperature of the gases at the furnace aperture and maintains constant superheat temperature of the steam as the load varies.
When the burners are tilted downward, the furnace gets filled completely with the flame and the furnace exhaust gas temperature is reduced as the furnace absorption is greater. This reduces the heat given to the superheater. The reverse is true when the burners are tilted upwards. The usual limit of tilt ± 30° is sufficient to provide 100°C difference in the furnace gas exit temperature.
Such burners promote quick ignition but it is a characteristic of pulverized fuel firing that it is extremely difficult to influence the tail of combustion. This cannot be done by burner agitation or by introducing secondary air at point further along in the path of the burning dust.