Compilation of interview questions on thermal engineering for students.
Interview Question # 1. Define engineering thermodynamics.
Ans. It is the science which analyses the problems related to the conversion of heat to work and vice-versa and changes in properties of the working fluids involved in such a conversion.
The thermodynamic study deals with a concept of ideal processes and most possible efficient operation of machine. This science establishes the fact that to what extent a particular process provides efficiency to the ideal conditions.
The study analyses thermal system and properties of the system and establishes a relationship between the various properties and parameter involved in system. It may be possible to know maximum work and efficiency of the system.
Interview Question # 2. What are the areas of thermal engineering?
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Ans. There are various areas of thermal engineering which are based on the study of engineering thermo-dynamics. The Engineering thermodynamics considers various thermal processes ideally possible and results of these processes are hypothetical, which are results to indicate scientifically how conversion processes are possible.
The thermal engineering is divided into various areas like:
(1) Heat Engines:
These are the devices in which heat energy is converted into mechanical energy by performing series of thermodynamic processes. The heat engines convert heat energy into useful mechanical work.
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These are various types of heat engines such as:
(1) Steam engine
(2) Steam turbine
(3) I.C. engine, and
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(4) Gas turbines.
(2) Energy Sources:
These are various sources of energy.
They are divided into two categories:
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(1) Conventional energy source
(2) Unconventional energy sources.
The conventional energy sources are fuels such as solid, liquid and gaseous fuels. The hydraulic energy is conventional energy.
The unconventional energy sources are:
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(1) Solar energy
(2) Wind energy
(3) Tidal energy
(4) MHD
(3) Refrigeration and Air-Conditioning:
These low temperature applications in which the system is brought to the temperature below atmospheric temperature. The system where the temperature is brought below the normal temperature.
The systems are known as:
(1) Refrigeration system
(2) Air-conditioning system.
In the refrigeration system, the temperature is brought below 0°C and article, foods, items, medices etc. can be maintained below atmospheric temperature.
In the air-conditioning system, the temperature of system brought to human comfort level and maintaining the system with proper humidity, odour etc. The system conditioned to human acceptable level.
This is study of transfer heat from one place to another.
The heat is transferred from one body to another using various modes like:
(1) Conduction
(2) Convection
(3) Radiation.
The transfer mass due combustion or diffusion or interaction is known mass transfer. The mass can be changing its phase in mass transfer process.
(5) Fuel and Combustion System:
This is area of thermal engineering in which the study of properties and combustion characteristics of various fuels is carried out. The combustion systems and methods are studied and analysed.
The compressors are the devices used for compression of air and fluids. The fluid is compressed to high pressure and temperature. This involves suction of fluid and increasing the pressure in return reducing the volume of fluid.
(7) Cryogenic:
This is the science dealing with behaviour of fluids at very low temperatures.
(8) Jet Propulsion:
This is the propulsion of jet resulting into movement object at high attitudes.
Interview Question # 3. Explain the intensive and extensive properties of matter?
Ans. Temperature, volume, internal energy, enthalpy, etc., which introduce useful concept of intensive and extensive properties in the study of thermodynamics.
An intensive property is that property which is not related to the mass of the matter with which it is associated. An extensive property is a property that is directly proportional to the mass of the matter with which it is associated.
If the working substance in a given condition is divided into two equal parts by mass, the values of the extensive properties will become half of the original values and the values of the intensive properties will be unchanged.
Pressure and temperature are examples of intensive properties while volume, enthalpy and internal energy are examples of extensive properties.
All specific properties are intensive properties as they are related to unit mass.
Interview Question # 4. Define perfect gas.
Ans. A perfect gas may be considered as the gas that obeys the laws of Boyle and Charles and the characteristic equation of a gas which is obtained by combining the above laws.
There is no gas which is perfect, but many gases can approach this standard within the temperature limits of applied thermodynamics. Such substances are Oxygen, Nitrogen, Hydrogen, Air, etc., may be regarded as perfect gases. They are known as real gases.
Interview Question # 5. Define vapour.
Ans. It may be considered as a fluid in its gaseous state, but at a temperature not far from its boiling point. It may contain liquid particles in suspension. When cooled or expanded in an engine, it readily condenses into its liquid state unlike a gas which remains in its gaseous state except under extreme conditions of temperature and pressure. The vapours do not obey laws of Boyle and Charles, though they may approximate under certain conditions. The behaviour of wet vapours cannot be determined by these laws.
Interview Question # 6. What is the effect of pressure on the boiling point of water?
Ans. When water, in a vessel open to atmosphere, is heated, the temperature of the water will increase until some of the water attains the temperature 100°C. Bubbles of steam will rise from the hottest parts of the vessel in contact with water.
These bubbles will rapidly bring up the temperature of the rest of the water at 100°C and are themselves being completely or partly condensed in the heat exchange involved. When the entire mass of water has attained the temperature of 100°C, the bubbles will rise to the surface and steam will be disengaged.
If the vessel is entirely closed except for a small steam outlet, the steam formed will occupy the space and drive away the air outside of the vessel leaving only water below and steam above. The temperature and pressure of both water and steam will be 100°C and 1.01325 bar absolute respectively. If the heat is supplied so rapidly that the rate of steam generation is faster than the rate of steam escape, the steam will accumulate in the vessel and the pressure will rise and hence, the temperature of water will rise.
When the pressure in the vessel reaches 2 bars, the temperature of water will be 120.2°C. At 4 bar steam pressure, the temperature of water will be 143.6°C. Thus, we notice that for each pressure there is a definite temperature at which water boils. The law connecting the pressure and temperature known respectively as the saturation pressure and saturation temperature of boiling water is called the pressure temperature relation of the boiling point.
Thus, we see that the temperature at which water boils depends upon the pressure in the vessel. For every pressure there is a definite temperature at which steam will begin to form. Water has got an infinite number of boiling points depending upon the pressure.
Interview Question # 7. State Boyle’s law.
Ans. This law was discovered by Robert Boyle in 1662 A.D. and it can be stated as follows:
The volume of a given mass of a perfect gas varies inversely as the absolute pressure when the temperature is constant.
Let P be the absolute pressure of the gas and V be the volume occupied by the gas at the above pressure. Refer fig. 2-1.
The above law states that V α 1/P, when temperature is constant.
∴ V = C/P where C is constant of proportionality
∴ PV = C.
This shows that the product of absolute pressure and volume of a given quantity of gas is constant, when the temperature does not change. This law was also discovered by Edmonde Mariotte, a French Physicist, in 1676,
If a certain gas is made to change its state from 1 to 2 without a change in temperature, then for the initial condition of the gas, we get P1V1 = C constant, by Boyle’s law. Similarly for the final condition of the gas, we get P2 V2 = C.
From the two relations, we get P1V1 = P2V2.
All ‘real’ gases obey this law very nearly and a perfect gas, by definition, obeys it exactly.
Interview Question # 8. What are the functions of a boiler?
Ans. The function of a steam boiler is to transfer heat generated by burning of fuel to water and thus to produce steam. The construction and appearance of a boiler depend upon the arrangements made for burning the fuel and effecting the transfer of heat. It is a heat exchanger used for converting water to steam by heating.
A boiler may be used for supplying:
(i) Steam to a steam engine or a steam turbine
(ii) Steam for industrial process work
(iii) Steam for heating installation.
Interview Question # 9. What are the advantages of high pressure boilers?
Ans. The major advantages of high pressure boilers are as under:
(i) There is a greater freedom for disposing of the heating surfaces and hence greater evaporation for a given size.
(ii) There is a reduction in the number of drums required.
(iii) It has table smaller diameter and therefore lighter tubes.
(iv) It is lighter for a given output
(v) The boiler can meet rapid changes of load without the use of complicated, or delicate control devices.
(vi) Using external supply of power, a very rapid start from cold is possible.
(vii) The tendency of scale formation is eliminated due to high velocity of water through the tubes.
(viii) Due to uniform heating of all parts, the danger of overheating, is reduced and thermal stress problem is simplified.
These boilers have disadvantages, that:
(i) The high cost of the pumping equipment,
(ii) The power required to run the pumps and
(iii) Safety of the boiler are to be kept in mind.
Interview Question # 10. What are the advantages and disadvantages of scotch boilers?
Ans. The following are the advantages and disadvantages of Scotch boilers:
Advantages:
(i) Compactness
(ii) Minimum amount of brickwork
(iii) Required head room is low
(iv) As the tubes are all of the same size, only one spare tube is necessary.
Disadvantages:
(i) Because of the large diameter, circulation difficulties are experienced while starting up.
(ii) As the internal furnaces are of fixed dimension it is not possible to meet the requirement of fuel changes without building a brick extension.
(iii) There is a practical limit for capacity and pressure.
(iv) Efficiency is less in comparison with the water tube boilers.
(v) Cleaning and inspection of boilers is difficult.
Interview Question # 11. What are the working fluids in thermodynamic system?
Ans. It is a fluid on which the thermodynamic processes are performed. The gases, vapours (steam) and liquids are employed as working fluids. There is no essential difference between a gas and a vapour.
A gas can be considered as the vapour of a corresponding liquid in a state far from liquefaction (heavily superheated steam), while steam is a real gas approaching the state of liquefaction.
We usually distinguish between saturated and superheated steam. Saturated steam is in dynamic balance with its liquid. According to its state, steam is referred to as dry saturated and wet saturated steam, which is a mixture of steam and liquid. When heat is added to dry saturated steam, its temperature rises and the steam becomes superheated.
The effect of the intermolecular forces and molecular size on the physical properties of the gas varies depending on its state. When gas is in a state in which the intermolecular forces and the volume of the molecules can be neglected (heavily superheated steam at a low pressure), the gas is called ideal gas, otherwise it is referred to as real gas. Therefore, one and the same gas may be assumed to be an ideal or real under different conditions.
A system in which a working substance undergoes thermodynamic processes and the properties of the substance are changed and energy transfer occurs is called to be a thermodynamic system.
Interview Question # 12. Define energy.
Ans. Energy is defined as the capacity for doing work. Energy can exist in various forms such as mechanical energy, heat energy, electrical energy and chemical energy. In all forms the unit of energy in the SI system is the joule. Engineers often use unit of energy as kilowatt hour. Kilowatt hour is the energy expended in one hour when power is one kilowatt. This unit is known as kWh, which is adopted for the sale of electrical energy.
1 kWh = 3.6 MJ.
Interview Question # 13. Define work.
Ans. If a system exists in which a force at the boundary of the system is moved through a distance, then work is done by or on the system. As soon as the force ceases to be moved then any work which was being done also ceases.
Work is, therefore, a transient quantity, being descriptive of that process by which a force is moved through a distance. Work, being a transient quantity, is therefore not a property. Work results due to pressure difference. Work is given the symbol W, and its unit is newton metre or joule.
Interview Question # 14. What is erosion in gas turbine?
Ans. In the gas turbine the gases enter the turbine at a temperature of 900°C to 1000°C and leave at 400°C to 500°C. Since the blades of the gas turbine operates under the conditions of high pressure and temperature. The hot gases supplied are not clean and contains particulates which would result into erosion of blades.
The contaminant in the gas turbine must be less in order to avoid corrosion of blades. The high premium fuels can be used such as natural gas. The liquid fuels such as gas oils are suitable. The LPG or CNG are suitable gaseous fuels.
Interview Question # 15. Define volume.
Ans. Volume is a property, being that property which is associated with cubic measure. The unit of volume is the cubic metre (m3) together with its multiples and sub-multiples. Sometimes litre may be used. 1 litre = 10-3 m3.
Although in the SI units the litre is a non-preferred unit, it is already in such common usage that its use will, no doubt, continue.
If the volume of a substance increases then the substance is said to have been expanded. If the volume of a substance decreases then the substance is said to have been compressed.
Specific volume is given the symbol v. The volume of any mass, other than unity, is given the symbol V.
Interview Question # 16. What are the functions of a chimney?
Ans. The chimney in a boiler installation has got one or more of the following functions:
(i) It produces the draught whereby the air and gas are forced through the fuel bed, furnace, boiler passes and settings. The air which carries the oxygen necessary for the proper combustion of the fuel is thereby furnished to the fuel bed.
(ii) It carries the products of combustion to such a height before discharging them that they will not be objectionable or injurious to the surroundings.
A chimney may be built either of masonry, steel or concrete. Strictly speaking the term chimney relates to a masonry structure while stack refers to a metallic one.
Interview Question # 17. Explain the central flow type of condenser?
Ans. In this type of the condenser, the suction pipe of the air extraction pump is located in the centre of the tubes which is responsible for the radial flow of steam. In this condenser better contact between the outer surface of the tubes and the steam is ensured. The pressure drop is reduced due to large passages.
Interview Question # 18. Define condenser efficiency?
Ans. There is no commonly accepted definition of condenser efficiency but a method adopted by Messrs. C. A. Parson & Co. has been widely used in Engineering practice.
Condenser efficiency = temperature rise of cooling water/vacuum temperature – inlet cooling water temperature
Interview Question # 19. What are the applications of I.C. engine?
Ans. The reciprocating internal combustion engines are used mainly in the automobile or vehicles, marine application such as ships, tractors, portable generating sets etc. whereas the rotary internal combustion engines are used for aeroplanes, power plants.
Interview Question # 20. What is the working medium of closed cycle turbine?
Ans. The working medium in case of closed cycle turbine can be air as used in the open cycle turbine. The air has been mostly used as the working fluid, but it is also possible to use some other gases. The gas used in the closed circuit system should have a high adiabatic index (ϒ), i.e., the ratio of the specific heat at constant pressure to the specific heat at constant volume.
The higher the value of this index, the lower will be the pressure ratio required for a compressor and turbine designed to give maximum efficiency at chosen values of the maximum and minimum temperatures in the system. The values of ϒ for mono, di and triatomic gases are 1.66, 1.4 and 1.3 respectively.
Thus a monatomic gas should preferably act as working fluid in the closed cycle. The density is also responsible for the selection. The higher density of the gas, the smaller will be the dimensions of the flow passages and hence smaller size of the unit.
The monoatomic gases suggested are argon, krypton and xenon having their relative densities as 1.38, 2.87 and 4.53 with respect to air. The use of carbon dioxide which pass through a triatomic gas has a density of 1.52 times that of air at the same temperature and pressure.
The specific heat of helium at constant pressure is about five times that of air. Therefore for each kg mass flow, the heat drop and hence energy dealt with in helium machines is nearly five times of those in case of air. For an equal percentage of loss of energy with the turbine work, the velocity of helium corresponds to 2.2 times that of air.
The surface area of the heat exchanger for helium can be kept as low as 1/3 of that required for the gas turbine plant using air as the working medium. If the temperature ratio is same and for same output the cross-sectional area required for helium is much less than that for air. Therefore, the size of the helium unit is considerably small.
Interview Question # 21. What are the advantages of closed cycle gas turbines?
Ans. The closed cycle gas turbine provides large number of advantages as below:
(1) The heating is done externally, i.e. the products of combustion do not mix with the working fluid.
(2) Since there is no mixing of fuel with air, any fuel of high calorific value may be used.
(3) There is no corrosion and accumulation of deposits of carbon or solids on the blades and nozzles of the turbine.
(4) There is no necessity of internal cleaning and regular maintenance.
(5) The air is precooled in the precooler before entering the compressor. Consequently the specific volume of air decreases. Also the pressure at the inlet to the compressor can be kept well above the atmospheric.
(6) Much higher pressure than the atmospheric can be maintained around the whole cycle. Hence for a given output the size of the compressor and the turbine are very small.
(7) Because the cycle works similar to reversible Ericsson cycle, it has a higher thermal efficiency.
(8) By making an inert gas like helium as the working medium, if can be made for materials in the turbine design, like molybdenum alloys.
(9) The waste heat of the combustion gases from the heater and reheaters can be further used up for heating water and which together with the hot water from the precooler and intercoolers can be utilised for hot water supply for industrial or domestic purposes.
(10) Because the air at a high pressure, has a higher heat transfer coefficient smaller heating surface is required in the intercoolers, precooler and the heaters.
Interview Question # 22. What are the advantages of gas turbine over steam turbine?
Ans. There are number of advantages of gas turbine over steam turbine such as:
(1) The gas turbine does not require any boiler. Hence the weight and space of gas turbine are less than those of a steam turbine. A gas turbine is more compact than a steam turbine of the same output and hence gives lower capital costs.
(2) It is quite useful in the regions, where due to scarcity it is not possible to supply water in abundance for raising steam.
(3) The gas turbine has been built to operate at the inlet temperature of 800°C and even more, while the steam turbine and boiler have been built for temperatures upto about 580°C. Now other things being equal in both the cases, higher the inlet temperature provides higher efficiency. Therefore, the efficiency of the gas turbine is much higher than that of a steam turbine.
(4) There is no likelihood of freezing in a gas turbine plant on winter nights as unlike in a steam turbine plant.
(5) The gas turbine plant is simple in design and construction. It is lighter in weight and there are few reciprocating parts.
Interview Question # 23. State the difference between steam engine and steam turbine.
Ans. The steam engine is a device used for developing mechanical power from high pressure steam. The steam turbine is similar to steam engine but it is a rotary machine.
In the reciprocating steam engine, reciprocating to and fro motion is imparted to the engine piston by the pressure of the steam and this reciprocating motion of piston is converted into rotary motion at the crankshaft through the medium of the crosshead, connecting rod and crank shaft.
The steam engines are found to be heavy and bulky. They operated at slow speed and full energy of steam is not utilized because of partial expansion of steam. These are intermittent in operation.
In the steam turbine, rotary motion is imparted directly to the shaft by means of high velocity steam jets striking the blades fixed on the rim of a wheel which is fixed to the shaft. The turbine is much simpler in mechanical construction, and it utilizes the kinetic or velocity energy of the steam instead of pressure energy.
The steam turbines allow expansion of the steam to the lowest exhaust pressure of the condenser pressure. They are steady flow machines and uses no valves during operation. Steam turbines may expand steam to 25 mm of mercury absolute pressure or less.
Interview Question # 24. What are the advantages of steam turbine over steam engine?
Ans. The main advantages of steam turbine over the reciprocating steam engine are as follows:
(i) These are high speed machines as compared reciprocating steam engine.
(ii) They are smaller is size
(iii) Requires less floor space
(iv) Steam the turbine is a rotary machine, perfect balancing is possible.
(v) Foundation of the turbine is lighter and smaller.
(vi) The ability of turbine to use high pressure and superheated steam and uniflow direction of steam flow through the turbine.
(vii) The working of the turbine is much smoother than that of the steam engine. The speed of rotation is uniform. The torque produced by the turbine is uniform and there is practically no vibration.
(viii) As no internal lubrication is needed, highly superheated steam can be used and exhaust steam contains no lubricating oil.
Interview Question # 25. How are steam turbines classified?
Ans. The classification of steam turbines can be made according to their:
(i) Position of Shaft:
This according to the position of shaft axis, which are horizontal or vertical.
(ii) Nature of Steam Supply:
This according to their nature of steam supply and use of steam they are high-pressure or low-pressure, and bleeder or extraction.
(iii) Direction of Steam Flow:
This according to the direction of steam flow, they are axial, radial, tangential, single-flow or double-flow.
(iv) Construction and Arrangement of Blades and Wheels:
This according to their construction and arrangement of blades and wheels, they are pressure compounded or velocity compounded.
(v) Number of Stages:
This according to their construction and arrangement of blades and wheels, they are pressure compounded or velocity compounded.
Interview Question # 26. What are the types of steam turbines?
Ans. Steam turbines are of three main types according to the working principles.
They are:
(1) Impulse Turbines:
In these turbines the power is generated by impulse of high velocity steam jets. These are the turbines in which complete process of expansion of steam occurs in stationary nozzles and the velocity energy is converted into mechanical work in the turbine blades.
The high velocity steam jets are obtained by expansion of the steam in the stationary nozzles and the steam passes at high velocity through the moving blades with no drop in pressure but with a gradual reduction in velocity.
In purely impulse turbines the rotary motion of the shaft is obtained by having high velocity jets of steam directed against the blades attached to the rim of the turbine wheel or rotor.
(2) Reaction Turbine:
A pure reaction turbine the drop of pressure with expansion and generation of kinetic energy takes place in the moving blades. The steam jets can leave the moving blades at greater velocities than those at which they enter these blades.
The passages through the moving blades are made convergent so that the steam expands while passing through them, which causes the steam to leave the blades at higher velocity. The power is generated by reaction of steam passing over the moving blades.
(3) Impulse-Reaction Turbine:
The modern reaction turbine uses both the impulse and reaction principles. The pressure drop is effected partly in the fixed guide blades which are designed to work as nozzles and partly in the moving blades which are designed that the expansion of the steam takes place in them.
The high velocity of issuing jet from the fixed guide blades produces an impulse on the moving blades and the jet coming out at still higher velocity from the moving blades produces a reaction. Therefore, part of the work is due to impulse and the remainder due to the reaction.
A very good example of reaction turbine is a Parson’s turbine. In a reaction turbine, the stationary, blades and the moving blades are virtually convergent nozzles so that the steam passing through them has a fall in pressure.
The speed of the moving blades is kept the same as the velocity of the steam that enters the blades. This ensures that the steam will flow into the blades without striking them.
Interview Question # 27. State the law of conservation of energy.
Ans. The law of conservation of energy states that “Energy cannot be created or destroyed”. The total energy in existence must, therefore, be constant. It is possible, however, to change from one energy form to another.
For any system exchanging energy with its surrounding, the following statement called an energy balance can be made by applying the conservation principle:
Energy entering the system + initial energy of the system = Energy leaving the system + final energy of the system.
The main source of energy in the world is heat transfer obtained by the combustion of the fuel. It is, therefore, necessary when applying the law of conservation of energy to consider both heat and work transfers.
Interview Question # 28. What is turbo-charging?
Ans. The supercharging is the process of supplying of air at high pressure to the internal combustion and turbo-charging is the process of supplying the supercharged air using compressor to the internal combustion engine. The compressor is driven by the gas turbine which operates on the exhaust gases coming out from the I. C. engines.
Interview Question # 29. What is a combined cycle power plant?
Ans. These plants are good choice to produce the energy because of high efficiency and use of low carbon content fuels such as natural gas, which reduces the greenhouse gases. The combined cycle power plants coupled with a Braytron cycle and Rankine cycle. It results into optimization of heat recovery from the gas turbine exhaust gas to maximize the production in the steam cycle.
Interview Question # 30. What are the assumptions in thermodynamic cycles?
Ans. The assumptions in the thermodynamic cycle are:
(1) It is assumed that the heat is rejected and added to the thermodynamic medium without any temperature difference between the heat source or heats sink and thermodynamic medium.
(2) It is considered that the expansion and the compression process are reversible isentropic processes. It means that no loss of heat is permitted during these processes i.e. the system is completely isolated. Since the process is reversible, it means that these are frictionless isentropic processes where the friction in reciprocating components is negligible.