UNIT III POWER PLANT ENGINEERING

1. What are the different sources of energy?

Hydro power, nuclear energy, fossil fuels, wind energy, solar energy, tidal power, and geo thermal energy.

2. Name four non-renewable sources of energy.

Liquid fuels like petrol, solid fuels like coal, gaseous fuels like natural gas and nuclear fuels like uranium, thorium.

3. Name some renewable sources of energy.

Renewable sources of energy are also called as alternative sources. They are solar energy, hydal energy, tidal energy, wind energy and geo thermal energy.

4. What is the difference between renewable and non- renewable energy sources of

energy?

Renewable sources of energy are not consumed and inexhaustible, examples are solar energy and wind energy. Non-renewable sources of energy are either consumed or converted into other forms. Examples are coal and petrol.

5. Give two applications of wind energy.

� Wind can be used to run the power plant to generate electricity � To pump water.

6. Name four solid fuels.

� Coke, Coal, Wood, and Peat.

7. Name four liquid fuels.

� Petrol, Diesel, Kerosene, and Furnace oil.

8. Name four gaseous fuels.

� Natural gas, producer gas, flue gas, blast furnace gas.

9. Name two nuclear fuels.

� Uranium and plutonium.

10. Name four nuclear reactor sites in India.

� Narora – Uttar Pradesh � Kalpakkam – Tamilnadu (Near Chennai) � Kakrapur – Gujarat (Near Surat)

11. What are the disadvantages of wind energy?

� Occupies large area � Noisy operation � Initial investment is high

12. Give two applications of nuclear energy.

� Nuclear power plants to generate electricity � Nuclear missiles (war heads)

13. Is coal as a fossil fuel?

� Yes, coal is a fossil fuel.

14. What is the other name for nuclear energy?

� The other name for nuclear for nuclear is atomic energy.

15. Define power plant.

� Power plant is a machine or assemblage of equipments that produces and delivers a flow of electrical energy.

16. What are the different types of power plants?

� Steam power plant � Hydel power plant � Nuclear power plant � Gas turbine power plant � Tidal power plant

17. Give the function of condenser in steam power plant.

� Condenser is used to condense the steam from the turbine and again fed to the boiler.

18. What is the function of moderator in a nuclear power plant?

� Moderator is used to slow down the fast moving electrons in the nuclear reactor,

which are produced during nuclear fission.

19. Give important factors to be considered for selecting hydroelectric power plant.

o Availability of water o Water storage o Water head o Distance form load centre.

20. Mention the reason for preferring steam power plant to other power plants.

� Less space required. � Initial cost is low. � Respond to changing load. � Continuous power generation.

21. What is a cooling tower? Give its uses.

� Cooling tower is an arrangement where cold water is sprayed to cool the water

coming out of the condenser.

22. What are the nuclear fuels used in the nuclear reactor?

� Uranium235, Uranium238, plutonium.

23. What is the function of penstock pipe?

� Penstock is closed pipe made of steel or concrete used for supplying water from surge tank to the turbine.

24. What is mean by nuclear fission?

� Nuclear fission is the process of splitting the nucleus of fissionable material like uranium into two or more fragments with release of enormous amount of energy.

25. What is the function of intercooler in gas turbine power plant?

Inter cooler is used to reduce the work of the compressor and increase the efficiency.

26. Name the different components of a gas turbine power plant.

Low pressure compressor, High pressure compressor, Intercooler, regenerator, Combustion chamber, low pressure turbine and high pressure turbine.

27. Briefly explain what radiation shielding means?

� A thermal shield is provided through steel lining and another shield of thick

concrete is also provided around the nuclear reactor. This is called radiation shielding and is used to protect the reactor. This is called radiation shielding and is used to protect the reactor against the harmful rays and fast neutrons.

28. What are the different types of hydro power plants?

� High head hydro power plants. � Medium head hydro power plants. � Low head hydro power plants.

29. State the disadvantages of steam power plant. � Erection requires long time. � Less efficiency. � Transportation of fuel is a problem.

30.Mention the application of gas turbine power plants.

� Petrochemical industries. � Used in air craft and ships. � Standby unit. �

31. Define steam turbine. � A steam turbine is prime mover which converts heat energy in the steam into

mechanical work.

32. State the main parts of a steam turbine.

o Nozzle o Rotor o Rotor blades

33. How steam turbines are classified?

� Impulse turbine � Reaction turbine

34. Give an example for reaction turbine.

� Parson’s turbine.

35. State the limitations of an impulse turbine.

� Suitable for low pressure steam � Out let velocity of steam.

36. What is the difference between impulse and reaction turbine?

� In a impulse turbine, the steam coming out of the turbine has only kinetic

energy, where as in reaction turbine, the steam out of the turbine has both pressure and kinetic energy.

37. Relative velocity of steam increases in reaction turbine. Give reason.

� The relative velocity of steam increases in moving blades of the reaction turbine due to the continuous expansion of steam.

38. What are the advantages of centrifugal pump?

o The cost of a centrifugal pump is less as it has fewer parts. o Installation and maintenance are easier and cheaper. o It is compact and as smaller size and weight.

39. What is meant by centrifugal pump? If the mechanical energy is converted, into pressure energy by means of centrifugal

force acting on the fluid, the hydraulic machine is called centrifugal pump.

40. What is meant by reciprocation pump?

If the mechanical energy is converted into hydraulic energy (or pressure energy) by sucking the liquid into a cylinder in which a piston is reciprocating (moving backward and forwards), which exert the thrust on the liquid and increases its hydraulic energy (pressure energy), the pump is called reciprocating pump.

41. What is meant by multistage pump?

To get the high head, number of impeller are connected in series on the same shaft is

called multistage pump.

42. What is meant by priming? Before starting the pump, air from suction pipe, casing and portion of delivery pipe

up to the valve is replaced by water. The operation of replacing the air with water in the pump is called priming.

43. Difference between single acting reciprocating pump and double acting

reciprocating pump? S.No Single acting Reciprocating

Pump Double acting Reciprocating Pump

1. It has one suction pipe and one delivery pipe.

It has two suction pipe and two delivery pipe.

2. One complete revolution of the crank there are one delivery stroke.

One complete revolution of the crank there are two delivery stroke.

44. Difference between Centrifugal Pump and Reciprocating Pump?

S.no Centrifugal Pump Reciprocating Pump 1. The discharge is continuous and

smooth The discharge is not continuous. It is fluctuating and pulsating.

2. It can be used for lifting highly viscous liquids.

It is meant for small discharge and high heads.

3. It can handle large quantity of liquid.

It handles small quantity of liquid only.

45.Name different types of conventional and non conventional power plants.

CONVENTIONAL NON CONVENTIONAL

Or Renewable or Primary or Non – Renewable or Secondary

Steam hydro electric

Diesel solar

Nuclear tidal

Geothermal

46. What is minimum wind speed and minimum tide height required for power

generation?

� Minimum wind speed: 18kmph

� Minimum tide height: 5m

47. Name the prime mover in steam power, hydro power plant.

� Steam power plant: steam turbine

� Hydro power plant: hydraulic turbine

48. What is water head and head race?

� Water head: Difference between head race level and tail race level.

� Head race: the level of water stored in the reservoir.

49. What are the classifications of turbine?

� Pelton wheel=>high head=> >70m

� Francis=>medium head=>15-70m

� Kaplan=>low head=> <15m

50. What is the purpose of surge tank?

� It helps to prevent the damage to penstock by water hammer.

51. Define water hammer.

� The pressure in the penstock is known as water hammer.

52. What is the function of moderator and indicate the materials?

� It is used to reduce the speed of fast moving neutrons.

� Materials: heavy water, graphite, beryllium.

53. What is the use of intercooler in gas turbine power plant?

� It is located in between L.p compressor and H.p compressor and it is used to

reduce the load of compressor.

54. What are ebb tides and flood tides?

� When the level of water is above mean sea level=>ebb tide.

� When the level of water is below sea level =>flood tide.

55. What is the use of turbine?

Turbine is a rotary engine that extracts energy from a working flood such as

steam, gas and water. a working flood possesses potential and kinetic energy into

mechanical energy.

56. What are the different types of turbine and compare them?

IMPULSE REACTION

It consists of separate fixed it consists of fixed blades and

Nozzles and rotating blades moving blades both acting as nozzles

Steam expands fully in nozzle it expands partly in fixed blades

Overall efficiency is low high

Very much suitable for small high power generation

Power generation

57. What is the use of pump and classify its type?

� It is used to rise or transfer fluids.

Application:

� Draining, sewage, irrigation, chemical industries

Positive displacement pump:

� Here fluid is forced (inlet) into finite space and then it is sealed by

mechanical means. Then the fluid is forced out (discharged).

� E.g.: reciprocating

Roto dynamic:

� Here free passage between the inlet and outlet without sealing.

� E.g.: centrifugal pump

58. What is the difference between centrifugal and reciprocating pump?

� CENTRIFUGAL PUMP: low head and high discharge

� RECIPROCATING PUMP: high head and low discharge

59. What is mean by multistage pump?

� It contains two or more impellers. it helps to develop high head and high

discharge.

60. What are advantages of centrifugal pump?

� It is reliable.

� It runs smoothly.

� They are easy to install.

� It is compact in size.

61. What are the classifications of centrifugal pumps?

� Radial flow.

� Mixed flow.

� Axial flow.

62. What is the difference between single acting and double acting reciprocating

pump?

� In single acting one inlet and one outlet and in double acting there are two

inlets and two outlets. Therefore the discharge is high in double acting

reciprocating pump.

63. Name the Gas, Diesel, Hydel, Thermal and Nuclear power plants in Tamilnadu?

Gas Power Plant

� Perungulam (Natural gas),

� Kovikalappal (oil),

� Basin Bridge (naphtha)

Diesel Power Plant:

� Vasvi (CMS India Limited),

� Samayanallur (Balaji power corporation Ltd)

� Samalpatti (samalpatti Power Corporation).

Hydel Power Plant:

� Kundah, Kadamparai, Mettur.

Nuclear Power Plant:

� Kalpakkam (Madrs Atomic Power Station),

� Kudankulam (Kudankulam Nuclear Power Plant)

Therm al Power Plant

� Ennore, Neyveli, North Madras and Tuticorin,

64. Define High pressure boilers.

High pressure boilers are used in power plants to generate steam at a high

pressure. The modern high pressure boilers used for power generation have steam

capacities ranging from 40 to 650 tonnes/h with pressure up to 200bar and maximum

steam temperature of about 6000C.

65. What are the special features of high pressure boilers?

o Provision of tubings and drums

o Forced water circulation

o Evaporation of water above critical pressure resulting in the saving of latent

heat

o Use of supersaturated steam for heating water.

UNIT IV I C ENGINES 1. What is an engine?

� An engine is a device used for converting heat energy into mechanical energy

by combustion of a fuel.

2. What are the types of heat engine?

� Internal combustion engine (I.C. Engine) � External Combustion engine (E.C. Engine)

3. What is mean by S.I. Engine? Why is it called so?

Petrol engine is called is as spark ignition, because the combustion of fuel takes place by means of a spark produced by the spark plug.

4. What is mean by C.I. Engine? Why it is called so?

Diesel is called as compression ignition engine, because the combustion takes place due to the heat produced by the compression of air-fuel mixture.

5. Spark plug is necessary to run a --------------------------- engine.

� Ans: Petrol

6. Give the main components of a petrol engine.

Cylinder, cylinder head, piston, connecting rod, valves, spark plug, crank shaft, cam

shaft and fly wheel.

7. Number of working strokes per minute for a four stroke cycle engines are -------------- the speed of the engine.

Ans: Half.

8. A petrol engine works on ----------------- cycle.

Ans: Otto

9. ----------------- is used to mix fuel and air in a petrol engine.

Ans: Carburetor.

10. What is a four stroke engine?

In a four stroke engine, one power stroke is completed for every stokes of the piston or during two revolutions of the crank shaft.

11. Diesel engine works on the principle of ------------------------------ cycle.

Ans: Diesel cycle.

12. What is the function of a carburetor?

To mix the fuel with air in correct proportion and to evaporate the fuel with fast

moving air. To regulate the supply of air -fuel mixture entering into the engine cylinders.

13. What is the fundamental difference between two-stroke and four-stroke engine?

In four- stroke engine one power stroke is obtained in two revolutions of crank shaft where as in two- stroke engine , one power stroke is obtained in each revolution of crank shaft.

14. Why fuel is injected in a C.I. engine?

The fuel used in C.I. engine cannot be vaporized and hence injected into the cylinder in the form of fine spray.

15. Define carburetor.

Carburetor is a device used for mixing the air with petrol in correct proportion.

16. Mention the types of ignition systems used in petrol engine.

Battery (or) coil ignition system. Magneto ignition system

17. State the function of choke in a petrol engine.

Choke is provided for easy starting of the engine. It allows rich mixture into the cylinder by reducing the amount of air present in the mixture.

18. What is the function of a spark plug?

Spark plug is a device used to ignite the compressed air fuel mixture by producing an electric spark.

19. The gap between the central electrode and earth electrode of a spark plug is ------

----

Ans: 0.4mm

20. The device used to supply correct quantity of fuel into the cylinder of a diesel engine is ---------------.

Ans: Fuel pump

21. Define fuel injector

Fuel injector is a device is device used to atomize the fuel and to deliver the fuel to the

cylinder of a diesel engine in the form of fine spray.

22. What are the types of cooling systems used in I.C. engine?

Air cooling. Water cooling.

23. Mention the types of water cooling on I.C. engines.

Natural circulation system Forced circulation system

24. Define lubrication.

Lubrication is the process of applying lubricant between the surfaces of contact of two moving parts.

25. Mention some engine parts that require frequent lubrication.

Cylinder, piston and piston rings. Main bearings Crank shaft Valves etc.

26. What are the types of lubrications for I. C. engines?

Mist lubrication. Wet lubrication.

27. Compression ratio of petrol engine is in the range of

(a) 2 to 3 (b) 3 to 5 (c) 7 to 10 (d) 16 to 20

Ans: (c) 7 to 10

28. Compression ratio of diesel engine is in the rang e of

(a) 8 to 10 (b) 10 to 15 (c) 16 to 20 (d) 20 and above

Ans: (c) 16 t0 20.

29. Fuel injector is used in (a) S.I. engines (b) C.I. engines (c) Gas engines.

Ans: (b) C.I. engines.

30.What is mean by internal combustion engine?

Internal combustion engine (I.C. engine) is a heat engine where combustion of fuel (petrol/diesel) with air takes place inside the engine cylinder.

31. What are the main parts of the internal combustion engine?

The main parts of an internal combustion engine are cylinder, piston, piston rings, piston pin, crank shaft, connecting rod, crank case, valves and flywheel.

32. How I.C. engines are classified?

I.C. engines are classified as spark ignition engines (S.I. engines) and compression ignition engines (C.I. engines). Another classification of I.C. engines are (1) Four stroke engine (2) two stroke engine.

33. What are the four stroke of an I.C. engine?

Suction stroke, compression stroke, expansion or power stroke, working stroke and exhaust stroke.

34. What is mean by stroke in an I.C. engine?

A stroke refers to the linear distance between the two extreme positions of the piston. This distance is measured parallel along the axis of the cylinder.

35. Name the two extreme positions occupied by the piston?

The two extreme positions occupied by the occupied by the piston in a cylinder are Top dead centre (TDC) and Bottom dead centre (BDC).

36. Name the important systems in an I.C. engine.

Fuel system, ignition system, cooling system, lubricating system and air system.

37. What is the main purpose of fuel system in an I.C. engine?

Storage and supply of fuel for combustion of engines is the main purpose of fuel system.

38. What are the important components of fuel system?

Fuel filters, fuel tank, carburetor (for S.I.engine), fuel pump and fuel injector (for C.I. engine).

39. What is the basic function of ignition system?

Ignition system is used in a spark ignition engine to ignite the air fuel mixture in the cylinder

40. What are the two types of ignition system?

(1) Battery ignition system (2 ) Magneto ignition system

41. Why cooling system is necessary in an I.C. engine?

Combustion of air-fuel mixture in the cylinder produces very high temperature in the range of 20000C. This heat may cause expansion of cylinder wall, cylinder head, piston and other parts resulting in abnormal ignition. Cooling system is used to maintain the temperature within limits so that normal ignition occurs.

42. What is the purpose of lubrication system used in I.C. engines?

Lubricating system is used to reduce the friction between the moving parts and thus reduces its wear and tear. It also cools and cleans the moving parts.

43. What is the compression ratio of an I.C. engine?

The compression ratio of an I.C. engine is defined as the ratio of maximum cylinder

volume to minimum cylinder volume.

44.

45.What is meant by scavenging in I.C. engine?

Scavenging refers to the process of removing burnt gases during exhaust stroke with the help of incoming charge and deflector.

46. State the purpose of flywheel.

Flywheel is used to sustain the movement of the piston even during non-power strokes.(Suction, compression and exhaust strokes)

47. What is meant by TDC?

TDC means top dead centre, it is the extreme position of the piston at the top end of the cylinder in a vertical engine.

48. What is meant by BDC?

BDC means Bottom dead centre, it is the extreme position of the piston at the bottom of the cylinder in vertical engine.

49. What is function of I.C. engine?

I.C. engine is used to produce mechanical energy by burning fuel such as petrol or diesel inside the cylinder.

50. What is the cycle used in SI engine?

The cycle used in SI engine is OTTO CYCLE.

51. What is the fuel used in SI & CI engine?

The fuel used in SI & CI engines are PETROL & DIESEL respectively.

52. What is the standard ratio of air-fuel mixture for an SI engine?

The standard ratio of air-fuel mixture for SI engine is 15:1.

53. What is the crank shaft revolution in 2 Stroke and 4 Stroke?

2 Stroke - one revolution. 4 Stroke - two revolutions.

54.What are the sequences in 4 Stroke engine?

Suction stroke, Compression stroke, Power stroke, Exhaust stroke

55. What is the use of piston ring?

The piston is used to lubricating oil on the cylinder walls

56. What is the use of connecting rod?

Connecting rod is used as the linear or reciprocating motion of the piston into the rotary motion of crankshaft.

57. What is the use of flywheel?

It is used to minimize the fluctuation of energy in crankshaft

58. What is the ratio of petrol engine and diesel engine?

Petrol engine- 7 to 10. Diesel engine- 15 to 20

59. What is the fundamental difference between 2-stroke and 4-Stroke engine?

In four stroke engine one power stroke is obtained in two revolutions of crankshaft where as in two stroke engine, one power stroke is obtained in each revolution of crankshaft.

60. Write Short notes on Fly wheel?

Fly wheel is a fairy large wheel mounted on the crank shaft. During the power stroke,

fly wheel stores excess energy and releases it during the other strokes. Thus fly wheel ensures minimum variation in speed.

61. What is clearance volume?

The volume of cylinder above the top of the piston when the piston is at TDC is called clearance volume.

62. What is gudgeon pin?

The pin which is used to connect connecting rod and piston is known as gudgeon pin.

63. Define Otto cycle.

Ina four stroke cycle petrol engine (also known as spark ignition engine), four strokes of the piston, namely suction, compression, expansion and exhaust take place in one cycle of operation in two revolutions of the crank shaft. This system of operation is known as Otto cycle.

64. What is Scavenging?

The process of cleaning or removing the burnt exhaust gas by the incoming compressed air-fuel (petrol) mixture is known as scavenging.

STEAM GENERATORS

1. How boilers are classified?

� According to flow of water and gases Fire tube boilers Water tube boilers

� According to pressure Low pressure boilers High pressure boilers 2. Mention the advantages of high pressure boilers?

� Rate of steam production is high � Steam produced at a pressure pf more than 890 bar. � Superheated steam can be produced.

3. State the main function of a boiler

� The main function of a boiler is to evaporate water into steam at a higher pressure. 4. How modern boilers differ from olden day boilers? Give four important points.

� Steam is produced at high rate. � Steam is produced at a higher pressure.

� Suitable mountings avoid the danger and provide safe operation and control. � Boiler accessories fitted increases the efficiency.

5. What is the use of an economizer in a high pressure boiler?

� Economizer extracts the heat from the hot flue gases going out of the boiler and preheats the water that is fed to the boiler.

6. What is the difference between mountings and accessories in a boiler?

� Boiler mountings are fitted in the boiler for safe operation and control steam generation where as boiler accessories increases the boiler efficiency of the boiler.

7. Name any two mountings of a boiler?

� Pressure gauge � Water gauge

8. What is a boiler?

� Boiler is a closed vessel in which steam is generated from water by the application of heat and the pressure being higher than the atmosphere.

9. State the main components of a boiler.

� Shell, furnace, chimney, manhole, super heater etc. 10. What is the purpose of super heater in a high pressure boiler?

� Super heater is used to increase the temperature of the steam above its saturation temperature.

11. Name any two steam boiler accessories.

� Economizer � Air preheater.

12. What do you understand by forced circulation boiler?

� In this type of boiler, water is circulated by a pump driven by a motor. � Example: La-Mont Boiler.

13. State the different types of safety valves used in a boiler.

� Dead weight safety valve. � Lever safety valve � Spring loaded safety valve.

14. State the function of air preheater.

� It preheats the air supplied to the combustion chamber by using the heat of the flue gases.

15. Give an example for a water tube boiler.

� Babcock and Wilcox boiler.

UNIT V REFRIGERATION AND AIR CONDITIONING SYSTEM

1. Define refrigeration.

� Refrigeration is the process of reducing and maintaining the temperature of a body below the general temperature.

2. What is refrigerator?

� Refrigerator is equipment used to reduce and maintain the temperature below the atmospheric temperature by removing the heat from the space continuously.

3. What is refrigerant?

� A refrigerant is the working fluid usually liquid or gas which is used in the refrigerator. A liquid whose “saturation temperature” is below the temperature to be produced by refrigeration is chosen as the refrigerant. Such a liquid will evaporate at lower temperatures and will absorb the heat from the surroundings.

4. Define capacity of refrigerator.

� Capacity is the rate at which heat can be extracted from the cold body, i.e, the rate at which refrigeration can be produced. The capacity is expressed in terms of tons of refrigeration.

5. Define one ton.

� One ton of the refrigeration is equal to the amount of refrigeration produced by melting 1ton of ice in 24 hours.

6. Give some examples for refrigerant.

� Ammonia, carbon-di-oxide, Freon-12, Chloro fluoro carbon, Methyl chloride etc. 7. Define COP

� COP is the ratio of heat extracted and work input. Heat extracted

� Coefficient of performance of a refrigerator (COP) = Work input 8. Mention the classification of refrigeration.

� Vapour compression refrigeration. � Vapour absorption refrigeration.

9. Mention the type of refrigerators.

� Primary refrigerators � Secondary refrigerators.

10. Give some properties of a good refrigerant.

� It should have low freezing point and boiling point. � It should be easily liquefied. � It should have high COP. � It should absorb high latent heat.

11. State the function of a compressor.

� Compressor is used to compress the low pressure vapour refrigerant. 12. Mention some applications of refrigeration.

� In preserving food, fruits and drugs. � Used in refineries.

� Manufacturing of ice. � In manufacturing industries

13. Define air conditioning.

� Air conditioning is the process of conditioning the air according to the human comfort irrespective of external conditions.

14. What is the purpose of air conditioner?

� Air conditioners control the temperature, moisture, cleanly ness and movement of indoor air. It cools the air when the weather is hot/ It warms the air when whether is cold. By controlling air movement, air conditioning brings fresh air into a room and pushes out stale air. This makes the air inside the room fresh and pure.

15. What is the application of refrigeration?

� In chemical industries for separating liquefying gases and vapours � In manufacturing ice. � For the preservation of perishable food items in cold storages � For cooling water. � For controlling humidity of air in the manufacture of papers � For the preservation of medicines � For the preservation of blood, tissues etc in hospitals. � For comfort air-conditioning in hospitals, theaters etc.

16. What is psychrometry?

� It is study of behavior of moist air (mixture of air and water & water vapour) together with their measurement and control is known as psychrometry.

17. What is dry air?

� Dry air is the mixture of nitrogen (77% by weight) and oxygen (23% by weight). 18. What is moist air?

� It is a mixture of dry air and water vapour the amount of water vapour vary with temperature. It increases with decrease in temperature and decreases with the increase in temperature.

19. What is water vapour?

� It is the moisture present in the dry air. The moisture content in air is an important factor in all air-conditioning system.

20. Mention four industrial applications of air conditioning.

� (1) Food industry (2) Photographic industry (3) Textile industry (4) Printing industry.

21. Define relative humidity.

� It is the ratio of water vapour in a given volume of air at given temperature, to the mass of water vapour present in the same volume under same temperature of air when it is fully saturated.

22. Define DBT.

� The temperature of air measured by the ordinary thermometer is called dry bulb temperature.

23. Define WBT

� The temperature of air measured by the ordinary thermometer when it is covered by a wet cloth is known as wet bulb temperature.

24. Mention the types of air conditioning.

� Comfort air conditioning. � Industrial air conditioning

25. What are the three methods of heat transfer?

� There are three methods of heat transfer by which the substances take place by direct contact; it is called conduction, convection and radiation.

26. Explain the three methods of heat transfer. Conduction

� When heat transfer takes place by direct contact between two substances, it is called conduction process of heat transfer.

Convection

� It is the process of heat transfer in which molecules of liquids and gases move due to the change in temperature and pressure. In convection the transfer of heat does not take place from substance to substance; instead it involves transfer of heated substance from one place to another.

Radiation

� It is the process of heat transfer in which transfer of heat takes place through electromagnetic waves, for example radio and T.V. Signals also travel through electromagnetic waves. In radiation process, energy can be transferred through empty space (vacuum), radiation does not require any substance or medium for transfer of heat from one place to another.

� Examples: Heat from sun to earth, heat from an electric heater.

27. Distinguish between sensible heat and latent heat.

� When heat is removed or added to a substance it causes temperature change in the substance, this heat is called sensible heat.

� When heat is removed or added to a substance and there is no change in temperature of the substance, but instead the physical state of the substance changes (i.e. from solid to liquid or gas) this is called Latent heat.

28. What is Enthalpy of refrigerant?

� Enthalpy of a refrigerant id defined as the quantity of heat required to convert one kilogram of vapour refrigerant at 00C into wet vapour at constant pressure. Enthalpy is denoted by h. The unit of enthalpy is KJ/Kg.

29. What are the refrigerants used in refrigeration & air conditioning?

� Air: Air is used as refrigerant in aircrafts. However, its COP is low.

� Ammonia (NH3): Ammonia is used as refrigerant in cold storage plants and ice manufacturing companies. Though ammonia has high refrigeration effect, it is not used in domestic air conditioning or refrigeration due to its toxicity and Inflammability (ability to be set on fire).

� Carbon dioxide (CO2): Carbon dioxide occupies less space. It is used in

refrigerators fitted in ships. CO2 is non-toxic and non-inflammable.

� Sulphur dioxide (SO2): SO2 forms sulphuric acid when it comes in contact with water. Sulphuric acid corrodes metals. Now-a-days it is not used.

� Freon-12: Freon-12 is a combination of fluorine and chlorine. It is non-corrosive,

non-flammable and non-toxic. It is used as refrigerant in domestic refrigerators, air conditioners and water coolers.

30. Define VAV and CAV.

� VAV (Variable Air Volume): VAV refers to an air conditioning system which varies the volume of air supplied into the room to sit the requirements.

� CAV (Constant Air Volume): CAV refers to an air conditioning system where the

volume of air supplied into room is constant.