Refrigeration and Air Conditioning | HVAC Interview Question and Answers part 2
Refrigeration and Air Conditioning | HVAC Interview , viva , oral Question and Answers part 2
31. Define COP
The performance of a refrigeration system is defined as the ratio of heat absorbed by the refrigerant to the work input to compress the refrigerant in the compressor.
32. Name some different types of refrigeration cycles
a. Vapour compression refrigeration cycle
b. Vapour absorption refrigeration cycle
c. Air refrigeration cycle
d. Steam jet refrigeration system
e. Thermo electric refrigeration system
33. What are the components involved in vapour compression refrigeration system?
- Expansion valve
34. What are the four processes in Air refrigeration cycle?
- Isentropic compression: Work is consumed during this process
- Reversible constant pressure cooling: Hot air from the compressor is cooled
- Reversible adiabatic expansion: pressure at outlet equal to refrigerated space
- Reversible constant pressure heating: Heat is removed from the space
35. Why super heating and sub cooling is preferred vapour compression cycle?
Super heating is preferred in practice because it ensures complete vaporisation of the liquid in the evaporator before it enters the compressor. Sub cooling of condensate is preferred because it reduces the vapour percentage after throttling so that refrigeration effect per unit mass is increased.
36. What are the advantages of vapour compression system?
- COP is very high as compared to other systems.
- Different cold temperature can be obtained by changing the evaporator pressure.
- Pressure in the condenser and evaporator results in higher heat transfer co-efficient.
- The system is compact in size
37. What are the advantages and disadvantages of vapour absorption system?
- As there is no moving parts, the operation is quiet and little wear
- Input to the system is mainly heat, which is low grade energy.
- Unlike vapour compression system where COP is not reduced drastically as load increases.
- Suitable for very large capacity (more than 400TR capacity)
- Not suitable for low capacity
- More precise equipments are needed
- Very low COP
- High initial cost
38. What is the principle of absorption refrigeration system?
In absorption refrigeration system the vapour is drawn from the evaporator by absorption into liquid having high affinity for the refrigerant. The refrigerant is expelled from the solution by application of heat and its temperature is also increased. This refrigerant in the vapour form passed to the condenser where heat is rejected and the refrigerant gets liquefied. This liquid again flows to the evaporator at reduced pressure and the cycle is completed.
39. What is the refrigeration effect of the refrigerant?
Refrigeration effect is the total heat removed from the evaporator by the refrigerant. It is called as Tonne of Refrigeration of kW.
40. Define sub cooling and super heating
The process of cooling the liquid refrigerant below its saturation temperature in the condenser is known as sub cooling
The process of heating the vapour above saturation temperature in the evaporator is known as super heating.
41. Compare the vapour compression and vapour absorption refrigeration systems?
43. Mention some important application of cryogenics
Food preservation, Medicine, Heat treatment, Transport refrigeration,
superconductivity and space programs.
44. Define COP of refrigeration.
The COP of a refrigeration system is the ratio of net refrigeration effect to the work required to produce the effect.
45. What is the purpose of throttling valve in vapour compression system?
The function of throttling valve is to allow the liquid refrigerant under high pressure and temperature to pass at controlled rate after reducing its pressure and temperature.
46. What are the difference between refrigeration and air-conditioning?
- Refrigeration is the process of providing and maintaining the temperature in the space below atmospheric temperature.
- Air conditioning is the process of supplying sufficient volume of clean air containing a specific amount of water vapour and maintaining the predetermined atmospheric condition with in a selected enclosure.
47. What is the unit of refrigeration?
The capacity of refrigeration is expressed in tonnes of refrigeration (TOR).
1 tonnes of refrigeration = 210 kJ/min (or) = 3.5 kJ/sec (kW)
A tonne of refrigeration is defined as the quantity of heat to be removed in order to form one tonne of ice at 0oC in 24 hours.
48. Define refrigeration effect.
The amount of heat extracted in a given time is known as refrigeration effect.
49. What is the advantage of air refrigeration system?
- The refrigeration used is air, is non poisonous, cheap and easily available.
- The system is highly reliable.
- The system is weight less and less space is required, therefore, air refrigeration is extremely useful for aircraft refrigeration.
- Air is non-flammable therefore no risk of fire.
50. What are the disadvantages of air refrigeration?
- The COP of the air refrigeration system is low therefore the running cost f the system is high.
- Large volume of air is required to handle, to compare with other system.
- Therefore it requires large compressor and expander.
- The freezing of moisture in the air during expansion is liable to choice up the valves.
51. What are the assumptions made for drawing TS diagram of refrigeration system?
- The condition of the vapour leaving the compressor is dry saturated.
- The compression of vapour in the compressor is isentropic
- There is no pressure loss in the system.
- The work required to drive the system is equal to the difference between the heat rejected in the condenser and heat absorbed in the evaporator.
52. What re the effects of under cooling?
- It increases the refrigeration effect therefore the COP increases.
- The mass flow rate of the refrigeration is less than that for the simple saturated cycle.
- The reduced mass flow rate reduces the piston displacement per minute.
- Power per tones of refrigeration losses due to reduction in mass flow rate.
- The increased efficiency may be offer some extent by the rise in the condenser pressure.
- Work input almost remains same.
- The heat rejection capacity of the condenser increases.
53. What are the effects of super heating?
- Supper heating increases the net refrigeration effect, but super heating requires more work input therefore super heating reduces the COP.
- No moisture contents in the refrigerant therefore no corrosion in the machines part.
54. What re the advantages of vapour absorption system over vapour compression system?
As there is no moving part in the system the operation is quiet and there is very little wearing. The maintenance cost is low. The system does not depend upon electric power. It can be built in capacities well above 1000 tones each. At reduced loads to absorption system is almost as efficient as of full load the COP of the compressor system decrease as the decrease. Absorption refrigeration system can operate at reduced evaporator temperature by increasing the steam which is supplied to generator with little decrease in capacity. The capacity of its compression system drops rapidly with lower evaporator temperature.
55. What are the properties of ideal refrigerant?
- It should have low boiling point and low freezing point.
- It must have low specific heat and high latent heat.
- It should have high thermal conductivity to reduce the heat transfer in evaporator and condenser.
- It should have low specific volume to reduce the size of the compressor.
- It should be non-flammable, non-expensive, non-toxic and non-corrosive.
- It should have high critical pressure and temperature to avoid large power requirements.
- It should give high COP to reduce the running cost of the system.
- It must be cheap and must be readily available.
56. What are the advantages of vapour compression system?
- The COP is better, because the cycle using vapour as refrigerant absorbs and rejects heat at constant temperature.
- The expander is eliminated.
- The temperature at which the heat is to be absorbed can be changed conveniently by altering the boiling pressure.
- The transfer co-efficiently is high
- The same refrigerant is used over and over again.
57. List the important industrial and commercial application of refrigeration.
- Food and milk preservation.
- Ice formation
- Comfort and industrial air conditioning
- Storage of liquid fuels used in rockets.
- Treatment of metals and processing in chemical and related industries.
- Processing of beverages and transportation of food below freezing.
- Medical and surgical aids especially in preserving human bloods and tissues.
- Oil refining.
58. How does humidity affect human comfort?
Human beings want to feel comfortable. They want to live in an environment that is neither hot not cold, neither very humid nor very dry. The desires of human body could not be adopted with high or low humidity. They feels comfortable only when they can freely dissipate their waste heat to the environment.
59. What do you mean by the “Infiltration” in heat load calculations?
The amount of heat load added due the air entering into the A/c system through small opening in the doors and windows, cracks in the walls etc., are termed as infiltration.
60. Define RSHF and RTH.
Room sensible heat factor is defined as the ratio of room sensible heat load to the room total heat load.
61. What are the important psychrometric processes?
a. Sensible heating and sensible cooling
b. Cooling and dehumidification
c. Heating and humidification
d. Mixing of air streams
e. Chemical dehumidification
f. Adiabatic evaporative cooling
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