1 2 3 (e) Heating and humidification: w DBT t t. An air washer can works as a year-round air conditioning system. Though air washer is a and extremely useful simple device, it is not commonly used for comfort air conditioning applications due to concerns about health resulting from bacterial or fungal growth on the wetted surfaces. However, it can be used in industrial applications. 4 5 Factors affecting comfort air conditioning The four important factors for comfort air conditioning are discussed as below:

6 7 The human body feels comfortable when there is no change in the body , i.e. When the heat stored in the body Qs is zero. Any variation in the body temperature acts as a stress to the brain which ultimately results in either perspiration or shivering. 8 9 10 Summer air conditioning systems (a) Simple system with 100 % re- circulated air: In this simple system, there is no outside air and the same air is recirculated as shown in Figure below it also shows the process on a psychrometric chart.

11 12 13 14 15 16 17 18 19 20 Winter Air Conditioning Systems In winter the outside conditions are cold and dry. As a result, there will be a continuous transfer of sensible heat as well as moisture (latent heat) from the buildings to the outside. Hence, in order to maintain required comfort conditions in the occupied space an air conditioning system is required which can offset the sensible and latent heat losses from the building. Air supplied to the conditioned space is heated and humidified in the winter air conditioning system to the required level of temperature and moisture content depending upon the sensible and latent heat losses from the building. In winter the heat losses from the conditioned space are partially offset by solar and internal heat gains. Thus in a conservative design of winter A/C systems, the effects of solar radiation and internal heat gain are not considered. Heating and humidification of air can be achieved by different schemes. Figure below shows one such scheme along with the cycle on psychrometric chart. As shown in the figure, the mixed air (mixture of return and outdoor air) is first pre-heated (m-1) in the pre-heater, then humidified using a humidifier or an air washer (1-2) and then finally reheated in the reheater (2-s). The reheated air at state‘s’ is supplied to the conditioned space. 21 22 The flow rate of supply air should be such that when released into the conditioned space at state ‘s’, it should be able to maintain the conditioned space at state ate I and offset the sensible and latent heat losses (Qs and Ql). Pre-heating of air is advantageous as it ensures that in the humidifier/air washer does not freeze. In addition, by controlling the heat supplied in the preheater one can control the moisture content in the conditioned space. The humidification of air can be achieved in several ways, e.g. by bringing the air in contact with a wetted surface, or with droplets of water as in an air washer, by adding aerosol sized water droplets directly to air or by direct addition of dry saturated or superheated steam. When air is humidified by directly adding dry, saturated steam, then the humidification proceeds close to the constant dry bulb temperature line. By applying energy balance across the conditioned space, at steady state, the sensible and latent heat losses from the building can be written as:

23 24 All year (complete) air conditioning systems: Figure below shows a complete air conditioning system that can be used for providing air conditioning throughout the year, i.e., during summer as well as winter. As shown in the figure, the system consists of a filter, a heating coil, a cooling & dehumidifying coil, a re-heating coil, a humidifier and a blower. In addition to these, actual systems consist of several other accessories such as dampers for controlling flow rates of re-circulated and outdoor (OD) air, control systems for controlling the space conditions, safety devices etc. Large air conditioning systems use blowers in the return air stream also. Generally, during summer the heating and humidifying coils remain inactive, while during winter the cooling and dehumidifying coil remains inactive. However, in some applications for precise control of conditions in the conditioned space all the coils may have to be made active. The blowers will remain active throughout the year, as air has to be circulated during summer as well as during winter. When the outdoor conditions are favorable, it is possible to maintain comfort conditions by using filtered outdoor air alone, in which case only the blowers will be running and all the coils will be inactive leading to significant savings in energy consumption. A control system is required which changes-over the system from winter operation to summer operation or vice versa depending upon the outdoor conditions.

25 26 Note: • Evaporative cooling systems are attractive for hot and dry . • Evaporative cooling systems are ideal for several industrial applications • In a direct evaporative cooling system, the lowest possible temperature is the wet bulb temperature corresponding to the outdoor air. • In a direct evaporative cooling system, cooled and humidified air is supplied to the conditioned space. • In an indirect evaporative cooling system, the air supplied to the conditioned space is at a lower temperature and at a ratio corresponding to the outdoor air • In multi-stage evaporative cooling systems, it is possible to cool the air to a temperature lower than the entering air WBT. • Evaporative cooling systems are environment friendly • Evaporative cooling systems offer lower initial and lower running costs. • Evaporative cooling systems are easier to maintain and fabricate. • Direct evaporative cooling systems are attractive in places where the summer design WBT is less than 24oC • Indirect evaporative cooling systems can be used over an extended range of climatic conditions

27 • A combination of evaporative cooling system with conventional air conditioning system can offer better overall performance • In winter air conditioning systems, heated and humidified air is supplied to the conditioned space • A pre-heater is recommended in winter air conditioning systems to prevent freezing of water in the humidifier and for better control. • When humidification is done using an air washer, the temperature of air drops during humidification • When humidification is carried out by adding dry steam, the temperature of air remains close to the DBT of entering air • An all year air conditioning system can be used either as a summer air conditioning system or as a winter air conditioning system. • When an all year air conditioning system is used during winter, the cooling and dehumidification coils are switched-off. • In an all year air conditioning systems, the blowers are always on.

28 1. Which of the following statement is correct?

(A) The human body can lose heat even if its temperature is less than the atmospheric temperature. (B) The increase in air movement increases the from the human body. (C) The warm air increases the rate of radiation of heat from the human body. (D) Both (A) and (B) Answer – D

2. The minimum temperature to which water can be cooled in a cooling tower is (A) point temperature of air (B) Wet bulb temperature of air (C) Dry bulb temperature of air (D) Ambient air temperature Answer – B 3. Which of the following statement is correct?

(A) The constant enthalpy lines are also constant wet bulb temperature lines (B) The wet bulb and dry bulb temperature are equal at saturation condition (C) The wet bulb temperature is a measure of enthalpy of moist air (D) All of the above Answer - D

29 4. During cooling and dehumidification, dry bulb temperature

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer - C

5. The wet bulb depression indicates ______humidity of the air.

(A) Absolute (B) Relative (C) Specific (D) None of these Answer – B

6. The sensible heat factor during the heating and humidification process is given by (where h₁ = Enthalpy of air entering the heating coil, h₂ = Enthalpy of air leaving the heating coil, and hA = Enthalpy of air at the end of humidification process)

(A) (hA - h2)/ (h1 - h2) (B) (h2 - hA)/ (h1 - h2) (C) (h1 - h2)/ (hA - h2) (D) (hA - h1)/ (h2 - h1) Answer - D

30 7. The conditioned air supplied to the room must have the capacity to take up

(A) Room sensible heat load only (B) Room latent heat load only (C) Both room sensible heat and latent heat loads (D) None of the above Answer – C

8. The index which correlates the combined effects of air temperature, relative humidity and air velocity on the human body, is known as

(A) Mean radiant temperature (B) Effective temperature (C) temperature (D) None of these Answer – B

9. The specific humidity during heating and humidification process.

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer - B 31 10. The relative coefficient of performance is (A) Actual COP/theoretical COP (B) Theoretical COP/actual COP (C) Actual COP × theoretical COP (D) None of these Answer - A 11. The wet bulb temperature at 100% relative humidity is ______dew point temperature.

(A) Same as (B) Lower than (C) Higher than (D) None of these Answer - A

12. Air conditioning means

(A) Cooling (B) Heating (C) Dehumidifying (D) All of these Answer – D 32 13. The specific humidity during dehumidification process

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer – C

14. The alignment circle is marked on the psychrometric chart at

(A) 20°C DBT and 50% RH (B) 26°C DBT and 50% RH (C) 20°C DBT and 60% RH (D) 26°C DBT and 60% RH Answer - B

15. Relative humidity is given by (where pv = Partial of water vapour, pb = Barometric pressure, ps = Pressure of saturated air, pd = Pressure at dry bulb temperature, pw = Saturation pressure corresponding to wet bulb temperature, and μ = Degree of saturation)

(A) 0.622 Pv / (Pb - Pv) (B) μ/[1 - (1 - μ) (Ps/Pb)] (C) [Pv (Pb - Pd)]/ [Pd (Pb - Pv)] (D) None of these Answer - B

33 16. During sensible cooling,

(A) Relative humidity remains constant (B) Wet bulb temperature increases (C) Specific humidity increases (D) Partial pressure of vapour remains constant Answer – D

17. The bypass factor (B. P. F.) in case of sensible heating of air is (Where td₁ = Dry bulb temperature of air entering the heating coil, td₂ = Dry bulb temperature of air leaving the heating coil, and td₃ = Dry bulb temperature of heating coil)

(A) (td₂ - td₃)/(td₃ - td₁) (B) (td₃ - td₂)/(td₃ - td₁) (C) (td₃ - td₁)/(td₂ - td₃) (D) (td₃ - td₁)/(td₃ - td₂) Answer - B

18. In order to cool and dehumidify a stream of moist air, it must be passed over the coil at a temperature

(A) Which lies between the dry bulb and wet bulb of the incoming stream (B) Which lies between the wet bulb and dew point temperatures of the incoming stream (C) Which is lower than the dew point temperature of the incoming stream (D) Of adiabatic saturation of incoming stream Answer: C

34 19. The specific humidity during cooling and dehumidification

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer: C

20. The bypass factor for a cooling coil

(A) Increases with increase in velocity of air passing through it (B) Decreases with increase in velocity of air passing through it (C) Remains unchanged with increase in velocity of air passing through it (D) May increase or decrease with increase in velocity of air passing through it depending upon the condition of air entering Answer: A

21. For unsaturated air, the dew point temperature is ______wet bulb temperature.

(A) Equal to (B) Less than (C) More than (D) None of these Answer - B 35 22. The temperature of air recorded by a thermometer, when its bulb is surrounded by a wet cloth exposed to the air, is called

(A) Wet bulb temperature (B) Dry bulb temperature (C) Dew point temperature (D) None of these Answer - A

23. The humidity ratio or specific humidity is the mass of water vapour present in

(A) 1 m3 of wet air (B) 1 m3 of dry air (C) 1 kg of wet air (D) 1 kg of dry air Answer - D

24. During adiabatic saturation process on unsaturated air ______remains constant.

(A) Relative humidity (B) Dew point temperature (C) Dry bulb temperature (D) Wet bulb temperature

Answer - D 36 25. Degree of saturation or percentage humidity is

(A) The mass of water vapour present in 1 m³ of dry air (B) The mass of water vapour present in 1 kg of dry air (C) The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure (D) The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure Answer - C

26. The humidification process, on the psychrometric chart is shown by

(A) Horizontal line (B) Vertical line (C) Inclined line (D) Curved line Answer – B 27. In case of sensible heating of air, the coil efficiency is given by (where B.P.F. = Bypass factor) (A) B.P.F. - 1 (B) 1 - B. P.F. (C) 1/ B.P.F. (D) 1 + B.P.F. Answer – B 37 28. The mass of water vapour present in ______is called absolute humidity.

(A) 1 m3 of water (B) 1 m3 of dry air (C) 1 kg of wet air (D) 1 kg of dry air Answer - B

29. The wet bulb temperature during sensible heating of air

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer – B

30. The relative humidity is defined as

(A) The mass of water vapour present in 1 m3 of dry air (B) The mass of water vapour present in 1 kg of dry air (C) The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure (D) The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure Answer – D

38 31. As relative humidity decreases, the dew point temperature will be ______wet bulb temperature.

(A) Same as (B) Lower than (C) Higher than (D) None of these Answer - B

32. In a psychrometric process, the sensible heat added is 30 kJ/s and the latent heat added is 20 kJ/s. The sensible heat factor for the process will be

(A) 0.3 (B) 0.6 (C) 0.67 (D) 1.5 Answer – B

33. It is desired to condition the outside air from 70% relative humidity and 45° C dry bulb temperature to 50% relative humidity and 25° C dry bulb temperature (room condition). The practical arrangement will be

(A) Dehumidification (B) Cooling and humidification (C) Cooling and dehumidification (D) Dehumidification and pure sensible cooling Answer - C

39 34. The difference between dry bulb temperature and wet bulb temperature, is called

(A) Dry bulb depression (B) Wet bulb depression (C) Dew point depression (D) Degree of saturation Answer - B

35. In a saturated air-water vapour mixture, the

(A) Dry bulb temperature is higher than wet bulb temperature (B) Dew point temperature is lower than wet bulb temperature (C) Dry bulb, wet bulb and dew point temperature are same (D) Dry bulb temperature is higher than dew point temperature Answer - C

36. In chemical dehumidification process,

(A) Dew point temperature decreases (B) Wet bulb temperature decreases (C) Dry bulb temperature increases (D) All of these

Answer - D 40 37. The relative humidity lines on a psychrometric chart are

(A) Vertical and uniformly spaced (B) Horizontal and uniformly spaced (C) Horizontal and non-uniformly spaced (D) Curved lines Answer – D

38. The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure, is called

(A) Humidity ratio (B) Relative humidity (C) Absolute humidity (D) Degree of saturation Answer - D Correct Answer

39. During dehumidification process, ______remains constant.

(A) Wet bulb temperature (B) Relative humidity (C) Dry bulb temperature (D) Specific humidity Answer - C

41 40. In case of sensible cooling of air, the coil efficiency is given by

(A) B.P.F. - 1 (B) 1 - B.P.F. (C) 1/ B.P.F. (D) 1 + B.P.F. Answer – B

41. The wet bulb temperature during sensible cooling of air

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer – C

42. Wet bulb temperature is the temperature of air recorded by a thermometer, when

(A) It is not affected by the moisture present in the air (B) Its bulb is surrounded by a wet cloth exposed to the air (C) The moisture present in it begins to condense (D) None of the above Answer - B

42 43. The difference between dry bulb temperature and dew point temperature, is called

(A) Dry bulb depression (B) Wet bulb depression (C) Dew point depression (D) Degree of saturation Answer – C

44. The specific humidity during humidification process

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer – B

45. During sensible cooling of air, specific humidity

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer - A

43 46. The dehumidification process, on the psychrometric chart, is shown by

(A) Horizontal line (B) Vertical line (C) Inclined line (D) Curved line Answer - B

47. The wet bulb temperature at 100% relative humidity is ______dry bulb temperature.

(A) Same as (B) Lower than (C) Higher than (D) None of these Answer – A

48. The human body feels comfortable when the heat stored in the body is

(A) Positive (B) Negative (C) Zero (D) None of these Answer - C

44 49. The vertical and uniformly spaced lines on a psychrometric chart indicates

(A) Dry bulb temperature (B) Wet bulb temperature (C) Dew point temperature (D) Specific humidity Answer – A

50. The process, generally used in summer air conditioning to cool and dehumidify the air, is called

(A) Humidification (B) Dehumidification (C) Heating and humidification (D) Cooling and dehumidification Answer - D

51. When the air is passed through an insulated chamber having sprays of water maintained at a temperature higher than the dew point temperature of entering air but lower than its dry bulb temperature, then the air is said to be

(A) Cooled and humidified (B) Cooled and dehumidified (C) Heated and humidified (D) Heated and dehumidified Answer – A

45 52. For air conditioning the operation theater in a hospital, the percentage of outside air in the air supplied is

(A) Zero (B) 20 (C) 50 (D) 100 Answer - D

53. The temperature of air recorded by a thermometer, when it is not affected by the moisture present in the air, is called

(A) Wet bulb temperature (B) Dry bulb temperature (C) Dew point temperature (D) None of these Answer - B

54. During humidification process, ______increases.

(A) Wet bulb temperature (B) Relative humidity (C) Dry bulb temperature (D) Specific humidity Answer – B

46 55. The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure, is called

(A) Humidity ratio (B) Relative humidity (C) Absolute humidity (D) Degree of saturation Answer – B

56. During dehumidification process, the relative humidity

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer – C

57. The wet bulb depression is zero when relative humidity is

(A) Zero (B) 0.5 (C) 0.75 (D) 1.0 Answer – D 47 58. During humidification process, dry bulb temperature

(A) Remains constant (B) Increases (C) Decreases (D) None of these Answer – A

59. The process, generally used in winter air-conditioning to warm and humidity the air, is called

(A) Humidification (B) Dehumidification (C) Heating and humidification (D) Cooling and dehumidification Answer – C

60. During heating and dehumidification process, dry bulb temperature

(A) Remains constant (B) Increases (C) Decreases (D) None of these

Answer – B 48 61. Dry bulb temperature is the temperature of air recorded by a thermometer, when

(A) It is not affected by the moisture present in the air (B) Its bulb is surrounded by a wet cloth exposed to the air (C) The moisture present in it begins to condense (D) None of the above Answer - A

62. During heating and humidification, the final relative humidity of air

(A) Can be lower or higher than that of the entering air (B) Is lower than that of the entering air (C) Is higher than that of the entering air (D) None of the above Answer - A

63. The curved lines on a psychrometric chart indicates

(A) Dry bulb temperature (B) Wet bulb temperature (C) Dew point temperature (D) Relative humidity Answer - D 49 64. On the pressure-enthalpy diagram, and desuper heating is represented by a horizontal line because the process (A) Involves no change in volume (B) Takes place at constant temperature (C) Takes place at constant entropy (D) Takes place at constant pressure Answer - D Correct Answer

65. When the temperature of the surrounding is higher than the temperature of the body, then the heat loss by from the body to the surrounding will be

(A) Positive (B) Negative (C) Zero (D) None of these Answer - B

66. The atmospheric air at dry bulb temperature of 15°C enters a heating coil maintained at 40°C. The air leaves the heating coil at 25°C. The bypass factor of the heating coil is

(A) 0.376 (B) 0.4 (C) 0.6 (D) 0.67 Answer – C

50 67. A human body feels comfortable when the heat produced by the metabolism of human body is equal to the

(A) Heat dissipated to the surroundings (B) Heat stored in the human body (C) Sum of (A) and (B) (D) Difference of (A) and (B) Answer - C

68. The sensible heat factor during cooling and dehumidification process is given by (where h₁ = Enthalpy of air entering the cooling coil, h₂ = Enthalpy of air leaving the cooling coil, and hA = Enthalpy of air at the end of dehumidification process)

(A) (hA - h2)/ (h1 - h2) (B) (h2 - hA)/ (h1 - h2) (C) (h1 - h2)/ (hA - h2) (D) (hA - h1)/ (h2 - h1) Answer – A

69. For air at a given temperature, as the relative humidity is increased isothermally, [GATE-2001] (a) the wet bulb temperature and specific enthalpy increase (b) the wet bulb temperature and specific enthalpy decrease (c) the wet bulb temperature increases and specific enthalpy decreases (d) the wet bulb temperature decreases and specific enthalpy increases Answer - (a, c)

51 70.

Air is cooled at constant pressure to make unsaturated air to saturated one. 71. The statements concern Psychrometric chart. [GATE-2006] 1. Constant relative humidity lines are uphill straight lines to the right 2. Constant wet bulb temperature lines are downhill straight lines to the right 3. Constant specific volume lines are downhill straight lines to the right 4. Constant enthalpy lines are coincident with constant wet bulb temperature lines Which of the statements are correct? (a) 2 and 3 (b) 1 and 2 (c) 1 and 3 (d) 2 and 4 Answer. (a) 52 72. When atmospheric air is heated at constant pressure, then which one is not correct. [GATE-2000] (a) humidity ratio does not change (b) relative humidity increases (c) dew point temperature does not change (d) wet bulb temperature increases Answer. (b)

73. During chemical dehumidification process of air [GATE-2004] (a) dry bulb temperature and specific humidity decrease (b) dry bulb temperature increases and specific humidity decreases (c) dry bulb temperature decreases and specific humidity increases (d) dry bulb temperature and specific humidity increase Answer. (b)

53 74

75

54 76. State of a wet vapour cannot be specified only by [IES-2011] (a) Pressure and temperature (b) Pressure and dryness fraction (c) Temperature and dryness fraction (d) Pressure and volume Answer. (a)

77. The equation φ = Pv/Ps is used to calculate the (pv = partial pressure of water vapour in moist air at a given temperature, Ps = saturation pressure of water vapour at the same temperature) [IES-1999] (a) relative humidity (b) degree of saturation (c) specific humidity (d) absolute humidity Answer. (a) 55 78. The wet bulb depression is zero, when relative humidity is equal to (a) 100% (b) 60% (c) 40% (d) Zero: [IES-2006] Answer. (a) 79. Evaporative air-cooler is used effectively when [IES-1995] (a) dry bulb temperature is very close to the wet bulb temperature (b) dry bulb temperature is high and relative humidity is high (c) dry-bulb temperature is low and relative humidity is high (d) dry bulb temperature is high and the relative humidity is low. Answer. (d) 56 80. What is the saturation temperature at the partial pressure of water vapour in the air-water vapour mixture called? [IES-2009, 2010] (a) Dry bulb temperature (b) Web bulb temperature (c) Dew point temperature (d) Saturation temperature Answer. (c)

81. In a cooling tower, the minimum temperature to which water can be cooled is equal to the [IES-1995; 2001] (a) dew point temperature of the air at the inlet (b) dry bulb temperature of the air at the inlet (c) thermodynamic wet bulb temperature of the air at the inlet (d) mean of the dew point and dry bulb temperature of the air at the inlet Answer. (c)

82. In a chilled-water spray pond, the temperature of water is lower than dew point temperature of entering air. The air passing through the spray undergoes [IES- 1999] (a) cooling and humidification (b) cooling and dehumidification (c) sensible cooling (d) dehumidification Answer. (b) In this case condensation of moisture takes place which results in fall in specific humidity ratio. Cooling and dehumidification take place.

57 83. When a stream of moist air is passed over a cold and dry cooling coil such that no condensation takes place, then the air stream will get cooled along the line of [IES-1996] (a) constant web bulb temperature (b) constant dew point temperature (c) constant relative humidity (d) constant enthalpy. Answer. (b) When a stream of moist air is passed over a cold and dry cooling coil such that no condensation takes place, then air stream is cooled along constant dew point temperature

84. If the specific humidity of moist air remains the same but its dry bulb temperature increases, its dew point temperature [IES-2011] (a) Remains the same (b) Increases (c) Decreases (d) May increase or decrease depending on its relative humidity Answer. (a)

58 85. In case A, moist air is adiabatically saturated and in case B, moist air is isobarically saturated. The saturation temperatures in cases A and B are respectively [IES-2002] (a) dry bulb temperature and wet bulb temperature (b) dew point temperature and wet bulb temperature (c) wet bulb temperature and dew point temperature (d) wet bulb temperature and dry bulb temperature Answer. (c)

86. When the wet and dry bulb temperatures are identical, which of the following statements is/are true? [IES-2001; 2003] 1. Air is fully saturated 2. Dew point temperature is reached 3. Humidity ratio is unity 4. A Partial pressure of vapour equals total pressure Select the correct answer from the codes given below: (a) 1 only (b) 1 and 2 (c) 3 and 4 (d) 1, 2, 3 and 4 Answer. (b)

59 87. When dry-bulb and wet-bulb temperatures are identical, it means that the (a) air is fully saturated and dew-point temperature has reached [IES-2000] (b) (b) air is fully saturated (c) (c) dew-point temperature has reached and humidity is 100% (d) (d) partial pressure of water vapour is equal to total pressure Answer. (a)

88. At 100% relative humidity, the wet bulb temperature is [IES- 1995] (a) more than dew point temperature (b) same as dew point temperature (c) less than dew point temperature (d) equal to ambient temperature. Answer. (b)

60 89. In a saturated air-water vapour mixture, the [IES-1993] (a) dry bulb temperature is higher than the wet bulb temperature (b) dew point temperature is lower than the wet bulb temperature (c) dry bulb, wet bulb and dew point temperatures are the same (d) dry bulb temperature is higher than the dew point temperature Answer. (c) In a saturated air-water vapour mixture, the dry bulb, wet bulb and dew point temperatures are the same.

90. For a given dry bulb temperature, as the relative humidity decreases, the wet bulb temperature will [IES-2011] (a) Increase (b) Decrease (c) Be the same (d) Depend on other factors Answer. (b)

61 91. To fix the state point in respect of air-vapour mixtures, three intrinsic properties are needed. Yet, the Psychrometric chart requires only two because [IES-1998] (a) water vapour is in the superheated state (b) the chart is for a given pressure (c) the chart is an approximation to true values (d) the mixtures can be treated as a perfect gas Answer. (b) Psychrometric chart is plotted for standard and as such only 2 coordinates are used to fix the state point. For other than standard atmospheric, some correction is required.

92. Which one of the following is correct? [IES-2008] In a sensible heating or cooling process (a) dry bulb temperature remains constant (b) wet bulb temperature remains constant (c) the humidity ratio remains constant (d) the relative humidity remains constant

Answer. (c) 62 93. When warm saturated air is cooled [IES-2000] (a) excess moisture condenses (b) excess moisture condenses but relative humidity remains unchanged (c) excess moisture condenses and specific humidity increases but relative humidity remains unchanged. (d) specific humidity increases and relative humidity decreases Answer. (b) RH 100% constant remains. 94. If air flows over a cooling coil, dehumidification of air will take place if the coil surface temperature is below the following of the entering air [IES-2011] (a) Wet bulb temperature (b) Dry bulb temperature (c) Dew point temperature (d) Adiabatic saturation temperature Answer. (c) 63 95. When moist air passes through a bed of silica gel, the [IES-2010] (a) Dry bulb temperature of air decreases (b) Dry bulb temperature of air increases (c) Specific humidity of air increases (d) It undergoes adiabatic saturation Answer. (b) When moist air passes through a bed of silica get chemical dehumidification process occurs. It increases dry bulb temperature of air due to latent heat of vapour.

96. Air at 35°C DBT and 25°C dew point temperature passes through the water whose temperature is maintained at 20°C. What is the process involved? [IES-2004] (a) Cooling and humidification (b) Sensible cooling (c) Cooling and dehumidification (d) Heating and humidification Answer. (c) As temp of shower (200C) is below DBT (350C) sensible cooling will occur. As temp of the shower (200C) is below Dew point temp (250C) some moisture of will condensed and form water droplets i.e. dehumidification. 64 97. For cooling and dehumidifying of unsaturated moist air, it must be passed over a coil at a temperature [IES-2002] (a) of adiabatic saturation of incoming stream (b) which is lower than the dew point of incoming stream (c) which lies between dry bulb and wet bulb temperature (d) which lies between wet bulb and dew point temperature of incoming stream Answer. (b) 98. In summer, air may be cooled and dehumidified by spraying chilled water to air in the form of mist. The minimum temperature to which air may be cooled is the [IES-2011] (a) Wet bulb temperature (b) Adiabatic saturation temperature (c) Apparatus dew point (D) Dry bulb temperature Answer. (c)

65 99. In the case of sensible cooling of air, the coil efficiency is given by (BPF = Bypass factor) [IES-1993] (a) BPF-1 (b) 1-BPF (c) BPF (d) 1 + BPF Answer. (b) Coil efficiency in the sensible cooling is = 1 - BPF 100. The main process which takes place in a desert-cooler is [IAS-2001] (a) sensible cooling (b) dehumidification (c) adiabatic saturation (d) cooling and dehumidification Answer. (c) In a desert-cooler water vaporize and latent heat of vaporization is cools the air. i.e. cooling and humidification.

101. Desert coolers are suitable for hot very dry outside conditions because (a) water is recirculated in the spray [IAS 1994] (b)(b) heat is neither added nor removed from the water (c) (c) wet bulb depression (t-t) is very large (d)(d) large quantity of air can be conditioned Answer. (c) 66

5. A passive method to keep the house comfortably warm by solar conditioning in cold climatic condition is to paint the: [IES-2005] (a) Eastern wall of the house by black paint on its outer side (b) Eastern wall of the house by back paints on its inner side (c) Southern wall of the house by black paint on its outer side (d) Southern wall of the house by black paint on its inner side 5Ans. (b) 9. The reason for a person feeling more comfortable on a warm day if seated in front of an electric fan is that the [IES-1999] (a) metabolic heat production is reduced (b) body loses more heat by convection and evaporation (c) body loses more heat by radiation (d) body loses more heat by evaporation and radiation 9Ans. (b) 10. On a summer day, a scooter rider feels more comfortable while on the move than while at a stop light because [IES-1998] (a) an object in motion captures less solar radiation. (b) air is transparent to radiation and hence it is cooler than the body. (c) more heat is lost by convection and radiation while in motion (d) Air has a low specific heat and hence it is cooler. 10Ans. (c) 12a As an index of comfort, the temperature of saturated air at which a person would experience the same feeling of comfort as experienced in the actual unsaturated environment is called the [IES-2010] (a) Comfort temperature (b) (b) Effective temperature (c) Wet bulb temperature (d) Soothing temperature 12a Ans. (b) It is the definition of Effective temperature (ET)

13. The effective temperature is a measure of the combined effects of [IES-1998] (a) Dry bulb temperature and relative humidity (b) Dry bulb temperature and air motion (c) Wet bulb temperature and air motion (d) Dry bulb temperature, relative humidity and air motion 13Ans. (d) The effective temperature is the combined effect of dry bulb temperature, relative humidity and air motion. 14. Effective temperature is that temperature of saturated air which gives the same degree of comfort as the air at given [IES-1993] (a) DBT, WBT and incidental solar radiation (b) WBT, incidential solar radiation and air flow rate (c) DBT, sol-air temperature and air flow rate (d) DBT, WBT and air flow rate 14Ans. (d)

15. Effective temperature depends on dry bulb temperature, and [IES-2006] (a) Wet bulb temperature only (b) Relative humidity (c) Specific humidity (d) Wet bulb temperature and air motion 15Ans. (d) 17. Which one of the following statements is correct? [IES-2005] The optimum effective temperature for human comfort is: (a) higher in winter than that in summer (b) lower in winter than that in summer (c) same in winter and summer (d) not dependent on season 17Ans. (b) 18. Which one of the following statements is correct? [IES-2004] (a) Effective temperature is the index which the correlates combined effects of air dry bulb temperature, air humidity and air movement upon human comfort (b) The value of effective temperature in winter and summer should be same for human comfort (c) Effective temperature and wet bulb temperature are one and the same (d) The value of effective temperature should be higher in winter than In summer for comfort 18Ans. (a)

19. Upon which of the following factors does the effective temperature for human comfort depend? [IES-2003] 1. Dry bulb temperature 2. Humidity ratio 3. Air velocity 4. Mean radiation temperature Select the correct answer from the codes given below: (a) 1 and 2 (b) 1, 3 and 4 (c) 2, 3 and 4 (d) 1, 2, 3 and 4 19Ans. (d) Important factors are the dry bulb temperature, relative humidity, air motion and surrounding surface temperature. Of these the dry bulb temperature affects heat transfer by convection and evaporation, the relative humidity affects heat loss by evaporation, air velocity influences both convective and evaporative heat transfer and the surrounding surface temperature affects the radiative heat transfer. 20. Consider the following parameters: [IES-2000] 1. Dry-bulb temperature 2. Humidity ratio 3. Air velocity 4. Solar radiation intensity Which of these parameters are taken into account for determining effective temperature for human comfort? (a) 1 and 2 (b) 1 and 4 (c) 2, 3 and 4 (d) 1, 2 and 3 20Ans. (d)

21. The difference between the comfort air conditioning and industrial air conditioning lies in the [IAS-1998] (a) equipment used (b) process adopted (c) indoor requirements (d) ambient conditions 21.Ans. (c) 75 76 4. Moist air at a pressure of 100 kPa is compressed to 500 kPa and then cooled to 350C in an after cooler. The air at the entry to the after cooler is unsaturated and becomes just saturated at the exit of the after cooler. The saturation pressure of water at 35°C is 5.628 kPa. The partial pressure of water vapour (in kPa) in the moist air entering the compressor is closest to [GATE-2008] (A) 0.57 (B) 1.13 (C) 2.26 (D) 4.5 Answer. (B) Volume change is one fifth and water vapour just compressed to one fifth volume so unsaturated vapour pressure= 5.628/5 = 1.1256 = 1.13kPa

77 5. In a sample of moist air at standard atmospheric pressure of 101.325 kPa and 26°C the partial pressure of water vapour is 1.344 kPa. If the saturation pressure of water vapour is 3.36 kPa at 26°C, then what are the humidity ratio and relative humidity of moist air sample? (a) 0.00836 and 1.32% (b) 0.00836 and 40% (c) 0.01344 and 1.32% (d) 0.01344 and 40%

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