CHAPTER NO.4 Psychrometry

C605.4-Explain Psychometric properties & calculate various parameters.

Necessity of Air conditioning

 Purpose of air conditioning is  1) To provide comfort conditions for human comfort.  2) To provide comfort in commercial places like office, restaurant ,shopping complex , banks etc.  3) To provide controlled conditions for industrial process.  4) To provide ultra clean atmosphere for precision work. Dalton’s low of partial pressure

• Dalton’s law of partial pressure statures that ‘the total pressure of mixture of gases equal to the sum of the partial pressures exerted by each gas when it occupies the mixture volume at there temperature of mixture’.

• According to Dalton’s law of partial pressure, • Pt = Pa + Pb + Pc.

Psychometrics properties of air

1. Dry air : It is a mixture of oxygen (20.91%) and nitrogen (79.09 %) by volume. 2. Moist air: It is mixture of dry air and water vapour. 3. Saturated air : Air which have maximum amount of water vapor. 4. Dry bulb temp.(DBT) : It is temperature of air recorded by ordinary thermometer with clean and dry sensing elements.(td or tdb) 5. Wet bulb temp.(WBT) : It is temperature of air recorded by thermometer when its bulb is covered with a wet cloth.(tw or twb) Psychometric properties of air 6. Wet bulb depression : It is the difference between dry bulb temp. and wet bulb temp. 7. Dew point temp.: It is the temperature of air recorded by thermometer when the moisture present in the air begins to condensed. 8. Dew point depression : It is difference between dry bulb temp. and dew point temp. of air. 9. Specific humidity or Humidity ratio (W) : It is defined as mass of water vapour in unit mass of dry air. w= mv/ma 10. Degree of saturation or Percentage humidity (  ) : It is the ratio of mass of water vapour in unit mass

Psychometrics properties of air

of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature.  = w/ws 11. Relative humidity : It is the ratio of actual mass of water vapour (mv) in a given volume of moist air to mass of water vapour (ms) in the same volume of saturated air at the same temp. and pressure.  = mv/ms 12. Enthalpy of moist air : (H) It is total heat contained in moist air. It is sum of sensible heat and latent heat of moist air.

Psychrometric processes

The process of changing and affecting the psychometric properties of the moist air are called psychometric processes. The processes are : 1. Sensible heating 2. Sensible cooling 3. Humidification 4. Dehumidification 5. Heating and humidification 6. Heating and dehumidification

Psychometric processes

7. Cooling and dehumidification 8. Cooling with adiabatic humidification. 9. Adiabatic mixing of two air streams. Properties Of Air

 Dry-bulb temperature  Wet-bulb temperature  Dew-point temperature  Relative humidity  Humidity ratio Dry-Bulb Thermometer Wet-Bulb Thermometer Condensation Occurs At Dew Point

Amount of moisture that a given amount of air is holding Relative Humidity = Amount of moisture that a given amount of air can hold Relative Humidity …compares moisture content to saturation

50% 100% (saturated) Humidity Ratio …compares water vapor to dry air, by weight Knowing Two Properties …lets you determine the remaining three  Dry-bulb temperature  Wet-bulb temperature  Dew-point temperature  Relative humidity  Humidity ratio Summer Design Conditions

 95°F DB (dry bulb)  78°F WB (wet bulb) “X Marks The Spot”…

78°F

72°F

95°F Psychrometric Chart

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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190

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110 Staturation Line 100

70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND OF PER DRY AIR 60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Dry Bulb Temperature

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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Constant Dry Bulb Temperature 120

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70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND OF PER DRY AIR 60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Humidity Ratio

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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Constant Humidity Ratio 120

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70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Humidity Ratio

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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Constant Humidity Ratio 120

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70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Relative Humidity

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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Constant Relative Humidity 120

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90%

70 80% HUMIDITY RATIO - GRAINS OF MOISTURE POUND OF PER DRY AIR

70% 60

60% 50

50% 40

40% 30

30% 20 20%

10 10% RELATIVE HUMIDITY

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Specific Volume

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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190

15.5 180

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130 15.0

Constant Specific Volume 120

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100

14.5 SPECIFIC VOLUME ft³/lb OF DRY AIR 80

70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60

14.0 50

30 13.5

13.0

1 2 .5

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Wet Bulb Temperature

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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85 190

180 8 5 WET BULB TEMPERATURE 170

80 160

- °F 150 80

140 Constant Web Bulb Temperature 75

130

75 120 70

110

70 100 65

65 60 80

70 55 60 HUMIDITY RATIO - GRAINS OF MOISTURE POUND OF PER DRY AIR 60 50 55

50 45 50

40 40 45

35 40 30 30 35 25 30 20 20 15 25 20 10 10 5 15 10 0 5

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Enthalpy

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

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Constant Enthalpy 140

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70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Constant Enthalpy and Web Bulb

PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury

200

85 190

180 85 WET

BULB TEMPERATURE 170

80 160

- °F 150 80

140 Constant Enthalpy 75 and 130 75 120 Constant Web Bulb 70

110

70 100 65

65 60 80

70 55 60 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60 50 55

50 45 50

40 40 45

35 40 30 30 35 25 30 20 20 15 25 20 10 10 5 15 10 0 5

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Typical Chart With Enthalpy Lines

PSYCHROMETRIC CHART 55 60 210 Lexington, Kentucky USA 1.3 BAROMETRIC PRESSURE 28.874 inches of Mercury 50 200 65

85 85 190 1.2

180 45 85 WET BULB TEMPERATURE - °F 1.1 170 60 80 80 160

40 1.0

150 80

140 .9 75 75 55 35 130 15.

0 75 .8 120 70 70 30 110 ENTHALPY - BTU PER POUND OF DRY AIR .7 50 70 100 SATURATION65 TEMPERATURE - °F 65 .6 90 25

65 14. 60 5 80 60

SPECIFI ENTHALPY - BTU POUND PER OF DRY AIR VAPOR PRESSURE - OFINCHES PRESSURE VAPOR MERCURY 20 90% .5 45 70 60 C VOLUME 55 80% 55 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR .4 15 70% 60 50 55 f t ³/ 50

lb 60% 14. OF 50 0 45 50 DRY 45 .3 10 50% 40 40 AI 40 45 R 40 35 40% 40 35 13.5 30 .2 30 5 35 30% 30 25 30 20 25 20 20% 13.0 15 25 20 .1 10 20 35

10 POINTDEW - °F 5 15 10% RELATIVE HUMIDITY 10 10 12.5 0 0 5 -20 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F

10 15 20 25 30 Typical Chart Without Enthalpy Lines

PSYCHROMETRIC CHART 55 60 210 Lexington, Kentucky USA 1.3 BAROMETRIC PRESSURE 28.874 inches of Mercury 50 200 65

85 85 190 1.2

180 45 85 WET BULB TEMPERATURE - 1.1 170 60 80 80 160

40 1.0 °F 150 80

F DRY AIR 140 .9 75 75 55 UND O O 35 130 ER P 15.0

75 .8 RATURE - °F E - BTU P 120 MP 70 LPY 70 N TE 30 NTHA 110 E .7 50 70 ATURATIO 100 S 65 65 .6 90 25

65 14.5 SPECIFIC VOLUME ft³/lb OF DRY 60 80 60 ENTHALPY - BTU POUND OFPER DRY AIR VAPOR PRESSURE - OF INCHES PRESSURE VAPOR MERCURY 20 90% .5 45

60 70 55 80% 55 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR .4 15 70% 60 50 55 50

14.0 60% 50 45 50 45 .3 10 50% 40 40 AIR 40 45 40 35 40% 40 35 13. 30 .2 30 5 5 35 30% 30 25 30 20 25 20 20% 13. 15 25 20 .1 10 20 0 35

10 POINTDEW - °F 5 15 10% RELATIVE HUMIDITY 10 10 12.5 0 0 5 -20 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F

10 15 20 25 30 Reading a Psychrometric Chart Practice State Point

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

State Point 70 F

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140 Constant Dry Bulb Temperature 130 120

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70 HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIRDRY OF POUND PER MOISTURE OF GRAINS - RATIO HUMIDITY 60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 Linric Company Psychrometric Chart, www.linric.com DRY BULB TEMPERATURE - °F Dry Bulb 70 State Point 60% Wet-bulb

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Wet-bulb temperature = ? F Wet-bulb Wet-bulb

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Wet-bulb temperature = 61 F Dew Point

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Dew point = ?

Dew Point

Dew Point Dew Point

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Dew point = 55.5 F

Specific Volume

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Specific volume = ? Specific Volume Specific Volume

Sea Level Chart Dry-bulb temperature = 70 F Relative Humidity = 60%

Specific volume = 13.6 ft3 / lb dry air Humidity Ratio

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Humidity ratio = ?

Humidity Ratio Humidity Ratio

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Humidity ratio = 0.0094 lb water / lb dry air or 7000 grains = 1 lb water 7000 x 0.0094 = 65.8

Enthalpy

Sea Level Chart Dry-bulb temperature = 70 F Relative humidity = 60%

Enthalpy = ?

Enthalpy

Sensible Heating Mixing Air Streams

ma1 Energy Balance h1 ma1 h1 + ma2 h2 = ma3 h3 w1

m a3 Mass Balance h3 w3 ma1 w1 + ma2 w2 = ma3 w3

By Algebra ma2 h2 h3 = w2 w3 = ma1 h2 – – h3 – h1 w3 – w1 ma2 w2

Cooling Cooling Systems Cooling Systems Evaporative Cooling