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CHAPTER NO.4 Psychrometry

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CHAPTER NO.4 Psychrometry 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 © American Standard Inc. 1999 Air Conditioning Clinic TRG-TRC001-EN Fog Occurs When Air Is Saturated © American Standard Inc. 1999 Air Conditioning Clinic TRG-TRC001-EN Relative Humidity …describes the degree of saturation 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 © American Standard Inc. 1999 Air Conditioning Clinic TRG-TRC001-EN Basic Concepts Saturation Line PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury 200 190 180 170 160 150 140 130 120 110 Staturation Line 100 90 80 70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60 50 40 30 20 10 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 200 190 180 170 160 150 140 130 Constant Dry Bulb Temperature 120 110 100 90 80 70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60 50 40 30 20 10 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 200 190 180 170 160 150 140 130 Constant Humidity Ratio 120 110 100 90 80 70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60 50 40 30 20 10 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 200 190 180 170 160 150 140 130 Constant Humidity Ratio 120 110 100 90 80 70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60 50 40 30 20 10 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 200 190 180 170 160 150 140 130 Constant Relative Humidity 120 110 100 90 80 90% 70 80% HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF 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 200 190 15.5 180 170 160 150 140 130 15.0 Constant Specific Volume 120 110 100 90 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 40 30 13.5 20 13.0 10 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 200 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 90 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 Constant Enthalpy PSYCHROMETRIC CHART Lexington, Kentucky USA 210 BAROMETRIC PRESSURE 28.874 inches of Mercury 200 190 180 170 160 150 Constant Enthalpy 140 130 120 110 100 90 80 70 HUMIDITY RATIO - GRAINS OF MOISTURE POUND PER OF DRY AIR 60 50 40 30 20 10 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 90 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 - OF INCHES 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.
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