PSYCHROMETRICS and ELEMENTARY PROCESSES (English Units)

1. PSYCHROMETRICS and ELEMENTARY PROCESSES (English Units) Ashley F. Emery University of Washington A F Emery

2. Psychrometrics The study of a mixture of dry air and water vapor Although precise thermodynamic relations are available for moist air, we will treat moist air as a mixture of ideal gases A F Emery

3. Why study psychrometrics? The degree of moisture has a strong effect on 1) heating, cooling, and comfort 2) insulation, roofing, stability and deformation of building materials 3) sound absorption, odor levels, ventilation 4) industry and agriculture A F Emery

4. Dry Air and Water Vapor Dry Air Component % by vol MW Nitrogen 78.084 28.0134 Oxygen 20.448 31.9988 Argon .934 39.9430 Carbon Dioxide .031 44.0100 Effective MW 28.9645 Water Vapor 18.0153 A F Emery

5. IDEAL GAS PV=mRT P = pressure lbf/sq. ft. V= volume cu. ft. m=mass lbm R=gas constant T=temperature R =F+460 A F Emery

6. Dalton’s Law partial pressures A F Emery

7. Mixture of Gases letting and remembering that we obtain A F Emery

8. IMPORTANTPROPERTIES Humidity ratio, W Humidity ratio is the mass of water vapor per unit mass of dry air. Units are Lbm/Lbm, grams/grams, or grains/lbm (7000 grains=1Lbm) A F Emery

9. IMPORTANT PROPERTIES Saturation is when the air contains the maximum amount of water vapor at its current temperature. The saturation pressure is taken from the steam tables at the moist air temperature. Saturated Humidity ratio, A F Emery

10. IMPORTANTPROPERTIES Relative humidity, Relative humidity is defined as the ratio of the partial pressure of the water vapor to the saturation pressure at the same temperature A F Emery

11. IMPORTANT PROPERTIES Degree of saturation, Degree of saturation is the ratio of the amount of water contained in the moist air to that which would be contained if the air were saturated A F Emery

12. IMPORTANT TEMPERATURES Dry Bulb = temperature of moist air at rest Dew Point = temperature at which the water vapor will condense out of the moist air. It is the temperature for which W is the saturated humidity ratio A F Emery

13. IMPORTANT TEMPERATURES Adiabatic Saturation Temperature, it is the temperature at which liquid water would evaporate into the moist air without any heat addition to the system satisfies A F Emery

14. IMPORTANT TEMPERATURES Wet Bulb Temperature, Is the temperature reached by evaporative cooling. A cotton sock is wrapped around a thermometer, saturated with distilled water. The water evaporates and the resulting temperature is called the wet bulb temperature. It is a close approximation to A F Emery

15. Thermodynamic Properties enthalpy, BTU/unit weight of dry air =0.24 T +W(0.45 T +1061.1) specific volume, cu. ft. /unit weight of dry air A F Emery

16. Example moist air at 80F dry bulb, 65F dew point, 14.696psia A F Emery

17. W 0 . 0132 m = = = 0 . 595 W 0 . 0222 s P 0 . 306 w f = = = 0 . 604 Pws 0 . 507 53 . 35 * ( 80 + 460 ) v = da 14 . 696 - 0 . 306 ) * 144 Example (continued) Example a) degree of saturation d) relative humidity e) enthalpy h=0.24*80 +0.0132*(0.45*80 + 1061.1) = 1153.8 BTU/lbm-da f) volume = 13.90 cu. Ft. /lbm-da A F Emery

18. W Psychrometric Chart h saturation line rh A F Emery

19. W Simple Heating 1 2 A F Emery

20. Simple Heating, solution 0.24*50+0.003*(0.45*50+1061.)=15.25 0.24*70+0.003*(0.45*70+1061.)=20.08 = 4.83 BTU/Lbm-da A F Emery

21. W Simple Heating and Humidification 3 1 A F Emery

22. W Simple Heating and Humidification, Solution 3 15.25 1 18.06 0.24*70+0.0108*(0.45*70+1061.1)=28.60 =0.0108-0.003=0.0078Lbm/Lbm A F Emery

23. W Simple Heating and Humidification, Solution 3 15.25 1 18.06 0.24*70+0.0108*(0.45*70+1061.1)=28.60 =28.60-15.25-0.0078*18.06 =13.21 BTU/Lbm-da A F Emery

24. W Dehumidification and Cooling 1 28.60 2 15.25 The answer is the same as for the previous problem since the end points are the same BUT how can we actually go from point 1 to point 2?? A F Emery

25. 1 1’ W 2’ 2 Dehumidification and Cooling, solution 1 to 1’ by cooling 1’ to 2’ by cooling and dehumidification 2’ to 2 by heating A F Emery

26. 1 W 2 Dehumidification and Cooling, solution 1 to 1’ by cooling 1’ 28.60 2’ 0.0108 59.2F 0.24*59.2+0.0108*(0.45*59.2+1061.1)=25.96 (25.96-28.60)=-2.64 A F Emery

27. 1 W 2 Dehumidification and Cooling, solution 1’ to 2’ by cooling and dehumidification 1’ 2’ assume that the water leaves at A F Emery

28. 1 W 2 Dehumidification and Cooling, solution 1’ to 2’ by cooling and dehumidification 1’ 2’ A F Emery

29. 1 W 2 Dehumidification and Cooling, solution 2’ to 2 by heating 1’ 2’ A F Emery

30. 1 1’ W 2’ 2 Dehumidification and Cooling, solution A F Emery

31. 1 1’ W W 2’ 2 Difference between Humidification and Dehumidification 2 1 Water is rejected at 27F Water is injected at 50F A F Emery

32. Adiabatic Mixing of 2 Streams 1 3 2 A F Emery

33. 1 2 W 3 3 1 2 Adiabatic Mixing of 2 Streams A F Emery

34. Adiabatic Mixing of 2 Streams, example 1 500 cfm at 60F dry bulb and rh=50% is mixed with 250 cfm at 80F dry bulb and 60F wet bulb. 3 2 A F Emery

35. Adiabatic Mixing of 2 Streams, example 1 3 2 68F db, 40% rh, 43.4F dp A F Emery