Use Of Psychrometric Charts
Three charts for the airwater vapor system are given as Figs. 121 to 123 for low, medium, and hightemperature ranges. Figure 124 shows a modified Grosvenor chart, which is more familiar to the chemical engineer. These charts are for an absolute pressure of 1 atm. The corrections required at pressures different from atmospheric are given in Table 122. Figure 125 shows a psychrometric chart for combustion products in air. The thermodynamic properties of moist air are given in Table 121.
Examples Illustrating Use of Psychrometric Charts In these examples the following nomenclature is used:
t = drybulb temperatures, °F tw = wetbulb temperature, °F td = dewpoint temperature, °F H = moisture content, lb waterflb dry air
AH = moisture added to or rejected from the air stream, lb waterflb dry air h' = enthalpy at saturation, Btuflb dry air D = enthalpy deviation, Btuflb dry air h = h' + D = true enthalpy, Btuflb dry air hw = enthalpy of water added to or rejected from the system,
Btuflb dry air qa = heat added to the system, Btuflb dry air qr = heat removed from system, Btuflb dry air
Subscripts 1, 2, 3, etc., indicate entering and subsequent states.
Example 2: Determination of Moist Air Properties Find the properties of moist air when the drybulb temperature is 80°F and the wetbulb temperature is 67°F.
Solution. Read directly from Fig. 122 (Fig. 126 shows the solution diagrammatically).
Moisture content H = 78 grflb dry air
= 0.011 lb waterflb dry air Enthalpy at saturation h' = 31.6 Btuflb dry air Enthalpy deviation D = 0.1 Btuflb dry air True enthalpy h = 31.5 Btuflb dry air Specific volume v = 13.8 ft3flb dry air Relative humidity = 51 percent Dew point td = 60.3°F
Example 3: Air Heating Air is heated by a steam coil from 30°F drybulb temperature and 80 percent relative humidity to 75°F drybulb temperature. Find the relative humidity, wetbulb temperature, and dew point of the heated air. Determine the quantity of heat added per pound of dry air. Solution. Reading directly from the psychrometric chart (Fig. 122),
Volume in Cubic Feet Per Pound of Dry Air
FIG. 121 Psychrometric chart—low temperatures. Barometric pressure, 29.92 inHg. To convert British thermal units per pound to joules per kilogram, multiply by 2326; to convert British thermal units per pound dry airdegree Fahrenheit to joules per kilogramkelvin, multiply by 4186.8; and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624.
Volume in Cubic Feet Per Pound of Dry Air
FIG. 121 Psychrometric chart—low temperatures. Barometric pressure, 29.92 inHg. To convert British thermal units per pound to joules per kilogram, multiply by 2326; to convert British thermal units per pound dry airdegree Fahrenheit to joules per kilogramkelvin, multiply by 4186.8; and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624.
Relative humidity = 15 percent Wetbulb temperature = 51.5°F Dew point = 25.2°F
The enthalpy of the inlet air is obtained from Fig. 122 as h = h1 + Di = 10.1 + 0.06 = 10.16 Btu/lb dry air; at the exit, h = h2 + D2 = 21.1  0.1 = 21 Btu/lb dry air. The heat added equals the enthalpy difference, or qa = Ah = h2  h = 21  10.16 = 10.84 Btu/lb dry air
If the enthalpy deviation is ignored, the heat added qa is Ah = 21.1  10.1 = 11 Btu/lb dry air, or the result is 1.5 percent high. Figure 127 shows the heating path on the psychrometric chart.
Example 4: Evaporative Cooling Air at 95°F drybulb temperature and 70°F wetbulb temperature contacts a water spray, where its relative humidity is increased to 90 percent. The spray water is recirculated; makeup water enters at 70°F. Determine exit drybulb temperature, wetbulb temperature, change in enthalpy of the air, and quantity of moisture added per pound of dry air.
Solution. Figure 128 shows the path on a psychrometric chart. The leaving drybulb temperature is obtained directly from Fig. 122 as 72.2°F. Since the spray water enters at the wetbulb temperature of 70°F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wetbulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows:
Inlet moisture H\ = 70 gr/lb dry air Exit moisture H2 = 107 gr/lb dry air AH = 37 gr/lb dry air Inlet enthalpy h = h1 + D1 = 34.1  0.22
= 33.88 Btu/lb dry air Exit enthalpy h = h'2 + D2 = 34.1  0.02 = 34.08 Btu/lb dry air
Then
Enthalpy of added water hw = 0.2 Btu/lb dry air (from small diagram, 37 gr at 70°F)
Example 5: Cooling and Dehumidification Find the cooling load per pound of dry air resulting from infiltration of room air at 80°F drybulb temperature and 67°F wetbulb temperature into a cooler maintained at 30°F drybulb and 28°F wetbulb temperature, where moisture freezes on the coil, which is maintained at 20°F.
Solution. The path followed on a psychrometric chart is shown in Fig. 129.
Inlet enthalpy h = h1 + D1 = 31.62  0.1 = 31.52 Btu/lb dry air Exit enthalpy h2 = h2 + D2 = 10.1 + 0.06 = 10.16 Btu/lb dry air Inlet moisture H1 = 78 gr/lb dry air Exit moisture H2 = 19 gr/lb dry air Moisture rejected AH = 59 gr/lb dry air Enthalpy of rejected moisture = 1.26 Btu/lb dry air (from small diagram of Fig. 122) Cooling load qr = 31.52  10.16 + 1.26 = 22.62 Btu/lb dry air
Note that if the enthalpy deviations were ignored, the calculated cooling load would be about 5 percent low.
Example 6: Cooling Tower Determine water consumption and amount of heat dissipated per 1000 ft3/min of entering air at 90°F drybulb temperature and 70°F wetbulb temperature when the air leaves saturated at 110°F and the makeup water is at 75°F.
Solution. The path followed is shown in Fig. 1210.
FIG. 123 Psychrometric chart—high temperatures. Barometric pressure, 29.92 inHg. To convert British thermal units per pound to joules per kilogram, multiply by 2326; to convert British thermal units per pound dry airdegree Fahrenheit to joules per kilogramkelvin, multiply by 4186.8; and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624.
FIG. 123 Psychrometric chart—high temperatures. Barometric pressure, 29.92 inHg. To convert British thermal units per pound to joules per kilogram, multiply by 2326; to convert British thermal units per pound dry airdegree Fahrenheit to joules per kilogramkelvin, multiply by 4186.8; and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624.
FIG. 124 Humidity chart for airwater vapor mixtures. To convert British thermal units per pound to joules per kilogram, multiply by 2326; to convert British thermal units per pound dry airdegree Fahrenheit to joules per kilogramkelvin, multiply by 4186.8; and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624.
FIG. 124 Humidity chart for airwater vapor mixtures. To convert British thermal units per pound to joules per kilogram, multiply by 2326; to convert British thermal units per pound dry airdegree Fahrenheit to joules per kilogramkelvin, multiply by 4186.8; and to convert cubic feet per pound to cubic meters per kilogram, multiply by 0.0624.
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