Question 3.3: Read the properties of moist air at 75 F db, 60 F wb, and st......
Read the properties of moist air at 75 F db, 60 F wb, and standard sea-level pressure
from ASHRAE Psychrometric Chart 1a (see Appendix E).
The intersection of the 75 F db and 60 F wb lines defines the given state. This point
on the chart is the reference from which all the other properties are determined.
Humidity Ratio W. Move horizontally to the right and read W = 0.0077 lbmv/lbma on the vertical scale.
Relative Humidity φ. Interpolate between the 40 and 50 percent relative humidity lines and read φ = 41 percent.
Enthalpy i. Follow a line of constant enthalpy upward to the left and read i = 26.4 Btu/lbma on the oblique scale.
Specific Volume v. Interpolate between the 13.5 and 14.0 specific volume lines and read v = 13.65 ft³/lbma.
Dew Point t_{d}. Move horizontally to the left from the reference point and read t_{d} = 50 F on the saturation curve.
Enthalpy i (alternate method). The nomograph in the upper left-hand corner of Chart 1a gives the difference D between the enthalpy of unsaturated moist air and the enthalpy of saturated air at the same wet bulb temperature. Then i = i_{s} + D. For this example i_{s} = 26.5 Btu/lbma, D = −0.1 Btu/lbma, and i = 26.5 − 0.1 = 26.4 Btu/lbma. Not all charts have this feature.