Engineering Manual o.. - HVAC.Amickracing

PSYCHROMETRIC CHART FUNDAMENTALSheat) line. The protractor nomograph, in the upper left corner,is used to establish the slope of a process line. The mechanicsof constructing this line are discussed in more detail in theSTEAM JET HUMIDIFIERS section.31.6 BTU/LBD60% RHThe various properties of air can be determined from the chartwhenever the lines of any two values cross even though allproperties may not be of interest. For example, from the pointwhere the 70F dry-bulb and 60F wet-bulb lines cross (Fig. 3,Point A), the following additional values can be determined:62.5°F DPBA67.5°F WB13.8 CF/LBC0.012 LB/LBE77°F DBC1830Fig. 4.26.3BTU/LBDB54°F DPA56% RH13.505 CF/LB70°F DB 60°F WBC1829Fig. 3.C0.0088 LB/LB— Relative humidity is 56 percent (Point A)— Volume is 13.505 cubic feet per pound of dry air(Point A)— Dew point is 54F (Point B)— Moisture content is 0.0088 pounds of moisture per poundof dry air (Point C)— Enthalpy (total heat) is 26.3 Btu per pound of dry air(Point D)— Density is 0.074 pounds per cubic foot (reciprocal ofvolume)Figure 4 is another plotting example. This time the dry-bulbtemperature line and relative humidity line are used to establishthe point. With the relative humidity equal to 60 percent andthe dry-bulb temperature at 77F (Fig. 4, Point A), the followingvalues can be read:— Wet-bulb temperature is 67.5F (Point A)— Volume is 13.8 cubic feet per pound of dry air (Point A)— Dew point is 62.5F (Point B)— Moisture content is 0.012 pounds of moisture per poundof dry air (Point C)— Enthalpy is 31.6 Btu per pound of dry air (Point D)— Density is 0.0725 pounds per cubic foot (reciprocal ofvolume)Figure 5 is the same as Figure 4 but is used to obtain latentheat and sensible heat values. Figures 4 and 5 indicate that theenthalpy (total heat) of the air is 31.6 Btu per pound of dry air(Point D). Enthalpy is the sum of sensible and latent heat (LineA to E + Line E to D, Fig. 5). The following process determineshow much is sensible heat and how much is latent heat. Thebottom horizontal line of the chart represents zero moisturecontent. Project a constant enthalpy line to the enthalpy scale(from Point C to Point E). Point E enthalpy represents sensibleheat of 18.7 Btu per pound of dry air. The difference betweenthis enthalpy reading and the original enthalpy reading is latentheat. In this example 31.6 minus 18.7 equals 12.9 Btu per poundof dry air of latent heat. When the moisture content of the airchanges but the dry-bulb temperature remains constant, latentheat is added or subtracted.31.6 BTU/LBDLATENT HEATB18.7 BTU/LBESENSIBLEHEATACFig. 5.77°F DB60% R.H.C1831ENGINEERING MANUAL OF AUTOMATIC CONTROL 42