Handbook of air conditioning and refrigeration / Shan K

2.22 CHAPTER TWO
Cooling and Dehumidifying Curves. The two cooling and dehumidifying curves plotted
on ASHRAE’s chart are based on data on coil performance published in the catalogs of U.S.
manufacturers. These curves are very helpful in describing the actual locus of a cooling and dehumidifying
process as well as determining the state points of air leaving the cooling coil.
2.10 DETERMINATION OF THERMODYNAMIC PROPERTIES ON
PSYCHROMETRIC CHARTS
There are seven thermodynamic properties or property groups of moist air shown on a psychrometric
chart:
1. Enthalpy h
2. Relative humidity �
3. Thermodynamic wet-bulb temperature T*
4. Barometric or atmospheric pressure p at
5. Temperature T and saturation water vapor pressure p ws
6. Density � and moist volume v
7. Humidity ratio w, water vapor pressure p w, and dew-point temperature T dew
The fifth, sixth, and seventh are thermodynamic property groups. These properties or properties
groups are independent of each other except that the difference in slope between the enthalpy h line
and thermodynamic wet-bulb temperature T* line is small, and it is hard to determine their intersection.
Usually, atmospheric pressure p at is a known value based on the altitude of the location. Then, in
the fifth property group, p ws is a function of temperature T only. In the sixth property group, according
to Eq. (2.33), � a � 1/v; that is, air density and moist volume are dependent on each other. In the
seventh property group, for a given value of p at, properties w, p w, and T dew are all dependent on each
other.
When p at is a known value, and if the moist air is not saturated, then any two known independent
thermodynamic properties can determine the magnitudes of the remaining unknown properties. If
the moist air is saturated, then any independent property will determine the remaining magnitudes.
Example 2.1. The design indoor air temperature and relative humidity of an air conditioned space
at sea level are 75°F (23.9°C) and 50 percent. Find the humidity ratio, the enthalpy, and the density
of the indoor moist air
1. By using the ASHRAE chart
2. By calculation
Determine also the dew-point and thermodynamic wet-bulb temperatures of the moist air. The
following information is required for the calculations:
Atmospheric pressure at sea level
Specific heat of dry air
Specific heat of water vapor
Enthalpy of saturated vapor at 0°F
Gas constant of dry air
Solution
14.697 psi (101,325 Pa)
0.240 Btu/lb�°F (1.005 kJ/kg�K)
0.444 Btu/lb�°F (1.859 kJ/kg�K)
1061 Btu/lb (2468 kJ/kg)
53.352 ft�lb f/lb m�°R (0.287 kJ/kg�K)
1. Plot the space point r on ASHRAE’s chart by first finding the space temperature T r � 75°F on