Handbook of air conditioning and refrigeration / Shan K

2.12 CHAPTER TWO
Thermodynamic Wet-Bulb Temperature
For any state of moist air, there exists a thermodynamic wet-bulb temperature T* that exactly
equals the saturated temperature of the moist air at the end of the ideal adiabatic saturation process
at constant pressure. Applying a steady flow energy equation, we have
h1 � (w* s � w1)h* w � h* s
(2.38)
where h1, h* s � enthalpy of moist air at initial state and enthalpy of saturated air at end of ideal adiabatic
saturation process, Btu/lb (kJ/kg)
w1,w* s � humidity ratio of moist air at initial state and humidity ratio of saturated air at end
of ideal adiabatic saturation process, lb/lb (kg/kg)
h* w � enthalpy of water as it is added to chamber at a temperature T*, Btu/lb (kJ/kg)
The thermodynamic wet-bulb temperature T*, °F (°C), is a unique property of a given moist air
sample that depends only on the initial properties of the moist air—w 1, h1 and p. It is also a fictitious
property that only hypothetically exists at the end of an ideal adiabatic saturation process.
Heat Balance of an Ideal Adiabatic Saturation Process
Psychrometer
When water is supplied to the insulation chamber at a temperature T* in an ideal adiabatic saturation
process, then the decrease in sensible heat due to the drop in temperature of the moist air is just
equal to the latent heat required for the evaporation of water added to the moist air. This relationship
is given by
cpd(T1 � T*) � cpsw1(T1 � T*) � (w* s � w1)h* fg
(2.39)
where T1 � temperature of moist air at initial state of ideal adiabatic saturation process, °F (°C)
h* fg � latent heat of vaporization at thermodynamic wet-bulb temperature, Btu/lb (J/kg)
Since cpa � cpd � w1cps, we find, by rearranging the terms in Eq. (2.39),
Also
Wet-Bulb Temperature
w* s � w 1
T 1 � T* � c pa
h* fg
T* � T 1 � (w* s � w 1)h* fg
c pa
(2.40)
(2.41)
A psychrometer is an instrument that permits one to determine the relative humidity of a moist air
sample by measuring its dry-bulb and wet-bulb temperatures. Figure 2.4 shows a psychrometer,
which consists of two thermometers. The sensing bulb of one of the thermometers is always kept
dry. The temperature reading of the dry bulb is called the dry-bulb temperature. The sensing bulb of
the other thermometer is wrapped with a piece of cotton wick, one end of which dips into a cup of
distilled water. The surface of this bulb is always wet; therefore, the temperature that this bulb
measures is called the wet-bulb temperature. The dry bulb is separated from the wet bulb by a radiation-shielding
plate. Both dry and wet bulbs are cylindrical.
When unsaturated moist air flows over the wet bulb of the psychrometer, liquid water on the surface
of the cotton wick evaporates, and as a result, the temperature of the cotton wick and the wet bulb