In describing air–vapor mixtures, two wet bulb temperatures are commonly
used: the psychrometric wet bulb temperature and the thermodynamic
wet bulb temperature. For moist air, the numerical values
of these two temperatures are approximately the same. However, in
other gas–vapor systems, the difference between the two temperatures
can be substantial.
The psychrometric wet bulb temperature is obtained when the bulb
of a mercury thermometer is covered with a wet wick and exposed to
unsaturated air fl owing past the bulb at high velocity (about 5 m/s).
Alternatively, the bulb covered with a wet wick can be moved through
unsaturated air. When the wick is exposed to unsaturated air, moisture
evaporates due to the vapor pressure of saturated wet wick being
higher than that of the unsaturated air.
The evaporation process requires latent heat from the wick and causes
the temperature of the covered bulb to decrease. As the temperature
of the wick decreases below the dry bulb temperature of air, the sensible
heat fl ows from the air to the wick and tends to raise its temperature.
A steady state is achieved when the heat fl ow from air to wick
is equal to the latent heat of vaporization required to evaporate the
moisture from the wick. This equilibrium temperature indicated by
a wet bulb thermometer or similarly modifi ed temperature sensor is
called the wet bulb temperature.
As mentioned previously, the movement of air past the wet wick is
essential, otherwise the wick will attain an equilibrium temperature
between T a and T w .
In contrast to the psychrometric wet bulb temperature, the thermodynamic
wet bulb temperature is reached by moist air when it is adiabatically
saturated by the evaporating water. The thermodynamic wet
bulb temperature is nearly equal to the psychrometric wet bulb temperature
for moist air.
A mathematical equation that relates partial pressures and temperatures
of air–vapor mixtures, developed by Carrier, has been used widely
in calculations to determine psychrometric properties. The equation is
In describing air–vapor mixtures, two wet bulb temperatures are commonly
used: the psychrometric wet bulb temperature and the thermodynamic
wet bulb temperature. For moist air, the numerical values
of these two temperatures are approximately the same. However, in
other gas–vapor systems, the difference between the two temperatures
can be substantial.
The psychrometric wet bulb temperature is obtained when the bulb
of a mercury thermometer is covered with a wet wick and exposed to
unsaturated air fl owing past the bulb at high velocity (about 5 m/s).
Alternatively, the bulb covered with a wet wick can be moved through
unsaturated air. When the wick is exposed to unsaturated air, moisture
evaporates due to the vapor pressure of saturated wet wick being
higher than that of the unsaturated air.
The evaporation process requires latent heat from the wick and causes
the temperature of the covered bulb to decrease. As the temperature
of the wick decreases below the dry bulb temperature of air, the sensible
heat fl ows from the air to the wick and tends to raise its temperature.
A steady state is achieved when the heat fl ow from air to wick
is equal to the latent heat of vaporization required to evaporate the
moisture from the wick. This equilibrium temperature indicated by
a wet bulb thermometer or similarly modifi ed temperature sensor is
called the wet bulb temperature.
As mentioned previously, the movement of air past the wet wick is
essential, otherwise the wick will attain an equilibrium temperature
between T a and T w .
In contrast to the psychrometric wet bulb temperature, the thermodynamic
wet bulb temperature is reached by moist air when it is adiabatically
saturated by the evaporating water. The thermodynamic wet
bulb temperature is nearly equal to the psychrometric wet bulb temperature
for moist air.
A mathematical equation that relates partial pressures and temperatures
of air–vapor mixtures, developed by Carrier, has been used widely
in calculations to determine psychrometric properties. The equation is
การแปล กรุณารอสักครู่..
