The wet-bulb temperature is the reading registered by a temperature sensor placed in a moist gas stream and covered with a wetted cloth or wick. This temperature is lower than that of the gas stream itself, and is the dynamic equilibrium value attained when the convective heat transfer to the sensor effectively equals the evaporative heat load associated with the moisture loss from the wetted surface. Another temperature is sometimes called the thermodynamic wet-bulb temperature. This is the limiting temperature reached as a gas cools on adiabatic saturation, and is more properly termed the adiabatic-saturation temperature to avoid confusion.
Figure 1 shows the temperature and humidity profiles about a plane moist surface exposed to a humid gas flowing over it.
If the Chilton-Colburn analogy between heat and mass-transfer processes is used in its original (1934) form, then the temperature difference between that of the wetted surface (TS) and that of the bulk gas (TG) becomes [Keey (1992)]:
where YS and YG are the humidities of the gas adjacent to the surface and in the bulk gas, respectively, Lu is the Luikov number, the ratio of the mass to the thermal diffusivity, ΔHVS is the latent heat of vaporization of the moisture at the surface temperature and CPY is the humid heat capacity (the heat capacity of unit mass of dry gas and its associated moisture-vapor content). The various φi are factors close to 1 to account for secondary effects: φH is the Ackermann correction factor to account for the influence of the evaporation on the convective heat transfer; φM is the humidity-potential coefficient introduced to correct for the embedded approximations in the use of linear humidity differences in describing the evaporation rate; φR is the fractional contribution of radiation to the heat transfer. The above equation for the temperature difference or wet-bulb depression is remarkably similar in form to that for the temperature drop on adiabatic saturation (TG – TGS):
The terms within the square brackets is sometimes called the psychrometric ratio, although other slightly different definitions appear in the literature which relate to the coefficient in the equation for the wet-bulb depression. By contrast, the psychrometric ratio does not appear in the adiabatic-saturation expression. Whenever the moisture is water and the gas air, the psychrometric ratio is almost unity. Commonly it is assumed, then, that the wet-bulb and adiabatic saturation temperatures are the same within the accuracy with which these temperatures can be measured in practice. On the other hand, should the moisture be a nonaqueous solvent (when the Luikov number would be much smaller than 1), then this identity between the two temperatures no longer holds. The wet-bulb temperature is now greater than the adiabatic-saturation temperature, and both are higher than the dewpoint value, as illustrated in Figure 2
Unlike the evaporation of a single droplet containing a mixture of volatile liquids, the evaporation of a mixture from a porous surface is normally nonselective and the composition does not change [Turner and Schlünder (1985); see Pakowski (1990)]. At a given composition there is a specific wet-bulb temperature. This temperature for a nonideal mixture (in the tested case, isopropanol-water) can be less than the wet-bulb temperatures of either pure component for a given water-vapor concentration, but it may vary monotonically with composition depending upon the vapor concentrations.
The wet-bulb thermometer is one of two sensors which make up a psychrometer to measure the humidity of a moist gas. Both sensors should be shielded from thermal radiation and exposed to a flow of the gas at a sufficient rate to ensure fully developed forced convection. A gas velocity of 5 m s–1 over the bulbs is normally sufficient. The wick surrounding the bulb must be kept wetted, and commercial psychrometers usually provide a formal drip-feed system [Wiederhold (1987)]. Special precautions are required at dry-bulb temperatures above 100°C, by preheating the wick water to a few degrees of the estimated wet-bulb temperature to avoid the wick drying out. For the same reason, wet-bulb temperatures are very difficult to measure with accuracy once the relative humidity falls below about 20%.
Keey, R. B. (1992) Drying of Loose and Particulate Materials, Pergamon, Oxford.
Pakowski, Z. (1990) Drying of solids containing multicomponent moisture, Advances in Drying, 5 Ch. 5, A. S. Mujumdar Ed., Hemisphere, Washington.
Wiederhold, P., (1987) Humidity Measurements, Handbook of Drying Technology, Ch. 29, A. S. Mujumdar Ed., Marcel Dekker, New York and Basel.