Transport Equations for Moist Air at Elevated Wet Bulb Temperatures

Abstract

In meteorological applications psychrometers are used both as a humidity transfer standard and as a measurement instrument. Unfortunately wet bulb temperature, t _wb, is not a thermodynamic property and consequently, in equation linking vapor pressure and temperature, the psychrometer constant, from now on called the psychrometer coefficient, a, must be experimentally evaluated. Both theoretical formulations and experimental results show that the psychrometer coefficient, a, depends on a number of parameters. In this work a thermodynamic model of the coupled heat and mass transfer formulation of an adiabatic drying process is derived to state the adiabatic saturation temperature, t _as. Derived equations are also used in a couple of calculated examples to show to the reader why some psychrometric relations tend to be less usable at high wet bulb temperatures. The authors have found, based on the calculations, that the past conclusions of experimental studies of adiabatic evaporation from a water surface in humid air may verify both an assumption that the apparent heat transfer coefficient, α _tot, is greater than the apparent mass transfer coefficient, , (i.e., α _tot >  and t _wb > t _as) as well as an assumption that the apparent heat transfer coefficient is smaller than the apparent mass transfer coefficient (i.e., α _tot <  and t _wb < t _as) although pure analogy considerations propose that the apparent heat transfer coefficient is smaller than the apparent mass transfer coefficient (i.e., α _tot <  and t _wb < t _as).

Keywords:

• Adiabatic drying
• Evaporation
• Mass transfer
• Psychrometer
• Radiation

Acknowledgment

This work was mainly done during Dr. Carl-Gustav Berg's participation in a panEuropean drying project called QUID (i.e., Quality in drying). The authors wish to express their gratitude to the European QUID project for its financial support.

Notes

^aOpen drying process is here to be understood as a drying process with a continuous flow of drying air in and out of the control volume of the drying process.