ASHRAE Journal - April 2010 - 24

fan testing to standard air. This residence is located in Phoenix, larger at low surrounding air velocities. While the figure only and testing was done in August 2003. The residence’s air con- goes to 20 fpm (0.10 m/s), extrapolation to zero velocity implies ditioning was operational. The original trial date did not permit that, at worst, an increase of 50% of the wet-bulb depression the testing to be done during times when the indoor conditions seems a conservative maximum deviation. As the air immediwere closer to SBSC and all testing was done in conditions less ately above the wet surface is totally saturated with water, the conducive to evaporation than SBSC. temperature, dew-point temperature and wet-bulb temperature A chart was developed to review the significance of the of that air layer are the same temperature. This principle permits assumed space conditions Figures 2a and 2b. The chart is nor- the generation of relative drying time (Figures 2a, 2b and 3a, malized and shows the relative time for a given drying process 3b). The relative drying times are fairly close, independent of to occur at the different space conditions anticipated in the the actual surface temperature. In this study, the test conditions bathroom. The chart is generated from first principles. As can were less conducive to evaporation than SBSC, and the use of be seen from the chart, the evaporation rate varies by about the “a” charts produce smaller, more conservative corrections. ±25% from the reference condition to the extremes. This chart The more conservative condition was used (Figure 2a) to corshowed that the exact space conditions were not a critical issue rect test data results to SBSC. However, recent studies by the in this case. A more detailed explanation follows. author with infrared sensor thermometers indicate the natural Natural drying, as state previously, is the process of evapo- drying wetted surface temperatures to be closer to the 50% ration of water from a wet surface without the interference of value used in the “b” charts. artificial heating, cooling, wind or forced convection. In this An explanation is needed. For Figure 3b, if a particular drysituation, the rate of evaporaing process takes one hour at tion of water from a wet surface SBSC, it will take 1.2 hours 18 is only proportional to the at 65°F (18°C) dry bulb, 35°F 16 difference in the water vapor (2°C) dew point; but only 0.8 14 pressure of the air immediately hours at 80°F (27°C) dry bulb, 12 above the wet surface and the 50°F (10°C) dew point. The F 10 air reservoir surrounding the condition of 66°F (19°C) dry 8 surface. This vapor pressure bulb, 26°F (–3°C) dew point, 6 difference drives the water should permit identical drying 4 from the air layer immediately times as SBSC. 2 above the wet surface to the air 0 60°F reservoir around it. The vapor Test Procedure 20°F –2 –20°F pressure of water vapor in air Two pieces of evidence to –4 increases with the dew-point consider were: –6 20 50 100 200 500 1,000 2,000 5,000 temperature of that air. Since • Water droplets were presVelocity, fpm the wet surface is evaporatent on the bathmat in the showing water, the surface is being er of the bathroom; and Figure 1: The deviation of the thermometer bulb wetted sock (wetted cooled to a temperature at or • Two washcloths at the surfaces) from thermodynamic wet bulb as a function of surrounding somewhat near the thermodycrime scene bathroom were air velocity. Study used an extrapolation to zero velocity at 60°F wet namic wet-bulb temperature of reported to be “fairly wet” at bulb of 50%. Chart from ASHRAE Standard 41.1-1986 (RA2006). the surrounding air. 1:30 p.m. on April 30, 1989. Deviations of wetted surface temperature from thermodyThe presence of water on the bathmat was the evidence namic wet-bulb temperature are partially addressed in ASHRAE with the most variables but was also the most valuable for the Standard 41.14 in a discussion of wet-bulb temperatures at defense. Many tests were done on sample shower mats at the various surrounding air velocities. Since the Lewis Number test residence. All of these tests had drying times less than 10 for water-wetted surfaces is actually 0.8945 and is less than 1, hours and were done during drying conditions less conducive the energy loss due to water diffusion is greater than the energy to drying than SBSC. One such test had a time averaged space gain from conduction. This effect tends to make the measured condition of 72.6°F (23°C) drying bulb, 53.3°F (12°C) dew wet-bulb temperature lower than the thermodynamic wet-bulb point and a drying time of six hours. Examining Figure 3a, a temperature. Since the rate of heat and mass transfer to sur- one-hour drying process at SBSC would take 1.3 hours at these rounding air at low air velocities is very small, adjacent radiant conditions, translating into less than a five-hour drying time. and conductive heat transfer can become significant, which Many other tests were made that also had bathmat drying tends to make the measured wet-bulb temperature greater than times of less than 10 hours. I urge the readers to try this test the thermodynamic temperature. themselves. Unfortunately, evidence rules would not permit me In Standard 41.1, this effect is quantified (see Figure 1). The to suggest the jurors do this test on their own, as they had to measured wet-bulb temperature is increased by a percentage make a determination based on the facts and opinions presented of the thermodynamic wet-bulb depression and the deviation is in court only.
Actual WB Temp. – Thermodynamic WB Temp. =% WB Depression

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April 2010

ASHRAE Journal - April 2010

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