The form of (10) justifies a linear combination of dry-bulb and dewpoint temperatures as an approximation to wet-bulb temperature, for the assumptions of relatively small
wet-bulb depression and relatively moist conditions.
Note that the difference between the dry bulb and wet bulb of the air is known as the
wet-bulb depression. In areas of high
wet-bulb depression, such as the American Southwest, evaporative heat rejection offers even greater energy efficiency by enabling significantly lower system temperatures.
For example, at the relatively modest
wet-bulb depression condition of 26.7[degrees]C/19.4[degrees]C (80.1[degrees]F/66.9[degrees]F) (dry-bulb temperature/wet-bulb temperature) with the temperature sensor exposed to a 4m/s (787 ft/min) air velocity, the radiation parasitic was shown to be almost twice the 7.6 mW (0.026 Btu/hr) parasitic budget required for a wet-bulb temperature measurement error of [+ or -] 0.05[degrees]C ([+ or -] 0.09[degrees]F).
c) IEC heat exchangers that use a portion of the leaving, conditioned dry air as scavenger air in an attempt to increase the
Wet-Bulb Depression Efficiency (WBDE), but at the expense of increased size and/or power consumption.
Of the other variables measured, knot area, ring count, ring angle, initial MC, heartwood percentage,
wet-bulb depression, and wet-bulb temperature were insignificant at the 95 percent confidence level and were not included in the final correlation.
This type of economizer can be beneficial in areas of high
wet-bulb depression, such as desert regions.
As expected, the cooling is more for the hot and dry conditions (larger
wet-bulb depression) as compared to the humid conditions for the regenerative cooler and the direct cooler.
A previous report (Simpson 2001) showed that the heat conduction equations developed by MacLean (1932) work well in calculating heating time estimates when the heating medium is saturated steam (in practice, a
wet-bulb depression of no more than about 2[degrees] to 3[degrees]F (1.1[degrees] to 1.7[degrees]C)).
Evaporative precooling (Figure 1) significantly enhances heat rejection potential when a
wet-bulb depression (difference between the dry-bulb and wet-bulb temperature) exists of 10[degrees]F (5.6[degrees]C) or more.
Among the three mills, all drying schedules started between 65[degrees] and 71[degrees]C with a 5[degrees]C
wet-bulb depression and ended between 79[degrees] and 85[degrees]C with a 16[degrees] to 21[degrees]C
wet-bulb depression.
The measured wet-bulb temperature is increased by a percentage of the thermodynamic
wet-bulb depression and the deviation is larger at low surrounding air velocities.
Shifting to a higher
wet-bulb depression may cause more drying defects, such as surface checks.