Dew Point

How “dew” you do? Tidbits about dew point based on research • • • • • • •

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Dew point is the temperature at which condensation begins (no big deal). Dewpoint is the quantitative measure of the moisture content, relative humidity (RH) is not (not so big deal, think about it). RH indicates a level (e.g. percent) of saturation for the gas, not the absolute measure of the moisture). Theoretically, the dew point temperature cannot exceed the ambient temperature. For instance, at a temperature of 70°F ambient air, the dew point of ambient air cannot exceed 70°F (sounds simple, isn’t it?). The dew point at a pressure other than atmospheric is called the pressure dew point (again not a big deal). When a dew point is below 32°F (freezing point), it is referred to as frost point. When you deal with a frost point, the moisture content is normally termed as “trace moisture” (sounds OK). As a rule of thumb, for each 20°F increase in temperature, the moisture-holding capability of gases approximately doubles (so that the relative humidity is reduced by one-half) (I didn’t know this for some time). Up to approximately 4,350 psig, the capacity of water absorption of air is nearly independent of the pressure. For example, at 77°F and 14.7 psig, 1 cubic meter of air can absorb 23 grams of water. So if you have 10 cubic meter of air at atmospheric condition, it will contain 230 grams of water. Now you increase the pressure, e.g. to 147 psig (10 times) so that the actual air volume now becomes 1 cubic meter (isothermal process (= constant temperature) process). That one cubic meter volume still can contain only 23 grams of water. So the balance (230-23=207 grams) is condensed to liquid water (I didn’t know that either). A simple guide for specifying compressed air drying equipment is to specify a pressure dew point slightly below (5-10°F) the lowest air temperature to which the piping is exposed to compensate for fluctuations in flow rate or occasional differences in ambient temperatures (makes sense). The correct selection of the pressure dew point is the single most important factor in determining the correct size of a dryer (because removing water vapor costs money)(good point). The lower the temperature of the inlet air, the lower the initial and operating costs (higher temperature means more water-holding capacity means more work is required to dry the air means bigger unit means more initial cost and more operating costs)(wise man said so).