A Guest Article by Jim Jagers
The other day I was conducting a training class, and we were
discussing evaporative cooling. Someone said they didn’t think evaporative
cooling would work very well in their area because the summer temperatures were
90°F plus with 90% RH. If you were to look at many psychrometric charts, you’d
see this point is, dare I say it, “off the chart”. To get a feel for this
consider a steam room has general temperature of 104°F and 100%RH. At 90°F with
90% RH the heat index is 122°F. It’s doubtful the temperature and humidity are
as bad at the same time as he imagined.
People generally associate high temperatures with high
humidity percentages. It’s more likely that high temperatures will be
associated with lower humidity percentages. At 80°F and 41%RH the heat index is
80°F. 80 degrees feels like 80 degrees. At this point there is approximately
0.009 pounds of moisture per pound of dry air in the atmosphere. If the
moisture content remained constant and the air warmed to say 90°F, the relative
humidity would actually drop to about 30%. Conversely, if the moisture content
remained constant and the temperature dropped to 70°F, the relative humidity
would increase to about 57%. This is because cooler air can hold less moisture
than warmer air, and relative humidity is the ratio of the moisture in the air
compared to the amount of moisture the air (at a specific temperature) can hold
expressed as a percentage.
People usually think of their air conditioner as providing
cool dry air in the summer, and it does because it does both sensible and
latent cooling. Sensible cooling lowers the temperature we sense, and latent
cooling removes the moisture. The air entering the coil may be 78°F and have
0.0101lbs of moisture per pound of dry air. The coil temperature may be 45°F
and thus the leaving air may be 60°F (It won’t be 45°F because the water in the
air is absorbing some of the cold). At this point the leaving air may have a
moisture content of 0.0062lbs per pound of dry air. This is a significant
reduction in moisture, and it is evidenced you water dripping from the
evaporator coil. The leaving air is much dryer than the entering air.
However, in relative terms the air coming off the evaporator
coil in the air handler has a relative humidity of 100% or close to it. Remember,
cool air can’t hold as much water as warm air. When the air entering the coil,
contacts the cold fins it cools rapidly. Condensation occurs when air can’t
hold the moisture it contains. At this point the air is fully saturated meaning
its relative humidity is 100%.
The point to this brief essay is, as I said at the start, relative
humidity is all relative - to the moisture in the air and the air temperature.
Warm air isn’t necessarily humid; cool air isn’t necessarily dry, relatively
speaking.
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