Relative Humidity – It’s all relative

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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