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Absolute and Relative Humidity


Professor Eugene Silberstein
July 15, 2009
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At a recent seminar a question was raised about the widely-acceptedstatement that forced air heating system dry out the air as it passesthrough the heat exchanger.

Is this, in fact, true?

Well, the answer is that it depends. On What? Glad you asked.

If, by drying out the air you are referring to an actual reduction in the moisture content of the air, the answer is no.

Ina nutshell... When air is heated, the amount of moisture contained inan air sample (as water vapor) remains the same. After all, if water isheated, what do we get? WATER VAPOR! So, if water vapor turns to watervapor (no change) the amount of water vapor in the air as it enters theheat exchanger is the same as the amount of water vapor in the airbeing supplied to the occupied space. The actual amount of water vaporin an air sample is referred to as ABSOLUTE humidity, and is measuredin grains per pound of air or as grains per some unit volume. It takes7,000 grains of moisture to make 1 pound of water.

So far so good? Great!

So....what does happen to the air and why do we get that feeling of dryness?Why does my furniture crack? Why do I get nosebleeds in my house?

Althoughthe ABSOLUTE humidity does not change, the RELATIVE humidity does.Okay, time for some quick definitions. As we mentioned, the actualamount of moisture in an air sample is referred to as absolutehumidity. Relative humidity is a little more involved.

Relativehumidity is the ratio of the amount of moisture in an air sample(multiplied by 100) divided by the air sample's ability to holdmoisture (at saturation). For example, if an air sample has the abilityto hold 80 grains of moisture but is only holding 40 grains, therelative humidity is (40/80) x 100 = 50%.

Okay, okay... But what does this have to do with that dry feeling on my skin? Hold on... we're getting there.

Now,as an air sample is heated, the air's ability to hold moisureincreases. Think of a kitchen sponge that is completely saturated withwater. By saturated I mean that, if you placed one more drop of wateron that sponge, it would drip from the bottom. Completely saturated.Now, the amount of water in the sponge is well, the amount of water inthe sponge and the total amount of water the sponge can hold is thesame as the amount of water in the sponge since we said that the spongeis completely saturated. The relative humidity is, therefore, 100%.

Now,wave your magic wand over the sponge to double its size. Now, thewaving of the magic wand does not increase the amount of water in thesponge, but only the size (and water holding capability) of the sponge.Now the relative humidity in the sponge is only 50%, since the sponge'sability to hold water has doubled, but the amount of water in thesponge remained the same.

We proceed.

As airis heated in a forced air heating system, the air's ability to holdmoisture increases while the actual amount of moisture in the airsample remains the same. This causes the relative humidity to drop.Since the air can now hold more moisture and our bodies contain a wholelot of moisture, the moisture from our bodies is released to the air.The same hold true with moisture in our furniture, floors, paintings,etc.

So, we use humidifiers on forced air heating systemsNOT to replace the moisture that the heating system took away, but toincrease the moisture content in an attempt to increase the relativehumidity.

Something to think about for all of you chillheads to think about. When air passes over the evaporator coil, haveyou ever notices the condensate dripping from the coil? Of course youhave! Well, as we mentioned before, as air is heated, its ability tohold moisture increases, right. Well, if that's true, when air iscooled, its ability to hold moisture decreases! This moisture is whatyou and I know to be condensate.

Have fun!