The cooling of the AC causes the dry air... if the AC is operated properly.
When the evaporator section (the cold heat exchanger) is operating at a
temperature below the dew point, the household air drawn through the cold
exchanger leaves condensed water on the surfaces of the cold exchanger.
Depending on air speed, exchanger design, and the incoming room-air
temperature, the air leaving the AC unit (inside the house) will be in the
neighborhood of 50 F with nearly 100% RH. As that air mixes with the air in
the room, the RH of that air goes down because the moisture content is
constant but the temperature is rising. If the air leaving the AC is 100%
RH at 50 F, and it is entering a room that is maintained at 70 F, then the
humidity drops to 50% RH. If the room temperature is maintained at 76 F
then the RH drops to about 40% RH.
These are fairly typical numbers for AC operation for fairly normal Midwest
USA buildings because the outside RH typically runs higher than 60% and
often runs between 80% and 90% RH. In a drier climate the air exiting the
AC can be a lower temperature because the heat transfer from the inside cold
exchanger to the outside hot exchanger (the compressor unit) is not carrying
as much heat of vaporization of the condensed water. If the cold exchanger
is not condensing as much water then it can run colder and therefore get the
air cooler. However, if the room air is maintained at 75 F and it gets down
to 20% RH, the dew point is about 32 F. If the cold exchanger gets that
cold (which it can) then the moisture in the air can freeze in the heat
exchanger. Ohio is typically humid enough that this rarely happens, but it
has happened at my house on warm (but dry) autumn days. Otherwise, with an
internal house temperature of 70 to 75 F, and internal RH of 50 to 60%, the
air leaving the internal AC exchanger is about 50 F.
A pet peeve I have is HVAC engineers who oversize the AC unit. This means
the compressor operates with too low of duty cycle. When the compressor
cuts out, the moisture clinging to the cold exchanger surface will evaporate
and add humidity back into the room. In addition, since dehumidification is
not taking place when the compressor is off, inside sources of moisture
(such as respiration and perspiration of people, cooking, bathing,
housecleaning) will not be removed. An oversized AC unit that keeps the air
at 75 F with only 50% duty cycle can leave the building air between 70% RH
and 80% RH and this feels quite "clammy" or "sticky" to me. I prefer the RH
around 50%.
Since I have hot-water heat, and therefore no ductwork in my house, I use
window AC units. In my two story house I have two 5000 BTU units upstairs
and two 5000 BTU units downstairs. They are Energy Star compliant and very
quiet. I run one up and one down pretty much continuously. When the
outside temperature is below 85-90 F this is all I need. When the outside
temperature gets around 88 F or higher, I turn on the other two units.
Since I pretty much always have two of the four units running, my household
humidity hovers around 50% RH and I find this very comfortable. The only
problem I have is some rooms are warmer than others. The humidity is pretty
low throughout the house because water vapor diffuses pretty fast due to its
low molecular weight.
I am the chair of the Trustees Commission at my church, and last year we
installed AC at the church. Rather than one large unit, we installed five
units, three cooling the zone with the most people (the sanctuary) and two
units cooling the other zones. The units are staged. The first stage in
the sanctuary just about runs continuously. If it is not able to maintain
the temperature, the second unit comes on. If that can't maintain the
temperature, the third unit comes on. When the sanctuary is empty of
people, and the outside temperature is not too hot, the second and third
stages might not come on at all, but the first stage will run almost
continuously. This keeps the humidity below 60% and that is very good for
the grand piano, the pipe organ, and other wooden objects (as well as good
for people). Had we installed a single large unit that ran with low duty
cycle, the humidity would run much higher, and worse, it would fluctuate.
If you have a $100,000 pipe organ, you don't want the humidity to fluctuate
very much, nor do you want it to get above 80%.
Since we have five separate compressor units (three staged for one zone and
two staged for the other zones) we can switch the ordering of the compressor
stages. The manufacture suggests we change the ordering either annually or
semi-annually so we can spread the work load over all compressors.
This is the first full summer to operate the AC at the church, and because
of the unusually hot summer we are giving it a good testing. So far we are
pleased. I am also pleased with the operation of my house, and I've been
doing that for over 30 years.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270 (office)
edmiston@bluffton.edu
-----Original Message-----
From: Daryl L Taylor
Sent: Tuesday, July 26, 2011 9:26 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] it's a dry heat
Bob at PC, I'm sure I'm showing my ignorance of AC's, but I thought the
biggest benefit of "air conditioners" was the ability to dehumidify the air
without the added heat of the mechanism; as in a regular room dehumidifier.
This indicates to me that the Nevada "dry heat" simply could not be "dried"
any further and you were faced with a limited amount of cooling from the
heat "exchange" to the exterior since the sun is so unforgiving out that
way.
Or do I have it backwards: the cooling of the AC CAUSES the dry air?
Daryl L Taylor, Fizzix & Astronomy Guy
Greenwich HS, CT