[pianotech] Clarification Question: i'll take a pass

[Dean May]

>>Relative humidity isn't absolute moisture content, it's temperature
related. 

Exactly

>>A/C exhaust is of a lower moisture content than the room air

This is not entirely accurate. Assuming constant barometric pressure, the
moisture content per pound of exhaust air vs. pound of room air (I'm talking
mass here, not volume) is the same (assuming steady state conditions). Think
about it, it is the same air, you aren't adding or taking away any moisture
at that point. The only difference is the temperature rises, making it
capable of holding more moisture, thus it goes to a lower RH by definition..


At the lower temp of the a/c exhaust then the RH would be higher, and at the
room temp the RH would be lower, exactly as your first statement above
predicts. 

In the real world you have more moisture being added to the room which
complicates the model some. But still, the air coming off the evaporator
coils is near 100% RH. It has to be or water would not be falling out of it.
By the time it gets 60 feet down the trunk to the vents it has warmed some,
but the RH is still pretty high and you don't want it blowing on the piano.
Only after it warms to your 75 degree room temp does the RH drop to
comfortable levels. 

Think about the reverse, the heating season. It may be easier to comprehend
the air coming off the furnace heat exchanger is going to be drier (lower
RH) at 120 degrees than it will be once it cools to the room temp of 75
degrees. Same deal, once the air leaves the heat exchanger (or a/c coils)
you aren't adding or taking away any moisture. It is the same air, yet the
RH levels change dramatically once the air takes on the room temperature. 


Dean

Dean May             cell 812.239.3359 

PianoRebuilders.com   812.235.5272 

Terre Haute IN  47802

 


-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ron Nossaman
Sent: Friday, August 28, 2009 8:45 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Clarification Question: i'll take a pass

Dean May wrote:
> Be careful about thinking the a/c will keep RH down. Conditioned air
coming
> off the A/C evaporator coils is at near 100 % RH. If that air is blowing
on
> the piano, i.e., the piano is too close to a vent, then the piano is going
> to pick up significant moisture, even if A/C is on full blast and the room
> is cold. 
> 
> Dean

Well, no. Relative humidity isn't absolute moisture content, 
it's temperature related. A/C exhaust is of a lower moisture 
content than the room air, because the cold coils condensed 
moisture from it as they cooled it. That's why an A/C produces 
water, which  is diverted to an outdoor drip, or drain.
Ron N