[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Fri, 22 Jun 2012, matt at noway2.thruhere.net wrote:

> As you said, the cost driver in air conditioning is moving 
> the refrigerant from the low pressure side (evaporator) 
> located in the air handler to the high pressure side (the 
> condenser).

ah. the condenser, not the compressor.

I keep replacing compressors in my cars. Each generation of 
cars have made is more difficult to replace the compressor. 
I have compressors burned into my brain.

> The energy usage will be largely impacted not only by 
> temperature, but by the ability for the condenser to 
> reject the heat into the atmosphere. This will be 
> reflected in the SEER (seasonal energy efficiency ratio) 
> of the unit.

I've read about SEER, but it only lasted till I clicked on 
the next link.

> Now, where things get tricky is that the ability to reject 
> the heat is also a function of humidity.  When the 
> humidity is high, air cooled systems, like your typical 
> residential unit become extremely inefficient because they 
> can't get the heat into the air and hence more energy is 
> required to move the refrigerant across the temperature 
> differential.
>
> Consequently, by running the AC when the condenser can 
> best reject the heat into the atmosphere you will lower 
> your costs.  This will occur when the the temperature and 
> humidity are lowest.

don't understand how humidity plays much of a role here. 
Even at 100% humidity at 30degC, the air is only 4% water

https://en.wikipedia.org/wiki/Dew_point

moist air is lighter than dry air (MW water=18, MW air=29). 
However the heat capacity of water vapor is about double 
that of air

https://en.wiktionary.org/wiki/humid_heat

so moist air would absorb slightly more heat for a fixed 
temperature rise than would dry air.

> There is one other wrinkle here and that is what people 
> think of when they refer to humidity.  The weather report 
> always refers to "relative humidity" which is a worthless 
> concept.  Cooling load, as well as human comfort, is 
> largely a function of what is called the wet bulb 
> temperature.

Yes. I plot the wetbulb along with the temp each day with 
rrdtool. The wet bulb temp stays pretty much fixed during 
the day, unless a new body of air moves in. The RH on the 
other hand drops as the temperature goes up during the day.

> This is the temperature that takes into account 
> evaporative cooling.  Wet bulb is very closely related to 
> dew point.  When the wet bulb temperature is low, even if 
> the relative humidity is high, you can still more easily 
> reject the heat into the atmosphere.

The thing I keep track of wrt humidity and A/C, is that when 
you're cooling air, first you have to remove the water by 
changing it to the liquid phase. This requires a lot of 
energy, without changing the air temperature (this energy is 
called latent heat). Water has just about the highest latent 
heat of vaporisation of any material.

https://en.wikipedia.org/wiki/Latent_heat#Table_of_latent_heats

During this phase of cooling, the A/C unit is producing 
liquid water out the drain hole. It's only when the wet bulb 
temp drops to that of the evaporator, that you start to pull 
out the heat to drop the temperature of the house air 
(called sensible heat). I believe this is why people 
advocate leaving the house closed with the A/C at a fixed 
temperature. You aren't letting in humid air from the 
outside, from which you have to remove the water.

> By comparison, water cooled units using cooling towers, 
> make use of evaporative cooling

ah yes, the hyperbolic cooling towers at power stations (eg 
Shearon Harris)

https://en.wikipedia.org/wiki/File:Shearon_Harris_Unit_1.jpg

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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