[TriLUG] OT: thermodynamics of A/C question

[Matt Flyer]

I should have mentioned this in my previous post, but I didn't think
about it.  You can go to this site (it is public)
http://chwreports.unc.edu.  It is the Chilled Water Department reporting
site for UNC.  We use it mostly as a quick and easy interface to get
snapshot performance and charting information without activating the
full SCADA client and for use with phones that don't support Active-X. 
For those interested in AC performance and efficiency there is a lot of
information that can be viewed and plotted such as dry bulb, wet bulb,
total cooling tons, kw per ton (efficiency), etc.  There is information
related to each of the four main plants as well as most of the campus
buildings.  If anyone has any questions about it or what any of it
means, feel free to ask.

For example here is a graph showing total campus usage for the last 24
hours relative to dry bulb and wet bulb temperature.
http://chwreports.unc.edu/pic2.php?time=1340450882&bldg[]=2654&bldg[]=2653&bldg[]=2533&timedate=2012/06/23%2007:26:26&interval=900&period=86400&dateForm=YYYY/MM/DD&source=tomkins

You can see how even as the dry bulb temperature went down, the cooling
requirements went up as the wet bulb temperature changed. 


On 06/22/2012 09:46 PM, Joseph Mack NA3T wrote:
> 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
>