Air conditioner condenser coil emissivity

oil pan 4oil pan 4 Posts: 741Solar Expert ✭✭✭✭
Real quick and dirty test with a window unit. This is a little half ton window unit that we use for sleeping during the day after working a 13hr night shift. We always run it on the low fan setting, that cools plenty and is still fairly quiet.
I checked the air conditioner discharge with my flir. The coolest spot was 52'F from 10 feet away. The evap coils air inlet temperature was 72'F (digital temp display).
That is a temperature drop of 20'F. After checking several times 19 to 21 degrees F seemed to be the temperature drop this A/C liked to produce with todays outside temperature.
I turned the air conditioner off. Took my can of flat black engine paint and sprayed each pass of condenser coil with opposing 45 degree sweeps the third sweep normal to the coils.
Powered the air conditioner back up and the inlet air was reading 75'F and the discharge was almost immediately went down to 51'F and only kept getting colder.
Once the evap inlet temperature was showing 72'F the discharge air was 44'F.
That is an 8'F temperature differential improvement, now its discharging air 26'F to 28'F cooler than the inlet temperature (the discharge air is staying between 44'F and 46'F from 10 feet away according to the flir. I am guessing the expansion valve is kicking in to prevent evap freeze up? (If it even has one)
I was expecting a 2'F to 4'F improvement, not 8'F.

Now this AC faces east, points at the house next door and is shaded by several trees and under the roof over hang. The top of the AC might see direct sun light for maybe an hour a day a little before noon.
If you have a west facing AC and get sun rays directly onto the condenser coils painting them black might be really bad. I will let someone else test that.

Now should everyone run out and paint their shinny condenser coils black? No because every mechanical engineers who design these things are well aware of ideal black body emissivity, so if they are not black, then it stands to reason they are not black for a reason such as really bad overall performance if facing direct sun light.

Now according to Boltzmann's law a 1 square meter typical black surface, at room temperature will give off around 400 watts of energy. A shinny aluminum surface will give off only 1/3 of that or less.

Then a few days later...
I tried it on another air conditioner, this time one that was a few years old.
The coldest air on the discharge it picked up before the paint was 52'F, then after the paint was 47.5'F.
This time I had the kill-a-watt meter plugged in. It was drawing the same wattage before and after. I expected it to draw a little less. It might be drawing <1% less but appears to be in the range of its normal fluctuations.
So this mod looks like it wont save any power aside from getting you down to temperature that much faster and cycling the compressor off that much sooner.

I should have known, the compressor should draw the same wattage unless the expansion valve kicks in to prevent freeze up. Since this is an older unit, it may not be in danger of freezing up the evap coils at in these temperature ranges.
It kept using around 540watts before and after, but now its moving more heat with those 540 watts. Moving more heat with the same energy input, more efficient, I can live with that.
I originally posted this here:
There have been some replies an another test on a much larger centeral A/C.

http://ecorenovator.org/forum/conservation/4540-air-conditioner-condenser-coil-emissivity.html

Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

Comments

  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    I am really surprised by this, since I would expect the major path of heat transfer from the condenser coils would be conduction to the fan driven air.
    It is possible that the "lumpy" finish provided by the light coat of paint is actually changing the air flow regime at the surface of the fins and that is making the big difference. If that is the case, you may have found a real breakthrough in design!
    If you have another A/C to test with I would suggest trying a coat of clear finish with a similar spray pattern.

    Also, how long did you wait between spraying the paint and starting the A/C? There is a remote possibility that some of the cooling came from evaporation of solvent from the paint layer.
    I expect that if this had really been a factor you would have noticed a decline back to the original performance by now.
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    oil pan 4 wrote: »
    I turned the air conditioner off. Took my can of flat black engine paint and sprayed ...
    <snip>
    Powered the air conditioner back up and the inlet air was reading 75'F and the...

    Is it possible that while the air conditioner was off, ice on the evaporator melted so the air conditioner was more efficient when you turned it back on?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SkiDoo55SkiDoo55 Posts: 414Solar Expert ✭✭✭
    Have you tried this using full contact or air probe thermometers? FLIR and IR thermometers operate most accurately when pointed at non-reflective surfaces such as flat black. I paint spots on air-conditioner tube black to get a more accurate reading without having to use contact thermometer.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
  • oil pan 4oil pan 4 Posts: 741Solar Expert ✭✭✭✭
    vtmaps wrote: »

    Is it possible that while the air conditioner was off, ice on the evaporator melted so the air conditioner was more efficient when you turned it back on?

    --vtMaps

    I already know the typical temperature drop you get out of these window units was only around 20'F from when I started taking readings the week before just playing with my flir.
    2 weeks later its still blowing air in the mid to upper 40 range.
    Plus the temperature probe is located down low where it would be the first thing to know. If the evaporator coils were full of ice and I turned the air conditioner off then back on minutes later the probe would be reading much cooler than the surroundings, this time the probe started back up reading afew degrees warmer.

    With the second test I didn't turn the air conditioner off and I waited till the smell of wet spray paint was long gone before taking readings on both tests. I knew the paint would evaporate and that would provide some cooling.

    I don't think this is a break through at all. Just using well understood scientific laws to try to get a heated coils of pipes to give up their heat faster.

    The main reason I had to get my self a flir i7 is because I have been using one at work for all types of industrial applications I am very well acquainted with such devices.

    Oh and the emissivity of aluminum is far worse than I originally thought, because I didn't bother to look it up. Shinny aluminum foil has an emissivity of 0.03, condenser coils are shinny like aluminum foil.

    Here is a very simple physics experiment demonstrating this concept, its so classic you might even say its down right Victorian:
    https://en.wikipedia.org/wiki/Leslie_cube

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    oil pan 4 wrote: »

    Oh and the emissivity of aluminum is far worse than I originally thought, because I didn't bother to look it up. Shinny aluminum foil has an emissivity of 0.03, condenser coils are shinny like aluminum foil.
    Once upon a time the fins were copper. Which when oxidized is not shiny at all. Maybe we lost more than just thermal conductivity as a result of the switch to aluminum?

    SMA SB 3000, old BP panels.
  • oil pan 4oil pan 4 Posts: 741Solar Expert ✭✭✭✭
    Copper will weather fairly quickly. When it oxidizes like this it typically turns black, which increases emissivity to some where above 0.9
    The aluminum stays shinny for years.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • JohannJohann Posts: 240Solar Expert ✭✭✭
    oil pan 4 wrote: »
    Copper will weather fairly quickly. When it oxidizes like this it typically turns black, which increases emissivity to some where above 0.9
    The aluminum stays shinny for years.

    Last time I checked, copper will turn green when it is oxidizing.
  • oil pan 4oil pan 4 Posts: 741Solar Expert ✭✭✭✭
    Eventually it will turn green after the dark brown to blackish phase.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • mike95490mike95490 Posts: 7,874Solar Expert ✭✭✭✭
    If the exchanger were a "radiator surface plate" I would expect painting with high emissivity paint to work well in the shade. But it's a finned forced air exchanger, and the fins re-radiate to adjoining fins, not improving things, and the "coating" would reduce the metal-air transfer. So, I think either you have some other effect happening, but at least it's good.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • waynefromnscanadawaynefromnscanada Posts: 3,009Solar Expert ✭✭✭✭
    mike95490 wrote: »
    If the exchanger were a "radiator surface plate" I would expect painting with high emissivity paint to work well in the shade. But it's a finned forced air exchanger, and the fins re-radiate to adjoining fins, not improving things, and the "coating" would reduce the metal-air transfer. So, I think you have some other effect happening, but at least it's good.

    My thoughts exactly! Plus, even if it did help, the difference compared to the vast amount of forced air heat transfer would be so small % wise, as to be not noticeable. Has to be something else going on in my opinion.
  • BB.BB. Posts: 27,789Super Moderators admin
    From an earlier post of mine in another thread--found one NASA report from 1983 (PDF file)... Has a whole bunch of information about colors/materials and their absorption and emission of heat.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • oil pan 4oil pan 4 Posts: 741Solar Expert ✭✭✭✭
    mike95490 wrote: »
    If the exchanger were a "radiator surface plate" I would expect painting with high emissivity paint to work well in the shade. But it's a finned forced air exchanger, and the fins re-radiate to adjoining fins, not improving things, and the "coating" would reduce the metal-air transfer. So, I think either you have some other effect happening, but at least it's good.

    If I paint a stripe of black pain down the fins and put the flir on it, the black stripe show up on the flir as being 20 to 40 degrees hotter then the surrounding shinny metal.
    Something is happening indeed.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    oil pan 4 wrote: »

    If I paint a stripe of black pain down the fins and put the flir on it, the black stripe show up on the flir as being 20 to 40 degrees hotter then the surrounding shinny metal.
    Something is happening indeed.

    All that means is that the flir reading is not being interpreted properly. Any IR measurement must be corrected for the emissivity of the surface or at least for the ratio of emissivity at the different IR frequencies being measured. Accurate nfrared temperature measurements on highly reflective (low emissivity) metal surfaces are difficult at best.

    Here is a quick experiment on the degree to which radiative cooling is important to the coil temperature:

    When you stick your hand in front of an incandescent light bulb you feel your hand being heated by radiation, and you can distinguish that from conduction or convection by putting a sheet of low absorption glass or plastic which is known to be IR transparent between you and the bulb.

    Try the same experiment with the cooling fins, keeping your hand outside the air stream or momentarily shutting the fan down. I suspect you will not feel any significant amount of radiated heat.
    SMA SB 3000, old BP panels.
  • oil pan 4oil pan 4 Posts: 741Solar Expert ✭✭✭✭
    It doesn't have to be a significant difference, just a difference.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

Sign In or Register to comment.