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Old 08-24-15, 05:07 PM   #1
oil pan 4
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Default Air conditioner condenser coil emissivity

I see a silvery condenser coil and I want to paint it black.

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.
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 the evap coils might freeze up.

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. Someone else can 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.

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Old 08-25-15, 09:18 AM   #2
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Interesting. Something I'm willing to try! The txv will throttle it back if there's not enough superheat, but thats ok. Most central outdoor units that have exposed coils are painted black, but it seems like all that are covered by louvers are bare aluminum. I don't think there are any current models that have exposed and black coils. Could be an affect of two things, aesthetic design louvers look better and hide factory dings ( and protect against hail), and if you can't see them why would the factory spend money painting the coils?
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Old 08-27-15, 02:01 AM   #3
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I haven't done this yet because I want good baseline measurements. Doing a little homework on it I've found there is potential. Using a boltzmann's law calculator and info from emissivity tables there can be a gain of about 360 watts emitted from 1sq meter, about the average sized 2-3 ton outdoor coil. The gain might be better because I believe the calculation is a flat plate vs a 1" thick finned coil.
I'm using .4 emissivity for oxidized aluminum and 1 for optic black. Temp of 120F converted to 321 Kelvin. Surface area 1 sq meter. 240 watts aluminum vs 602 watts pure black.
Free Stefan Boltzmann Law Calculator Online | Calculator@TutorVista.com
Table of Emissivity in the IR Spectrum
I will be using rustoleum high heat bbq flat black which wont be '1' more like upper .9but 3M black velvet coating 9560 series optical black rates as 1 for anyone looking.
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Old 08-27-15, 09:20 PM   #4
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A good BBQ flat black or engine paint flat black should be around .95e.
New shinny reflective aluminum coil fins could be as low .1e or lower, aluminum is the 2nd most lustrous metal.
I suspect the heat exchanger fins might be shinnyed up some to make them more appealing to the customer.
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Old 08-28-15, 07:52 PM   #5
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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.
Its 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.
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Old 08-29-15, 08:25 AM   #6
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I'll do some tests today on my central today. Because window unit coils are smaller, less dense and lower seer the black paint might make up with extra subcooling. Keeping my fingers crossed my central gets the same gains as you!
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Old 08-30-15, 12:35 PM   #7
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It may not help the centeral air unit as much except for on those really hot days.
But any improvement is still an improvement and any HVAC improvement you can get for the cost of a can of black spray paint will likely pay for its self many times over.
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Old 08-30-15, 11:17 PM   #8
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My thoughts the same, a small improvement on my biggest power load will pay dividends.
Did my Ruud 2 ton, took it apart and sprayed both inside and outside of the condenser coils with bbq flat black. My units sit on the dark and shady side of the yard so no threats of enhance solar heat gain. Morning time I get the same 23f differential as before, hard to improve on that. Late afternoon, under the heaviest load, I did see an improvement of 2 degrees, originally 17f differential now 19f! Still taking notes on it. My units already do a good job subcooling the liquid line and thats my guess for the small improvement. I was curious if Id see any improvement. Time permitting I'll do the same on my 2 ton American Standard. Its outdoor coil is only 2 months old, still shiny. Maybe there will be a greater improvement.
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Old 09-20-15, 05:16 PM   #9
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Any updates on this?
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Old 09-20-15, 08:25 PM   #10
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3 weeks later and everything still is working for me.
Its still summer in Not Mexico.

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