Air conditioner condenser coil emissivity

oil pan 4 · 08-24-15, 05:07 PM

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.

08-24-15, 05:07 PM	#1
oil pan 4 Land owner Join Date: Nov 2012 Location: NM Posts: 1,026 Thanks: 12 Thanked 127 Times in 107 Posts	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.