If air conditioner efficiency improves when it rains outside, why don't all air conditioners include water sprayers to help cool them?

submitted by magnora7 from self.science

The water from rain helps the heat dissipate from the outside radiator on central A/C systems, according to an article I read. https://www.integrityair.com/is-my-air-conditioner-performance-affected-by-rain/

It makes sense, it evaporates out a lot of heat so the overall system is more efficient.

So why are air conditioners not designed to have a water hose to lightly spray on it all the time? Wouldn't this improve efficiency enough to offset the cost of the water spray?

all 37 comments
sorted by:
top
newinsightfulfun

[–]Tom_Bombadil 5 insightful - 1 fun5 insightful - 0 fun6 insightful - 1 fun -  (13 children)

My educated guess:

This addition would likely require regular cleaning so contamination (dust, pollen, etc.) doesn't accumulate in the radiator cooling fins.

Plus the evaporation rate depends:

  • on the RH of the air.
  • exposure to sunlight
  • Air temp.
  • Air flow

So you'd want a way of controlling the rate of water flow; adjusting for these floating variables.

Also, the water would generally need to be drawn through the sides, and blown up/out. This isn't an difficult obstacle, but it would change the design.

Plus, the liability of water damage from:

  • poor installation
  • leakage
  • neighbor kids screwing with it
  • Damage/wear from increased thermal cycling (cold water of hot surfaces) literally rinse and repeat.
  • etc.

Plus the warranty risks associated. Adding water to anything creates problems.

Also, it reduces profitability for the energy companies.

It's a good idea. On hot days you could momentarily boost you efficiency by spraying down you external ac unit, but it will evaporate quickly.

If your ac fins are bent then you could buy one of these to straighten bent fins and increase the air flow.

You could also winterize your windows to reduce leakage and basically add an extra insulating later to your windows.

This is all shooting from the hip guesses.

I'm not an AC engineer...

Good luck!

Edit:.

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

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low.

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (10 children)

Interesting analysis.

Maybe it could only be activated on super-hot days, when the efficiency gains would be particularly high? And the high heat would also guarantee the water would all evaporate quickly so it wouldn't hang around for too long, which seems the source of a lot of the problems.

[–]Tom_Bombadil 3 insightful - 1 fun3 insightful - 0 fun4 insightful - 1 fun -  (9 children)

And the high heat would also guarantee the water would all evaporate quickly so it wouldn't hang around for too long

Not necessarily. It depends on the weather conditions.

If it's a humid day then the evaporation rate drops quickly,. Adding water on a 90 deg day with 85 deg dew point won't get you much.

All machine efficiency depends on your ∆T. So it will always work better on hot days with cooler water. If it's extremely humid then the evaporation rate will be lower.

Evaporation absorbs the most heat energy. 1cal raises 1g of liquid water 1deg C.
The latent heat required to evaporate 1 g of 100 deg C water to 1g of 100 deg C steam is 540 Cal/g. Same situation with ice to water is 80 Cal/g.

Do you have insulation in your attic? Check the R value for efficiency.

If not, then that helps quite a bit. Also, attic ventilation. Get a attic fan that you can use to increase ventilation. Particularly at night when the outside is much cooler.

Moving air transfers heat away if. Static air insulates (tiny pockets is air in Styrofoam). Sometimes the attic fan works against you. It depends.

[–][deleted] 3 insightful - 2 fun3 insightful - 1 fun4 insightful - 2 fun -  (1 child)

This guy thermodynamics. 👍

Did u study chemistry or engineering?

[–]Tom_Bombadil 1 insightful - 2 fun1 insightful - 1 fun2 insightful - 2 fun -  (0 children)

I'm a wainwright, with a cobbler minor.

;-)

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (4 children)

Yeah I might get an attic fan, I was looking in to that today too. My temp gun read out 165F on the attic ceiling, and 145F on the floor. Pretty dang hot.

I tried a little experiment and put a gallon of water on the external unit. It cooled it from 140F to 95F, and it evaporated in about 90 seconds because of the heat. Then about 20 minutes later it was back up to 140F.

From a cost perspective, a gallon of water costs 1 cent. 1 cent of electricity is 100Wh. I think it'd be very hard to save 100Wh with one gallon going from 80F to 140F, not sure how many Joules (or watt-hours) that is.

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (2 children)

Try the same experiment with a mister than can be directed at the cooling fins.

Dumping a gallon of water is a waste.

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

I sprinkled the gallon around the entire thing, I didn't just dump it

[–]Tom_Bombadil 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (0 children)

I'm not sure that I've ever heard anyone say they've sprinkled a gallon.

I'm sure you did, cause it was hot AF, and you were on a mission to fix it! ;-)

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Definitely go with the attic fan.

The attic fan should blow the air in the attic to the outside.
There should be a separate vent in the attic that allows air to be drawn from the outside in.
The fan will create negative pressure (it's actually not negative) inside the attic, which will draw in cooler air.

The same applies in your house with window fans. Open windows in separate rooms/areas. Blow the hotter air out of the house (from the hot room, which is typically upstairs or a sun lit room) and let the colder exterior air draw in from the other room.

Blowing cool air into the house is much less effective.

Allow the negative pressure to work for you.

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

So I looked it up, it takes 2.45 Wh to raise the temperature of one gallon by one degree F. So if the water goes from 80F to 120F, that's 98 Wh. That's remarkably close to the 100 Wh break-even.

So if you had access to cheap water, or had expensive electricity, this might actually save money and use less energy overall. Or if the water delta is more than 40 degrees, that'd made it cost effective too.

[–]Tom_Bombadil 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (0 children)

It's way better than that.

That method is for heating water. Evaporation draws many times more heat energy away. Many.

The transition from liquid to gas is 540x the energy to raise the same mass 1 deg C.

You get the best results from evaporation. Your body cools when sweat evaporates. If it doesn't evaporate then you're hot and miserable.

Fans don't cool, but increase the rate of thermal transfer (increased air mass in physical contact per unit time), which also increases the evaporation rate. Moving air can also hold more moisture (from memory, needs verification).

[–]FormosaOolong 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

Hey, thanks for the link to that tool! Mine came super bent and I had no idea such a thing existed

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

No prob! Glad to help. ;-)

[–]FormosaOolong 3 insightful - 1 fun3 insightful - 0 fun4 insightful - 1 fun -  (7 children)

Probably mold. It's the last thing you'd want to be growing inside your unit to spew all over your house. This is also why it's essential to run an AC on fan only for 20-30 mins before you shut it off, so that any condensate on the coil evaporates.

[–]Tom_Bombadil 3 insightful - 1 fun3 insightful - 0 fun4 insightful - 1 fun -  (0 children)

Mold typical isn't a problem outside. Mold hates sunlight.

Water damage to foundations is a fucking disaster, and should be avoided at all costs.

This is also why it's essential to run an AC on fan only for 20-30 mins before you shut it off, so that any condensate on the coil evaporates.

This is true, but it applys to the inside coils. The address to sets.

Evaporation (inside): draws heat into it from evaporation.
Condensation (outside): compressed to give off heat to the exterior.

This is why portable ac is crap. Both inside.

Cool the exterior condensation coils.
You could shade the outside ac. The fan will blow crap up under it. There's generally a good reason that there things aren't common.

Consider a low hanging trellis...? With thick beams for Max shade...?

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (5 children)

This isn't the part that draws in air though. It's just the part that gets rid of the heat. So even if it filled with mold it wouldn't affect the air supply. The air that you breathe is re-circulated within the house only. But that's a good point about the mold. I guess (I would hope) it would dry out between runs, when it shuts off for 30 minutes or whatever.

[–]FormosaOolong 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (4 children)

Yes, that's true. I'm not an engineer of any kind, but my lame user inspections of my two units (mold makes me sick so I peek in there a lot) indicates that the only thing separating the indoor recirculating parts and the outdoor bits where the outlet fan is, is a thick piece of styrofoam. I would hate to have mycotoxin creep.

Of course the box outside is exposed to all the elements, and includes channels that with the correct pitch ditches the water it accumulates. So there's no reason I guess they couldn't make some kind of misting system that repurposes the water eliminated from the indoor space. I'd pass on that model myself though, as I'm already OCD with my current units.

ETA: All that said, you still may have a million-dollar patent right there!

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (2 children)

Haha. I see what you mean.

I'm simply imagining a small hose that does a tiny mist to simulate rain, just above the outside unit, only while it's running, and only when the outside temp is over 85F.

I think that might avoid the mold issues.

[–]FormosaOolong 2 insightful - 2 fun2 insightful - 1 fun3 insightful - 2 fun -  (1 child)

Noice! Now go collect all your money!

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Haha I might try it out on my AC. I mean I could literally just spray it with my hose sprayer by hand and measure the efficiency difference I think, to test it out. Then build a little control circuit with one of those electronic water hose switch boxes and a hose to the spigot

If it works that'd be amazing. I guess the efficiency from rain is already proven. It's just a matter of if it's more cost effective for the water use vs energy savings

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

The inside coil causes interior air to condense. If this condensed moisture doesn't drain adequately, or if it evaporates again then it can remain humid and mold can build up.

You could add a fan with a timer for ventilation, similar to what you find in bathrooms (a daily timer).

[–]happysmash27 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (2 children)

It might be more efficient in terms of electricity, but wouldn't it be less efficient in terms of water usage? That's a big problem in draught-prone areas, and might result in even more energy usage in places which use desalination plants for water.

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

But just from a cost perspective, if you can use 10 cents of water and save 30 cents of electricity, then that's a net benefit cost-wise. Which means it consumes less energy overall, which is better for the earth

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Water is cheap. A cool interior is priceless.

Edit:

even more energy usage in places which use desalination plants for water.

If you live in one of these areas then evaporative ac is available.

[–]wendolynne 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (1 child)

there are such things: https://en.wikipedia.org/wiki/Evaporative_cooler

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

It's kind of similar but not exactly the same. It's like a combination of both. I never thought of it that way though, interesting link thanks

[–]samwhiskey 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (1 child)

I have worked in the hvac industry in the past and hated it so much that I'm not even going to answer this question.

Well, maybe a little. It's not more efficient. You have an undersized system, dirty system, too small of a liquid line, not charged properly, not enough return air. A properly designed, clean system will do everything it needs to do all the time. The rain lowers the head pressure and changes the evaporation and condensation temperatures of the refrigerant and can actually make it cool less that it should. Proper evaporation of the refrigerant in the evap coil is necessary to pick up the excess heat from the return air flowing across the coils. Mess with that system and you have less heat being picked up by the refrigerant in the gas state.

Damn, drug me into this...

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

The rain lowers the head pressure and changes the evaporation and condensation temperatures of the refrigerant and can actually make it cool less that it should.

Oh, I hadn't thought of that. Thanks for that information. Amazing how complicated this stuff is...

[–]magnora7[S] 1 insightful - 2 fun1 insightful - 1 fun2 insightful - 2 fun -  (2 children)

Just a passing thought, I was wondering if anyone had any ideas.

[–]FormosaOolong 1 insightful - 3 fun1 insightful - 2 fun2 insightful - 3 fun -  (1 child)

the hick solution: http://blog.pcpools.com/wp-content/uploads/2012/07/garden-hose-with-sprinkler.jpg

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Haha that's a good idea, I might actually do it with one of those

[–]basedaf 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

I'm not like an expert, but air conditioners remove moisture the air which creates a cooling effect, more moisture like when its raining means easier to remove said moisture? So you'd have to have a humidifier going but that wouldn't be effective outside, so its not really possible? Just a guess in hell

[–]magnora7[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Good guess but it doesn't work quite like that. It's very interesting, check it out: https://3.bp.blogspot.com/-QXc1_R_aCCg/V3VnKmxM91I/AAAAAAAAARc/0nYP3Gw9niIMJdPSxD_IK0DZFuD64N9MgCLcB/s1600/CR-BG-Central-Air-Browser-Illo.jpg

We're talking about removing heat from the condenser, #3 in the diagram. Which is normally done with just dry air outside, and a fan, and a radiator. But if you sprayed some water on there too, the heat might leave even more quickly...

[–]Snow 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

I don't use the air conditioner when raining. The rain drop down the temperature already.