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Old 07-11-12, 02:14 PM   #1
benpope
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Default AC Evaporative Cooling

With the high temperatures and low humidity over the last few weeks, Central Arkansas has started to feel more like West Texas. The typical summer here sees temperatures in the high 90s with humidity around 85%. In the past three weeks, temperatures have been over 100 and humidity about 25%. My AC use has gone from high to really high, but the “dry heat” poses an opportunity that I didn’t have before: evaporative cooling. I’m not going to install a swamp cooler yet (we’ll see what sort of tricks climate change has in store) but, I can use evaporation to help my AC run more efficiently.



This chart shows temperature reduction of an evaporative cooler based on air temperature and relative humidity. As you can see, cooling efficiency goes down as humidity goes up, so they would be more aptly called desert coolers than swamp coolers. I think the chart is based on a cooling pad that is about 80% efficient. Spraying directly onto the coils is going to be less efficient, say 30 to 50%, so instead of seeing a 20 degree drop when air temp is 105 and RH is 25%, I may see a 10 degree drop, not as good but it will still be helpful. Overall, I guess that a mister on the AC may save 10% on the electric use, but I am going to test to see.

I was inspired by two things: a new drip irrigation system that I installed over the past month and a picture of a commercial AC mister. The commercial model uses a valve with a paddle to start water flow to three misters when the AC fan turns on. I immediately thought of the spare toilet fill valve sitting in my shed. Turned upside down, it would turn on when the float was up. Direct the water to a mister or series of them and I would be in business.

A regular fill valve won’t work—they require a lot more force to turn off than provided by gravity alone. So, I took a trip to the local Home Despot and found this flapperless fill valve. I turned it upside down and gravity was enough to turn off the valve. $50 later I had a valve, a new length of drip irrigation pipe, several 3 gph misters, and an assortment of connectors to get everything to work together. $20 was for the ˝ inch irrigation pipe, much of which I will use to expand my drip irrigation setup.

Summary of jury-rigged install:
  1. I unhooked the flapper arm and business portion of the valve and rotated it 180 degrees. I had to cut off a bit of plastic intended to prevent this.
  2. Attached the water supply to valve with a length of ˝ inch irrigation pipe, a barb fitting, and a compression fitting.
  3. Remove the cap from the valve outlet and silicone into place a length of ˝ inch irrigation pipe.
  4. Zip tie the contraption to the AC housing.
  5. Punch a hole in the pipe and attach irrigation mister. I am starting with one and will later test with two and three.

This took me much longer than it should because I didn’t realize that a ˝ inch compression fitting is different from a regular ˝ inch connection. Now I know.

Initial tests were promising. The fan blew the valve arm up and water flowed to the mister. Then the valve went down but water continued to flow.I think it is a problem with the vacuum breaker, but I am not sure. The valve works in the same way as a normal toilet valve. Thoughts on this?

Then my jury rigging chickens came home to roost. It sprung a leak. Then another. I shut it down, took everything apart, cleaned and applied new silicone to all of the connections.

Everything will be dry now, so I will take a look this evening, take some pictures, and continue the saga tomorrow.

Once everything is up and working I will test it by running the AC and timing how long it runs to drop indoor temperature from 78 to 76 in the heat of the day.

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Old 07-11-12, 05:23 PM   #2
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Sounds like an interesting setup. I hope to hear much more.

I believe others have contemplated things like this in the past but didn't due to problems such as sediment build up on the heat exchanger coils from the evaporation of tap water.
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Old 07-11-12, 09:23 PM   #3
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Get a bank of whole house water filters? Even if it's not used for the intended purpose you'd have nice water coming from the faucet and shower.

Interesting idea benpope. I wish it were merely 100° here with 25% humidity. Or even 95° with 25% humidity.
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Old 07-12-12, 12:02 PM   #4
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Thanks, gentlemen. I got some pictures together that you can look at here. Water comes in from the left, goes through the valve, and down to the mister at the bottom of the first picture. Close-ups of the valve follow.

Last night I cleaned out the valve and put new silicone on the connections. Tested it out this morning and the valve is shutting off correctly.

Thanks for pointing out the problem of mineralization; it is something I meant to address in the first post. Water here is very soft, so there should be little problem with mineral buildup. When it does become a problem, I will be able to get the scale off with a liberal application of a vinegar solution. If that doesn't work, then a stronger acid solution or coil cleaning spray should work. The whole house filter is a good idea--I'd like to get one when funds permit to take out chlorine.

We had a cold front come through earlier this week (unseasonably cool high in the low 90s) but the heat will be back in a week or two. That gives me time to get the kinks worked out.
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Old 07-12-12, 12:16 PM   #5
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Have you watched in operation yet? Does the misted air get pulled through the heat exhanger? Should you add another two misters so you have one on each side?
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Old 07-12-12, 04:13 PM   #6
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I asked a local HVAC guy about the water mist systems and he said he thought it would corrode the coils to have water sprayed on them. Then he suggested I feel the temperature coming out of a supply register after the a/c was running for awhile. Then spray the condenser down with water and go back inside and feel the air again. I did that, the air was coming out noticeably warmer. I'm not sure why and it doesn't make sense to me. I searched some Youtube videos of those misting systems and the comments from those in the HVAC industry seemed to imply that they were a bad idea for the corrosion aspects as well as if you get the condenser too cold you could freeze up the evaporator and which would cause the refrigerant to slug the compressor.

I originally thought the idea was more about spraying colder underground temperature water that was below ambient air temp on them more than the evaporative cooling idea.

I'm not trying to discourage you, just unsure about the whole idea myself. I thought of trying it since my A/C is 26 years old, 8.5 SEER, and has a small condenser coil so I might see a bigger improvement and the risk is less since its might need replacement soon anyway. ..but that decision is hard because my upcoming electric bill will be about 300kwh($45) and I don't keep the house that cool anyway, even with the record outdoor temps we've had here.
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Old 07-16-12, 12:01 PM   #7
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No chance to work on it over the weekend, but I do have some updates.

Once I turned it on I realized that I had bought a 7gph fan sprayer instead of a 2gph mister. In its current location, the sprayer puts out an arc about a foot wide and 2 inches tall. Because the flow rate is so high, most of the water hits the coils and drips down. I will go back to the hardware store and get three misters which will provide coverage on all sides over a broader surface area.

The ideal spray pattern will hit all sides with a flow rate low enough to be immediately evaporated. I shouldn't see any water dripping down the coils and should see steam in the air column leaving the unit. With the fan sprayer, I only see water pooling at the base of the condenser.

Looking over more of the theory, "Analysis of current generation air conditioning equipment (this was the 9 SEER unit era) indicates an increase in air conditioning efficiency of approximately 1.2% per °F (0.7% per °C) reduction in condenser inlet temperature." This means that evaporative cooling has great potential for efficiency gains when temperatures are high and humidity is low.

On a side note, I am in a good climate for a mini split heat pump. However, right now I have a 13 SEER 80 AFUE ducted system that works fine. Since I don't know how long I will be in this house, I am unwilling to drop the money on it right now. Also, even though the costs are greater, I am going to see better returns air sealing and adding wall and crawlspace insulation to my 1930 house.
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Old 07-18-12, 06:42 PM   #8
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There are too many variables to accurately gauge effectiveness by seeing how quickly the house temp goes down. It would be better to check amp draw with an inexpensive clamp-on ammeter ($20 or less on ebay). Do this after the unit has stabilized, then switch on the misting system and check after stabilized again. The system will also go up in btu/hr capacity when you do this since the temperature "lift" from evaporator temp to final condenser temp will be less. You could measure the actual efficiency gain by comparing the power draw to the calculated btu/hr removed by the system if you measure airflow and enthalpy before and after the evaporator, but that may be more work than you want.

I manually wetted a swamp cooler pad strapped around my condenser to test this idea and got a 5-7% lower amp draw depending on outdoor temp/humidity. I didn't calculate the btu/hr to get an actual efficiency gain but it was probably around 10%, allowing for slightly higher btu/hr capacity from the lower condensing temp.

The other factor is water usage, since water is a precious resource. To get the best bang for the buck, I plan on using a swamp cooler pad only on the lower third of my condenser, kept wet whenever the condenser is on by a circulator pump pumping condensate water from the indoor unit. It will dribble water onto the top of the pad and excess will be collected in a tray channel at the bottom, just like a swamp cooler. I can add nearly mineral-free water from my reverse osmosis system if the condensate isn't enough, probably with a modified toilet fill valve. This way I will use only 1/3 the water, very little of which is makeup water I pay for. I won't have to worry about mineral buildup, which is harder to completely remove than you might think. Spaced away from the coil far enough, there won't be any liquid on the condenser to cause corrosion problems down the road, either.

Why only the bottom third? My condenser coil has multiple flow paths in the upper 2/3, but all the refrigerant then flows through a single final loop in the bottom third. This way, there is less airflow restriction.The full flow of ambient air will do the initial cooling/condensing in the upper 2/3, but the compressor will only "see" the lower condensing temperature of the water cooled lower 1/3, regardless of the temp of the upper section. This assumes of course that the system is charged properly so the refrigerant is still partially a gas by the time it reaches the lower third. Otherwise, there will be no power draw benefit, only extra subcooling.
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Old 07-18-12, 07:40 PM   #9
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Quote:
Originally Posted by Mobile Master Tech View Post

...
I can add nearly mineral-free water from my reverse osmosis system if the condensate isn't enough, probably with a modified toilet fill valve. This way I will use only 1/3 the water, very little of which is makeup water I pay for. I won't have to worry about mineral buildup, which is harder to completely remove than you might think. Spaced away from the coil far enough, there won't be any liquid on the condenser to cause corrosion problems down the road, either.
Hi,

If you collect rain water, its nearly mineral free.

Gary
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Old 07-20-12, 09:14 AM   #10
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MMT,
Thanks for the advice about measuring electricity. Since that is what I want to reduce, that is what I should measure. I don't have a loop ammeter, but I do have a house KW meter. It will "tare", so I can set it for base load and then do my AC tests. There will still be variability, so the clamp meter will be better. I'll see if I can get one for a reasonable price. I am able to do some rough calculation of ambient air temperature, coil temperature, and air output temperature as well as register temperature with my IR thermometer and regular thermometer, so that will provide some information about what is going on in the system.

There is a lot of discussion of this idea over at hvac-talk.com with most of the guys poo-pooing the idea as too complicated, too many problems, too much room for end user error, etc. My feeling is that they are a rather unimaginative lot, but most people don't want imaginative from their HVAC tech.

I have looked more into the mineral content of my water. According to the water company, our water has dissolved solids at a rate of 40 ppm. Rainwater has less than half that, about 17 ppm. However, the rates of problematic dissolved minerals are much closer to rainwater: Ca 6.7 ppm (5.5 ppm for rainwater) and Mg 0.98 ppm (0.74 for rainwater). Still, I assume that sedimentation will eventually be a problem, so I get back to only running it in the hottest temperatures. I will test it in the high 80s or low 90s, but don't plan on running it all of the time.

Thanks for pointing out the increased capacity issue. I have a 2 ton system and a very small house. The unit was likely oversized to begin with and after air sealing and adding more insulation it is definitely oversized. I need to model with HEED or something similar to get a more accurate idea of needed sizing. The HVAC-talk.com guys were saying that most systems are set to run optimally between 90* and 95* F, so I plan on only using it only when temperatures are above that. What do you think?

As I have gone on with this project, I came to similar conclusions as you to set up a partial swamp cooler around the condenser. I was concerned about airflow and I think your solution to cover only part of the condenser is an elegant one. If I were installing a new system I would try to go this route. It would make a lot of sense in a hot, dry climate. According to this study on the CoolNSave, most of the cooling effect comes from water evaporating off of the coil, not from pre-cooling the air. However, a swamp cooler-AC would probably do nearly as good a job only cooling the air instead of coating the coils. Gary's suggestion to use rainwater is duly noted.

Now from theory to practice...

I couldn't find misters at the hardware store, so I ordered five 0.8 gph misters from an irrigation supply company. I won't be able to test the system until these arrive. However, I did some basic tests with the mist attachment to my garden hose to get an idea of spray patterns. It looks like the mister works best when pointed up parallel to the coils. Most of the mist is pulled in by the fan, but a portion collects on the cage around the coils. There is no water vapor coming out of the stack, so all of it either evaporates in the warm air, collects on the coils and drips down, or overshoots and is carried off by the wind. I will use four misters, one per side, and move the valve to the back of the condenser. Hot weather has returned (105* F today), so the misters can't come soon enough!

I'll keep everyone updated and I look forward to more from MMT.

-Ben

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