Sunday, May 25, 2008

Homemade Air Conditioner Version 3

I reworked the previous version. The design is basically the same. I just improved the construction.

The main construction materials used were:

• Aluminum repair roll ($10-15)
• Nuts, bolts, screws and washers ($10-15)
• Durable waterproof silicone: GE Silicone II - for adhesion to steel, aluminum, and wood - it's the type of high quality silicone used for aquariums (~$15)

I also used wood I had lying around. So that did reduce the price a bit. Total cost: $35-45.

Like before, the heat exchanger is made from straight sections of aluminum bent into a U-shaped channel. To maintain the gap around the edges I used slender lengths of wood, 1.8 cm wide x 1.8 cm thick. The lengths of wood, and the 2x4 shown also serve to strengthen and maintain the shape of the exchanger (Figure 1 and 2 ). A copious amount of silicone was used to seal all gaps around the edges and bolts to prevent water from entering. I also coated the exposed faces of the wood with silicone as I didn't want the water to be in contact with it.

Figure 1

Figure 2

The width of the heat exchanger is 25 cm. The flow length (along the U-shape) is approximately 60 cm. The inside gap is 1.8 cm.

Attached to the heat exchanger is a duct (Figure 2) which feeds air into the heat exchanger by way of a fan (Figure 3). The sides of the duct are oriented so that the wood grains run in the direction of the air flow. This helps reduce friction and pressure drop of the air before it enters the heat exchanger.

Figure 3

The top plate of the duct has a hole in it which is slightly larger than the fan diameter at the lip, allowing the fan to sit snugly inside (Figure 4). The gap between the top plate and duct is sealed with silicone to prevent air from escaping while the fan is running. The junction at the duct and heat exchanger interface is also sealed with silicone to remove external gaps, to prevent air from leaking.

Figure 4

A rubber mat is placed on the bottom of the container (30 cm diameter, 34 cm high) to prevent the aluminum from being scratched by the bottom of the steel container (Figure 5). The heat exchanger + duct + fan assembly is then placed inside the container (Figure 6). Figure 7 shows a close-up view of the air exit.

Figure 5

Figure 6

Figure 7

Supports (Figure 8) are used to keep the unit vertical. Ice, in addition to water, can then be placed inside the container (Figure 9). This serves as the cooling medium. You can also use ordinary tap water without adding any ice. In my area the average temperature of tap water in summer is roughly 20 degrees Celsius.

I made 40 bags of ice outdoors, on one really cold night this past winter (it was minus 20 degrees Celsius), and stored them in the freezer (Figure 10). This way I won't have to make, or buy any ice. Alternatively, you can also leave water outside overnight, allowing it to cool, and use it the following day if it's hot out.

Figure 8

Figure 9

Figure 10

With ice-water temperature between 10-12 degrees Celsius and ambient (room) temperature between 20-28 degrees Celsius, the air cools 3-7 degrees between inlet and outlet. To achieve greater cooling the gap of the heat exchanger would have to be smaller than 1.8 cm to increase the heat transfer rate. The drawback to this is that the air will flow through the exchanger at a slower rate, due to the greater pumping effort required. In the previous version the gap was roughly 1 cm, but the exiting air speed was less than ideal, even with the fan running at maximum. So I would need a more powerful fan (possibly a blower) to compensate for a smaller gap, and maintain suitable air speed.

I found that you need frequent water exchange or a constant supply of ice to maintain a low cooling-medium temperature. One way to do this without too much hassle is to have a big tank of water set up outside to cool overnight, and insulated during the day so that it doesn't heat up. You can then circulate the water from this tank though the container, via a small pump, to maintain the "sink" temperature for the heat exchanger. You can size the outdoor tank so that this lasts you all day and so that it has enough time to cool down during the night. Or you could run a hose from a water spigot to your container, and turned on low, while simultaneously draining it into a nearby drain, thereby keeping the "sink" water cool.

A note on condensation: On humid days water will likely condense on the inside of the heat exchanger, and pool on the bottom. So you may have to pour it out every few hours or so, depending on how much water condenses (it probably won't be a lot). And it's also a good idea to place old newspapers underneath the container to catch condensate accumulating on the outside of the container.

** Update July 09, 2008

Instead of using a metal container I switched to using a cooler (capacity 15 litres). The heat exchanger fits nicely inside and is held upright by horizontal wooden supports. The cooler works better than the container because it's insulated. To insulate the top area I just placed a towel around to help keep the "cool" in. See Figures 11-14. Surprisingly, the temperature drop was better than with the metal container. With ice-water temperature of 7 degrees Celsius, and ambient temperature of 26 degrees, the exiting air temperature was 16 degrees! Very decent.

Figure 11

Figure 12

Figure 13

Figure 14

This unit works great for local cooling. Better than a fan but not as good as a store-bought A/C. But if you just want to keep yourself cool then just put it beside you, and it's all you need.

Have a look at Cost To Make Ice For Homemade Air Conditioner.

Other related posts:

Homemade Air Conditioner Version 1

Homemade Air Conditioner Version 4


Anonymous said...

I'd have to agree about the blower, they are especially efficient at overcoming static pressure - which the aerodynamics of this project have a large degree of, especially with a narrower gap. The only drawback is they tend to be a bit on the loud side. I'm willing to bet you would get a better temperature difference though. :) Good job on the project!

Anonymous said...

Great job! Now you need to dig a well and use a small pump to circulate cool water through your unit. Almost FREE cooling year round!! Put some paint on your unit and it almost looks like one of these portable units for sale!

Colin said...

Pretty impressive. Have you tried using a salt-water/ice mixture yet?

Vittorio said...

I did some experiments last year with adding salt. I found you need to add a huge amount to get an appreciable level of additional cooling. Not a big deal if you use rock salt, the kind used for deicing in winter, as it's pretty cheap for large quantities. But it would probably be pretty corrosive for the container/heat exchanger. Also, I'm not sure how the silicone would hold up in a salty environment.

Inboulder said...

Oh geeze, talk about over engineering and making things difficult. Using the same materials, and without having to make a bunch of water proof seals, I could put together something with more cooling potential in 2 minutes. Just take the fan and have it blow into the bucket. It'll have higher airflow and larger cooling effect than this method.

Vittorio said...

^^^ I tried that. Blowing the fan into a bucket with the same ice-water/ambient temperature is *not* as effective as using the heat exchanger. The output air is about 2-3 degrees C warmer using your proposed method.

Anonymous said...

How about going to a junkyard, getting a junk automotive radiator and using a small pump to pump the ice water from the bottom of an icewater bucket through the radiator and then back into the top of the bucket. Blow the fan through the radiator. Even better, use this method with a well as a previous poster said, with ice to augment on really hot days.

Vittorio said...

I think with a radiator you need a stronger fan than what I have since the fins are generally spaced so closely together, and the air would barely push through. Also, it could be tricky getting a good air-tight seal around the radiator if there's not already a solid frame around it.

kul said...

interesting stuff there. I'm doing the reverse. I'm supposed to preheat air going into a drying oven. the concept is the hot air out of the oven would then be used to heat the fresh air in the heat exchanger before it goes into a heater box and then into the oven. The idea is to save on energy heating the air coming into the oven. Any ideas, Vitto or anyone??

Vittorio said...

^^^ Maybe you can use solar heat to preheat the air. One way is to make (or purchase) a flat plate solar collector to heat a heat-transport fluid such as water + propylene glycol, which runs through a heat exchanger which is immersed in "X" medium, such as water, in order to heat it up. This heated "X" medium can be used to preheat the air going into the oven, using another heat exchanger like the one shown in this post. So basically, instead of using cold water as "X" medium you would use heated water, and the air would come out warmer instead of colder.

A solar collector can get quite warm, up to 80 deg C from my own tests this summer, which should get pre-heated air temp close to that, assuming you have good heat exchangers (two of them). The tricky part is adjusting the flow rate of your heat-transport fluid to make this as efficient as possible. Here's a link which may give a better idea:

Now, if your air coming out of the oven is greater than the temperature reached in a flat plate solar collector, then you obviously need to change the collector design, and make it a parabolic dish (or trough) which can get your temperature up to several hundred degrees (scroll down in the link provided above, to see description). But you have to keep it pointed at the sun. Furthermore, your "X" medium can no longer be water since its boiling point at atmospheric pressure is 100 deg C. In this case you can probably use the same liquid as before as "X" medium, which is propylene glycol.

The other option you can consider using, if your oven is heated with natural gas or whatever, is to use an Economizer, which would use the waste heat from the hot flue exhaust to preheat the air going into the oven. For the basic concept have a look at:

The payback period should be a few years for all of these, assuming you use the oven constantly.

Anonymous said...

Gee whiz people. What a contraption. All you need is a transmission cooler from a junkyard. A submersible aquarium pump and a electric fan (car). RadioShack or any sells a 12 volt converter for the fan. Buy or find a $2 ice chest from a convenient store and put the radiator and fan on top and hook up the aquarium pump. The dollar store sells ice packs, the blue ones, for cheep and get about 8 of them and rotate as they melt. Not much detail but you get the idea. If you have ice in a bucket it melts a lot faster than in an ice chest. Get it good. Also junkyards have all this stuff for cheep.