Another Cheap Homemade AC

Materials used:

• Aluminum repair roll ($10-15)
• Nuts, bolts, and washers ($5-10)
• Waterproof silicone ($5-10)

Total cost: $20-35

The heat exchanger is made from straight sections of aluminum (cut from the repair roll) bent into a U-shape, and bolted together, as shown (Figure 1, 2, 3). Silicone was used to seal the gaps along the edges and around the bolts.

Figure 1

Figure 2

Figure 3


Air enters one end of the heat exchanger, cools down as it passes through the U-shaped channel, and exits the other end.

The cooling medium is ice-water (Figure 4).

Figure 4


The heat exchanger is placed inside the container (Figure 5).

Figure 5


The water level inside the container is deliberately kept low (less than half full) because a smaller amount of water gets colder for a given amount of ice. The trade-off with this is that there is less heat exchanger area in contact with the water, meaning the air doesn't cool as much as it passes through.

The balance is in using a volume of water such that you keep the water temperature as low as you can (for a fixed amount of ice), while providing sufficient heat exchanger area (in contact with the water) to obtain decent cooling of the air.

A fan is connected to the intake side of the heat exchanger via a plastic bag taped at the two interfaces. It's crude but nevertheless, it's an easy way to accommodate the transition and ensures that air enters the exchanger without leaking. See Figure 6.

Figure 6


The ice-water temperature was approximately 4.5 degrees Celsius.

The temperature drop of the exiting air was approximately 8 degrees Celsius**.

The fan speed was set on low.

If you don't have any ice available you can fill the container up completely with water and the air will be cooled very close to the water temperature. Tap water in the summer, in my area, is about 20 degrees Celsius. This will provide sufficient cooling on really hot days. Also, a larger volume of water will cool for a longer period of time. The container I used holds about 20 liters of water, enough cooling for a few hours.

Technical Notes:

• The cross-section of the heat exchanger is 1 cm x 25 cm. This yields a high wetted perimeter relative to cross-sectional area, inducing turbulence and improving heat transfer in a predictable way. To calculate this I used equations for internal flow from my heat transfer book.

• Lower fan speed results in greater cooling since the air stays in contact longer with the heat exchanger.

• The cross-sectional area of the heat exchanger is roughly 4 in2. This keeps the back-pressure reasonably low on the fan. An opening less than 1 cm wide can be used to achieve even greater cooling, but care has to be taken to keep flow (pumping) loss to a minimum. With a smaller cross-sectional area the fan can't push the air through as fast, meaning you have to set the fan speed higher, or get a stronger fan to push the air through at the same speed.

• Flow length also contributes to pumping loss, although it's not as strong an influence as cross-sectional area. It too must be balanced with the desired exit temperature and allowable fan power.

** With the heat exchanger immersed in the ice-water, I measured the exiting air temperature as 10-11 degrees Celsius. With the heat exchanger not immersed, I measured the exiting air temperature as 18-19 degrees Celsius.