How to keep a small room cool using wet curtains?

Question

I hypothesize that keeping curtains wet continuously will help in cooling a room through rapid evaporation with an indoor ceiling fan set at full speed. So how to accomplish this, and will it really keep the room cool for the following parameters:

• The room is closed to prevent hot air from outside coming inside the room during day time. The door of the room is open facing another bigger room which has only one small window open for the same reasons. The bigger room is generally cooler (we don't know why) and has one small window open which supplies fresh outside air passing through green plants. And the bigger room also has a full speed ceiling fan.

• The area is a humid tropical region - coastal India

• The ceiling fan is at full speed

• The size of the room is 15ft * 15ft (4.6m * 4.6m), the height is 16ft (4.9m)

• The dark curtain to make wet is on one wall having a 10ft * 5ft (3m * 1.5m) size

Based on the above I need to know:

1. Will a wet curtain really help in reducing the temperature of the room or keep us cool during daytime?

2. If yes (I assume yes after some research on this) then how to maintain wet curtains without manually keeping the curtain wet? I envision keeping it wet using an irrigation drip system, but are there any simpler mechanisms?

Answer 1

I think it could work with a minor tweak. If you stick the curtain to the outside of the window and keep it wet (including the surface of the window), it will cool the glass from the outside, which in turn will cool the air on the inside.

You will probably end up with condense on the inside (which would prove that it works) but compared to the original idea, that would be a minor problem. In fact it will lower the humidity inside making it more comfortable. In some countries it is quite common to have a small drainage under the window to dispose condense.

Here's a small example section drawing:

If you use a white cloth, it could keep the window more or less transparent (especially while wet) providing light while also partially reflecting some heating sunlight.

A dark curtain would not be a good idea. It may accelerate the rate of evaporation, but only because it becomes hotter, so that defeats the cause.

I guess some experimenting is worthwhile to find the ideal thickness and material for the cloth. If this proves to work out well, it may be interesting to develop an adaptive irrigation system specifically for this cause.

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Update: Experiment results

I finally did some experiments last summer (2nd of August, 2018). The humidity of the air on that day was 65%

My setup was with a table, having two shallow drawers under two glass plates, I had a digital thermometer for each. (this was taken in the morning while the table was still standing in the shade of a neighbors tree).

The thermometers would differ between 0.1 and 0.2 degrees Celsius when having the sensors in the same place.

I moved the sensors to approximately the middle of each drawer under the glass. The top thermometer display is in the drawer on the right.

Then I added the sheet to the left (I chose the left because it would be the first to get sunlight when the tree-shadow moved far enough. I also had given the left drawer the thermometer that gave the highest temperature. Not wanting to Bias the results in favor of what I would like to see.

The start was a little unfair off course:

I kept the sheet wet by spraying it every now and again.

And the results were quite startling!

63.8 degrees Celsius! I never would expect it to get that hot! Any way, compared to 48.4, the difference is about 24%. Now I guess my experiment is not like a window, it's almost flat, lightly slanted in the direction of the sun, so in real life with a vertical window I guess the difference would be a little less.

Later I did the same experiment with a dry sheet. The results were:

The difference, 49.4 vs 61.1 is about 19%, so the water is only contributing about 5% compared to the dry cloth. Note that the air humidity was 65%, so this 5% may be more if you live in a dryer climate, but less if the humidity is higher.

Sorry for taking so long to do the experiment and then the delay to report the results... better late than never I guess.

Answer 2

With the given parameters you originally gave, the idea won't work.

To provide continued cooling, the water blown onto the curtains needs to be able to evaporate water. Without a substantial ongoing supply of fresh air that is below its maximum humidity (for its temperature), the humidity will quickly reach an equilibrium point and then will no longer cool at all.

If you were to use plenty of fresh air (that isn't already at its maximum humidity) blown over a wet curtain, then you would obtain a supply of cooled air using evaporation. A heat exchange or heat pump system could still make use of that, but unfortunately both those systems tend to be either inefficient or expensive.

Fresh air feels cooler to our skin when it is able to evaporate some sweat, so simply having a flow of fresh air that goes past the inhabitants may be the simplest solution.

Now that you've updated your question, I think it's worth pointing out the following:

• Hot air rises, so if the larger room has a higher ceiling, then that could explain the difference. Otherwise, is there a difference in the solar load through windows, or even on the roof (a roof angled to face the sun will be hotter than one that isn't).
• If you use a fabric curtain you are likely to have mould/algae issues. A longer lasting solution is probably to use something that is allowed to grow algae but is able to be cleaned, such as stainless steel or plastic. Water doesn't flow as well over those surfaces, but I suspect your ability to evaporate water is going to be heavily limited anyway by your supply of fresh air rather than the surface area of the water.
• Ceiling fans may bring hotter air down and make the temperature feel higher than if you used ground level pedestal fans instead.
• The volume of air leaving the building is equal to the volume entering the building. This means you could vent the hotter air from the ceiling, and allow fresh air (where ever it is drawn in) to flow past a wet object, cooling it on the way into the building. This technique, combined with fans directed onto the inhabitants, would be your optimum solution in my opinion.

Answer 3

Evaporative coolers are common in dry hot climates. It's different from what you are proposing. It's a box on the roof that has coarse pads (commonly coir) with a small pump that keeps them wet. Air is sucked thorugh the pads, and exhausted into the house.

At the cost of more energy, you can use the now cooler air to cool the input air of a second unit. In principle with stages you can get the incoming air into the building down to the dew point.

In humid climates this doesn't work well. Moving the air through a wet pad doesn't cool it. Indeed, if the air is truly saturated it will warm up from mechanical losses.

If you climate gets cooler at night, then you may get some relief by using the cooler night air to cool a mass of water, then use that mass of water for cooling during the day.

Answer 4

Due to your humid climate, an evaporative cooler is not likely going to be very effective. A 100% efficient evaporative cooler will only cool down to the "wet bulb" temperature:

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

For direct evaporative cooling, the direct saturation efficiency, measures in what extent the temperature of the air leaving the direct evaporative cooler is close to the wet-bulb temperature of the entering air.

...

The same equation indicates why evaporative coolers are of limited use in highly humid environments: for example, a hot August day in Tokyo may be 30 °C (86 °F), 85% relative humidity, 1,005 hPa pressure. This gives dew point 27.2 °C (81.0 °F) and wet-bulb temperature 27.88 °C (82.18 °F). According to the formula above, at 85% efficiency air may be cooled only down to 28.2 °C (82.8 °F) which makes it quite impractical.

Using Surat, India as a representative coastal city, the current temperature is 26c/79f and the web bulb temperature is 24.5c/76f, so even if your evaporative cooler were 100% efficient, it would only drop the temperature by a degree or two, but it would also increase the humidity inside your home, making it less comfortable.

This heat index chart shows the relationship between relative humidity and perceived temperature:

Answer 5

Sorry for the short answer, I can't comment yet due to reputation rules:

You're also going to have to deal with the heat generated by the ceiling fan. If it's electric, then it has a motor and generates heat. :/ It may actually be more heat than you can store in water through evaporation.

Have you tried putting the water on your roof? If the air is not as saturated as it can be, you may get some cooling from the evaporation there; at the very least, it will offset the heat from the sun.