Thursday, April 14, 2016

Swamp Coolers

A few years ago, a young friend of mine moved to Southern California with her husband and new baby. The housing they were living in was fair, but when the air conditioner died in July, the landlord struggled to find someone to fix or replace it. Having been around babies in the Summer, I know how uncomfortable they are in the heat and how miserable that makes parents; she needed a way to cool at least one small room so her daughter could have a place to sleep comfortably.

I did some research and found several sets of plans for a home-made swamp cooler that she could afford to put together for less than $50 and were enough to cool a small room. It worked well enough and long enough to get them through until the AC got fixed, and her husband ended up making a few for neighbors with the same problem. While this wasn't a true “emergency”, it's still an example of how a bit of knowledge and some simple skills can make rough times easier to get through.

What is a Swamp Cooler? 
It's a way to cool air using the evaporation of water and a fan. They're simple, easy to make, and quite energy efficient, but they don't work everywhere, especially in areas of high humidity.

Swamp coolers work by passing air through a mesh or media that is kept wet. Using the same principle that your body uses to cool itself by sweating, the water evaporates and carries excess heat with it. In the time of the Pharaohs they used slaves with fans to move air past containers of water or wetted blankets hung over doorways, so it's not new technology. 

Water is an odd chemical from a lot of standpoints, but the one aspect that makes swamp coolers work is its high latent heat of  evaporation (enthalpy of vaporization). Put into layman's terms, water requires a lot more energy to convert from liquid to vapor than most other common chemicals -- on the order of three times more energy than ethyl (grain) alcohol and six times more than a lot of hydrocarbons. This means that if we can get water to evaporate, it will carry away a lot of heat.

The sticky point is getting that water to evaporate, which brings us to another term: Dew Point. If you get your weather forecast from the radio or TV you probably don't see the dew point listed, but it will be in all aviation forecasts and most of the internet weather sites. Dew point is the temperature at which the humidity already in the air will start to condense (100% relative humidity), creating fog and dew. Aviators need this information to avoid flying into fog, which is why it is part of a METAR (Meteorological Aerodrome Report).

Swamp coolers can cool air down to almost the dew point, but never below it. That means that if you live in an area like mine where the relative humidity runs 50-80% in the Summer, causing dew points of 75-80° F, the best output from a swamp cooler (or cooling tower) is going to be 80-85° F. That's not much help for staying comfortable, but usable in industrial settings. Those of you who live in more arid climates, where relative humidity tends to stay under 20% will see more cooling since the dew point will be under 50 ° F. Sorry Florida, swamp coolers don't work well in actual swamps. Don't I know it! -- Erin)

Here's a chart of dew points at various temperatures and humidity levels: 
How Do You Make One?
The simple DIY version I found for my friend was a 5 gallon bucket with a foam pad for the wetted media, a 12V DC fan from a dead computer case, and a small solar water fountain pump from Harbor freight. Full instructions are found here, but the concept is simple:
  1. Mount a fan or two in the lid of the bucket, blowing air into the bucket. 
  2. Cut vent holes in the sides of the bucket, as many as you want. 
  3. Line the bucket with a fibrous mesh (humidifier filters work best).
  4. Place a pump in the bottom of the bucket and connect a hose to the top of the mesh 
  5. Add water, usually 2 but no more than 3 gallons. 
  6. Turn on the power.
The pump moves water over the fiber mesh while the fan is blowing air through it. As the water evaporates, it cools the air coming out of the vent holes. Expect to go through a few gallons of water every day. so make sure you've got the extra water available. It doesn't have to be potable water, but the cleaner it is, the longer your filter mesh will last and the fewer problems you'll have with impurities getting volatilized and spread around your living areas.

Use your imagination to replace parts you may not have. 
  • Wood chips or shavings (think pet bedding) or loose straw will hold water while allowing air flow, so long as you don't pack them too tight. 
  • A towel or piece of blanket held up with bent coat hangers will hold water. 
  • 12V DC fans are easy to find; if you don't have a computer cooling fan around, look for an automobile blower fan in a junkyard or check out one of the larger truck stops for a 12V DC fan. 
  • Plastic trash cans can replace the bucket, 
  • Solar power is a good option because you'll need the cooler most when the sun is shining.

Knowing how something works is the first step to making it work.

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