Swamp Coolers: Find the Right Size

ESTIMATING COOLING CAPACITY

3 main considerations for swamp cooler sizing:
1. Airflow
2. Cooling capacity of the evaporation
2. Cabinet size: blade size and tank size

Swamp Cooler Sizing: Air Flow

A swamp cooler cools mainly by how much air it can blow. So, we decide how big a swamp cooler should be based on how much air it can move.

Look at the cooler’s CFM (Cubic Feet per Minute) or CMM (Cubic Meters per Minute) rating. This number measures the amount of air a swamp cooler can move per minute.

Use half the airflow for the area you want to cool as a starting point. This is less than the amount of airflow needed by an outdoor fan to cool the same-sized area. That’s because swamp coolers have the additional cooling power of evaporation.
For example, if you’re holding an outdoor event in a tent that’s 10′ wide x10′ long x10′ high (1,000 cubic feet), you’ll need a cooler with at least 500 CFM (1,000 cubic feet divided by 2 = 500).
In international measurements: If you’re holding an event in a tent that’s 3m x 3m x 3m (27 cubic meters) you’ll need a cooler with at least 13.5 CMM (27 cubic meters divided by 2 = 13.5).

This estimate works well for spaces like garages, warehouses, and outdoor sheds. It’s also quite accurate for places that have three or four sides, like a patio or a tent. But the less enclosed an area, the more airflow you’ll want from your swamp cooler. More open areas are hard to estimate, but you can use the basic formula as a starting point. The more open the area, the more additional airflow you’ll need.

A unit with more airflow won’t increase the evaporative cooling effect, but it will increase the amount of breeze you’ll feel. By itself, moving air can be up to 8º cooler than air that is still.
If a manufacturer or retailer only gives you CFM or CMM, you may want to convert it to measurements you’re more comfortable with. Here’s how: 1 cubic foot × 35.315 = 1 cubic meter; 1 cubic meter × .0283 = 1 cubic foot.

Swamp Coolers Need Fresh Air

Swamp coolers need fresh air to provide the most cooling. Because they add moisture to the cooled air, if this same air goes through the system again it will pick up even more moisture. If this keeps up, the air will pick up so much moisture it will no longer be able to provide any cooling. At that point, you’d be experiencing a muggy, “swampy” mess.

The simple solution is to provide fresh air — at least a 2 square foot opening for every 100 square feet / .186 square meters for every 9.3 square meters you want to cool.

This makes portable coolers well-suited to partly enclosed outdoor areas. A tent, cabana or patio with an open side or a shed, trailer or booth with open windows can contain the cooled air and still have plenty of fresh air for the cooling system. But it also makes it harder to estimate a cooler’s cooling capacity.

BTUs Measure Evaporative Cooling Potential

The most important measurement in swamp cooler sizing is its CFM / CMM. But a few manufacturers also measure their coolers’ BTU ratings, which can be helpful as a general guideline. BTUs (British Thermal Units) attempt to measure the evaporative aspect of a swamp cooler’s cooling capacity.

BTUs are the most important measurement of an air conditioner’s cooling capacity, but not of a swamp cooler. Since a swamp cooler’s ability to cool is based on the level of humidity in the air (less humidity = more cooling), swamp coolers with more BTUs won’t necessarily provide more cooling. They will have the POTENTIAL for more cooling, though.

That potential will be more available on hot, dry days than on more humid days. On a hot, dry day a swamp cooler with fewer BTUs can provide more cooling from evaporation than a swamp cooler with more BTUs that has that same amount of airflow on a humid day.

BTU ratings are not as precise for swamp cooler sizing as they are for air conditioners. Swamp coolers need lots of fresh air in the area they are cooling to help balance humidity levels. This is different from an air conditioner that needs an enclosed area to more precisely control the cooling level. It’s easier to figure out how much cool air you need for an enclosed room that isn’t affected by humidity. But for outdoor areas that are partly closed or completely open and affected by humidity, it’s a bit trickier.

Air Flow and BTUs

Once you find a swamp cooler with the right amount of air flow, see if it has about enough BTUs for the size of area you want cooled. Don’t follow exact numbers on this. Use it as an estimate, only. Following exact numbers and expecting an exact amount of cooling will only give you a headache, especially if you’re trying to cool a more open area.

If your area has significant corners or alcoves that the air from one swamp cooler may not reach easily, consider using two smaller units – one sized for the larger area and one sized for the corner or alcove area.

This chart shows how many BTUs are suggested to cool certain sized areas with an air conditioner. It will give you a general starting point with swamp cooler sizing. These numbers will vary by manufacturer and the actual conditions of your area.

AREA TO BE COOLED-
SQUARE FEET
AREA TO BE COOLED-
SQUARE METERS
CAPACITY NEEDED –
BTUs PER HOUR
100 – 1509.3 – 14.05,000
150 – 25014.0 – 23.36,000
250 – 30023.3 – 27.97,000
300 – 35027.9 – 32.58,000
350 – 40032.5 – 37.29,000
400 – 45037.2 – 41.810,000
450 – 55041.8 – 51.112,000 (1 TON)
550 – 70051.1 – 65.014,000
700 – 1,00065.0 – 92.918,000
1,000 – 1,20092.9 – 111.521,000
1,200 – 1,400111.5 – 130.123,000
1,400 – 1,500130.1 – 139.424,000 (2 TONS)
1,500 – 2,000139.4 – 185.830,000
2,000 – 2,500185.8 – 232.334,000
Area sizes and suggested BTUs to cool them. A loose guideline for swamp coolers. Numbers vary based on climate and conditions of the area cooled.

Manufacturers often use a kind of shorthand to refer to the sizes of their units. Any unit with a cooling capacity that is a multiple of 12,000 BTUs is said to provide so many “tons” of cooling. A 12,000 BTU unit is called a “1-ton” unit. A 24,000 BTU unit is called a “2-ton” unit, 36,000 BTUs is called a “3-ton” unit, and so on. These are in the chart.

When you purchase or rent a portable swamp cooler, consider the conditions of the area you’ll be cooling, including the weather and size of the area. If you use a sales or rental consultant, giving them this information will help them advise you on swamp cooler sizing and the type of unit for your needs.

Swamp Cooler Cabinet Size

Fan blades and water tank sizes

The cabinet houses the fan, pump, motor, and water tank, with the water tank and fan blades using the most space and being the biggest factors in the cabinet size.

Smaller swamp coolers

Smaller sizes, best used for “spot cooling” or “small area cooling” use fan blades smaller than 16 inches / 40 cm. 12 inches / 30 cm is most common. These small units seldom list their blade size in the product description. They usually look like a tall rectangle under 3 feet / 1 meter tall and less than 2 feet / 61 cm wide and deep.

Their cabinets are sleeker than larger-sized units. These are more visually appealing than the larger units. They range from as little as 250 CFM all the way up to 3,800 CFM.

Since they are small their water tanks are rarely more than 5 gallons / 19 liters. With the average water use of a swamp cooler at 3-15 gallons per hour, the water tank would need to be filled frequently.

Larger swamp coolers

Larger portable swamp coolers have fan blades that start at 16 inches / 40 cm and go up to 48 inches / 122 cm and more. They look like big-screen TVs or huge computer monitors with a big fan in the back. They need large cabinets to house these large fans and water tanks of 10 to 65 gallons / 30 to 250 liters. Their cabinets may be as small as 24 inches / 60 cm square by 42 inches / 105 cm, but are usually over 3 feet / 1 meter square and deep, and can get much larger. Weights start at just under 100 lbs / 45 kg and can range up to several hundred pounds / kilograms.

They are powerful, moving thousands of cubic feet per minute, and capable of cooling large areas. Yet they use very little electricity — they rarely draw more than 10 amps at 115 volts (1150 watts, or about as much as a microwave oven). So even most of these large coolers can run on regular household currents.

Other Outdoor Cooling Methods

Swamp coolers combine two of the most effective outdoor cooling methods. But, as mentioned above, they have some limitations. You can maximize their cooling by combining them with other methods or devices, or other options may be more effective.

Take a look at our comparison page or our pages on other outdoor cooling methods:

  • Outdoor fans combine well with swamp coolers, ensuring adequate air flow and distributing the cool air from evaporation.
  • Misting fans are another kind of device that combines evaporation and wind chill to cool outdoor areas.
  • Outdoor shades are a nearly universal solution that work well with any outdoor cooling method, including swamp coolers.
  • Misting systems use a similar technology as swamp cooler, but in an open system that can cool larger areas.
  • Portable air conditioners are especially suited to enclosed areas (tents, sheds, etc.) and special circumstances.