To size an evaporative cooling system, you first need to find the CFM (cubic feet per minute) requirements.  CFMs are essentially just the volume of air in your building, the standard being that you want to move the entire volume of air within one minute.

Finding your CFMs:

• Length X Width X Height = CFM (As a rule, we typically use 12 for the height value in this equation.)

• Let's assume a 30'x48' structure:
  30 X 48 X 12 = 17,280 CFMs

Using this you can size your fans, inlet shutters, and evaporative cooling wall.  Alternatively, you can size and install a positive pressure cooler and outlet shutters.

Sizing the Fans (Negative Pressure System):

With a negative pressure system, exhaust fans pull air out of the building.  This air is replaced by cool air from outside the building coming in through the inlet shutters. 

A typical exhaust fan arrangement with a negative pressure system is two fans.  This increases the efficiency of your airflow.  So:

• 17,280 (your total CFMs) / 2 (number of fans) = 8,640 CFMs per fan

This means each of your two fans should have a CFM rating of over 8,640 to have standard air exchange rates.

Sizing the Evaporative Cooler (Negative Pressure System):

Evaporative cooling walls are most often installed along an end wall, and as such, it they need to be sized to fit accordingly with the available space.

• 17,280 (your total CFMs) / 250 (4 inch pad thickness) / 3 (standard pad height in feet) = 23.04

This means you will need 23' of cooling pad at 3' in height.  In a 30' building this gives you plenty of room.

But what happens when you need a larger system than your building is wide?

In this situation, you will need either to move to thicker cooling pads, or a taller system.  Sometimes proper sizing will require both.

• Assume a 20'x 72' structure:
  (20 X 72 X 12 =17,280 CFMs) / 250 / 3 = 23.04

This is exaclty the same measurement as before, only now we have just 20' of wall space to put 23' of pad.  What do we do?

• Increase the height of the pad.
  17,280 / 250 / 4 (height of pad in feet) = 17.28

• Increase thickness of pads from 4" to 6"
  17,280 / 400 (6 inch pad height) / 3 = 14.4

Either of these solutions will work, as long as the shutters can be sized accordingly.

Sizing the Shutters (Negative Pressure System):

When an evaporative cooler is present, properly sizing the inlet shutters can be a little more complicated. There are three factors to consider when sizing your inlet shutters:

1. You need to size the shutters for the proper airflow.
2. You also need to closely match the height of the shutters with the height of the evaporative cooling wall.  This will ensure ease of installation on the cooling wall.
3. You will need to ensure that enough shutters are present to properly expose the cooling pads to the incoming air.

So, using our CFM measurement from above:

• 17,280 / 2 = 8,640 CFM

This corresponds best with 2 48" shutters.

However, if we use our first example we only have a 3' tall system.  This will generally be too large a disparity between shutter height and pad height to effectively seal the evaporative cooling system around the inlet shutters. 

Additionally, this shutter arrangement would only provide 8' in pad exposure to incoming air.  On a 20'+ system this is far to little exposed pad surface area to effectively cool your building.

To properly size your inlet shutters, it may be best to divide total CFMs by the CFM rating of the closest size shutter to your pad height and work from there.  This will give you the number of shutters you will need in that size and rating.

• 17,280 / 5,000 (CFM rating of a 36" shutter, ) = 3.5

Four 36" shutters should work. Now let's make sure they propely cover the cool cell.

• 4 (number of shutters) X 3 (width in feet) = 12 feet of shutter (NOTE: If the shutters are not a whole foot measurement, such as 40", then: Number of Shutters X Width in Inches / 12.)

There should be no more than a couple feet of unexposed pad along an evaporative cooler.  This means shutters can be spaced +/-2' in from each end, and then +/-2' apart.

• 2 (space between shutters in feet) X 5 (number of spaces) = 10 feet of acceptable additional space

Now add the amount of shutter coverage and the acceptable spacing distance.

• 12 + 10 = 22

This is perfect for our system.  Differently sized systems may require trial and error calculations to correctly size all components of your system.