# Calculating Humidification Loads for Economizer Cycle Systems: Humidification Basics Part 5

RL Deppmann
/
October 3, 2016

Now that we’ve learned how to calculate a basic humidification load, we need to understand how to do the same when an economizer cycle is part of our design.

Why is that so important? Because nearly every commercial building in the continental United States is required (per DOE) to incorporate either an air- or water-side economizer. If your state hasn’t already incorporated the ruling into its commercial building code, it’s just a matter of time before it does. And that’s a good thing, because using cool outside air to maintain building temperature is an excellent way to reduce cooling costs.

## Proportioning Return and Outside Air

Airside economizers work by mixing outdoor air with building return air in order to maintain the supply temperature to the space – typically about 55°F. Proportions of indoor and outdoor air will vary, of course, depending on what the outdoor air temperature is. The table below from the Dristeem Design Guide shows the percentages of outside air that can be mixed with return air to maintain a 70°F space using one of three mixed-air temperatures.

## Determining Outside Air Quantity

As with natural and mechanical ventilation, we must first determine the quantity of outside air in order to calculate what our humidification load will be. Again, there are excellent programs out there that do all the legwork for us, but to impart true understanding, we’re going to work through an example as if such aids don’t exist.

Let’s say we’re calculating humidification loads for a non-critical application with an economizer cycle, using the following parameters:

### Desired Space conditions

The formula for determining the quantity of outside air in an economizer cycle is:

V2 = VAH ÷ ([A ÷ B] + 1)

where:

VAH = V1 + V2

V1 = volume of return air (cubic feet per minute)

V2 = volume of outside air (cubic feet per minute)

A = temperature difference between mixed air and outside air

B = temperature difference between return air and mixed air

Plugging our numbers in, this is what we get:

V2 = 12,000 cfm ÷ ([35°F ÷ 15°F] + 1) = 3,600 cfm of outside air