ASHRAE 62.1 Standards and Air Systems Practice Test

The concept here is how the outdoor-air requirement for a zone relates to the total supply and the outdoor-air fraction in that supply. The zone needs a certain amount of outdoor air, call it Vbz. If the supply air to the zone has an outdoor-air fraction Ez, then the outdoor-air portion in the total supply Voz is Voz × Ez. To meet the zone’s outdoor-air requirement, this product must equal Vbz:

Voz × Ez = Vbz

Solving for Voz gives Voz = Vbz / Ez.

For example, if the zone requires 200 cfm of outdoor air and the supply is 50% outdoor air (Ez = 0.5), you need Voz = 200 / 0.5 = 400 cfm of total supply air. The outdoor portion is 400 × 0.5 = 200 cfm, which satisfies the requirement.

If Ez increases toward 1, the total supply needed decreases toward the required outdoor-air amount; if Ez is smaller, you need more total supply to deliver the same outdoor-air quantity.