Ventilation Education – Calculating Exhaust Fans for a Tunnel Dairy Barn

Dairy farmers need to know a lot about a lot. Besides knowing about their animals, they need to know about nutrition, medication, reproduction, electricity, mechanics, manure, plumbing, accounting, and that’s just scratching the surface. When it comes to barn ventilation, J&D Manufacturing can shed some light on how to calculate what you really need.

Calculating the number of barn exhaust fans needed for a tunnel ventilated dairy barn is easily done by using a simple formula.

Barn width x average height x desired velocity in MPH x 88 feet per minute (fpm) = total cubic feet per minute (cfm) needed.   


Barn width: 120’
Average barn height: 15’
Desired Velocity: 7 MPH

120’ x 15’ x 7 MPH x 88 fpm = 1,108,800 cfm needed

The next step is to choose the barn exhaust fan and divide the cfm performance of that fan by the total cfm needed to get the number of fans you’ll need to put in the barn. Let’s say you want the Magnum, and you’re running single phase. That fan throws 42,652 cfm at 0.10” static pressure (SP).

1,108,800 / 42,652 = 25.996 (round to 26)

Total cfm produced by 26 Magnums = 26 x 42,652 cfm = 1,108,952 cfm

However, you’re not done yet. You must make sure you have an adequately sized inlet. If you’re going to exhaust that much air out, you’ll need an inlet for air to come in and replace what is exhausted.

The inlet requirement calculation is your total cfm produced by fans / 600 cfm per sq. ft. That will give you the total square feet of inlet you need to ‘service’ your exhaust fans.

1,108,952/ 600 = 1,848 square feet. This can be done using end wall & side wall curtains, and existing scrape alley garage doors. But remember – air travels the path of least resistance, so don’t forget to keep the center feed alley door shut! Let’s say the intake wall (end opposite of the exhaust fans) of this example barn has a 16’ x 16’ center feed alley door with curtains on both sides. Each curtain section is 12’ x 50’ in length, giving us a total of 1200 sq. ft. of inlet.

As you can see, we still need an additional 648 sq. ft. minimum of intake. The end wall curtains alone will not provide enough inlet, and starving the fans of air will increase the static load at which the fans are working against. Operating at greater static pressures not only decreases fan longevity and performance but is also detrimental to the animals.

Since you are working with a tunnel ventilated barn, you can use a section of each of the side walls to achieve the remaining 648 sq. ft. of inlet that is needed. This works great on tunnel ventilated facilities, but we do not recommend side wall curtains as inlets on cross ventilated systems. Looking back at the example barn, you would need a 12’ x 30’ section of curtain on each side wall (12×60 = 720) to provide the remaining inlet square footage needed. Now we will have 1200 + 720 = 1920 sq. ft. total of inlet, or just a little more than absolutely necessary to accommodate for possible air obstructions.

Understanding how to calculate the number of barn exhaust fans needed, and how to make sure to adequately size the inlet will allow you to go further in your research when upgrading, retrofitting, or building new. J&D provides an abundant amount of fan performance data that will allow a dairy farmer to compare and contrast number of fans needed, energy usage, labor and maintenance requirements, etc. It’s increasingly important to have fan performance data on any barn exhaust fan you’re considering. You can always #RequestTheTest from J&D, and we encourage you to get performance data on every fan before making a purchase. There is a reason why some companies don’t provide fan performance data.

Note this example doesn’t include circulation fans. If Barnstormers, Panel Fans, or other recirculation fans are added, that changes the calculation. Let J&D be your barn ventilation experts. Contact us with any questions. We won’t just tell you what to buy, we will tell you how we came up with our recommendation.

#EducateTheCustomer #RequestTheTest #JandDProud