To ensure proper air quality, base the diameter of the tube on the fan’s air-throwing capacity, not its size, says Brotzman, who works with fellow veterinarian Ken Nordlund and UW-Madison agricultural engineering colleagues to provide farms with design specifications for supplemental positive-pressure ventilation systems.
One farm they worked with did not follow the design specs they were given. They ended up with the tube being the same diameter as the fan.
“So if the fan was a 12-inch fan, they put a 12-inch tube in (which wasn’t) designed to meet the air produced by that fan,” Brotzman recalls. “Although another 12-inch fan may have worked, that one was producing more air than what should have gone through that type of tube.”
Bottom line: When you’re going over the blueprints with the builder or ordering fans yourself, focus on the cubic feet per minute of airflow provided by the fan, not its diameter in inches.
“We certainly recommend that they go to someone who’s been through one of our training programs or come to our group (at the vet school) to have a system designed,” Brotzman says.
Find out more at: www.thedairylandinitiative.vetmed.wisc.edu
The tubes used in a positive-pressure ventilation system have holes in them. Making sure these holes are properly sized and spaced is important for good air distribution along the entire length of the tube.
Again, this goes back to making sure that you account for static pressure, aperture ratio and discharge coefficient during the design process.
“The holes need to be designed so that we have a balanced discharge speed throughout the whole length of the tube and so the air coming from the tube goes a certain distance,” Brotzman says.
The material that the tube is made of is an important consideration, too.
Tubes can be made of hard-walled material, like a PVC duct or a smooth-interior drain tile duct, or a flexible plastic or cloth-like material. However, when the wind is whipping around, the tubes — particularly those made of flexible material — tend to sway more and thus become more prone to breaking or developing tears.
Hasel Farms in Lake Mills, Wis., headed off this problem by hanging an extra chain in the center of each tube, says Heather Schuld, calf manager at the farm. This has helped keep the tube from swaying so much.
“We ordered the two-cable tube and added a chain at the center of each tube which went from each cable to the rafters to help (stabilize) the tube,” Schuld says. “If the tube is swinging in the wind,” Schuld says, “it’s not going to be doing 100 percent of the job.”