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The two pillars of barn ventilation and why they matter to you

Dan Veeneman for Progressive Dairyman Published on 07 April 2017

If you ask around, most dairy farms have some sort of ventilation system in their barn. These systems will vary from region to region and farm to farm. For some, it could be a simple naturally ventilated barn with sidewall curtains that allows the natural breeze into the facility. There may not be a single fan in the barn.

For others, it could be a large, low-profile, cross-ventilated facility with a sidewall full of large exhaust fans that run continuously for months on end. Though these two concepts look completely different, both of these producers are relying on some sort of ventilation strategy to perform two crucial functions in their barn. The two pillars of any good ventilation system are air speed and air exchange.

Pillar one: Air speed

Air speed is the pillar of ventilation that is the most “visible” of the two. You can feel it and you can measure it with a simple air speed meter. When we talk about air speed for a dairy facility, the goal is to generate sufficient air speed throughout the facility to properly cool the cows in times of heat stress. To cool cows, research suggests that air speeds need to be 500 feet per minute (fpm) or greater at the cow level. You want to achieve these air speeds before cows are heat stressed, as it is more efficient to keep cows cool than it is to try to cool them once they are hot.

The goal is to target this air speed in all of the areas where the cows are going to spend any amount of time: the bedding area, the feedbunk and the holding area. As in the example, a naturally ventilated barn will rely on the forces of nature to achieve these air speeds. In an ideal situation, this can work out, but more often than not, additional fans are needed in the target areas to achieve the required air speed. Depending on the facility design, there are a wide variety of recirculation fans that can be used to increase the air speed if the natural breeze does not meet the 500 fpm requirements. Depending on the size, number of blades, throw distance and air flow patterns, recirculation fans will be spaced at anywhere from 20 to 60 feet. The fan manufacturer should be able to supply you with the suggested spacing to achieve the desired air speed. It is recommended that all circulation fans blow in the same direction to avoid dead spots where air streams meet.

In a cross-vent or tunnel-vent barn, exhaust fans are used to pull the air out of the barn. The correct number of fans to achieve a desired air speed in a tunnel- or cross-vent barn is determined using the formula below (see Equation 1).

Equation 1

(Barn cross section area* x Target air velocity) / CFM rating of fan = N fans

*Cross section area is calculated as (barn width x [wall height + (peak height - wall height) ÷ 2])

Again, the fan manufacturer should be able to provide you with the CFM rating of the fans in question. Look for test data that is certified by an independent laboratory or agency.

Pillar two: Air exchange

The other pillar of ventilation in dairy facilities is air exchange. Air exchange is the process of forcing air out of the barn and replacing it with clean, fresh air from outside. Stale barn air is full of dust, ammonia, moisture and other gases. If stale air is not replaced with fresh air, it can lead to a host of health-related issues. For this to happen, there has to be an inlet of fresh air as well as a way for the stale air to escape. Air exchange is not as measurable or visible as air speed. The best and simplest way to test for air exchange is to do a smell test. Stale air will stink, and fresh air will not. When we are talking air exchange, there are two targets that ventilation systems should aim to achieve. In the winter, your ventilation system should be achieving a 15-minute air exchange. This means all the air in your facility should be changed over in 15 minutes. In the heat of summer, you should be targeting a one-minute air exchange.

If we look at the naturally ventilated barn, it is hard to mandate or force the air exchange. However, you will need to ensure there are adequate openings for an inlet of fresh air, as well as a way for the stale air to escape. In a fully enclosed system like a tunnel- or cross-vent barn, there is greater control over the air flow. This allows us to target air exchange rates in the same way we targeted air speeds. You can determine the number of fans required to achieve the desired exchange rate by using the formula below (see Equation 2).

Equation 2

([Barn length / Target minutes per air exchange] x Barn cross section area*) / CFM rating = N fans

*Cross section area is calculated as (barn width x [wall height + (peak height - wall height) ÷ 2])

When we look at fans through the lenses of air speed and air exchange, you can see that not all fans are equal. Circulation fans are great at increasing the air speed in desired areas, and if they are all blowing the same direction, they can also aid in air exchange. Exhaust fans serve as the sole purpose of air exchange and air speed in tunnel- and cross-ventilated barns, so it is important that the number and size of fans be calculated correctly. It is important that every fan in a dairy facility accomplishes at least one of these two objectives. If not, then all that fan is doing is costing you money on your electrical bill each month. Evaluate your fans against the two pillars of ventilation. Are they achieving what you need?  end mark

Dan Veeneman
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