While natural ventilation is the most prevalent type of barn ventilation system seen in continental climates, interest in mechanical ventilation systems is growing. This is due to a desire to have more consistent ventilation year-round, with an emphasis on combating summer heat stress. One of the downfalls of natural ventilation is that the wind doesn’t always blow, and even with heat abatement methods in place, such as fans over the stalls and soakers at the bunk, cows still bunch in the heat of the summer because of inadequate air exchange. These problems have led many farmers to look at mechanical ventilation options for their barns. Because these barns are more enclosed, light is more evenly distributed in the barn, and consistent air speeds can be maintained, leading to a reduction or elimination of cow bunching behavior.
Mechanical ventilation systems rely on controlled inlets and outlets to exchange air in the barn, with a goal of providing a minimum of four to eight air changes per hour (ACH) in winter and 40 to 60 ACH in summer. Additionally, air speeds of 200 to 400 feet per minute should be achieved at 1.5 feet above the stall surface in over 90% of the stalls when the inside of the barn reaches a temperature-humidity index (THI) of 68. These mechanical ventilation systems are categorized as positive-, negative- or neutral-pressure systems, with the main differences being how fans are used to bring air into or out of the barn for ventilation purposes.
Positive pressure
The positive-pressure systems people are most familiar with are positive-pressure tube ventilation (PPTV) systems. These systems are supplemental to natural ventilation and do not pressurize the barn. They’re primarily found in calf barns where they are designed to provide a minimum year-round ventilation rate of four ACH without drafting the calves. However, they can also be fitted for poorly ventilated freestall barns as an additional method for bringing fresh air into the space and delivering cooling air speeds to the cows.
Positive-pressure hybrid barns are relatively new and rely on both natural and positive-pressure ventilation. For most of the year, these barns are naturally ventilated but switch to positive pressure in the summer when the sidewall curtains are closed and the fans, spaced approximately 10 to 12 feet along both sidewalls and angled toward the head-to-head stall platform, are turned on. The inside of the barn is pressurized, which forces air out of an oversized ridge and the endwalls to create air exchange. Positive-pressure hybrid systems are best suited for four-row, head-to-head freestall barns.
Neutral pressure
Neutral-pressure barns rely on fans to bring in fresh air and exhaust stale air out of the barn. This concept has been used in some calf and heifer facilities. Compared to other mechanical ventilation options, this option tends to have increased running and maintenance costs, since fans act as both the air inlet and outlet, but it has the advantage of fans running at lower speeds, where they are more energy efficient.
Negative pressure
The most common type of mechanical ventilation system is a negative-pressure system, which can be broken down into tunnel or cross ventilation. Tunnel ventilation is where curtain inlets are located on one endwall with exhaust fans located on the opposite endwall so that air travels down the length of the barn, parallel to the feed alley. Cross ventilation is where curtain inlets are located along one sidewall and exhaust fans on the opposite sidewall, with air moving across the width of the barn, perpendicular to the feed alley. Barn layout, preference for maintaining fans, running costs and plans for expansion all factor into whether tunnel or cross ventilation is better suited for a facility.
No matter what type of mechanical ventilation system is chosen, it must adhere to the following criteria:
- It must provide sufficient air exchange to remove heat, dust, noxious gases and moisture from the barn – four to eight ACH in the winter and 40 to 60 ACH in the summer – and provide sufficient air exchange of at least 1,500 cubic feet per minute (CFM) per cow during the summer.
- Air speeds of 200 to 400 feet per minute at 1.5 feet above the stall surface must be provided in at least 90% of the stalls, with strategically placed fans or baffles.
- The system should work as well in winter as it does in summer.
- It must be economical to operate.
