Natural Ventilation with Supplemental Positive Pressure Tube Ventilation
Natural ventilation is the preferred method of ventilation for youngstock facilities. Mechanically ventilated facilities are not recommended for calves in climates similar to Wisconsin’s since it is extremely difficult to produce a reliably-sized inlet at winter ventilation rates. Inlets typically become very small to match the required exhaust fan rate, and these small inlets deliver so little air that it is almost impossible to find consistently good air quality throughout the barn. Summer ventilation rates in negative pressure facilities are feasible with calves, but the concern with these systems is whether or not the systems are regulated well enough to avoid chilling calves during cool nights.
The other issue with negative pressure systems is where to locate the inlets and the exhaust fans. Placing the inlets on the nursing calf side of the barn subjects the youngest calves to the freshest, but coldest air. Exhausting the air out of the nursing calf side of the barn exposes the youngest calves to the oldest calves’ pathogens and contaminants. In addition, at least three levels of ventilation rates, with the ability for finer control of the system at varying heat and humidity levels, will need to be accounted for.
Natural ventilation is advantageous because natural forces are used to ventilate buildings, reducing costs for both fans and electrical power. Natural forces include wind moving through, against and over buildings, and thermal buoyancy of warmed air rising inside a building. However, natural ventilation has a number of shortcomings, especially in calf housing when winds are still. Wind roses that summarize wind conditions throughout the year are available for most parts of the United States, and can be accessed through the USDA at the following website: http://www.wcc.nrcs.usda.gov/ftpref/downloads/climate/windrose/.
When the wind is still, naturally ventilated barns are dependent upon thermal buoyancy for ventilation. Unlike adult cows, calves do not generate sufficient heat to effectively warm the air that surrounds them to allow for thermal buoyancy to occur, thus natural ventilation becomes insufficient.
Further limitations of natural ventilation occur when outside air is warmer than the air inside of the barn – a situation that occurs for a period of several hours almost every day as the sun warms the air outside the barn more quickly than inside. During these periods of time, air entering the cooler interior of the barn through eaves will rise and leave the barn without good mixing near the floor.
Because of these occasional limitations with natural ventilation, we have advocated the use of positive pressure tube ventilation systems to supplement naturally ventilated calf barns.
The supplemental positive pressure tube systems are usually sized to provide four changes of interior air per hour. This ventilation rate assumes a “normal” stocking density and is recommended as the minimal winter ventilation rate by Bates and Anderson3. The tube fan never stops, running 24 hours a day, 365 days per year. If designed properly, the tube system(s) will deliver fresh air without a draft into calves’ microenvironments. Fresh outside air from the tube system(s) is evenly distributed around the barn and exits passively through the typical ridge and eave openings.
During the coldest period of the year, the sidewalls of the barn can be closed except for the eaves and ridge opening. As the weather warms, the sidewalls are opened more and more to allow winds to enter the barn. In warm weather, the sidewalls should be completely opened. With opened sidewalls in windy conditions, the fresh air exiting the tubes gets carried away by the winds entering the barn. While the tubes are not as effective in these conditions, it is preferable to let the tube fan run continuously rather than stopping and starting the tube fan depending on outside wind conditions since they will continue to deliver the minimum year-round ventilation rate.
Concerns are frequently raised about whether the tube fan(s) should be shut down during cold weather. We had an opportunity to compare winter temperatures between two identical naturally ventilated calf barns on the same dairy, one with a supplemental tube system and the other without. Over a two-week period, the average temperatures were identical at 23⁰F (-5⁰C). However, the barn with the tube would usually get 2 degrees colder during the middle of the night and 2 degrees warmer during the middle of the day (see adjacent chart). This showed that the tube system results in a modestly higher ventilation rate that causes the barn interior to track slightly more closely with the outdoor ambient temperature, both up and down.
Heating the barn volatizes gases from manure and urine as well as promotes bacterial growth, which requires higher ventilation rates. In addition, calves will have to face the cold at some point in their lives, and introducing them to the cold before weaning may lessen some of the stress associated with weaning and being introduced to group housing if they were not already housed in groups.
Typical PPTV systems are relatively inexpensive and require modest electricity for operation. For example, a tube system in a 100 foot by 35 foot (30.5 m by 10.7 m) barn might require a single 20-inch (51 cm) fan. Depending on the materials chosen, the fan and tube might cost $1,000 USD plus the design of the system, and installation and wiring costs, bringing the total to approximately $2,000 USD. The 20-inch (51 cm) fan may consume 500 watts or 0.5 kWh of electricity, which would yield 12 kWh per day or 4,380 kWh per year. If electrical costs are $0.10 USD per kWh, the annual electrical costs would total ~$438 USD per year.
The new PPTV systems are not the tubes of the 1970’s. In the traditional 1970’s tube system, the fan and tube were recirculation devices designed to mix air and equalize temperature within a barn. Typically, the tube fan would be located about 3 feet (0.9 m) inside of the barn wall and near to an intake louver. The discharge holes were usually located to discharge air straight out of the sides of the tubes at the 3:00 and 9:00 positions, and the diameter of the holes was of minimal concern. While these systems were effective in equalizing temperature within the space, they also recirculated pathogens within the barn – this is not what we are recommending!
The “new generation” tube systems distribute small quantities of 100% fresh air from outside of the building into the microenvironment around the calf without drafting the calves.
The fans are mounted on an exterior wall to draw fresh outside air into the barn. The fan(s) is chosen to change the interior air of the barn at the minimum winter ventilation rate of four air changes per hour. There is usually one fan and tube for approximately every 25 to 30 feet (7.6 to 9.1 m) of building width with the tubes running parallel to the length of the barn. While there are general recommendations to limit the length of individual tubes to a maximum length of 100 feet (30.5 m), we have monitored excellent performance of 250-foot (76.2 m) long tubes.
The diameter of the tube relative to the capacity of the fan is critical. The tube should be sized so that the calculated velocity of air in the inlet portion of the tube is less than 1,200 feet per minute (6.1 m/s)4. When the proximal air speed is greater than this, air discharge becomes less uniform, there will be greater noise, and in more severe cases, the tube will flutter and flap near the fan. In most systems, the diameter of the tube will be larger than the fan on which it is mounted.
The tubes can be made of a variety of materials that range from very inexpensive clear polyethylene, moderate cost woven polyethylene or vinyl, and relatively expensive PVC or drainage pipe with smooth interiors. Each material has advantages and disadvantages related to the cost, durability, and manufacturing options for discharge hole sizing and location. We find the best overall value with the moderately priced woven polyethylene tubes that are supported with double-cable supports and have anti-condensation treatment. These tubes cost between $5 to $12 USD per linear foot ($16 to $40 USD per linear m), depending largely on the diameter of the tube.
The diameter and spacing of the holes are specifically designed for each installation. The fundamental requirement is that the tube delivers fresh air to the calves without creating a chilling draft, which is described using the technical terms “throw distance” and “still air.” Still air is defined as air moving at a speed of less than 60 feet per minute (0.3 m/s) or less than a foot per second 2. The throw distance of air from a tube is determined by the static pressure inside of the tube and by the diameter of the holes or perforations in the tube4. At a given static pressure, air exiting a larger diameter hole will travel further away than air exiting from a smaller diameter hole.
The desired throw distance will be determined by how high the tube is located above the floor and how far to the side the air needs to travel. “Still air” should be achieved at a point approximately 4 feet (1.2 meters) above the floor. The location of the discharge holes is specified by clock positions such as 5:00 and 7:00, depending on the height to the bottom of the tube from the floor and the desired width of the throw pattern. The throw distances to the desired points of still air are calculated using trigonometry while the diameters of the discharge holes are sized based on these distances and the estimated static pressure within the tube.
These calculations require the use of principles of fluid mechanics that are beyond the scope of this website. Training sessions on how to operate a spreadsheet used to design PPTV systems are offered periodically through The Dairyland Initiative Workshops. If you are looking for assistance in designing a PPTV system, please contact a Certified Consultant or PPTV trainee.
Bates, D.W. and Anderson, J.F. Calculation of Ventilation Needs of Confined Cattle. 1979. JAVMA 174:581-589.
Lago, A., McGuirk, S., Bennett, T., Cook, N., and Nordlund, K. Calf Respiratory Disease and Pen Microenvironments in Naturally Ventilated Calf Barns in Winter. 2006. J. Dairy Sci. 89:4014–4025.
Wathes, C. M., Jones, C. D. R. and Webster, A. J. F. 1983. Ventilation, Air Hygiene and Animal Health. Vet. Rec. 113:554–559.
Managing Curtain Sidewalls
Managing curtains on calf barns and adult cow barns alike can become tricky, especially in spring and fall when the air is often cool and damp, with intermittent periods of warm weather. Unfortunately, there are not any well-researched rules for managing curtains, but we can apply some basic principles to managing calf barn curtains.
The goal is to provide ventilation while not creating chilling wind on the calves. Given that a calf’s thermal neutral zone (the ambient temperature range in which calves do not expend additional energy to cool off or stay warm) is 50 to 78° F (10 to 26° C) at birth and 32 to 73° F (0 to 23° C) at one month old, a starting set of guidelines for curtain management in nursing calf barns might be as follows:
Below ~40° F (~4° C), curtains should be completely closed, but the eaves and ridge opening should remain open. A well-designed supplemental positive pressure ventilation system should be installed and run continuously throughout the year to provide the minimum ventilation rate regardless of curtain position.
Between ~40 and 75° F (~4 and 24° C), adjust the top curtain opening “as needed” based on outdoor conditions such as wind, rain, and sun.
Curtains should be completely open when the inside temperature of the barn exceeds ~75° F (~24° C).
Older calves have a lower critical temperature of around 20° F (-7° C), so the above general guidelines should be adjusted downward accordingly. Wet, dirty, and matted hair coats on calves will not insulate as well as clean coats, so these calves will require more energy to stay warm before the mercury drops below their normal lower critical temperature, and require the above guidelines to change as well.
In the summer, we use the term “humid” to describe “damp” air, but either way we are talking about water in the air. While curtains can be adjusted to protect calves from cold wind and keep some warmth in the barn, humidity cannot be kept out of the barn effectively with curtains or ventilation systems. Calves and bedding can certainly be kept dry during rain and snow by raising the curtains, but this will not prevent moist air from getting into the barn. Actually, we can easily increase the humidity within the barn by raising curtains on damp days due to a lack of ventilation and added moisture to the air from urine and spilled water and milk. So dampness, apart from actual rain or snow, is not a good reason to close the curtain on a calf barn, particularly during the spring and fall.
Mechanical controllers can be very helpful by automatically adjusting the curtains in relation to the inside temperature of the barn, and fancier systems can also account for wind and precipitation. While there may be a few mismatched weather events that require an override of the mechanical system, automatic curtain openers can help decrease the chore of opening and closing curtains, and help ensure that calves get the fresh air they need.