Did you unpack and install the inlet openings that came with your tunnel ventilation fans? They were probably in the box, but are hard to see, and easily overlooked. Tunnel ventilation air inlets must be installed properly - at the end of the barn opposite the fans - in order for the system perform effectively. Inlet size determines air exchange rate, operating static pressure, and air speed. Placement determines distribution of fresh air in the animal space. Improper size and placement can result in short circuiting of air, dead spots, increased energy consumption, and inadequate air exchange. Don't worry if you mistakenly threw them away, they can be easily replaced and installed to improve air quality and cow comfort.
The primary objectives of a tunnel ventilation system are: 1) provide a rapid air exchange - to control moisture, gas, temperature, and pollutant levels in the animal space - and 2) provide a breeze to improve the cow's ability to get rid of heat. Designed, installed, and operated properly tunnel ventilation systems can play a significant role in keeping cows comfortable and productive during hot weather.
Ideally tunnel ventilation fresh air inlets are located uniformly across the width of the barn end wall opposite the fans. For end wall inlets, two square feet of inlet area per 1,000 cubic feet per minute (cfm) of fan capacity is recommended. For example, a fan delivering 25,000 cfm needs approximately 50 square feet of inlet area. A stall barn tunnel ventilation system using four 25,000 cfm fans (100,000 cfm total) requires 200 square feet of inlet opening, or a clear opening approximately 5.25' x 38' uniformly across the width of the barn. Six 25,000 cfm fans (150,000 cfm total), need 300 square feet of inlet, or an opening of approximately 8' x 38'. Wall studs, floor joists, beams, and grates block inlet opening, so more end wall or mow floor covering may need to be removed to provide the recommended inlet.
Some stall barn layouts do not allow end wall inlets to be used, so air needs to be drawn in through sidewall openings. Sidewall inlets tend to send incoming air toward the middle of the animal space, away from the cows, creating dead spots at the front end of the stall rows. Recommended opening for sidewall inlets is 3.5 square feet per 1,000 cfm - divided between both sides - to encourage incoming air to 'turn' sooner and provide better air distribution in the feeding alleys.
Existing window and door openings rarely satisfy the necessary inlet requirements. A 3' x 3' window - completely removed - nets only nine square feet of opening. That 8' x 10' door at the end of the service alley may provide 80 square feet of inlet, but it tends to send air rushing past the end of the cow that doesn't breathe - offering little benefit for the cows. When a uniform inlet across the width of the barn cannot be installed, openings in-line with the stall rows and feed alleys are favored.
Along with an anemometer to measure air speed throughout the animal space, I use soap bubbles to track the path of moving air. Blowing bubbles does add some fun to the job, but it also quickly points out dead air spots and unplanned inlets. A few summers ago I was invited to evaluate the tunnel ventilation systems installed in 23 dairy stall barns. Fan capacity for all the systems met or exceeded the requirements based on the building dimensions and number of cows. Two of 23 had enough total inlet space - properly located - to accommodate the fan capacity and provide maximum benefit for the cows.
Providing too little inlet forces the fans to work harder, consuming more energy to remove less air from the building. One evaluated system had potential fan capacity of 150,000 cfm, but only 80 square feet of inlet area. Window inlets in line with each feed alley sent air sailing down the length of the barn in excess of 8 mph. However, due to a lack of air exchange and poor air distribution, the middle alley and stall beds were damp. After some mow floor boards were removed to increase the total inlet area and provide more uniform distribution, the floors and stall beds soon dried out, and the fans sounded relieved!
Designed, installed, and operated properly, tunnel ventilation systems can play a major role in keeping cows comfortable and productive during hot weather. Providing enough fan capacity is important, but properly sized inlets - located to supply uniform distribution throughout the animal space - are essential to provide a more comfortable space for the cows.
If you feel your tunnel ventilation system isn't performing as well as it should, check the size and placement of the inlets to see if they may be affecting performance. For more information on tunnel ventilation system design contact your local Penn State Extension office and request Fact Sheet G-78, "Tunnel Ventilation" or download a copy from our web site. If you still can't find the inlet opening that came with your fan, send me a self-addressed stamped envelope and I will gladly send you a replacement. Over 300 square feet of clear opening can easily be folded and sent in a standard envelope.