Mechanical ventilation options for dairy barns
May 12, 2012
Dairy producers have many options for mechanically ventilating dairy barns. Options include conventional mechanical-ventilation, tunnel-ventilation and cross-ventilation. Each system has strengths and weakness that need to be considered when deciding what to install. Well managed systems can be effective and energy efficient.
Whatever system is used, it is important to remember the reason for providing ventilation air exchange. In cold weather, air exchange between inside and outside the barn removes moisture, gases and airborne pathogens. During very cold weather, some managers restrict ventilation to keep manure from freezing to the floor. The restricted air exchange can lead to increased humidity, cold and clammy conditions, condensation on cold surfaces and unhealthy conditions for the cows. The recommended minimum continuous ventilating rate for a 1,400-pound cow is 50 cubic feet per minute per cow (CFM/cow).
In warm and hot weather, ventilating systems need to provide enough air exchange to remove animal heat and solar heat that gets into the barn. Hot weather ventilation removes moisture, gases and airborne pathogens too. The minimum recommended hot weather ventilating rate for 1,400-pound cows is 500 CFM/cow. Mixing fans are sometimes used to help reduce heat stress by creating a draft past the cows. Sprinklers that wet the cows and high pressure misting systems can provide evaporative cooling; however, they increase the humidity level and temperature-humidity index in the barn.
Conventional mechanical-ventilation is commonly used in tie-stall barns where ceiling or wall inlets bring fresh outdoor air in and distribute it while exhaust fans blow warm moist air out. These systems have temperature controllers to adjust the airflow rate and size of powered inlets. The inlets are key to fresh air distribution and uniform conditions throughout the barn. Powered inlets are mechanically adjusted as ventilating rates change to provide uniform fresh air distribution. Gravity inlets can be used as well.
Tunnel-ventilation is used to ventilate dairy barns in mild and hot weather. Air enters at one end of the barn and is exhausted out the other through fans. Tunnel-ventilation works best in barns with ceilings. The ventilating rate for tunnel-ventilated barns is based on the cross sectional area of the barn and providing an average air velocity down the barn of at least 2.5 miles per hour. Some barns are designed to provide air velocities up to 7 miles per hour. Airflow going down feed and freestall alleys and driveways reduces tunnel-ventilation effectiveness at cow level.
Tunnel-ventilation can be used in modified naturally ventilated barns. Most naturally ventilated barns do not have ceilings so producers install baffles in the rafters to reduce the cross sectional area and move the airflow closer to the cows. For tunnel-ventilation to work in naturally ventilated barns the open ridge and sidewalls must be closed. Some producers install chimneys with dampers in the ridge that are closed when the barn is tunnel-ventilated. Controllers can be used to adjust the inlet opening and fan flow rates based on indoor temperatures.
Tunnel-ventilation is not recommended for use in cold weather because it creates large temperature, moisture and gas concentration differences between the inlet and the outlet at low cold weather ventilating rates. The size of the temperature differences depends on insulation levels, cow numbers and the airflow rate. In cold weather at minimum ventilating rates, it is common for air to warm by 20 degrees F or more as it travels from the inlet end to the exhaust end.
Fresh air tube system in a calf barn with automatic feeders.
Cross-ventilation is another ventilating option for minimizing heat stress in hot weather. The air flows from one side of the barn to the other. Cross-ventilation is used in barns that range from 200 to 500 feet wide depending on the number of freestall rows. Interior baffles from the ceiling to within 7 feet of the floor are installed over freestalls to increase the air speed in the stall area at cow level. Hot weather ventilating rates in cross-ventilated barns exchange the barn air every one to four minutes to minimize temperature rise from inlet to outlet. In very cold weather at minimum ventilating rates, large temperature rises and gas concentration increases can be produced between inlet and outlet sides of the barn, especially if the system is managed to prevent manure from freezing in very cold weather.
Dairy producers now have more mechanical-ventilating system options to investigate when building new or remodeling existing barns. Generally, as systems get more automated and complicated, costs increase and managers need to understand how the system and controllers work to manage them effectively. Properly matching fans, inlets and controllers is critical.
For additional information on cow housing and comfort, check the University of Minnesota Extension Dairy webpage at www.extension.umn.edu/dairy and click on the "facilities" link.