Tunnel Ventilation - Fad or Fan?

Richard Stowell 
Extension Agricultural Engineer

I have received numerous questions from dairy producers this year about tunnel ventilating freestall barns. Unfortunately, very little information is available on system and cow performance with these systems and almost all of this information pertains to summertime use in renovated tiestall (generally bank) barns. The following summarizes my observations from visits to several first-generation tunnel ventilated freestall barns and discussion with peers outside Ohio.

System description: 
In a tunnel ventilation system, the barn functions as a duct. The basic design (see figure) draws air from one end of the barn using a bank of exhaust fans and brings in fresh air through large openings in the opposite endwall (typically, alley doors serve as air inlets) Ideally, air flows at a constant velocity or as a uniform wall of air through the length of the barn.

The system design is fairly straight-forward in that the required airflow rate (fan capacity) is simply a product of the desired velocity and the barns cross-section area [Qfans = v x Ac ]. Generally, new systems are designed to generate 350 feet per minute (4 mph) with all fans operating. For perspective, a three-row gable barn requires roughly 300,000 cfm (a bank of fans is required). Most systems place sets of fans on thermostats to allow for varying the airflow.

Preliminary evaluation: 
The primary advantages of well-designed tunnel ventilation systems are that they can always provide more than the minimum recommended hot weather air exchange rate (assuming power to the fans is maintained) and keep interior air moving noticeably. To reap a tangible financial advantage, a tunnel ventilated barn must be able to maintain milk production for enough additional days each summer compared to a naturally ventilated building to offset the annualized installation and operating costs of the fans. Tunnel ventilation systems seem quite attractive this year. But keep in mind that as of August 10th, Columbus had experienced 50% more cooling degree days than normal.

Designed systems flush stale air from a barn quite adequately. Recognize that air naturally flows toward and down alleys that are free of cows and that dead air zones exist immediately downwind of side openings, along sidewalls (i.e. outer row of stalls in tail-to-tail arrangements), and leeward of any obstructions (including other cows). This implies deviations from the basic system design disrupt desired airflow patterns. It also means the only way to adequately evaluate the environment is to leave the comfort of the feed alley and walk among the cows.

Research and experience show that to handle increasingly higher levels of heat, cows need: 
1) Adequate air exchange, followed by 
2) Direct flow of air past their bodies (especially in holding pens and at feed bunks), and 
3) Supplied evaporative cooling (and eventually air conditioning if it were affordable). 
Regardless of the ventilation system selected, these criteria need to be addressed when planning a new freestall barn. Other items to consider with tunnel ventilation include:

* Tight construction is required, may protect perimeter stalls from the weather. 
* A well thought-out plan for cold and mild weather ventilation is essential. 
* Response to power outages must be immediate. 
* Fly (and possibly bird) control is improved with steady airflow. 
* Bunching of cows may be reduced (cow behavior issue possibly related to light/shade levels). 
* On really hot, muggy days, air movement alone will not sustain milk production. 
* Evaporative cooling pads may be used as alternative to sprinkler/mister systems. 
* There are limits to barn length and width. 
* Spacing between barns is less critical. 
* Planning of traffic patterns and  management of doors is very important.