Optimize tunnel ventilation

In October 1998, Dale and Colleen Mattoon purchased Pine Hollow Dairy in Genoa, N.Y. The dairy came complete with two long, narrow poultry barns which had been converted to free-stall barns. Each barn featured a ventilation-limiting 10-foot insulated ceiling and, to complicate matters, one of the 180-cow barns blocked air flow to the other barn, further hindering air movement.

"I knew we had some severe ventilation problems, and I knew we had to fix them," says Dale Mattoon. After investigating his options, Mattoon invested in tunnel ventilation. While this summertime ventilation system is most often found in tie-stall or stanchion barns, it also can work successfully in free-stall barns with careful installation and management.

At Pine Hollow Dairy, tunnel ventilation not only corrects the ventilation problems in both free-stall barns, but it also improves air flow to the holding pen and parlor. "There's no way we could function without tunnel ventilation," Mattoon says. "I'm glad we have it."

If you're considering installing tunnel ventilation in your free-stall barn, be sure to review the following key management principles and experiences from Mattoon and other producers to ensure that it works on your dairy.

1 Provide good air exchange and air movement.

The goal of a well-designed tunnel ventilation system is to provide sufficient air exchange and air flow over the cows' bodies. To accomplish this goal in a free-stall barn, aim for a minimum air exchange rate of 1,000 cubic feet per minute (cfm) of fresh air per cow, says Curt Gooch, extension agricultural engineer with Cornell University's PRO-DAIRY program.

In addition, you need to move air past the cows at a rate of 400 to 600 feet per minute. This amount of air movement creates a 4-mile-per hour breeze through the building, notes Dan McFarland, Penn State extension agricultural engineer in York, Pa. However, "the breeze becomes a draft in cooler weather," he adds. Thus, only use tunnel ventilation during warm weather.

(To help you design a system which meets these goals, please see "How to design tunnel ventilation in a free-stall barn," on page 27.)

2 Manage fans properly. In a tunnel-ventilated barn, "tunnel" fans are placed on one endwall of the barn. These fans create a negative pressure which draws fresh air through openings on the opposite end of the barn. (See the graphic below.)

To maximize airflow across the cows' bodies, position the tunnel fans close to the barn's floor. Then, stack additional fans above the first row. Avoid placing fans above the center door on a drive-through free-stall barn, Gooch says. "Fans placed in this position may cause air to move down the center and high above the feed alley," he says, failing to properly move air at cow level.

It also is a good idea to use an automatic control system to adjust the fans as weather conditions change during the summer. To do so, place temperature-control sensors in each barn to monitor the temperature at cow level. The sensors trigger the system to turn on "banks" of fans - several fans which are controlled together - at a specified temperature. At Pine Hollow Dairy, a temperature-integrated variable automatic control system turns on two, then four, and finally all six fans when the barn temperature reaches 70 F.

3 Manage sidewall curtains.

In order to create a true "tunnel" for air to move through the barn, you must close the curtains on the sidewalls when the system is operating. However, this often creates poor air movement in the stalls along the outside walls.

To correct this problem, John Merrell, who operates a 1,050-cow dairy with his family in Wolcott, N.Y., raises the sidewall curtains about 4 to 6 inches to let a little air in along the sidewalls. This strategy improves air flow along the outside row of stalls without significantly hindering the effect of the tunnel system.

As a guideline, open the bottom of the curtain about 4 to 8 inches, McFarland recommends.

In addition to closing the sidewalls, you must close all floor and ceiling openings, such as the ridge vent. At Elkendale Farms in Genoa, N.Y., Ken Patchen recently built an addition onto an existing free-stall barn. The barn contains a ridge vent to facilitate natural ventilation during cooler weather. Patchen can close the curtains on the ridge vent to prevent air from being drawn in through the ridge vent when the tunnel system is on.

Remember, tunnel ventilation is designed to improve ventilation and air flow in free-stall barns which do not receive good wind exposure during the summer. Therefore, your tunnel ventilation system should allow you to easily switch to natural ventilation - a system which makes use of an open peak and big, open sidewalls - during cooler times of the year, Gooch says.

4 Move air down around the cows.

Air tends to take the path of least resistance. "One of the problems with tunnel ventilation is the air works its way to the center of the building," Merrell says. Furthermore, when air flow collides with cows or other obstructions, it rises to go past them, creating a lot of air movement near the ceiling.

This two-fold problem - air movement down the center alley and near the ceiling - creates a challenge when tunnel ventilating a free-stall barn. To overcome this, some producers have installed air deflectors or baffles (see photo on page 25) in their free-stall barns. An air baffle redirects the air back down to cow level.

At Oakwood Dairy, partners Ted O'Hara, Paul Colgan and Bill Morgan installed baffles in a free-stall barn at the 1,250-cow dairy near Auburn, N.Y. The baffles "disturb" the airflow and force it to come down around the cows, Morgan says. In addition to baffles, they also lowered part of the ceiling in one area of their free-stall barn.

Place air baffles laterally across the barn at no more than 100-foot intervals, Gooch recommends. For example, the air baffles at Elkendale Farms are spaced about 50 to 70 feet apart, Patchen says.

5 Add a cooling system. Heat stress research has shown that air movement alone won't sufficiently cool cows. Thus, it's a good idea to combine tunnel ventilation with a supplemental cooling system, such as sprinklers, misters or evaporative cooling.

At Oakwood Dairy, the partners installed a high-pressure misting system which releases a fine mist into the air. As the mist evaporates, it cools the air, thus reducing the air temperature in the free-stall barn, Morgan says.

A sprinkler system, on the other hand, applies water droplets directly on the cows' skin. As air flow from the tunnel system passes over the cows, it removes the moisture. "Air exchange accompanied by a breeze can help evaporate moisture from the cows' skin and enhance cooling," McFarland says.

How to design tunnel ventilation in a free-stall barn

A well-designed tunnel ventilation system should provide proper air exchange and air movement. Curt Gooch, extension agricultural engineer with Cornell University's PRO-DAIRY program, provides the following example for a 500-cow free-stall barn to help you design a tunnel ventilation system which meets these goals:

Step 1. Determine total fan capacity.
Fan capacity must examine air velocity and air exchange. First for air velocity, multiply the cross-sectional area of the barn that is perpendicular to airflow (in square feet) by the desired air speed (an air speed of 400 to 600 feet per minute is recommended) to arrive at a product with units of cubic feet per minute.

In this example, the cross-sectional area of the barn is 1,730 square feet.

1,730 square feet
x 500 feet per minute
= 865,000 cfm

Next, check to see if the total fan capacity determined above will provide adequate air exchange in the free-stall barn. To accomplish this, multiply the number of cows in your barn x 1,000 cfm, the air exchange rate needed per cow during the summer.

1,000 cfm
x 500 cows
= 500,000 cfm air exchange rate

Use the larger of the two values calculated in Step 1 to determine the required fan capacity. Thus, the fans would need to supply 865,000 cfm in this example.

Step 2. Determine size of air inlets.
Use the total fan capacity from Step 1 to determine the size of the air inlets on the endwall opposite the tunnel fans. Size air inlets to provide a minimum of 1 square foot of area for every 400 cfm of fan capacity needed. Do not to exceed 700 cfm per 1 square foot of inlet area.
865,000 cfm
÷ 400 cfm per 1 square foot
= 2,162 square feet inlet area

Step 3. Purchase tunnel fans.
Choose large-diameter fans (48 to 60 inches) that have a high efficiency rating (cfm/watt) at 0.15 inch of static pressure. Use fan performance charts provided by the manufacturer or a certified fan testing laboratory to determine a fan's efficiency.

How tunnel ventilation works in a free-stall barn

The goal of a tunnel ventilation system is to provide proper air movement and air exchange during the summer. In order for the system to operate effectively in a free-stall barn, you must seal all sidewall, ceiling and floor openings to create a "tunnel."

Here's how a tunnel ventilation system operates in a free-stall barn:

1. Tunnel fans placed on one endwall of the barn create a negative pressure in the barn.

2. This negative pressure draws fresh air through openings in the opposite endwall of the barn.

3. The fresh air travels the length of the barn at a velocity of 400 to 600 feet per minute.

4. The tunnel fans exhaust the "stale" air.