Chapter 9 - Natural Ventilation in Hot Weather

In warm or hot weather, naturally ventilated buildings rely almost completely on wind-induced ventilation. Ridge openings allow heated air near the interior roof line to escape. Ridge openings are also high enough to capture wind effect pressures, which drive ventilation airflow. During calm, hot weather, circulation fans may be needed to provide air flow (wind chill) over the birds. Some naturally-ventilated houses will have a mechanical or tunnel ventilation system installed to assure there is sufficient air exchange and air movement during hot weather.

Circulation fans

The primary purpose of circulation fans is to provide air movement over the birds in order to remove body heat. The faster the air moves, the more heat is removed from the broilers. Circulation fans should produce an air speed of at least 400 ft/min at broiler level. It is essential that every bird be exposed to adequate air movement. Broilers in dead-air spots, such as the corners of the building, are often the first to die during extreme hot conditions.

The shape and size of a fan’s coverage area are partially determined by the type of fan. Generally, a belt-driven axial fan will produce 400 ft/min or better air speed over an area fifteen times the fan diameter in length by five times the fan diameter in width. For instance, air emanating from a standard 36-inch circulation fan with a ½-hp motor travels in an egg-shaped pattern. The dimension of this area is approximately 45’ x 15’. On the other hand, a direct drive fan will produce the 400 ft/min over an area approximately twenty times the fan diameter in length by three to four times the fan diameter in width.

In some coastal areas and mountain ridge locations, breezes are prevalent enough that additional air movement is only needed in the center of the house. A single row of fans can be installed down the middle and/or near the ends of the house to minimize dead air zones.

There are two principles which should be considered when sizing and placing mixing fans for cooling:

1. Maximize coverage of broilers with air speeds sufficient to give a cooling effect. For floor-raised broilers, this is expressed as the floor area covered by air speeds of 200-500 ft/min. Fan type, placement, and orientation affect air speed distribution.
2. Uniform, fast air speed over the coverage area is desirable. Broilers will crowd into the areas with highest air speeds in hot weather, which causes additional heat stress and mortality from piling. Some fan arrangement provides better coverage of the floor with uniformly high air speeds.

The type and placement of mixing fans greatly affect the air velocity distribution. For floor-raised birds, the velocities within a foot of the floor are of interest.

• Titling mixing fans toward the floor about 10° increases the total area covered with air velocities from the fan, but it also increases the maximum air velocity near the floor. If the highest velocities are not above 600 ft/min, this trade-off may be beneficial.
• Increasing the height of the fan above the floor from 40 inches to 60 inches reduces both maximum velocity and the total area to be covered by desirable air velocities. Place the fans on winches so their height can be adjusted.
• Turning propeller fans to blow downward results in more floor area covered with desirable velocities, but a large area is covered with air velocities above 600 ft/min, which may cause excessive bird crowding in hot weather.
• Ceiling paddle fans blowing downward can cover large areas with desirable velocities without the excessive air velocities associated with propeller fans blowing downward. This is the result of spreading the airflow over a larger fan blade area. Use higher-powered paddle vans since low-power (150 W or less) ceiling fans cover much less floor area with desirable air velocities.

Other placement strategies for mixing fans including the following:

1. Keep fans away from the sidewall. Otherwise birds tend to move toward the fans and pile against the sidewall.
2. Fans should blow with the direction of any strong prevailing wind at the site to increase the area covered by each fan.
3. To minimize dead air spots, provide air movement in the ends and corners of the house with smaller fans, if necessary.

These examples demonstrate that providing complete coverage of indoor floor area with high air speeds using mixing fans can require many fans. For buildings where wind environment is not favorable, mixing-fan costs should be compared to the cost of fans and energy use for tunnel ventilation, which provides fresh air exchange in addition to uniform high air speeds at bird level.