Chapter 9 - Desired Air Exchange Rates

The ventilation system has to exhaust the heat entering the building through the walls and roof, and the heat produced by the birds. There are separate calculation methods for conventional and tunnel ventilation. The conventional approach bases the calculation on heat removal. For tunnel ventilation, the calculation is based on the desired air velocity through the building.

When designing a hot weather ventilation system, the acceptable difference between inside and outside temperature must be defined. At first though, the choice might be to maintain inside house temperature within a degree or two of the outside temperature. Although this would be ‘ideal,’ it is not practical or necessary in many situations. For example, if a producer wants to ensure that it would never be more than a degree warmer inside than it is outside, it would require that 771,000 cubic feet of fresh air be brought into the house each minute. This would be roughly equivalent to the air-moving capacity of nearly forty, 48-inch exhaust fans (20,000 CFM each).

Over the years, designers of ventilation systems have found that under most conditions a ventilation rate based on a house temperature 5°F degrees warmer than the outside temperature is sufficient to minimize heat stress in poultry housing. For example, to make sure that the house temperature would not exceed 95°F on a 90°F day, approximately 154,000 cubic feet of air would have to be brought into the house each minute (approximately eight, 48-inch fans). When evaporative cooling is used, the allowable temperature difference might be 6°F; the larger temperature rise minus the cooling effect will still be comfortable for the broilers.

Tunnel ventilation air exchange rates

The goal of tunnel ventilation is to provide a high air velocity or windchill cooling effect on the broilers by pulling air through the poultry house, like a wind tunnel. Tunnel ventilation is used only during hot weather and requires its own fans, inlets, and controls in addition to those provided for the conventional ventilation system. It is an additional ventilation system.

Tunnel ventilation often results in higher ventilation rates than required for heat removal, especially with well-insulated, relatively-short buildings (< 300 feet). However, for longer buildings or those housing large numbers of broilers, the tunnel ventilation air exchange rate may be very close to or even less than the rate required for heat removal.