Chapter 9 - Mechanical Ventilation Systems

Mechanical ventilation systems use fans to bring air into a building and are especially appropriate when a narrow control of temperature is desired, such as with young broilers, or in special applications, such as light-controlled pullet and layers houses. Well-designed systems provide for adequate air exchange capacity and uniform air distribution.

The major types of mechanical ventilation are positive-pressure systems and negative-pressure systems. The two differ in whether the house interior is at a higher (positive-pressure) or lower (negative-pressure) static pressure than outside static pressure. Air movement is in response to pressure differences with air moving from regions of higher pressure to lower pressure. Resistance to airflow causes a static pressure drop, measured in inches of water gauge (iwg). Air enters the building at a speed dependent on the static pressure difference between the inside and outside of the inlet, which is affected by the dimensions of the inlet.

In positive-pressure ventilation systems fans push air into a building which creates a higher static pressure within the structure. This forces air to leave the building through any opening including doors, windows, exhaust outlets, and building cracks. Positive-pressure ventilation systems tend to have less uniform air distribution. Ducts with carefully sized and positioned holes are often used to create proper air patterns. However, duct systems are costly to design and maintain and require additional fan power to overcome resistance to airflow through the duct.

Sometimes positive-pressure ducts are used as a supplement to negative-pressure systems to distribute fresh or tempered air specifically to where it is needed. Ducts are more commonly employed during cold and mild weather when air exchange capacity through the duct is significantly lower than summer’s high air exchange rates.

Positive-pressure systems can force moist air into walls and attic spaces, causing condensation and water damage and, potentially, frozen-closed doors and windows in cold weather. Positive-pressure systems are uncommon in poultry houses because of the disadvantages mentioned.

Circulation fans are installed within a building to overcome stagnant air problems or to increase air velocity at bird level. These fans do not contribute to the air exchange. A well designed mechanical ventilation system does not need circulation fans. Circulation fans in a poorly-insulated, leaky house help prevent temperature and moisture stratification through improved air distribution. They are usually used in naturally ventilated buildings and discussed later in the natural ventilation section.

There are two approaches to negative pressure mechanical ventilation – conventional and tunnel. In each case, fans exhaust air from the building creating a lower static pressure inside the building compared to outside conditions. This draws air into the building. Conventional systems are designed based on removing bird heat in the summer and on removing moisture and contaminants in the winter. Tunnel systems are a hot weather strategy designed to create the desired air velocity at broiler level and to remove broiler heat. In each case, fans are selected to provide the desired air exchange rate and inlets are designed to provide good air distribution.

Large broiler houses generally use 36-inch and 48-inch diameter, single-speed fans. In a conventionally ventilated house fans are either uniformly spaced or banked in groups of two or three along one or both sidewalls while inlets are located along one or both sidewalls at the eaves. Tunnel-ventilation systems, often combined with evaporative cooling, may be used during hot weather. In tunnel ventilation, exhaust fans are located on one end of the building and inlets are grouped at the other end.

Fans

The air exchange capacity of a mechanical ventilation system is provided by fans. Fans discharge a volume of air per minute from the building and, in concert with inlets and a static pressure difference, cause fresh air to enter the building to replace the exhausted air.

An exhaust fan creates a slight vacuum within the structure compared to outside static pressure. The static pressure difference required to ventilate a building is very small – on the order of 0.05-inch water (pressure is often measured as a depth of water in a column). This can be visualized as the amount of suction needed to draw water 5/100 of an inch up a straw. This may not seem like a lot of suction, but it is enough to create sufficient airflow to properly ventilate a building. Static pressure should be maintained within a reasonably constant range. Creating a static pressure difference requires relatively tight building construction, however, and not all poultry buildings meet this criterion. Mechanical ventilation buildings need a static pressure gauge (manometer) so the operator can verify that desired static pressure (0.05 – 0.08-inch water) is being maintained.

Fans for the poultry house ventilation are belt- or direct-drive propeller fans and are designed for providing large volumes of air against low airflow resistance. Poultry house fans require totally enclosed motors for protection from dust and gas damage. In a conventional system, fans are often banked, or installed side by side, in sets of two to four fans approximately every 50 – 100 ft along one or both sidewalls of long broiler houses. Some producers locate summer fans on or near one end wall for tunnel ventilation applications.

The resistance to airflow that must be overcome by fans is affected by ventilation inlets and fan shutters and guards. Additional pieces of equipment, such as wind protection devises, evaporative pads, or light traps, further restrict airflow. Fan airflow capacity is influenced in turn by static pressure, which is most effective when kept at 0.05 to 0.08 inches water gauge (iwg) across the broiler house inlets. This is monitored as part of the ventilation system control, but it only represents one component of the static pressure difference against which the fan must operate. Total resistance along the airflow path from outside to building interior and back outside, can be as high as 0.20 iwg if the fan is moving air through evaporative pads or exhausting air into strong winds. Obstructions within twenty fan-diameters’ distance downstream of the fan should be minimized. For example, a 36-inch fan should have no obstructions within 60 ft of its exhaust side. Light trap hoods violate this rule, but they are often necessary for light-controlled poultry houses.