Modern poultry barns often have two, and sometimes three, sets of fresh air ventilation inlets that are specialized for conditions ranging from cold, to mild and hot weather. [image source: Eileen Fabian]
Most poultry in the USA live in buildings where the interior environment is kept at comfortable temperatures year-round. Inside modern facilities, the heating, cooling and ventilation is mechanized and computer-controlled. When thinking about preventing or minimizing respiratory diseases in poultry, we should immediately realize the importance of ventilation. Exposure to highly concentrated ammonia and dust particles can result in upper respiratory tract inflammation that renders the birds much more susceptible to pathogens. To reduce the chance of respiratory disease, birds should always have access to good-quality air. Making sure building ventilation is adequate will reduce ammonia levels. High levels of ammonia will damage the airways of the birds making them more susceptible to opportunistic pathogens like viruses and bacteria.
In negative-pressure mechanical ventilation systems, fans, inlets, any supplemental heaters, and controllers must coordinate properly for appropriate air exchange and air distribution to occur. Inlets are often overlooked. We want to emphasize here that inlets are perhaps the most important part of the ventilation system for maintaining uniformly comfortable and healthy conditions throughout the poultry house. Air inlets in poultry houses over the past several decades have typically been constructed of hinged baffle boards that are placed fairly evenly around the perimeter of the building. Opening and closing of these inlets is achieved via mechanized cable or rod configurations that adjust inlet openings a set amount depending on ventilation system needs. More modern inlets offer improved baffle and inlet housing geometries to overcome limitations of the simple “baffle on a hinge” design. There are often two sets of ventilation inlets in modern mechanically (fan or power) ventilated poultry houses: one set for hot weather and the other set for cold and mild weather conditions.
Interior of cage-free hen house showing eave inlets featuring baffle board on a hinge design along with large (black) tunnel ventilation, hot-weather inlet (the black is an assembly for light control. [image source: Eileen Fabian]
Air inlets are responsible for providing good air distribution and direction of fresh air movement throughout the poultry house. If the inlets are poorly designed or are not correctly managed, animal performance and well-being will suffer, independent of the quality of the fans. Exhaust fans create the negative pressure difference between the interior and exterior of modern, tightly-constructed poultry houses. By creating this slight vacuum in the building (of about 0.05 inches of water measured with a gage called a manometer), the outside atmospheric pressure literally pushes air into the house through any opening in the building shell. For the air inlets to be effective, it is important to keep the house as airtight as possible so that incoming fresh air flows through the ventilation inlets. If the house is not tight, fresh air will enter via any pathway (holes, cracks, open doors) leaving you with limited means to control the air flow pattern in the barn.
Functions of an air inlet
- Provide fresh air throughout the building
- Control direction of air flow
- Maintain sufficient air velocity
Inlets need to be adjustable structures. And just like fans, the inlet adjustment is achieved via the computerized ventilation system controller. It is necessary to modify the inlet opening according to the season and throughout the day as outdoor temperature and weather conditions change. In addition, birds of different ages require different ventilation approaches. This is a good time to note that hot air rises and cold air falls (thermal buoyancy). At least some inlets are ideally positioned high in the poultry house during winter to allow cold, incoming fresh air to mix with warmer barn air along the ceiling before entering the area occupied by the animals. Inlets also dictate the air direction and, in concert with the whole ventilation system, provide suitable air velocity. During cold weather or when raising young birds, position inlets so the incoming cold air travels along the ceiling of the building, with enough speed to mix with the warm air. If the speed is insufficient, the cold air will drop onto the animals, chilling them, without properly mixing with the warmer air of the building.
As a rule of thumb, the inlets should provide an open area of 1.7 square feet per 1,000 cfm (cubic feet per minute) of exhaust fan capacity. Inlets smaller than that may cause the fans to be overwhelmed by high static pressure difference; inlets larger than that may result in an incoming air speed too slow for proper air mixing. Avoid placing obstructions to the air jet both before and after the inlet opening to avoid interfering with air mixing. If the air is traveling through any portion of the building (for example through the eave or attic) before reaching the inlets, make sure that there is no restriction to the air flow. Provide two to three times the area of the full inlet to avoid creating a restriction to air entering the inlet. Remember that we must be able to control air flow and speed at the inlet opening! The inlet is the control point.
Tips for cold weather. The cold weather ventilation goal is to maintain good air quality in the poultry building. This is crucial to controlling ammonia, humidity, dust, and reducing any other air contaminants. A relatively small amount of fresh air is exchanged for stale air exhausted from the facility to avoid removing too much heat from the building. It is difficult to evenly manage inlet openings under cold weather conditions when they need to be opened only a fraction of an inch. Due to small irregularities in our buildings and the inability of tiny fresh air jets to have enough momentum to provide proper air mixing, good performance of very small inlet openings is difficult to achieve. One strategy that can work for traditional style hinged-board inlets is to manually close every other (or every third) inlet to allow a larger opening and more air flow at each open inlet compared to barely cracking every inlet open. In practical terms, more uniform fresh air flow can be achieved by opening every other inlet half an inch than by opening all inlets just one quarter of an inch. Another option is to install attic inlets in the ceiling for minimum ventilation or cold weather needs. More recent interest in advanced inlet design has shown better performance with airfoils and systems that manage both low flow and moderate flow of fresh air.
Cold weather ventilation is challenged by the low volume and momentum of the incoming tiny fresh air jet. Two ways to overcome this are making sure inlet air jets can be larger by good design or management so that fresh air will reach and mix across the entire poultry house. Top image shows thin air jets from two example inlets directing air across ceiling. Every other inlet is closed in the middle image so that the one, larger air jet has more momentum to travel further across the ceiling. The bottom image has an engineered inlet design that offers inlet jets capable of moving further across an air space during minimum ventilation.
Tips for hot weather. The goal of hot weather ventilation is to provide a large amount of air flow to not only remove bird body heat from the building but to also offer cooling convection breezes to the birds. During warm and hot conditions, the building can be turned into a wind tunnel where outdoor air flows directly onto fully feathered birds to create a breeze for enhanced cooling. Inlets to a “tunnel ventilated” poultry house will be very large, practically floor-to-ceiling openings with an open area equivalent to the cross-sectional area of the building, plus about 20% to account for lost area due to obstructions from building columns and bird wire covering the opening.
Inlets play a very important function in poultry house ventilation design. Some argue that inlets are probably the most important aspect of the whole ventilation system due to their role in providing uniform air distribution throughout the house. When uniform delivery of fresh air is achieved, many of the common complaints related to drafts, hot spots or cold spots in the barn, and poor air quality are minimized. Inlet design is evolving to correct the shortcomings of simple hinged baffle designs and offer inlets that work well in low air flow rates used during for cold weather while also performing well during warmer conditions. Many poultry barns also incorporate tunnel ventilation during hot weather for thermal comfort by opening a second set of large inlets to provide high quantities of fresh air flow. A good understanding of inlet design and function is worthwhile since all are in agreement that good air quality and comfortable conditions are essential for welfare and productivity of birds.