A negative pressure, or exhaust, ventilation system is common in most mechanically ventilated animal buildings. Fans are used to lower the pressure inside the building creating a slight vacuum; hence the term "negative" pressure ventilation. Air moves from an area of higher pressure (outside) to an area of lower pressure (inside) through planned and unplanned inlets. This pressure difference is what drives ventilation air flow. The fresh air mixes with the air already in the room and dilutes the moisture, dust, gases, heat, and pathogens. This stale air is then removed from the building by fans. For example, a dairy cow will add 4 to 6 gallons of moisture per day to the barn air by respiration alone. Additional moisture is added to barn air from manure evaporation and any water spillage or barn washing functions. During cold weather, the ventilation objective is to remove this excess moisture from the building. During warm weather the primary objective changes to body heat removal from the building. More fan capacity and inlet area are needed as ventilation rates increase.
Figure 1. A two-way side wall self-adjusting baffle inlet assembly can be made on site and is the focus of this fact sheet. The bottom slide opening allows a small fraction of the air to travel down the sidewall for condensation control in poorly insulated block wall barns in winter.
Design guidelines calculate rates for cold, mild, warm, and hot weather rates. In operation, the entire system must adjust according to seasonal and daily changes in temperature, weather, wind, and animal needs. So in practice the ventilation system must adjust to an infinite number of conditions.
Adding another fan to the building is often used as the solution for improving ventilation. While total fan air exchange capacity is important, a fresh air inlet system is essential for proper air distribution and mixing in all animal-occupied areas. Most good inlet designs work toward a common goal: Directing inlet air into the room at 800 to 1000 feet per minute (fpm). Designed and installed properly, inlets provide fresh air evenly throughout the room without causing drafts.
The self-adjusting baffle inlet is a simple inlet design which has proven particularly effective in retrofit situations.
Inlets direct air across the ceiling, down the wall, or a combination of both. An inlet air jet traveling along a building surface like this will "throw" further into a room than an isolated jet directed into free air. Ideally the surface should be reasonably smooth and without obstructions such as beams, ceiling joists, plumbing, and light fixtures which can interfere with the air jet direction and speed. Deflected air jets result in drafts and limited air distribution. The area of good jet air mixing is typically 18 feet. This is why buildings more than 18 feet wide require inlets on both sidewalls.
Provide fresh air inlets throughout the structure since animals are housed throughout the structure. For example, many dairy barn ventilation systems introduce air along only one sidewall of the barn and exhaust it, with fans, on the opposite wall. As air moves across the dairy barn it picks up moisture, gases, heat, pollutants, and dust. Producers have noticed reduced feed intake in the stall row on the fan side which is likely due to poorer air quality on this side. Air distribution throughout the barn will be improved by introducing fresh air along both sidewalls of the building.
Fresh air inlets do not have to be continuous along each side. Inlet assemblies spaced evenly along the wall will also provide good distribution. In practice, keeping the space between inlets below 8-feet seems to provide more predictable air mixing and reduces "dead air spots" between 4-to 6-foot long inlet assemblies. The air speed exiting the inlet should be similar at all locations. As with other systems in the barn, such as feeding, watering, and lighting, inlets require initial adjustment and periodic maintenance to function properly.
A general guideline for sizing inlets is to provide 1.6 to 2.0 square feet of inlet area for every 1,000 cfm of fan capacity. For example, a dairy barn needing 60,000 cubic feet per minute (cfm) of fan capacity requires about 96-120 square feet of inlet area, distributed evenly throughout the animal area. From this guideline the inlets will usually provide the desired inlet air speed of 800 to 1000 fpm at a proper static pressure difference (0.04 to 0.08 inches water) so that the negative pressure ventilation system functions properly.
Inlet Opening Adjustment
Inlets to modern mechanical ventilation systems are not fixed-size openings. Inlet size control is essential and can be simple or elaborate, manual or automatic. The inlets need to be adjusted according to the number of fans operating so that inlet opening size matches fan air exchange capacity. Less frequent adjustment is needed during hot or cold weather since the variation in outside temperature is relatively low; i.e. fans are not cycling on and off as often. More adjustment is required during the spring and fall, however, when daily outside temperatures may vary considerably and the ventilation system has to frequently turn fans on and off to maintain a desirable indoor temperature.
Inlets closed too far increase the static pressure difference which provides more resistance against which the fans must exhaust air. This may prevent the desired air exchange. If inlets are too far open the result is too low inlet air speed, which results in drafts and poor air distribution. Manually adjusted inlet systems are all too often not adjusted properly or frequently enough in a mechanically ventilated barn to be effective and are, therefore, not recommended. The more sophisticated automatic inlet control systems use a manometer to measure and respond to the barn static pressure difference. Inlets are opened and closed using a cable and winch assembly to maintain a relatively constant static pressure regardless of the number of fans operating. These baffle inlets are usually bottom hinged and direct air along the ceiling. Automatic inlet control may be mechanized as outlined above or automatic; as described in the rest of this fact sheet.
Self-Adjusting Baffle Inlet Assembly
A simple automatic inlet system uses a weighted baffle, often made of rigid insulation board or curtain material, which "self-adjusts" according to static pressure influences (Figures 1 and 2). Top-hinged baffle assemblies are common. As the fans exhaust more air, the inlet opens wider to maintain a relatively constant static pressure difference within the structure. When the ventilation exchange rate is reduced by fans turning off, the inlet narrows in response to the reduced air flow. This maintains the desired inlet air speed. One advantage of self-adjusting baffles is that their operation can be visually inspected.
Figure 2. Effective self-adjusting baffle inlets are manufactured by several sources and are readily available for immediate installation. Three examples are shown here
Self adjusting inlets are a particularly good solution for providing controlled air inlets in retrofit situations. They offer marked improvement over manually adjusted inlets (or no inlets) in a simple design which has proven to work. One advantage of the self-adjusting baffle inlet assembly is that it can be built in a "comfortable" workshop and installed in the barn with relative ease. Figures and construction detail drawings included here show design and dimension recommendations.
Air discharging off the baffle inlet assembly will benefit from being as close to the ceiling as possible. When installing larger baffle inlet assemblies, in order to provide a larger cross-sectional area, increase inlet width before increasing height. For example, to provide 4 square feet of inlet area, a 1 foot high by 4 feet wide inlet assembly would be preferable to a 2 foot by 2 foot assembly. As mentioned earlier, air flowing along a surface will travel faster and further into a room to mix better with room air. When directed along the ceiling, this air jet will help destratify room air bringing warm air back down to the livestock.
Air supply to inlet assemblies
Inlet air may be drawn from the attic space or mow area, especially during colder weather (Figure 3; Construction detail shown in Drawing set A). The air drawn from this area is less affected by wind than inlets in a sidewall which are directly exposed to outside wind pressures. The buffering action of the attic or mow allows the inlets to perform more predictably by preventing wind gusts from directly entering the animal area.
Figure 3. Cross section of self-adjusting baffle inlet which draws air from attic or mow. Detail shows protection of air passage from being blocked by insulation or mow contents (i.e. feed, bedding, equipment).
Cut an opening to accept the inlet assembly through the ceiling into the attic space or hay mow floor. For hay mows, some inlets will be placed under high-traffic areas of equipment, people, and material movement. Protect any exposed opening with a rugged metal mesh or grating capable of withstanding the anticipated traffic load (concept shown in Figure 4).
Figure 4. Protect mow inlet openings in traffic areas.
Provide adequate air flow to the attic or mow so as to not restrict fresh air to the inlets. One rule of thumb is to provide two times the required inlet area to the attic or mow than what is provided by the inlets. In the above example, if a total of 120 square feet of inlet area is provided to the animal area, then supply 240 square feet of opening for fresh air entry into the attic or mow. For an attic, this is typically satisfied by opening up the eaves, then covering it with 1" x 2" hardware cloth or bird netting. Eave openings on the windward side of the attic may be closed during cold weather to minimize wind effects as long as enough total inlet area is provided by the remaining inlets. Hay mows may not have eaves so provide air through additional endwall louvers or open windows or doors.
During warm weather, a concern is that attic or mow air is significantly warmer than outside air. However, this is not usually the case since the air exchange is so great during this time that any temperature increase caused by passing through the attic or mow is minimal. Fresh air may be taken directly from outside during hot weather if the inlet is designed and installed to provide this feature.
In swine and veal calf facilities, the self adjusting inlet is designed to bring air directly from the preheat alley into the livestock area as shown in Figures 5 and 6 (Construction details in Drawing Set B). Fresh air openings to the preheat alley need to follow the rule of thumb in supplying about twice as much area as that provided by the total inlet area to all livestock rooms. An option is to be able to draw air either from the preheat alley or attic space as shown in Figure 7 (Construction details in Drawing Set C in fact sheet G-93).
Figure 5. Self-adjusting baffle inlet for use with preheat alley adjacent to livestock room.
Figure 6. Cross section of self-adjusting baffle inlet used with preheat alley.
Figure 7. Cross section of self-adjusting baffle inlet with option for using attic air or preheat alley air.
A trial and error procedure may be necessary to provide air flow control to the entire set of inlets. Weight the hinged baffle boards to provide uniform air distribution throughout the ventilated space. Without weights, often the inlets closest to the fan will open more widely than those furthest away. In normal use, the baffle maximum opening should be about 45° from its closed vertical position. The "secret" to making this system work properly is to weight each inlet so that all the inlets open approximately the same amount at 0.05- inch static pressure difference. Turn on the fans to the mild weather ventilation rate, adjust static pressure by adding weight to each baffle using metal rods, bolts and washers, or wood strips until the inlets are open the same amount no matter where they are located in the barn. This is typically an adjustment made at installation and further adjustments are not necessary.
Inlets located too close to exhaust fans can cause short circuiting of fresh air directly out the fan. Self adjusting inlets can be placed as close as four feet from fans, but the opening size should be limited to prevent short circuiting. A "stop" may need to be installed to limit the travel of the baffle. This can be as simple as a nail pushed through a hole in the end board of the inlet assembly.
Proper management and maintenance of the inlet system is necessary. Inspect inlet assemblies regularly for proper operation. Air flow to and from the inlet should not be restricted. Keep inlets clean and free of debris. Automatic adjusting weighted baffles should provide smooth dependable operation and will be an improvement over fixed inlet openings.
Air, like some people, takes the path of least resistance. Broken windows, open hay chutes, manure gutter openings, open doors, and all those cracks in the wall provide equal-opportunity inlets for the air. These "unplanned" inlets generally produce "dead air spots", drafts, uneven temperature and humidity levels throughout the barn, and undesirable air quality. All these poor air flow characteristics can be improved through the installation of "planned" inlets. At the same time, those old "unplanned" inlets need to be blocked (broken windows fixed, cracks sealed) or managed better (doors closed).
In most dairy tie-stall barns an inlet assembly that has a 6" x 48" opening at the top can be used (Figures 1 and 3). The inlet assemblies are located at intervals along each "long sidewall" of the room to be ventilated. These 4-feet long inlet assemblies are typically spaced 8 feet on center (4' between inlet assemblies). In a tiestall dairy barn, the center of each inlet assembly may be placed in line with every other stall partition to provide up to 500 cfm/cow. A 60-stall barn (30 cows per side) would require 30 inlet assemblies (15 per side). To ensure adequate air flow, provide twice the inlet area required for the animal space to the attic/ mow. For each 6" x 48" baffle (2 sq. ft. of inlet), provide 4 square feet of inlet area to the attic through the eave or endwall louvers. The two-way selfadjusting inlet is desirable in block wall barns.
Many swine rooms use a version of this self-adjusting inlet as air enters off a preheat alley. The inlet runs the entire width of the room and is usually located above a horizontal shelf which directs air flow out into the room rather than blowing air directly into the first set of pens. The baffle is often a curtain material weighted with a metal rod in a hemmed bottom pocket. Problems in air distribution occur during cold weather when the curtain does not open evenly along the entire room width or when it flaps and tears. A shelf within 12- inches of the ceiling will encourage air to flow farther along the ceiling and mix with warm room air before dropping into the pens. A set of three rigid-board baffle boxes can offer more control of air entry than the flexible curtain material while providing the same air flow and cross-sectional area of inlet.
The self adjusting inlet baffle is ideal for providing adequate air volume and mixing for mechanically ventilated veal calf rooms which currently operate with manual inlet control. Multi-room buildings usually have a service or preheat alley from which air can be drawn into the calf room (Figures 5, 6, and 7). Currently, many calf room air inlets consist of manually adjusted panels in the feed aisle door. As fans cycle on and off or variable speed fans ramp up and down in speed, the fixed opening does not accommodate the changing air flow needs and proper air distribution is not maintained. One inlet assembly over the feed aisle entry door is advisable. A second and third assembly to each side can provide warmer weather ventilation when some air flow over the calves is desirable for cooling. See Fact Sheet: Ventilation Improvements for Veal Calf Housing Using 50-Calf Room Example for more inlet design criteria and examples.
Proper air exchange capacity combined with uniform air distribution throughout the animal area are goals of a good ventilation system. Inspect your inlet system. It is often the cause of poor air quality and uneven air distribution in barns. This fact sheet provides guidance on retrofitting animal housing with self adjusting baffle inlet systems to provide more uniform air distribution throughout the facility. It is an inlet system that producers can "see" working as air flow moves the baffle. Good air quality is essential for animal health and production and provides benefit to human health and productivity.
Inlets for Mechanical Ventilation Systems in Animal Housing
Ventilation Improvements for Veal Calf Housing Using 50-Calf Room Example