VENTILATION AND COOLING IN ADULT CATTLE FACILITIES
The Cow and Her Thermal Environment
Dairy cows generate a lot of heat. A cow milking 120 lbs (54 kg) of milk per day generates about 6,300 BTU (British Thermal Units) per hour – twice as much heat as a cow producing only 40 lbs (18 kg) of milk per day (3,300 BTU/h), and 19 times the 330 BTU/h a human produces at rest. While cows are quite cold tolerant, they are heat stressed at a temperature that most humans find comfortable; their thermoneutral zone is in the range of 40 to 70 °F (4 to 21 °C). Therefore, when designing a comfortable thermal environment for dairy cattle, it needs to function independent of human intervention. Cows cannot wait for us to turn the fans on!
We often keep cattle too warm in the winter, compromising air quality so that workers are not chilled. In the summer, we do not activate cooling systems at a low enough temperature.
The challenge for barn design in the winter is to achieve sufficient turnover of air within the barn to obtain good air quality. This will limit the risk for respiratory disease, and typically means that we need to ventilate the barn at around 4 air changes per hour. Meanwhile, during summer, the requirement for clean fresh air to ventilate the barn continues at a greater rate of around 40 to 60 air changes per hour.
We know that cows are susceptible to the combination of both heat and humidity. To account for both, we typically use the Temperature Humidity Index (THI), which adjusts temperature to account for the impact of high humidity to describe climatic conditions.
THI is calculated as:
THI = (dry bulb) Outdoor Temp oF – (0.55- (0.55 x (Relative Humidity %/100)) x (Outdoor Temp oF – 58)
THI takes into account the impact of relative humidity (RH) on the cow. Research suggests that cow behavior and performance is impacted at about a THI of 68. At 20% RH, this would be at 75 oF (24 oC), but at 90% RH, cows would be stressed at 69 oF (21 oC). Thus the more humid it is, the lower the ambient temperature at which the cow will experience heat stress.
Click on the link to view the latest THI Heat Stress Chart in “Heat hits cows sooner than we thought”, by R. B. Zimbleman and R. J. Collier. Reprinted by permission from the April 25, 2011 issue of Hoard’s Dairyman. Copyright 2012 by W.D. Hoard & Sons Company, Fort Atkinson, Wisconsin 53538.
Producers may be skeptical of the impact of heat stress on their cows. Dairy records can be used to determine if there is a significant effect on cow performance from to decide whether or not to invest in cow cooling measures.
The tell tail signs of heat stress to look for in the dairy’s records are:
- Reduction in milk production – Use daily milk weights to look for drops in milk production of more than 5 lbs (2 kg) per cow per day during periods of warm weather. Do not use monthly tests as the impact of heat stress can come and go between tests.
- Decline in conception rate – Look for a decline in conception rate of 5 points or more during the summer months compared to the annual average conception rate.
- Increase in lameness – Look at hoof health records for an increase in sole hemorrhage and sole ulcer fractures two months after the main heat stress period, usually observed in the early fall.
- Behavioral changes such as bunching in the pens during periods of warm weather.
Any or all of these signs indicate a heat stress problem worthy of attention.
Heat Stress Check List
- Your cows could be heat stressed if:
- Daily milk weights drop by more than 5 lb (2 kg) per cow during a warm weather period
- Conception rate drops by 5 or more points during the summer months
- Lameness due to sole ulcers spikes in the early fall about two months after heat onset
- Cows bunch away from side and end walls in the summer
Bunching is a common complaint during the summer, particularly in freestall barns where cows gravitate toward the center of the barn and away from sidewalls. There is a simple explanation for this behavior – cows are grazing animals and are hard-wired to seek shade when hot. Bunching cows are trying to tell us that they are hot and that the current heat abatement strategies are failing to cool the cows sufficiently.
Cows lose heat through conduction, convection, radiation, and evaporation. However, as ambient temperature approaches body temperature, heat loss through evaporation becomes the main mechanism for heat loss – through sweating and thermal panting. When cows lie down, about a third of their body loses access to the ventilation provided in the barn. Cows accumulate heat rapidly while lying down (about one degree F (0.5 oC) per hour of rest) and dissipate heat when they stand (about a half a degree F (0.25 oC) per hour). As temperature increases, the number of lying bouts per day stays the same, but lying bout duration decreases. Daily lying times may rapidly fall to as low as 6 hours per day during heat stress as cows stand more and thermal pant to cool. Cows may exhale more than 4 gallons (15 liters) of water from her lungs per day!
A cow at about a THI of 68 fitted with a vaginal temperature logger and an activity logger determining when the cow is standing (upper horizontal yellow line) and lying (lower horizontal yellow line). Notice how the cow’s core body temperature increases when she is lying down and decreases when she is standing up.
Summer cooling strategies must focus on:
- Reducing the rate of heat accumulation while resting
- Increasing the rapidity of heat dissipation while standing
This may be achieved through a combination of ventilation and targeted heat abatement strategies. Remember, ventilation refers to the displacement of contaminated, stale air from the barn through the arrival of fresh outside air. Good ventilation certainly helps cooling, but for adequate heat abatement, the additional provision of fast moving air at cow level and use of water to either cool the air before it reaches the cow or cool the cow directly, are required.
The Dairyland Initiative’s Priorities for Ventilating a Barn
We are still struggling to come to a consensus when it comes to how to ventilate a dairy barn. Should we use natural, cross, tunnel, hybrid, or positive pressure delivery systems to ventilate the barn? Design variation within a system can be as great as it is between systems, so here is our priority list for all types of ventilation systems:
- The #1 priority is to provide fresh air to the cow’s lying area
We need to provide fast moving air in the lying area in the summer and a gentle breeze in the winter. The two main ways of achieving this goal are with baffles or fans. Baffles are ideally suited for cross ventilated barns where a single baffle can redirect air below it over the stalls for the entire length of the barn. However, in the winter, baffles serve to trap stale air between them and reduce the efficiency of winter ventilation. The challenge in tunnels is even greater as air is always drawn to the feed and stall alleys, away from the cows. Baffles are ineffective in tunnel ventilated barns since they only impact a few cows lying below them across the barn. Fans over the stalls are necessary whatever the means of ventilation is to provide fast moving air where the cows are located. To optimize the number of cows exposed to cooling air speeds of about 400 ft/min (4.5 mph) (2 m/s), most 48 to 50-inch (122 to 127 cm) panel fans need to be spaced 20 to 24 feet apart (6.1 to 7.3 m). 72-inch (183 cm) cyclone fans need to be spaced 60 feet (18.3 m) apart. These resting space fans may also be used at low speeds in the winter to facilitate air flow across or along the barn. Once we have provided fast moving air in the lying space, we can use natural or mechanical ventilation to displace contaminated stale air in the barn by bringing in fresh outside air.
- The system should work as well in the winter as it does in the summer
Most mechanically ventilated barns are designed around optimizing summer ventilation with a ‘hope for the best’ approach for winter ventilation. We think it is important that whatever the season, sufficient fresh air should be brought into the barn to ensure good respiratory health. As tunnel barns get longer (>400 to 500 feet or >122 to 152 m) and cross ventilated barns get wider (>8 rows of stalls), the expectation that small amounts of air will find their way efficiently from the inlet to the fans is overly optimistic. A minimum ventilation rate for winter of 4 air changes per hour (ACH) can be achieved with natural ventilation with open eaves and an open ridge. While 4 ACH may theoretically be achieved in new mechanically ventilated barns, there is plenty of opportunity for uneven airflow, trapped air, excessive humidity, and poor air quality to occur. This is most commonly seen in wide body cross ventilated barns where air gets trapped between the baffles and/or diverts to cross alleys. In addition, the high negative pressure from within the barn may force air to enter through some fan openings rather than exhaust out, thus stalling fans and limiting air exhaust. Producers often complain of fogging in the winter in these types of barns for the simple reason that the air is not moving through the barn as predicted. When fan capacity is increased to decrease the fog, excessive air inlet speeds result, which often freeze the alleys on the inlet side. Bringing air in through other inlets, such as the roof ridge may help, but winter ventilation is a significant challenge in these wide-body barns. These difficulties lead us back to more conventional 4- or 6-row barns which can ventilate during the winter through natural ventilation and ventilate in the summer with or without some mechanical assistance. These hybrid barns have adjustable opening side walls, a ridge vent that may be opened and closed, and a lower roof pitch (usually 2 in 12) with the ability to control fan capacity through proper fan selection and variable speed drives. These barns have the advantage of being able to be placed closer together (about 60’ or 18.3 m apart), somewhat independent of orientation.
- Do not assume that faster moving air is better
Mechanical systems may be specified with a focus on air speed, the provision of a set amount of air per cow (e.g. 1,000 ft3/min or 1,700 m3/hr for an adult cow), or on air changes per hour. Most commonly, cross ventilated barns aim for air speeds of 5 to 7 mph (8 to 11 kph) below the baffles while tunnels may push above these specifications, a far cry from the 2.5 mph (4 kph) we used to recommend. Designers know that the air does not distribute evenly across the cross sectional area of the barn. Because of this, they tend to design for much higher air speeds in the alley to be sure that there is acceptable air movement over the cow’s lying area. However, designing for higher air speeds in the alley may not guarantee sufficient air speeds over the resting space. If the air moves too fast, it may actually draw more air away from the cow space! We prefer an approach where we ensure fast moving air in the cow space by installing fans over the resting area to promote cooling. With that in place, all we need to do is ventilate the barn at an adequate ventilation rate, which can be done with a naturally ventilated barn with shades to prevent bunching in some locations, or a tunnel or cross ventilated barn in other situations, specified to provide a minimum target of about 50 ACH in the summer and 4 ACH in the winter.
Provide Fast Moving Air in the Lying Area
While baffles in a mechanically ventilated barn may achieve fast moving air over the lying area in the summer, they serve to trap stale air between them in the winter. Instead, fans need to be installed above the resting space in both naturally and mechanically ventilated barns, and in lactating and dry cow pens, to ensure the provision of fast moving air in the summer and a gentle breeze in the winter.
There are many misconceptions about air movement and speed related to fans. The following summarizes two fundamental concepts regarding air coming out of a fan: air entrainment and air discharge speeds.
When air is discharged from a fan or through an inlet, it immediately contacts the “still” air within the room. Some of the room’s air becomes “entrained” into the air jet, causing the jet to slow down and widen, creating an “angle of entrainment” within the air jet. The angle of entrainment is 22 to 24 degrees, and is well-defined near the fan but becomes billowy with greater distance from the fan.
This phenomenon creates a cone of fast moving air leaving the fan that widens at an increasing distance from the fan.
Air speeds discharged from the fan
Air discharged from a fan will degrade in speed with increasing distance from the fan. The following diagram and table were developed by Greenheck Fan Corporation (2001) to illustrate the angle of entrainment and expected air speeds at various distances from their fans.
Predicted air speed by distance from a typical 3-foot (0.9 m) and 4-foot (1.2 m) fan
|Fan Diameter||3 ft||4 ft|
|11,000 CFM||20,000 CFM|
|Distance from Fan, ft||Air Speed, ft/min||Air Speed, ft/min|
|Fan Diameter||0.9 meters||1.2 meters|
|5,191 Liters/sec||9,439 Liters/sec|
|Distance from Fan, m||Air Speed, m/s||Air Speed, ft/min|
Fan suppliers should provide ‘throw distance’ measurements so that fans can be spaced to optimize the provision of fast moving air.
How fast does air have to move to cool the cow?
There is very little data to tell us how fast the air needs to travel for efficient cooling.
Using a wet skin model, Berman (JDS 91:4571, 2008) showed that still air and air moving at 100 ft/min (0.5 m/s) failed to cool the skin. However, at air speeds of 200 to 400 ft/min (1 to 2 m/s), cooling was achieved for a period of about 10 minutes. Therefore, we recommend air speeds of 200 to 400 ft/min (1 to 2 m/s), preferably 400 ft/min (2m/s), to facilitate cooling when the skin is wet. Fast moving air may also facilitate cooling when the skin is dry, although the effect will be lessened.
Change in wet surface temperature when exposed to air moving at different speeds
Using the guideline of maintaining air speeds of 400 ft/min (2 m/s) in the cow’s resting space, it is clear that traditional recommendations for spacing fans 10 fan diameters apart are erroneous since cooling air speeds of >200 ft/min (1 m/s) discontinue 15 to 20 feet (4.6 to 6.1 m) from typical 36-inch (0.9 m) and 48-inch (1.2 m) fans. Our observations of cow behavior and heat stress support that conclusion.
With roof supports typically at 10 to 12 feet (3 to 3.7 m) on center, we recommend spacing 48-inch to 51-inch (1.2 to 1.3 m) fans above the stalls at 20 to 24 feet (6.1 to 7.3 m) intervals, turned from one side of the stall platform to the opposite side, angling the fan to target the stall below the next adjacent fan in line. Each row of stalls should have fans above them. In head-to-head pen layouts, two fans can be mounted on opposite sides of the roof support posts, or they may be staggered on each side of alternating support posts. Greater distances may be used when fan throw distance is known to exceed the above general guidelines. For example, 72-inch (183 cm) cyclone fans may be spaced every 60 feet (18.3 m).
Alternating post mounting of fans in a head-to-head pen layout
Double fan mounting in a head-to-head layout
The direction of the fans should be in line with the prevailing winds. The fans should be activated above 65 degrees F (18 °C). Alternate fans can be activated at this lower temperature as long as all of the fans are on at 70 degrees F (21 °C).
Positive Pressure Tube Systems
Positive pressure tube delivery systems can successfully be installed over tie stalls and freestalls to assist in cooling cows by providing fast moving fresh air over the resting area.
The systems carry with them the advantage that they bring fresh outside air directly down on top of the cows as they occupy the stalls rather than recirculating contaminated air from inside the barn. These systems will supplement natural ventilation with additional air exchanges, but will not be able to achieve the desired ventilation rate of 40 air changes per hour or more in the summer alone, so sidewalls must remain open to capture prevailing winds.
Challenges of installing positive pressure tube systems over stalls:
- Access to the outside wall for fan location
- Suspension of the tubes and mounting height; the tubes need to be out of the way of machinery used around crossovers and out of reach of cows
- The impact of cross winds on the air jets from open sidewalls
- Fan capacity and tube length to cover long distances
Pen lengths may exceed several hundred feet, making the task of supplying sufficient air along the length of the tubes challenging. For these reasons, there have been relatively few installations. However, the approach is feasible for pens of up to around 100 stalls, about 128 feet (39 m) long, with considerable cost savings for fan installation and running costs when compared to traditional approaches.
For explanation purposes only, a head-to-head layout with a 17-foot (5.2 m) stall platform with 25, 48-inch (1.2 m) wide stalls, and 14-foot (4.3 m) crossovers at each end, a 40-inch (102 cm) diameter tube could be mounted on a 30-inch (76 cm) diameter fan with a capacity of about 10,000 cubic feet per minute (CFM) (17,000 m3/h) at a height of 9 feet (2.7 m) above the stall. The system would supply air at 400 ft/min (2 m/s) at 4 feet (1.2 m) above the stall surface with two rows of 5-inch (13 cm) holes located every 49 inches (1.3 m), equates to approximately one set of holes per stall. For a larger pen with a second run of 25 stalls, it would be feasible to add another fan installation and double the length of one tube from the end wall and only punch holes in the second half of the tube to supply air to the run of stalls furthest from the fan. This would potentially cover the cool air needs for a 100-stall, 2-row head-to-head pen with only four fans.
In the installation below, the installer chose to use 4-rows of fans to layer fast moving air over the entire stall surface.
A tube system installed above head-to-head stalls in a freestall facility
Once fast moving air has been provided in the resting space, the barn may be ventilated to provide the target air changes per hour with a variety of natural, mechanical, and hybrid options.
Provided that the site constraints for natural ventilation are minimal, we have a strong desire to retain some option for natural ventilation in adult cow barns. However, if the location reduces the ability for the barn to ventilate naturally, then we believe mechanical ventilation systems are necessary for the maintenance of cow health and well-being.
Natural ventilation is an effective, least cost option for many situations. Fresh air enters the barn and stale air leaves largely as a result of a difference in wind pressure across the building, and to a lesser extent, a difference between the inside and outside temperature. Wind blowing across the open ridge creates a suction effect, drawing warm moist air out of the barn and pulling fresh air in through the eaves. On still days, thermal buoyancy drives air out through the ridge opening, a process called the ‘chimney effect’.
The chimney effect is facilitated by the temperature difference between the inside and the outside of the barn. During the night at low wind speeds (<10 mph or <16 kph), the inside temperature of the barn will be 1.5 to 4 degrees higher than outside, while at higher speeds, the difference may only be about 2 degrees. The use of roof insulation to increase this temperature difference has been explored. However, insulation with an R value up to 14.3 yielded relatively modest differences in temperature difference of less than 2 degrees – equivalent to increasing the stocking density of the barn from 1 to 1.2 cows per stall in a 4-row barn. Roof insulation has therefore not been viewed as a significant benefit in all but the most severe of climates.
In the summer, the ridge opening plays a relatively minor role. The ability to capture winds from the south and southwest (in North America) through open sidewalls is of much greater importance.
Key Criteria for Natural Ventilation
In order to achieve adequate air movement for optimal natural ventilation, four key criteria must be met:
- Locate the building free of windshadows
In most of North America, barns should be oriented east-west in order to capture the common prevailing southwesterly winds. See the Natural Resources Conservation Service website for regional wind rose maps at http://www.wcc.nrcs.usda.gov/climate/wind-data.html. Because most of the prevailing winds in North America come from the southwest, close proximity of nearby structures to the south and west of a naturally ventilated barn are therefore a potential problem for the optimization of natural ventilation because of the windshadow effect.
To find the minimum distance between the windward obstruction and the building to be naturally ventilated (Dmin), the following equation has been suggested:
Dmin = 0.4*(height of the obstruction in feet or meters)*[(length of the obstruction in feet or meters)0.5]
For example, a building placed downwind from a structure 13 feet (4 m) high and 96 feet (29.3 m) long would need to be 0.4 x 13 x 960.5 = 5.2 x 9.8 = 51 feet away [0.4 x 4 x 29.30.5 = 15.5 m]. For a 30 feet (9.1 m) high, 250 feet (76.2 m) long barn, the separation distance should be at least 190 feet (57.9 m).
These distances for larger buildings are not viable with current construction standards, the footprint available for barns on most sites, and the cost of linking the barns together. Stowell has a rule of thumb for minimum building separation distance – take the square root of the height x length of the building. For example, a 30 feet (9.1 m) high and 500 (152.4 m) feet long building would yield a minimum separation distance of 125 feet (38.1 m) – still more than the typical 100 feet (30.5 m) we commonly see in the industry.
Barn orientation also impacts heat stress. When barns are oriented north-south rather than the preferred east-west, there will be greater solar exposure along the west side of the barn during the afternoon hours, creating bunching issues and reducing the usage of the outside row of stalls.
Sun angles of an east-west oriented freestall barn for August 21, 40 degrees north latitude (Omaha-Springfield).
Sun angles of a north-south oriented freestall barn for August 21, 40 degrees north latitude (Omaha-Springfield).
From ‘Heat Stress Abatement in Naturally Ventilated 4-Row Freestall Barns (Head-to-Head Stalls) Using TeeJet Turbo Jet Nozzles.’ JP Harner, JF Smith, G Boomer, and M Brouk.
- Adjust the sidewall opening so that at least half of the sidewall surface area can be opened in the summer, and 1 inch per 10 feet (2.5 cm per 3.0 m) of building width in the winter.
In the winter, the eave inlets should be able to be opened 1 inch for every 10 feet (2.5 cm per 3 m) of building width on each side of the barn. Under conditions of minimal wind speed and mild temperatures, natural ventilation is driven by thermal buoyancy and the chimney effect. Eave openings less than 0.5 inches per 10 feet (1.3 cm per 3 m) of width will fail to provide for the minimum ventilation requirements of four air changes per hour. This should be viewed as the minimum opening year round, no matter what the weather conditions are. Any sign of condensation will indicate a need for an increased opening of the eave inlet. Ideally, the eave should never be closed completely.
In the summer, at least half of the total area of the sidewall should be opened. Curtain sidewalls are preferred so that the whole wall can be opened in the heat of the summer and the inlet area controlled in the winter. Curtains that are located two thirds of the way up the side wall keep the material out of the way of the bedding in the winter and allow the opening of the upper third to be easily adjusted to create an inlet size that is half of the ridge opening on each side of the barn so that the total eave opening equals the total ridge opening.
For naturally ventilated barns, sidewall height typically ranges between 14 and 16 feet (4.3 and 4.9 m), allowing 12 feet (3.7 m) of opening to facilitate air flow. With large sidewall openings, 3 to 4 feet (0.9 to 1.2 m) of roof overhang is recommended to shield the opening from snow and driving rain.
- Ensure that the ridge is sufficiently open to draw air out of the barn
The ridge opening width should be at least 6 inches (15 cm) wide for barns up to 30 feet (9 m) in width, and 2 inches wide per 10 feet of building width (5 cm per 3 m of building width) for barns wider than 30 feet.
An appropriately sized ridge, coupled with the correct animal density, typically does not result in a lot of precipitation entering the barn. However, the opening can be modified to reduce precipitation entering the barn if it is a problem.
Some barns are fitted with adjustable ridge ventilators that can be limited during severe weather. The most common ridge ventilator is one using a PVC pipe and nylon cord to raise and lower the pipe.
The simplest ridge modification without compromising the opening is to add a vertical baffle on either side of the ridge opening. This will significantly reduce snow blow-in, but will not completely eliminate it. The vertical upstand is typically sized at 1.5 to 2 times the ridge opening width.
Other solutions to reduce precipitation coming into the barn include the installation of a ridge cap or the use of an overshot roof.
Ridge caps run the risk of limiting air flow through the ridge, so they must be designed correctly. With these designs, vertical baffles deflect the air over the ridge cap. These are often used when bedded areas are located below the ridge rather than a concrete alley. The total ridge opening width must be held constant as the air moves under and around the cap.
An alternative ridge cap used in Ohio dairy herds appears to preserve airflow while preventing the entry of precipitation into the barn. For a typical 20-inch (51 cm) opening, a cap is located 20 inches (51 cm) above the roofline with a 4-inch (10 cm) baffle. The cap overlaps the ridge opening by 6 inches (15 cm) on either side.
Use of an overshot roof has become popular in recent years. However, in our experience, these openings have not solved the concerns over precipitation, and depending on the direction of the wind, may reduce the draw of air through the ridge and direct snow into the barn.
Ridge modifications can significantly add to the cost of the building. For example, an overshot roof compared to a simple ridge may cost about $60 USD per foot (0.3 m) more.
The influence of the ridge on ventilation has been explored during the summer. Barns with a sealed insulated ceiling with no ridge outlet have been compared with those with a traditional ridge opening design. When wind blew perpendicular over an open ridge, additional ventilation was achieved, which was not influenced by making the ridge opening wider than 2 inches per 10 feet (5 cm per 3 m) of building width. At 10 mph (16 kph), the increase in ventilation was about 20%.
- The pitch of the roof should allow at least 1 unit of rise for every 4 units across (1 in 4)
The vertical separation between the eave and the ridge impacts the pressure differences generated by thermal buoyancy and the chimney effect. Most commonly, new naturally ventilated cow barns are built with a 4 in 12 roof pitch. Adequate slope is essential if the air is to flow unimpeded toward the ridge opening for winter ventilation. This flow is facilitated by a relatively smooth lining to the ceiling, unencumbered by cross beams and roof trusses. Minimize the depth of purlins used to 4 inches (10 cm) if uncovered, or line deeper purlins to avoid air becoming trapped between them.
Roof slopes with less pitch (e.g. a 2 in 12 roof pitch) will not stop natural movement of air toward the ridge, but will reduce it.
Natural Ventilation Requirements Check List
- Free of windshadows
- A sidewall opening of at least 50% in the summer and a minimum of 1 inch per 10 feet (2.5 cm per 3 m) of building width in the winter
- Open ridge 2 inches per 10 feet (5 cm per 3 m) of building width
- Recommended I in 4 roof pitch with smooth ceilings
Supplemental Cooling in Naturally Ventilated Barns
Naturally ventilated barns require additional measures to enhance cooling in the summer. We have already discussed the provision of fans over the resting space to provide fast moving air where the cows lie down, but water may also be used to enhance cooling by soaking the cow directly or using a fine mist or evaporative cooling pad to cool the air before it reaches the cow.
Water Soaking Strategies
Effective cow cooling is achieved with a combination of wetting her skin and exposing it to moving air. Because cows produce very little sweat on their skin, water must be used to wet the skin for optimal cooling. This is best accomplished with large water droplets – not mists. Optimal cooling range air speeds are from 200 to 400 ft/minute (1 to 2 m/s) (Berman, 2008).
Using a wet skin model, Berman (JDS 91:4571, 2008) showed that still air and air moving at 100 ft/min (0.5 m/s) failed to cool the skin. However, at air speeds of 200 to 400 ft/min (1 to 2 m/s), cooling was achieved for a period of about 10 minutes.
At the higher air speed, rewetting needs to occur more frequently as the skin dries and air speed alone has minimal effects. Relative humidity (RH) of the moving air also has a substantial effect on evaporation rate and cooling of the cow. Increases in RH of 10% will reduce the effectiveness of evaporative cooling substantially. (Berman, 2006).
Wet surface temperature changes when e
Soakers have been installed in holding areas, parlors, parlor exit lanes, and over feedbunks in freestall pens because thoroughly wetting the cow is a great way to improve evaporative heat loss. There are controller units to change soaking times and intervals at different ambient temperatures.
However, soaking in the pens along the feed bunk is problematic. The additional water in the alley causes wet manure to be transferred to the freestall bedding, increasing the risk of mastitis. In sand bedded barns, the extra water leads to sand settling in transfer channels and collecting pits, which leads to pumping problems. Also, water is wasted when cows are not at the bunk (19 hours per day!).
One idea to improve soaking efficiency is to develop soaker stations around the pens where cows can voluntarily enter all day long and be soaked by activating an optic sensor when they desire. New soaker control units are now available to facilitate this approach (e.g. Edstrom Cool SenseTM Motion Cooling System with dual motion sensors and temperature activation).
Low-pressure sprinklers (15 to 20 psi, 103 to 138 kPa, or 1 to 1.4 bar) may be used along the feed bunk in the pens, set to provide 0.03 gallons of water per square foot (1.2 liters of water per square meter) of wetted area per sprinkler per cycle above temperatures of 70 degrees F (21 °C). The wetted area in freestall pens should be set to cover the area 6 to 8 feet (1.8 to 2.4 m) behind the feed line, and the water supply should be sized to provide the necessary flow rate of water.
We recommend wetting cycles have soakers on for 0.5 to 1.5 minutes every 10 minutes at temperatures above 70 degrees F (21 °C). However, soaking frequency may need to be increased during periods of severe heat stress. At 85 degrees F (29 °C), sprinklers should be on every 5 minutes.
The nozzles on the water line are typically suspended 6 to 12 inches (15 to 30 cm) above the top of the headlocks, 5 to 6 feet (1.5 to 1.8 m) above the cow alley, and 12 to 18 inches (30 to 46 cm) behind the feed line. The nozzles used in the barn should spray water in a 180-degree arc, and they should be spaced according to their spray diameter, which is usually 6 to 8 feet (1.8 to 2.4 m). Avoid the use of nozzles that create fine mists. Droplets need to be large to penetrate the hair coat and cool the skin of the cow. Always check the alignment of the nozzles to make sure that the water is actually landing on the cows’ backs, and use nozzles with check valves to prevent the distribution line from draining after each cycle.
Recommended pipe diameter for different TeeJet nozzle capacities based on feed line length. The nozzle capacity influences the time required to apply 0.05 inches of water per square foot (1.4 cm of water per square meter) per on-cycle.
|Nozzle Capacity (gallons per minute)|
|Pipe Diameter (inches)||0.5 gpm||1.75 gpm||1.0 gpm||Inlet Water Demand (gpm)**|
|Feedline Length (feet)||Number of Nozzles*||Feedline Length (feet)||Number of Nozzles*||Feedline Length (feet)||Number of Nozzles*|
|On-cycle time to apply 0.05 inches of water per square foot||2.5 minutes (150 seconds)||1.7 minutes (100 seconds)||1.25 minutes (80 seconds)|
|Pipe Diameter (cm)||TeeJet Turbo Nozzle Capacity (Liters per minute)||Inlet Water Demand**
(Liters per minute)
|1.9 Liters per minute||2.8 Liters per minute||3.8 Liters per minute|
|Feedline Length (m)||Number of Nozzles*||Feedline Length (m)||Number of Nozzles*||Feedline Length (m)||Number of Nozzles*|
|On-cycle time to apply 1.4 cm of water per square meter||2.5 minutes (150 seconds)||1.7 minutes (100 seconds)||1.25 minutes (80 seconds)|
*Assume nozzle spacing is 8 feet (2.4 m) on center using agricultural spray nozzles with a minimum of 20 psi (138 kPa or 1.4 bar) pressure at the outlet of the nozzle.
**Water demand based on a maximum of 5 feet per second (1.5 meters per second) flow velocity in the pipe.
From ‘Heat Stress Abatement in Naturally Ventilated 4-Row Freestall Barns (Head-to-Head Stalls) Using TeeJet Turbo Jet Nozzles.’ JP Harner, JF Smith, G Boomer, and M Brouk
Diagram of sprinkler system components
Diagram from KSU Extension Bulletin
- Thermostatically controls start of sprinkler system
- Control multiple zones with solenoid valves
Filter: 50-micron canister filter that meets required flow capacity
Electric Solenoid Valves
- Match sprinkler pipe size and flow rate (nozzle gallon per minute times the number of nozzles, or nozzle liter per minute times the number of nozzles)
- Use “normally closed” solenoids
- Lower water pressure produces a larger water drop size to soak through the hair and down to the skin of the cow
- Adjustable pressure reducers drop pressure to the recommended 15 to 20 psi (103 to 138 kPa or 1 to 1.4 bar) in holding pens and feed alleys
Nozzles and Tips
- Use tips to provide 0.5 to 1 gallon per minute (1.9 to 3.8 liters per minute), low pressure, and large droplet size
- 10 psi (69 kPa or 0.69 bar) check valves keep lines full between water cycles
- Clamp-on “saddle” type nozzle bodies clamp over pre-drilled 3/8-inch (0.95 cm) holes in S40 PVC pipe with a maximum sprinkler line length of 180 feet (54.9 m). Use 1-inch (2.54 cm) pipe with 0.5 gallon per minute (1.9 liters per minute) nozzles.
- Threaded nozzle bodies screw directly into 1/4-inch (0.64 cm) pipe-thread tapped holes in S80 PVC or steel pipe. Used for large pipe sizes at least 1-inch (2.54 cm) in diameter.
- Threaded nozzle caps require a wrench to clean the nozzle, but are cow safe, while “quick-caps” do not require tools for cleaning
- Holding pen nozzles should have check valves of 6 to 8 psi (41 to 55 kPa or 0.41 to 0.55 bar) with capacity to provide 1 gallon per 150 square feet (1 liter per 3.7 square meters)
Teejet Technologies – www.teejet.com
Edstrom Industries – www.agselect.com
Nelson Irrigation Corporation – www.nelsonirrigation.com
Senninger Irrigation Inc – www.senninger.com
Edstrom Industries – www.agselect.com
Meter-Man Inc. – www.meter-man.com
FarmTek – www.farmtek.com
Misters and Evaporative Cooling
When water is used to cool the air moving toward the cow, conditions of relatively low humidity are required. Misters can be added to fans to help cool the air cone leaving them, or evaporative cooling pads can be used to cool the air stream as it is drawn through the pad. This type of cooling can be modestly effective under conditions of low humidity with temperature drops of more than 10 degrees F possible. However, with relative humidity greater than 55%, the temperature drop may be less than 1-degree F, making this type of cooling ineffective (see Berman study below).
In climates where humidity frequently exceeds 60%, evaporative cooling is less reliable making soaking the cow directly the preferred method.
Efficacy of evaporative cooling at different relative humidity % (Berman, JDS 89:3817, 2006) with the cooled air at 65% RH
|Ambient Temperature||Relative Humidity %|
|At 93 oF||15||25||35||45||55|
|Temperature Drop (degrees)||24||18||12||7||1|
Mechanical ventilation systems may use positive pressure or negative pressure systems. Positive pressure systems have made a resurgence in cattle facilities over recent years with a focus on providing minimum and transitional ventilation rates through the use of tube delivery systems. However, summer ventilation rates approaching 40 to 60 air changes per hour are generally achieved using negative pressure systems such as tunnel or cross ventilation systems.
When should mechanical ventilation be used?
The following situations would make mechanical ventilation more desirable than natural ventilation:
- The barn has significant wind shadows
- The barn must be oriented north-south rather than east-west
- The barn has more than 4 rows of stalls
- Multiple barns are planned parallel to each other
Mechanical Ventilation Check List
- Use mechanical ventilation if:
- Site has significant wind shadows
- Barn must be oriented north-south to fit the location
- Barn has more than 4 rows of stalls
- Where large dairies plan to put multiple barns parallel to each other
How do we specify a negative pressure ventilation system?
Tunnel and cross ventilation systems can be specified in a similar manner using the below guidelines.
- Ensure that there is sufficient fan exhaust capacity
Currently, there are competing philosophies for designing mechanical ventilation systems. One approach specifies systems based on air changes per hour (ACH) with a target of 4 ACH during the winter, 15 to 20 ACH during transitional periods in the spring and fall, and 40 to 60 ACH during the summer. An alternative approach seeks to ventilate based on estimates of body mass. Traditionally, estimates of 36, 120, and 335 CFM per 1,000 lbs of body weight in winter, spring/fall, and summer are used (61, 204, and 569 m3/h per 454 kg of body weight). More current estimates for summer rates target 1000 CFM per adult cow. The third approach is to base the ventilation design around a target air speed. This has been increased over the years from 2.5 to around 6 mph (4 to 10 kph) through the area of the barn (or the area beneath the baffle in a cross ventilation barn).
Since air does not travel through the barn uniformly and we have already prioritized installing fans over the lying area to provide fast moving air in the resting space, we recommend an approach which focuses on providing sufficient air changes per hour.
The interior volume of the barn must be calculated first by multiplying barn width, length, and height in feetmeters to yield the volume in cubic feetmeters (or in meters to yield cubic meters).
To calculate the exhaust capacity, multiply the volume of the barn (in cubic feet or cubic meters) by the target number of ACH (e.g. 40 to 60 ACH for the summer) and divide by 60 to yield the required cubic feet or cubic meters per MINUTE.
To determine the number of fans needed to supply the target exhaust capacity, use the data available at http://bess.illinois.edu/index2.htm from the University of Illinois BESS lab (or other accredited test chamber) to select appropriate fans at the required static pressure – typically 0.10 to 0.15 inches H2O (25 to 37 Pa) for most negative pressure systems.
Note that baffles, if they are used, raise the static pressure within the barn according to the equation:
Estimated static pressure per baffle = (target air speed e.g. 528 ft/min) /4000)2 i.e. ~ 0.017 inches H2O per baffle
Fan location does little to distribute the air within the barn, the fans should be located based on convenience opposite the inlets.
- Locate the inlets correctly and ensure that they are the correct size
The inlet determines where air will be distributed and the size of the inlet determines the entry speed of the air. For incoming air to mix well with air already inside the barn, we recommend an entry speed of 400 to 500 ft/min (2 to 2.5 m/s). This is also the maximum entry speed for use with evaporative cooling pads.
To determine the total inlet area in square feet, divide the calculated exhaust rate in CFM by the target incoming air velocity (400 or 500 ft/min). For metric units, multiply the exhaust rate in m3/h by 602, and then divide by the incoming air velocity (2 to 2.5 m/s). To determine the range in inlet opening, use the required exhaust rate for winter and summer, and make sure that the width of the opening in winter (minimum of 1 inch (2.5 cm)) is feasible.
Again, inlet location is critical for the distribution of air within the barn, but the inlet location differs between tunnel and cross ventilation systems. Air will always try to follow the path of least resistance, which is not necessarily where the cows are!
Inlet location and air distribution in tunnel and cross ventilation systems
It is clear that the difficulty of providing fast moving fresh air in the cow pens has not been entirely solved by either tunnel or cross ventilation systems. In both systems, air still attempts to follow the path of least resistance, which is above or around the cows. For that reason, even in mechanically ventilated systems, we use fans above the resting area to deliver fast moving air to the cows.
Improved air distribution in cross ventilated barns has been achieved by:
- Distributing the inlet along the entire length of the barn
- Using baffles 7 to 8 feet (2.1 to 2.4 m) above stall rows to redirect the air down on the cow every 40 to 50 feet (12.2 to 15.2 m)
- Putting the roof pitch at 0.5 in 12 or 1 in 12 with a minimum 15-foot (4.6 m) sidewall height
However, the baffles only redirect air flow for about 10 feet (3 m). Air becomes trapped between them, creating moisture and poor air hygiene issues, especially in the winter and transitional periods. Baffles are best used over the middle of a head-to-head stall layout (or perhaps above the curb on the inlet side of the platform).
Location of baffles in a cross ventilated barn over head-to-head stalls
Some cross ventilated barns have explored additional inlets in the roof at the ridge to improve air quality, especially in the winter, and have had success with them.
Distributing air in tunnel ventilated barns is more difficult than in cross ventilated barns because of the inlet location and the tendency for air to travel down the alleys. Baffles impact very few cows and are not recommended in this type of barn. Air distribution in tunnel ventilated barns has been improved by:
- Building with a lower roof pitch at 2 in 12, limiting the chimney effect for natural ventilation, but bringing air movement closer to the cow
- Building a false ceiling about 18 feet (5.5 m) above the floor, preventing the barn from being naturally ventilated
- Using additional fans to redirect air flow down on the cows in the stalls
Tunnel ventilated barn with a false ceiling and fans over the stalls rather than baffles. This barn does not have a natural ventilation option.
The main problem with mechanically ventilating barns is not in the summer, but in the winter. In large barns, it is an immense challenge to move air effectively in and out of the barn because of low air speeds, mixing of air, and entrapment of air within micro-climates within the barn.
A hybrid tunnel barn with a 2 in 12 roof pitch, adjustable ridge opening, and sidewalls that can be opened and closed depending on weather conditions
The relative merits and weaknesses of both tunnel and cross ventilation systems are summarized in the comparison table below:
Comparison of tunnel and cross ventilation systems
|Along the length of the barn||Air flow direction||Across the width of the barn|
|Usually 4 or 6 rows||Rows of stalls||Can be designed with 4 to 16 rows with 8 to 12 rows being the most common|
|South end of a north-south oriented barn or on the east end of an east-west oriented barn||Usual fan location (to avoid fans working against prevailing winds)||East end of a north-south oriented barn or on the north end of an east-west oriented barn|
|1 in 12 or 2 in 12||Roof pitch||0.5 in 12 or 1 in 12|
|12 to 13.5 feet (3.7 to 4.1 m)||Typical sidewall height||15 to 16.5 feet (4.6 to 5 m)|
|Usually longer than a cross (maximum of about 400 feet or 121.9 m)||Air flow distance||Usually shorter than a tunnel|
|At the end wall or along the sidewalls at one end of the barn, providing less even air entry distribution (may add inlets half way down)||Inlet location||Along the entire length of the barn, providing evenly distributed air entry over a greater distance (may also draw air through the ridge)|
|Problems with air flow along the feed and stall alleys once the air enters the barn because of air taking the path of least resistance||Air distribution||Air travels perpendicular to the alleys with potentially better distribution of air in the cow pen, but transfer lanes are still a problem|
|Influence air flow over very few stalls (not recommended)||Use of baffles to redirect the air toward the cow||Function well to distribute air at high speeds over a row of stalls along the length of the barn|
|More restricted space to provide necessary surface area||Use of evaporative cooling pads||Better designed along the inlet for even distribution|
|Roof pitch and openings can be suitable for natural ventilation in winter/spring/fall||Natural ventilation option||Wide-body barns usually have a low roof pitch, and sidewall location of fans precludes use as an inlet|
|Potential for natural ventilation and improved air flow with lower risk for freezing||Winter ventilation||Air distribution problematic at low air changes per hour; frozen alleys along the inlet side of the barn is common|
|Largely independent of barn, but the transfer lane must be managed as a potential inlet||Location of the milking center||Problematic since frequently located at the air discharge side of the barn. Transfer lane may also serve as an inlet.|
|Optional natural ventilation in an emergency||Energy dependence||All day energy needs requiring a back-up generator and emergency plan|
|Compatible||Compatibility with organic bedding||Air speeds may create problems with moving bedding, which could cause dust and air hygiene problems|
|Poorer control of light intensity in barns with a natural ventilation option||Photoperiod||Potential for better control of light intensity|
|Generally barns are of traditional width, but they may be spaced closer together (50 to 60 feet apart (15.2 to 18.3 m)) vs. naturally ventilated barns (100 to 125 feet apart (30.5 to 38.1 m))||Footprint||Potential to increase the number of cows housed in the available space in wide-bodied barns; more of a square-like layout|
It is important to note that, according to OSHA standards, fans within 7 feet (2.1 m) of the floor or working level must be guarded. Also, the guard openings must not be greater than 0.5-inch (1.3 cm) in width.
Holding Area Cooling
Milking center holding areas have been identified as one of the highest risk locations for heat stress on modern dairy farms (Collier, 2006, JDS). Modern holding areas are typically ventilated using natural ventilation to move fresh air into the holding area with recirculating fans to create air velocity on the backs of wetted cows. These systems can work well provided that the holding area is well exposed to prevailing winds and that there is an adequate number of recirculating fans.
However, many holding areas are not sufficiently exposed to winds, and although recirculating fans create air velocity on the cows, the air can become increasingly hot and humid as it gets recycled within the space.
There are two primary reasons for poor ventilation in most holding areas. First, holding areas are frequently located within a complex of buildings that do not allow for good natural ventilation, and second, significant periods of “calm” conditions exist in some parts of the United States.
Thus, milking center holding areas require special attention for cooling strategies. Cooling this area of the farm should be addressed before all other areas. Cows should cool while thermal panting and standing since they increase their body surface area available for cooling. However, if cows are collected together in a tight space with poor air flow, cooling is compromised. Providing the necessary space and adequately sizing the parlor for efficient cow throughput will help decrease heat stress within the holding area.
Holding areas are often built between existing buildings, limiting natural ventilation and increasing heat stress
(Picture of holding area squeezed in between buildings)
Mechanical ventilation systems pose their own problems in locating fans and inlets in milking centers and the holding area. It is almost impossible to achieve effective air distribution between cows when they are tightly packed together in a small space. Even well designed holding areas for natural ventilation will not be ventilated whenever winds are calm or not coming from the prevailing direction.
The combination of fast moving air speeds from well positioned fans and soaking systems is the most effective method to cool cows in the holding area.
Holding Area Soakers
Soakers should be provided to wet the cow before entering the parlor and be activated at temperatures above 70 degrees F (21 °C). Nozzle spacing can be made equal to the spray diameter and placed in a grid pattern to cover the holding area using nozzles with at least 25 gallons (95 liters) per hour rated to deliver 1 gallon per 150 square feet (3.8 liters per 13.9 sq m) of holding area space within a 1-minute period. Cycles can be 1 minute every 5 minutes or 2 minutes every 10 minutes. If larger nozzles are used, spray patterns should overlap by 10 to 25 percent. Pressure in the distribution line should be 15 to 20 psi (103 to 138 kPa or 1.03 to 1.38 bar), and the water supply should provide 1 gallon (3.8 liters) per 10 cows per cycle. All equipment should be about 9 feet (2.7 m) above the floor.
Soakers in the parlor exit lanes and the parlor itself have been used on some farms, but cannot be relied upon to replace the recommendation for soaking before milking at this time.
Sprinkler nozzle requirements based on holding pen capacity
|Holding Pen Capacity (cows)||Typical Holding Pen Size(feet by feet)||Water Required (gallons)*||Minimum Flow Rate (gal per min)**||Number of 360-Degree Nozzles Required***|
|40||24 by 32||20||10||14|
|60||24 by 42||25||12||20|
|80||24 by 50||30||15||27|
|100||32 by 48||40||20||34|
|120||32 by 56||45||23||40|
|160||32 by 75||60||30||54|
|200||32 by 96||80||40||68|
|300||32 by 144||120||60||102|
|400||32 by 192||150||75||136|
|500||32 by 240||200||100||170|
|Holding Pen Capacity
(number of cows)
|Typical Holding Pen Size
(meters by meters)
|Minimum Flow Rate
(liters per minute)**
|Number of 360-Degree
|40||7.3 by 9.8||75.7||37.8||14|
|60||7.3 by 12.8||94.6||45.2||20|
|80||7.3 by 15.2||113.6||56.8||27|
|100||9.8 by 14.6||151.4||75.7||34|
|120||9.8 by 17||170.3||87.1||40|
|160||9.8 by 22.9||227.1||113.6||54|
|200||9.8 by 29.3||302.8||151.4||68|
|300||9.8 by 43.9||454.2||227.1||102|
|400||9.8 by 58.5||567.8||283.9||136|
|500||9.8 by 73.2||757.1||378.5||170|
*Assumes application of 0.025 gal of water per cycle per square foot of pen area (0.095 liters of water per cycle per 0.09 square meters of pen area)
**Flow rate based on a 2-minute on cycle with 10 minutes off
***Assumes nozzles have an 8-foot (2.4 m) spray diameter and 0.5 gpm capacity
‘Reducing Heat Stress in the Holding Pens.’ JP Harner III, JF Smith, MJ Brouk, and JP Murphy, Kansas State University.
Currently, we rely upon the use of recirculation fans to recycle air at high speeds over the cows in the holding area. Cooling air speed recommendations most commonly used suggest 1,000 cubic feet per minute (CFM) (1,699 m3/h) per cow through recirculation fans activated at 65 degrees F (18 °C). This equates to 10 cows per 36-inch (90 cm) fan and 20 cows per 48-inch (120 cm) fan.
Traditional fan installation for holding area cooling
Traditional fan recommendations based on holding pen capacity
|Holding Pen Capacity (cows)||Typical Holding Pen Size(feet by feet)||Total Fan Capacity Required (cfm)||Number of30- to 36-inch Fans||Number of48-inch Fans|
|40||24 by 32||40,000||4||Not Recommended|
|60||24 by 42||60,000||6||Not Recommended|
|80||24 by 50||80,000||8||Not Recommended|
|100||32 by 48||100,000||10||Not Recommended|
|120||32 by 56||120,000||12||Not Recommended|
|160||32 by 75||160,000||16||8|
|200||32 by 96||200,000||20||10|
|300||32 by 144||300,000||30||15|
|400||32 by 192||400,000||40||20|
|500||32 by 240||500,000||50||25|
|Holding Pen Capacity
(number of cows)
|Typical Holding Pen Size
(meters by meters)
|Total Fan Capacity Required (liters per second)||Number of 0.76 to 0.9 meter Fans||Number of 1.2 meter Fans|
|40||7.3 by 9.8||18,878||4||Not Recommended|
|60||7.3 by 12.8||28,317||6||Not Recommended|
|80||7.3 by 15.2||37,756||8||Not Recommended|
|100||9.8 by 14.6||47,195||10||Not Recommended|
|120||9.8 by 17||56,634||12||Not Recommended|
|160||9.8 by 22.9||75,512||16||8|
|200||9.8 by 29.3||94,389||20||10|
|300||9.8 by 43.9||141,584||30||15|
|400||9.8 by 58.5||188,779||40||20|
|500||9.8 by 73.2||235,974||50||25|
‘Reducing Heat Stress in the Holding Pens.’ JP Harner III, JF Smith, MJ Brouk, and JP Murphy, Kansas State University.
Larger diameter fans are more energy efficient than several smaller fans. However, operational cost and efficiency varies across brands (8.3 to 18.6 CFM (14 to 32 m3/h) per watt). Poor maintenance can reduce fan efficiency by 40% or more. Each cooling season, the fan blades and grills should be cleaned, oil applied (if required), and any damages repaired. The fan alignment and orientation should be checked, belts tightened, and thermostat cleaned and calibrated as well.
Unfortunately, using recirculation fans for cooling is very inefficient. The throw distance to air speeds less than 400 feet/min (2 meters/sec) is typically less than 15 to 20 feet (4.6 to 6.1 m) for most fans, so many cows in the holding area do not receive air flow sufficient to effectively cool them. This approach serves merely to recycle warm humid air within the holding area, rather than bringing fresh air in from the outside.
Air movement vs. distance from the fan in a holding area with recirculation fans installed along a sidewall
Air movement vs. distance from the fan in a holding area with recirculation fans installed over the length of the holding area
Horizontally-positioned, 50- and 72-inch (130 and 180 cm) fans have recently been marketed, which improve throw distance and produce extremely large volumes of air movement (e.g. VES Environmental Solutions’ Cyclone Fan). These fans are beginning to be used in many holding areas. However, they also suffer from merely recycling hot, humid air.
Supplemental Positive Pressure Tube Ventilation and Cooling Systems
First applied to naturally ventilated calf barns, positive pressure tubes are useful for providing both the delivery of fresh air and effective heat abatement air speeds. While recirculation fans can be used in conjunction with soakers to effectively cool cows, they do not provide ventilation – the delivery of fresh, clean air into a space. Furthermore, the constraints of low ceilings and the crowd gate often result in fans being installed intermittently and at an angle, thus missing some cows entirely and blowing hot, dirty air off of other cows directly onto the cows behind them.
A positive pressure tube system delivers fresh outside air of lower temperature, humidity, particulate matter, and bacterial loads at heat abatement air speeds to cows who spend a significant portion of their day in the holding area. With careful calculation to ensure consistent performance across the holding area and adequate throw distance to cooling air speeds of 200 to 400 ft/min (1 to 2 m/s), positive pressure tube ventilation and cooling systems are a better use of fan and electricity dollars than recirculation fans. In fact, these positive pressure systems will use fewer fans and less power than a well-designed recirculation fan heat abatement system.
Positive pressure ventilation and cooling systems shower cows with fresh air at heat abatement air speeds more effectively than recirculation fans
In general, fan and tube systems are spaced every 6 to 10 feet (1.8 to 3 m), stretching across the holding area and above the crowd gate. Sprinkler heads can be located between the tubes for additional cooling benefit. Fans are typically 30-inch (76 cm) or larger belt-driven models, capable of around 11,000 CFM (18,689 m3/h). Multiple fan and tube systems are installed to provide about 60 to 90 air changes per hour in order to displace hot, humid air along with dust and microbial contaminants at heat abatement air speeds from the holding area.
(Stylized video of PPV & cooling tubes in the Holding Area)
Positive pressure ventilation and cooling tube system design schematic
During the winter months, one in every four or five tube systems can be exchanged for a tube with many smaller holes to avoid drafting the cows while maintaining adequate air exchange rates. These holes should direct most of the air to the sides. The remaining summer systems would be turned off for the season.
A positive pressure tube installation in a large holding area
Positive pressure tube ventilation and cooling systems should be designed by someone trained to create these systems that balance the physical properties of the fan and tube while recognizing the needs of the animals it will serve. For assistance in designing a positive pressure ventilation and cooling system, please contact one of the trained individuals who can be found here.
Key features of holding area installations:
- Aim for 60 to 90 air changes per hour in the holding area (estimate one 10,000 CFM (17,000 m3/h) fan per 200 to 250 square feet (19 to 23 sq m)).
- Use the largest available tube fans, typically 30 to 36 inches (76 to 91 cm) in diameter, moving about 10,000 CFM at 0.17 inches H2O (17,000 m3/h at 42 pascals) static pressure. Avoid costly variable speed controllers since maximum fan capacity is diminished. Fans manufactured with two speeds can be used with the stated fan capacities. Approximate cost is about $900 USD per fan.
- Avoid hoods, but make sure there is at least a 3-foot (0.9 m) roof overhang protruding over the outside wall where the fans are located. Fit curtains below the fans and close the curtains when the positive pressure tube system is running to avoid recirculating air from the holding area. Curtains may be opened in winter.
- Enclose the area between the fans to also avoid recirculating air from the holding area. (picture)
- Align fans across the holding area, but some installations may be done parallel because of space and access concerns.
- Aim for one tube for every 6 to 10 feet (1.8 to 3 m) to cover 80% of the holding area, positioned at about 9 to 11 feet (2.7 to 3.4 m) above the ground, and allow for 4 feet (1.2 m) above the crowd gate controls (some arms may need to be modified). Approximate cost of a 40-foot (12.2 m) tube would be about $800 USD per tube at about $20 USD per linear foot.
- Size the tube to maintain air speed in the proximal end of the tube at about 1,200 ft/min (6.1 m/s), and use a double cable support system at 3:00 and 9:00 with an optional cable at 12:00 to reduce sagging when the tubes are turned off.
- Size the holes to deliver air speeds of 400 ft/min (2 m/s) at 5 feet (1.5 m) above the floor. Holes are typically at the 5:00, 6:00 and 7:00 positions, and can be staggered to increase the area covered.
Staggered hole alignment improves air distribution
9. Holes near the fan require deflectors to align the air jets vertically. We recommend deflectors for the first 3 rows of holes.
Deflector installation in the part of the tube closest to the fan
10. Select anti-condensation treatment when ordering winter tubes.