Many tie stall or stanchion barns do not provide adequate ventilation.
Proper ventilation is needed to remove moisture and manure gases year-round, as well as excess animal heat during mild weather and the hot summer months.
Proper ventilation results in a healthier and more productive environment for both cows and people in the building.
This paper describes inexpensive methods to provide satisfactory exhaust ventilation in existing dairy barns.
Any exhaust-type mechanical ventilation system must include the following three components to work properly: fans, inlets, and controls. When most dairy producers consider improving ventilation in their barn, the first component they think of is a fan. However, fans serve only as outlets for the ventilation system. Fans must be combined with properly sized and distributed fresh air inlets in order to work properly. Thermostats control the number of operating fans to regulate ventilation.
Fans and Thermostats
Fans are the driving force in providing the required air exchange. Fans should be selected to provide the required winter, mild weather, and summer ventilation rates based on the number of animals in the barn. The minimum ventilation rates for heifers and cows are given in Table 1. For most situations, the required ventilation rates are met by operating a group of fans in three stages.
The winter ventilation rate is supplied by a single continuously running fan. When the internal barn temperature rises to approximately 40 to 45 ° F (4 to 7°), additional fans are activated by a thermostat to provide the additional airflow needed for the mild weather rate. The summer airflow is provided by operating an additional fan(s) along with the winter and mild rate fans.
Table 1. Minimum Ventilation Rates for Dairy Barns
||cfm per animal
|Calves 0-2 months
|2 - 12 months
|12 - 24 months
* All fans should be rated at 1/8 inch of static pressure. Ventilation rates based on recommendations given in the Dairy Housing and Equipment Handbook (MWPS-7, http://www.mwps.org).
Fan selection and static pressure
Fans should be selected to provide the required airflow while working against a specified static pressure. In exhaust mechanical ventilation systems, fans create a lower air pressure inside the building than outside. This air pressure difference forces fresh outside air to enter the building through the inlets, and is referred to as static pressure. The air moving capacity of fans decreases as static pressure increases as shown in Table 2. The maximum airflow rate occurs at 0 inch static pressure (also called free air), but is not used for exhaust ventilation applications.
In addition to the static pressure difference, the air moving capacity of fans is influenced by: blade diameter, motor size, fan speed, housing type, and the use of louvers. Direct drive fans (fan blades mounted to the motor shaft) turn at the same speed as the motor. Belt driven fans turn at a speed that is determined by motor speed, and the diameters of the sheaves or pulleys. Belt driven fans are not recommended for the winter and mild weather fans since more maintenance is required. If the summer ventilation rate is to be supplied by a single large fan then it may be difficult to obtain a direct drive fan with enough capacity. Consider using two smaller direct drive fans to provide the additional summer airflow.
Table 2. Sample Performance Curves for Exhaust Fans
||Airflow in Cubic Feet Per Minute (cfm) at the Indicated Static Pressure
||(inches of water)
Louvers reduce the air moving capacity of a fan. Louvers are not recommended for the continuously running winter fan. If louvers are used on the mild weather fan(s) then check the airflow ratings of that particular fan with the louver installed for proper sizing. Routine maintanance is needed for louvers. Lubricate hinges with graphite annually and clean dust from the louvers as needed. Dirty and rusted louvers can reduce fan performance by 40%.
Fans should be selected to give the required airflow (cfm) at 1/8 inch of static pressure. Do not select fans based on motor size, or blade diameter. Always look for the AMCA (Air Moving and Conditioning Association) certification or some other reliable testing certification to assure valid fan ratings. Single-speed fans are recommended over multiple-speed fans since they have more reliable static pressure characteristics.
Ventilation Rates Provided By Existing Fans
Estimating the amount of airflow provided by existing barn fans is one of the greatest challenges in improving the ventilation system of a dairy barn. Generally, the airflow is not given on the fan. If an airflow rating is listed on the fan then it is most likely the "free air" rating. That rating cannot be used to estimate airflow at 1/8 inch of static pressure. The best option is to contact the fan dealer or manufacturer and obtain the air moving capacity of the fans. Provide the following information to the fan dealer:
- blade diameter (inches),
- motor size (hp),
- motor speed (rpm),
- size of sheaves if the fan is belt driven, and
- a description of any louvers or shutters that are used.
The motor size and speed are generally given on the nameplate. Blade diameter and sheave size can be determined by direct measurement. After determining the rated fan capacity of the fans, compare the ventilation rates with the recommendations given in Table 1. If the motor size and speed are not given on the nameplate, then replace the fan with a new one. In many cases the existing fans in a dairy barn are so old that a producer should consider replacing them with new properly sized fans.
Caution: The fan curves in Table 2 are given for example purposes only. If the fan speed, motor size, and blade diameter are known, then the values in Table 2 can only provide a rough guess of the air moving capacity of a fan. Always obtain manufacturers ratings for existing fans if possible. Use existing fans to supply mild weather or summer requirements instead of the continuous winter rate. The winter ventilation rate is critically important, and should be provided by a new rated fan in many cases. More variation can be tolerated for the mild and summer ventilation rates.
The winter rate fan should not be controlled by a thermostat to ensure that it will run continuously. In older barns that lack proper insulation, a normally closed safety thermostat is sometimes installed for the winter fan. The safety thermostat is set at about 33-34 ° F so that the fan will turn off if the barn temperature falls to 33-34 ° F.
The mild rate fans should be controlled by a thermostat to turn on at about 40-45 ° F. Maintaining the barn at about 40 ° F will help to control condensation in older barns. Manual control is typically used for the summer fan, although thermostats are becoming more popular to avoid human error. Some energy savings can be achieved during the summer by operating the summer fan(s) with a thermostat set at 60 ° F. During cool summer evenings the ventilation rate will be reduced automatically to the mild rate. Use an insulated panel to cover the summer fan during winter.
Ideally, all fans should be located on the sidewall of the barn opposite the prevailing winter winds. Fans should be equally spaced and 10 feet from doors or windows. However, in an existing barn one should make sure that at least the continuously running winter fan is located on a sheltered wall. Strong winter winds can over-power a small winter fan. Satisfactory air distribution can be achieved with uneven fan placement in existing buildings if the inlets are properly sized to match the airflow rate, and are evenly distributed around the barn.
Fresh Air Inlets
Fans and inlets must be used together for the ventilation system to work properly. In some dairy barns plenty of fan capacity is available, but either there are no planned inlets or they are not used properly. Fresh air inlets must be evenly distributed around the building and sized according to the amount of airflow required.
In new buildings, thermostatically or gravity controlled inlets can be used to adjust the total inlet area each time the airflow rate changes. Such a system is often called a multi-setting inlet system and provides the best ventilation control.
Information for the design of new mechanical ventilation systems can be found in the following Midwest plan Service publication: Mechanical Ventilating Systems for Livestock Housing (MWPS-32). This publication is available from the University of Minnesota (see MidWest Plan Service, http://www.mwps.org).
A two setting inlet system is recommended for improving ventilation in existing dairy barns because it is the simplest and least expensive. One inlet setting is used for the winter and mild weather ventilation rates, and another is used for summer ventilation.
Inlet system for winter and mild weather ventilation
The inlets for winter and mild ventilation rates must be sized to satisfy the following condition: inlet velocities should be 800 fpm at the mild weather rate, and not less than 200 fpm at the winter rate.
A simple rule of thumb that can used to calculate the total inlet area required to provide the correct velocities is: provide one square foot (1 ft2) per 800 cfm at the mild weather ventilation rate. Application of this rule is demonstrated by using a 40-cow barn as an example.
Based on Table 1 the mild weather rate needed for 40 cows is 6,800 cfm. The total inlet area required to give a velocity of 800 fpm is calculated as shown below:
Winter & Mild Inlet Area
= Mild Rate / 800 cfm/ft2
= 6,800 cfm / 800 = 8.5 ft2.
Location of Winter and Mild Rate Inlets
Fresh air should not be drawn directly from the outside through the top of the sidewall or over the plate during winter or mild weather. If winter inlets are located in the sidewall then winter winds will blow into the barn and over-power the ventilation system. Instead, the air must be drawn from the attic space or the hay mow which will substantially dampen out the effects of winter winds.
Most tie stall and stanchion barns are 32 to 36 feet wide. Therefore two rows of inlets, one along each sidewall, are sufficient. Barns that are greater than 38 feet wide but less than 48 feet require an additional row of inlets down the center of the barn. Buildings wider than 48 feet (typically free stall barns) need four rows of inlets to insure adequate fresh air distribution.
Check Existing Inlets
Most dairy barns that were built with well planned inlet systems use a continuous slot inlet along each sidewall. Determine if the existing inlets provide the required opening area for the winter and mild ventilation rates by direct measurement.
Add Inlets if Needed
Many older dairy barns do not have a designed fresh air inlet system. Two methods of adding inlets will be discussed:
- a bored hole inlet system; and
- a home-built box inlet system.
The use of these two methods will be demonstrated for a 40-cow barn in the following sections. Several other methods of adding inlets can be used as long as the principles of inlet distribution and sizing are used.
Caution: Barns that have large cracks around walls or doors, or large leaks will not benefit as much from the addition of air inlets as a relatively tight barn. Leaks in the barn must be corrected to attain the desired level of air mixing and distribution. If doors or windows that are not part of the designed inlet system are opened then static pressure will be reduced and the ventilation system will not perform as desired.
Bored Hole Inlet System:
A bored hole inlet system provides a simple way to add inlets to a two-story dairy barn that has a mow floor. If a ceiling is installed beneath the hay mow floor then a box inlet system should be used. Inlets are formed by drilling a row of evenly distributed holes along each sidewall using a hole saw and drill. A header board is required in the hay mow to prevent loose hay from plugging the holes. Typically two or three inch diameter holes are used. A two inch hole provides 0.02 ft2 of inlet area, and a three inch hole gives 0.05 ft2. The total number of holes required is determined as follows:
Number of Inlet Holes = Winter & Mild Inlet Area / Area Per Hole.
Previously, it was calculated that the total inlet area for a 40- cow barn was 8.5 ft2. Therefore, the total number of three inch inlet holes required is 170 (8.5 / 0.05 = 170), or 85 holes per sidewall.
If the 85 holes are evenly distributed along each sidewall of the barn then good air distribution will be achieved. If two inch holes were selected then 425 holes would be required. Obviously, the three inch holes are prefered since fewer holes are required.
Home-Built Box Inlet:
A box inlet system should be used in dairy barns that have a well insulated ceiling and attic space or a ceiling in addition to a mow floor. A box inlet is also a good way of adding inlets to mechanically ventilated free stall barns that require three or four rows of inlets. Either commercial or home-built box inlets may be used to provide inlets. The main advantage of a home-built box inlet is the low cost.
The construction of a home-built box inlet is as follows. The length of the box inlet is 22.5 inches (assuming the joists are spaced 24 inches on-center), and the width is 6 inches. The baffle board should measure 14 inches by 32 inches to assure that a jet is formed along the ceiling. The "cold side" of the baffle board should be insulated with 1/2 inch foam board insulation to reduce condensation problems. The baffle board can be hung from the box inlet using eye hooks, eye screws, chains, and cotter pins. The opening thickness is the distance between the baffle board and the ceiling. The amount of inlet area provided by the box inlet varies with the size of the opening thickness. The range of inlet areas that can be used with a home-built box inlet is given in Table 3. For example, using an opening thickness of 1.5 inches gives an inlet area per box inlet of 0.6 ft2.
TABLE 3. Range of Inlet Areas For a Home-Built Box Inlet.
|Total Inlet Area Provided Per Box Inlet
Box inlets should be evenly spaced about 12 to 16 feet apart. The long side of the inlet should run in the same direction as the sidewall. If box inlets are used in a barn with a ceiling and a mow floor then a header board will be required to prevent hay from plugging the box inlet. In stall barns, a row of box inlets are typically installed over each feed alley. In the 40-cow barn example, the total inlet area required was determined to be 8.5 ft2. A row of 20 tie-stalls that are 4.5 ft wide is about 90 ft long. Determine the number of box inlets needed per row using the following equation:
Number of inlets
= [ row length (ft.) / 14 ft. ] - 1
= [ 90 ft / 14 ft. ] - 1 = 5.4
Since the result is between 5 and 6 inlets, provide 6 inlets per row to be conservative. The total number of inlets in the barn would be 12. The inlet area needed per box inlet is calculated as follows.
Inlet Area Needed per Box Inlet
= Winter & Mild Inlet Area / Number of Inlets
= 8.5 ft2 / 12
= 0.708 ft2 (use 0.7 ft2)
Rounding off the inlet area per box to the nearest tenth of a square foot indicates that each box inlet must provide 0.7 ft2 of inlet area. From Table 3, it can be determined that an opening thickness of 1.75 inches is needed to provide the required inlet area.
Bring Fresh Air Into the Hay Mow or Attic
Fresh air must be able to enter the attic space or hay mow for the winter and mild rate inlet system to be effective. Screened louvers, or covered ridge ventilators can be used as inlets into the attic. Provide attic openings that are approximately twice the size of the total opening area for the winter and mild rate inlets system. For the 40-cow barn example, the attic space needs about 17 ft2 of opening area. Louvers should be screened with 1/2 inch square wire mesh to keep birds out. A hood may be needed to prevent snow and rain from blowing into the attic space if a large louver is used.
Inlet system for summer ventilation
The inlet area required for summer ventilation is determined in a similar manner as for the winter and mild weather ventilation. The inlets are sized to provide an inlet velocity of 800 feet per minute. The main difference is that the inlets that are used for the winter and mild ventilation rates are included in the summer inlet system. Therefore, the amount of additional inlet opening needed for summer ventilation is determined by subtraction. Using a 40-cow barn as an example, the summer ventilation rate needed is 20,000 cfm (Table 1). The total inlet area required for summer is:
Total Summer Inlet Area
= Summer Rate / 800 cfm/ft2
= 20,000 cfm / 800 = 25.0 ft2.
It was previously determined that 8.5 ft2 of inlets are needed for the winter and mild rates. Therefore, the additionl inlet area needed for summer ventilation is 16.5 ft2 (25.0 - 8.5).
Location of Summer Inlets
In the summer, most of the fresh air should be drawn into the barn directly from the outside. Air from the attic space or hay mow is heated by solar energy during the day. Fresh air can be brought in over the top plate or directly through the sidewall using adjustable vents. The inlets should be well distributed around the barn in the same manner as the winter and mild rate openings.
Check for a Slotted Inlet System and Openings in the Eaves
In barns with slotted inlet systems, one or both slots can be adjusted to increase the opening area for summer ventilation. Large openings under the eaves can be opened to allow air to flow into the building from beneath the eaves. If such a system exists, check to make sure that the available opening area matches the required summer airflow. If only one of the slots can be opened for summer ventilation, then the summer fan(s) should be located on the opposite wall.
Adding Summer Inlets
Adding summer inlets to old dairy barns is often difficult. However, two simple methods can be used in many barns -- use of existing windows, and addition of sidewall vents. Many other methods of adding summer inlets also exist. In some cases, summer air can be brought over the eaves and into the barn in a way that is similar to a conventional slotted inlet system. In any case, the inlet area must match the required airflow.
Use of Windows:
In many dairy barns a row of evenly spaced windows is located on one or both sides of the barn. These windows can be used as designed inlets by calculating how much the windows should be opened to match the summer airflow rate. If only one row of windows is available then the fans should be located on the opposite wall. The use of rows of windows to provide the additional summer inlets is demonstrated for a 40-cow barn.
A 40-cow dairy barn has 16 windows -- 8 per sidewall. The winter and mild rate inlet system consists of two rows of bored holes that provide the needed 8.5 ft2 of opening. To match the summer ventilation rate of 20,000 cfm an additional 16.5 ft2 of inlets are needed. Since 16 well distributed windows are available, each window will be opened the same amount to provide 16.5 ft2 of inlet area. The inlet area needed per window is 1.03 ft2 (16.5 ft2 / 16 windows). The next step is to calculate how much each window should be opened to provide 1.03 ft2.
Three types of windows are generally used: double-hung, sliding and casement windows that open by pulling the window forward. Since the area per window is known, the following equations can be used to determine the amount to open each type of window (d).
Double-Hung and Sliding Window:
d (in.) = 12 x Opening Area ft2) / W (ft.)
d (in.) = 12 x [Opening Area (ft2) / (W + H) (ft.)]
In the above equations W is the width of the window opening, and H is the height of the open portion of a casement window.
For the present example, the windows are assumed to be of the casement type with a width (W) of 2 ft and a height (H) of 1.5 ft. Therefore each window should be pulled open 3.5 inches to provide the needed 1.03 ft2 per window.
Adding Sidewall Vents:
Adjustable vents can be installed in the sidewalls of dairy barns that have non-masonry sidewalls. These vents are designed to open a measured amount in the same way as a pull-type casement window. The vents should be equally distributed along both walls. The opening thichness, d, is determined the same way as for a casement window.
Summertime Tunnel Ventilation
In dairy barns with masonry walls or barns that are partially underground, it may be impossible to add sufficient inlets for summer ventilation. Tunnel ventilation can be used to provide summer ventilation in these cases. Some dairy producers consider a tunnel ventilation system because they want the higher air velocities for their own comfort. However, electricity costs are generally higher for tunnel ventilation systems. For this reason, tunnel ventilation is considered to be the system of last resort. A well designed conventional exhaust system will generally perform better at lower cost.
Tunnel ventilation involves exhausting all of the air through one end of the barn using a bank of fans and drawing the air into the building on the opposite end through an intake duct, doors or windows. Tunnel ventilation should never be used for winter or mild weather ventilation since the temperature inside the barn will be very cold near the intake end of the barn but grow progressively warmer near the exhaust fans. Use a conventional exhaust system for the winter and mild ventilation as described previously.
Sizing Fans for Tunnel Ventilation
Fan sizing for a typical tunnel ventilation system is based on the cross sectional area of the building and a selected design velocity of 220 fpm (feet per minute) as shown below.
Fan Size = Building Width (ft) x Ceiling Height (ft) x 220 fpm.
Using the above equation indicates that a total fan capacity of 62,000 cfm is used for any dairy barn that has a ceiling height of 8 ft and a width of 35 ft. If the barn houses 40 cows then the ventilation rate per animal is 1,550 cfm per cow or more than three times the airflow required in a conventional system (see Table 1). However, the additional airflow will not make the barn much cooler than a conventional exhaust ventilation system. The main benefit is the cooling effect caused by higher air velocities around the animals and the operator.
An alternative method of determining the total fan capacity for a tunnel ventilation system is to provide 1,000 cfm per cow. Sizing the fans in this manner would limit the ventilation rate to about twice the recommended summer ventilation rate (Table 1). The average velocity would be reduced to about 150 fpm.
Inlet Design For Tunnel Ventilation
The inlet area required for tunnel ventilation is determined by dividing the ventilation rate by 400 fpm. If the fans are sized to provide 62,000 cfm, then the inlet area needed is 155 ft2. An inlet area of 53 in x 35 ft is needed to provide adequate intake area for such a system. Such a large opening is often provided by building a duct across the end of the hay mow that can be opened to the housing area by a hinged door. The external intake for the duct can be provided using louvers that provide the same amount of area as the barn inlet (i.e. 155 ft2).
Some producers simply open doors and windows on one end of the barn to provide the needed intake area for tunnel ventilation. However, blowing rain can easily enter the animal housing area, and flies tend to be a problem.
Control of Tunnel Ventilation
Manual control is common with tunnel ventilation systems. The tunnel ventilation fans must be covered with insulated panels during fall, winter, and spring. The fans for the winter and mild ventilation rates can not be used with tunnel ventilation and should be turned off. If unseasonably cool summer weather occurs then a tunnel ventilation system can significantly over-ventilate a barn. For this reason, some producers install a thermostat on one or two of the fans used in the tunnel ventilation system to reduce airflow when barn temperature falls to 60 ° F or below.
Insulation and Condensation
The recommended amount of insulation for the ceiling of a stall barn is R-25 which is equivalent to about 8 inches of fiberglass insulation. The walls should be insulated to R-14 which can be provided by the combination of 3.5 inches of fiberglass and internal sheathing. A 6 mil polyethylene vapor retarder should be installed below the inside surface (or warm side) of the wall. Proper insulation of dairy barns helps to conserve animal heat, but the most important function of the insulation is to prevent condensation during extremely cold weather.
Dairy barns that have an uninsulated mow floor require a minimum of two feet of hay to provide the required insulation for heat retention and moisture control. Condensation will often form on solid block or masonry walls during extremely cold weather even when the barn is properly ventilated. The only way to prevent condensation on the walls is to add insulation to the block walls. Some dairy producers in northern Minnesota have added a layer of rigid insulation to the outside of their stall barn walls with some success.
In order to significantly improve the ventilation in a dairy barn one must provide:
- at least three stages of fans that are rated to deliver the winter, mild weather, and summer ventilation rates at 1/8 inch of static pressure;
- two sets of properly sized and distributed fresh air inlets; and
- thermostats to control the operation of the fans. Tunnel ventilation can be used to provide adequate ventilation during hot summer weather if the needed inlets for a conventional exhaust ventilation system can not be installed.
The diagrams for many of the inlets presented in this article were originally developed by Donald W. Bates, Professor Emeritus and Extension Agricultural Engineer; and John F. Anderson, Professor of Large Animal Clinical Sciences.