Glen J. Arnold
Michael A. Veenhuizen
Ventilation is the air exchange process that brings fresh air into a livestock building, thoroughly mixes the air, and exhausts moist contaminated air from the building. A ventilation system is more than just a fan. A properly designed and installed ventilation system includes air inlets, air outlets, and controls. The fan is the "heart" of a mechanical ventilation system. It provides the driving force for the needed air exchange in livestock facilities. Without adequate air exchange, animal performance can suffer while moisture and manure gases will corrode metal building components and equipment and sensing elements on thermostats, electrical switches and controls.
Livestock producers can better manage building environments by understanding the factors influencing ventilation fan performance.
Required ventilation rates vary with number and size of animals, stocking density and inside and outside temperatures. During cold weather, minimum air changes are needed to remove moisture and harmful gases while conserving heat. As outside temperatures increases, additional airflow is needed to remove excessive heat.
Seasonal air exchange requirements require two or more fans and controls to manage the environment. A continuously operating minimum ventilation fan should be used during cold weather to control moisture and harmful gas levels. This provides a more uniform and better control of indoor building temperature than a larger fan operated by a timer or a large variable speed fan on a low setting. Optimum building temperatures are listed in Table 1. Provide additional fans during mild and hot weather to control temperature increases.
| Table 1. Optimum temperatures for livestock housing. | |
|---|---|
| Animal | Desirable temperature, F |
| Lactating sow | 50-70 |
| Litter-newborn | 90-95 |
| Litter 3 weeks old | 75-85 |
| Pre-nursery (12-30 lbs) | 75-85 |
| Nursery (30-50 lbs) | 70-80 |
| Nursery (50-75 lbs) | 60-70 |
| Growing-finishing (75-220 lbs) | 50-70 |
| Gestating sows | 50-70 |
| Boars | 50-70 |
| Dairy Cows | 40-60 |
| Calves, floor level, bedded | 40-60 |
| Calves, raised stalls | 60-70 |
| Beef cows | 40-60 |
| Horses | 50-70 |
| Rabbits | 40-60 |
| Layers | 55-70 |
Fans can be axial flow or centrifugal. Axial flow fans pass air straight through the fan parallel to the fan blade drive shaft. Centrifugal fans (squirrel cage fans) bring air in through a center inlet, the air makes a right angle turn and is discharged perpendicular to the fan axis.
Axial flow fans are designed to operate at low static pressures (typically less than 0.5 inches of water column) and are commonly used for livestock building ventilation. The initial cost is less and performance is influenced less by dirt buildup on the fan blades. Axial flow fans are either propeller fans, tubeaxial, or vane-axial fans.
Propeller fans are the most common. Propeller fans have propellershaped blades mounted in a circular ring or orifice plate and a drive motor. Blade tip clearance is an important factor in propeller fan performance. A small, uniform clearance is preferred to prevent air from flowing back around the propeller. These fans move large volumes of air at low static pressure.
Tube-axial fans consist of a tube-shaped housing, a propeller-shaped blade, and a drive motor. Tubeaxial fans can operate at higher static pressure because of a larger hub and reduced blade tip clearance, so less air flows back through the fan. Tube-axial fans are typically used in low and medium pressure duct air distribution systems.
Vane-axial fans are similar in design and application to tubeaxial fans. The major or difference is that air straightening vanes are added either in front of or behind the blades to reduce the circular motion of the air. This results in a somewhat more efficient fan capable of operating at a slightly higher static pressure.
The amount of air a fan moves depends on the blade diameter, blade shape, fan speed (revolutions per minute, rpm), motor horsepower, and housing.
Two most common measurements used to describe the characteristics of a fan are blade diameter and motor horsepower. These are useful measurements but without performance characteristics (airflow and static pressure), they give only very general indicators of fan capacity.
For example, test results comparing the performance of thirtynine commercially available thirty-six inch diameter fans show the variability in fan performance for similar size fans. These fans were operated at 0. 1 inches static pressure. The air delivery capacity of these fans varied from 6,400 to 10,000 cfm. Similar tests of thirty-nine commercially available forty-eight inch diameter fans showed that air delivery at 0. 1 inches static pressure varied from 4,100 to 23,000 cfm (BESS, 1992).
These large variations in fan capacity for similar diameter fans significantly affects the selection process of fans and should provide clear evidence that fans should not be purchased or evaluated strictly by the fan blade diameter.
Airflow rate (cfm) and static pressure are closely related for fans and ventilation systems. The air moving capacity of a fan (cfni) is directly affected by the system static pressure. As the resistance to airflow (static pressure) increases, the delivered airflow capacity decreases. Hence, a fan delivers more air against a low static pressure than against a high static pressure. The relationship between static pressure and airflow delivery for a specific fan is represented by a tabulation of performance values or fan performance curve. Table 2 gives example airflow rates at different static pressures for several different fans.
Fan performance data is used to determine the airflow rate a fan will deliver when operating at a given static pressure. Ventilation fans are typically selected based on the air delivery capacity at 0.1 to 0.125 inches of static pressure. Performance data is often reported for zero inches of static pressure (free air) a condition that is rarely encountered in actual applications. A ventilation fan must deliver the desired airflow capacity at the ,rated static pressure. Fan performance data should be reported with all attachments included such as shutters, guards, and hoods. Select fans rated to perform at pressures slightly above the operating range to assure adequate airflow.
For more reliable performance of cold weather fans, select a fan that performs consistently across a wide range of static pressures, 0 to 0.25 inches of water column. The effect of wind gusts and pressures can cause the static pressure across the fan to vary over this range.
| Table 2. Typical rating tables for exhaust fans These are
just examples. Actual performance values from the fan manufacturer should
be used when selecting fans. Air Delivery in Cubic Feet per minute (clm) at Indicated Static Pressure | ||||||||
|---|---|---|---|---|---|---|---|---|
| Diameter | RPM | HP | 0" | 1/10" | 1/8" | 1/4" | 3/8" | 1/2" |
| (Free Air) | ||||||||
| 8" | 1650 | 1/30 | 400 | 316 | 289 | |||
| 10" | 1550 | 1/30 | 594 | 457 | 413 | |||
| 12" | 1550 | 1/30 | 730 | |||||
| 12" | 1600 | 1/12 | 1198 | 1073 | 1033 | 827 | ||
| 16" | 1140 | 1/12 | 1673 | 1440 | 1374 | |||
| 16" | 1725 | 1/3 | 2534 | 2392 | 2353 | 2142 | 1890 | 1635 |
| 18" | 1140 | 1/6 | 2686 | 2460 | 2395 | |||
| 18" | 1725 | 5/8 | 4065 | 3920 | 3880 | 3862 | 3445 | 3195 |
| 21" | 1140 | 1/4 | 3812 | 3599 | 3540 | |||
| 21" | 1725 | 3/4 | 4914 | 4770 | 4740 | 4510 | 4320 | 3920 |
| 24" | 855 | 1/3 | 4691 | 4310 | 4180 | |||
| 24" | 1140 | 7/8 | 6254 | 5990 | 5920 | 5470 | 4810 | 4220 |
Fan capacity can be affected by fan blade diameter, fan speed (rpm), motor horsepower, and louvers or shutters. Table 2 shows the difference in capacity for similar diameter fan blades, due to variations in motor horsepower and fan speed.
Louvers or shutters used to close the fan opening when not in operation can also restrict airflow. Shutters typically reduce airflow by 10% to 25%. For minimum ventilation fans that operate continuously, shutters should not be used because they reduce efficiency and add to the cost. Shutters are essential for most mild weather and hot weather fans. If no shutter is installed on the fan, its opening will act as an air inlet when the fan is shut off causing poor air distribution and drafts.
Guards typically reduce airflow and efficiency by less than 5%. Guards are essential for the safe operation of the fan. Guards also protect the fan from being damaged by large objects that can strike the blades. Some fan manufactures rate fans both with and without shutters or guards.
It is important to evaluate the fan's airflow capacity with all the appropriate attachments in place. Table 3 gives example airflow values for a small diameter fan with and without shutter to show the effect on fan performance.
| Table 3. Fan capacity with and without shutters for 7-1/2 inch diameter fan, 1/ 15 hp motor, 3400 rpm | |||||||
|---|---|---|---|---|---|---|---|
| Air delivery in cfm's at indicated static pressures | |||||||
| Shutters | 0" | 0.05" | 0.10" | 0.125" | 0.15" | 0.20" | 0.25" |
| NO | 588 | 569 | 546 | 533 | 518 | 481 | 313 |
| YES | 286 | 258 | 236 | 236 | 269 | 301 | 202 |
Fan ratings also depend on motor speed. Table 4 shows that as rpm values are reduced, so is the air delivery of variable speed fans. At low speeds, variable speed fans may not maintain sufficient static pressure to deliver reliable air exchange against changing wind pressures. More reliable airflow rates are provided by single speed fans which are less affected by wind pressures. This is important to remember when selecting continuous running fans that provide the minimum cold weather ventilation rate.
| Table 4. Fan capacity of a 14 inch diameter fan with 1/8 hp motor at different rpm. | |||||||
|---|---|---|---|---|---|---|---|
| Air delivery in cfm's at indicated static pressures | |||||||
| RPM | 0" | 0.05" | 0.10" | 0.125" | 0.15" | 0.20" | 0.25" |
| 1675 | 2172 | 2112 | 2028 | 1988 | 1932 | 1840 | 1480 |
| 1355 | 1775 | 1622 | 1473 | 1385 | 1298 | 1005 | 585 |
| 855 | 1 | 110 | 813 | 273 | |||
| 586 | 4 | 82 | 83 | ||||
Wind pressure blowing against a building also affects fan performance.
| Table 5. Wind effect on static pressure. Fan exhausts against the wind with no hood or wind shielding. | |
|---|---|
| Wind Speed MPH |
Static Pressure in Water |
| 5 | 0.02 |
| 10 | 0.05 |
| 15 | 0.10 |
| 20 | 0.20 |
| 25 | 0.28 |
If the wind is blowing in the same direction as the fan airflow, then a negative pressure or suction effect can reduce the pressure across the fan increasing airflow capacity. However, wind blowing against the fan air increases the static pressure the fan has to operate against, (Table 5). For example, when a twenty-five mile per hour wind is blowing into the fan, the pressure from the wind is 0.28 inches of water. This is added to the operating pressure of the building of about 0.06 inches static pressure to give a total static pressure of 0.34 inches of water. Fan capacity will be greatly reduced by the wind pressure. To reduce the effect wind has on fan performance, locate fans on the leeward side or provide an adequate weather hood to reduce wind effects.
Fan efficiency, or the "cfm per watt ratio", is a measure of the number of cubic feet of air moved per minute (cfm) per watt of electrical power input. Use this factor to compare the efficiencies of different fans.
Efficiencies of commercially available fans can vary by more than a factor of two. In a fan test of thirty-nine commercially available thirty-six inch diameter fans the "cfm per watt ratio" varied from 8.3 to 17.4 cfm/watt.
Energy efficient fans are usually more expensive. However, energy efficient fans can save the extra cost of the fan in about 2 to 3 years. Good quality fans can last more than ten years, the money saved is more than the cost of the fan.
If fan efficiency ratings are not available, the following guidelines can be helpful.
Fans operating in livestock housing environments are exposed to large amounts of dust and moisture. Maintenance practices can have a significant effect on fan performance. Dust on fan blades has only a slight effect on fan performance, but dust accumulations on shutters and guards could reduce air flow by as much as 40%.
Shut off electric currents for fans before checking and servicing. Also, check fuses for a tight fit and wiring for deterioration and good connections.
Clean fan blades, shutters, and guards regularly and lubricate fan shutters with graphite to reduce dust accumulations. A pressure washer can be used to clean the fan, housing and hood thoroughly. Be sure the fan motor has a totally enclosed housing so water and dirt cannot get into the motor windings. Otherwise, take the motor off and clean it separately. Most fan and motor bearings are sealed and don't require oil. Oil should be used sparingly, when necessary, as too much oil attracts dust and soaks into motor windings.
After cleaning and servicing the fan, place a sticker on the fan housing with the current service date. Locate the sticker and check regularly as a reminder to perform regular maintenance. If you cannot read the date or see the sticker, it is definitely time to clean the fan.
Livestock environments also include corrosive gases, especially in buildings with manure storage. The combination of high moisture, gas concentrations, and dust can create a corrosive environment. Fan blades and fan housings should be made of non- corrosive materials such as high density plastic and fiberglass. Protect controls from the effects of the environment with air-tight enclosures or by locating them in a separate room and utilizing remote sensors.
For more information, obtain a copy of MWPS-32, Mechanical Ventilating Systems for Livestock Housing available through Ohio State University Extension county offices.