Fan performance and efficiency for animal ventilation systems
On this page
- Fan capacity
- Static pressure
- Fan ratings
- Factors affecting fan capacity
- Performance testing
Fans are a necessary component in a mechanical ventilation system. They are the driving force that provides needed air exchange in facilities housing poultry and livestock. Fan selection is the important first step in designing a mechanical ventilation system, and it is very important to choose fans that are performance-tested and energy efficient.
The amount of air a fan moves depends on the diameter of the blades, shape of the blades, speed at which the blades turn (revolutions per minute or rpm), horsepower (hp) of the motor, design of the shroud, and other attachments such as louvers. These combined factors establish the air moving capacity of a fan. Fan capacity is measured in terms of the cubic feet, or volume, of air moved per minute (cfm).
Figure 1. This illustrates operation of a negative pressure ventilation system (A), a positive pressure ventilation system (B), and a neutral pressure system (C).
When a fan operates, it creates a static pressure difference between the inside and outside of a building. This air pressure difference can be measured with a manometer in inches of water.
Depending on the type of ventilation system, there may be a negative, positive, or neutral pressure inside the barn compared with the outside (figure 1). The negative pressure system (A), where air is exhausted with fans and enters the barn through inlets, is by far the most common system. Figure 1 (B) shows a positive pressure system, used with certain types of furnaces, to supply both airflow and heat in a livestock unit. A neutral pressure system (C) exhausts from and blows air into the barn simultaneously, creating an approximate zero pressure difference. Most of the commercial heat exchangers marketed today for use in animal buildings use a neutral pressure system.
The air moving capacity of a fan (cfm) depends on the size of the operating static pressure difference. Table 1 shows the variation of cfm values with static pressure and the ratings for a group of commercial fans. The maximum cfm value occurs at zero inch static pressure, or "free air." As static pressure increases, cfm values are reduced until a point is reached where there is no airflow. What static pressure should be used for selecting livestock and poultry ventilation fans? The common recommendation is 1/10" - 1/8" static pressure. This is because most systems operate in the range of 0.03 (3/100)" to 0.10 (1/10)" static pressure. So, by choosing pressures slightly above the operating range, sufficient airflow is assured.
Table 1. Sample performance data for exhaust fans
|Airflow in cubic feet per minute (cfm) at the indicated static pressure|
|Fan diameter (inches)||Fan speed (rpm)||Motor size (hp)||0||1/10||1/8||1/4|
|(inches of water)|
Factors affecting fan capacity
Table 1 shows fan capacity differences among similar diameter blade fans due to such variations as motor horsepower and rpm. Louvers or shutters (used to close the fan opening when not in operation) can also restrict airflow. Some fan manufacturers rate fans both with and without shutters to indicate the effect on fan performance. Table 2 compares cfm values in both instances for a small capacity fan.
Table 2. Fan capacity with and without louvers for a 7 1/2-inch diameter blade fan, 1/15-hp motor turning at 3400 rpm
|Air delivery in cfm's at indicated static pressures|
Fan ratings also depend on motor speed. Table 3 shows that as rpm values are reduced, so is the air delivery (cfm) of variable speed fans. At low speeds there is concern whether or not variable speed fans can maintain sufficient static pressures to provide reliable air exchange against any wind pressure present.
Table 3. Fan capacity of 14-inch diameter blade with 1/8 -hp motor turning at the four indicated rpm
|Air delivery in cfm's at indicated static pressures|
More reliable airflow rates are provided by single speed fans since they have better pressure characteristics. This is important to remember when selecting continuous-running fans (providing the minimum ventilation rate). This is especially important if the barn has manure storage below a slatted floor and the associated "pit" fan(s).
Figure 2. This seal indicates that the fan has been tested under a Standard Test Code of the Air Movement and Control Association, Inc. (AMCA).
The fan ratings given in Tables 1 through 3 were obtained using laboratory equipment and standards established by the Air Movement and Control Association, Inc. (AMCA). If a fan manufacturer tests fans following these procedures, an AMCA label (figure 2) and certification can be listed in the company's literature. Fan manufacturers also can use other procedures that give valid results. The important point is to select a fan that delivers the desired cfm against 1/10" - 1/8" static pressure.
Another characteristic that is becoming important when selecting fans for an animal ventilation system is energy efficiency. This is expressed as airflow per unit of input energy, or cfm/watt. The energy efficiencies of fans are being tested by various fan manufacturers and at least one independent lab, the Bioenvironmental and Structural Systems Laboratory (BESS) at the University of Illinois Agricultural Engineering Department. Tests by this lab on 36-inch diameter blade fans showed a wide variation in both fan performance (6,400 to 13,000 cfm) and energy efficiencies (8.3 to 18.6 cfm/watt) at 1/10" static pressure.
Figure 3. Test data for 36-inch fans that provide 9,000 to 9,900 CFM.
Figure 3 compares the energy efficiency for six fans from this group that have performance ratings between 9,000 and 9,900 cfm at 1/10" static pressure. As can be seen, significant differences do exist in the cfm/watt ratings (8.4 to 18.6) for fans that deliver approximately the same airflow.
Choosing fans with high energy efficiencies is most important for the mild and warm-weather fans since more air is being moved when these fans are operating. Small continuous-running fans need to have good performance ratings, since their most important job is to guarantee that the air exchange will occur under almost all conditions to insure good air quality in the facility.
As an example, let's take the most energy efficient 36-inch diameter blade fan of the fans shown in figure 3 and compare it to the least efficient of that group. We can see in Table 4 that at 1/10" static pressure the most efficient fan has a 18.6 cfm/watt energy efficiency value, while the least efficient fan only has an efficiency of 8.4 cfm/watt. Using the cfm ratings and efficiencies at 1/10" static pressure and assuming a rate of $0.10 per kWh (electric cost) and 120 days (24 hrs/day) of operation for a warm-weather fan, one would save roughly $190 per year ($340 vs. $150) by using the more energy efficient fan (Figure 4). Typically, the more energy efficient fan would cost an additional $150 to $250, but the payback with the above assumptions would be only 1 or 2 years. If one assumes a 10-year fan life, that would result in a $1,900 gross savings over the life of the fan!
Table 4. Performance and efficiencies of most and least efficient of the 36-inch blade fans shown in Figure 3*
|0.04" static pressure||0.10" static pressure|
|* Bioenvironmental and Structural Systems Lab's handbook entitled "Agricultural Ventilation Fans – Performance and Efficiencies." July, 1993 edition.|
Figure 4. Comparison of average cost for a high and low efficiency fan
Maintenance of fan systems
Fans operating in animal housing units are exposed to vast amounts of dust and moisture, and accumulate dirt on blades, louvers, and shrouds. Dirt on fan blades has little or no effect on fan performance, but dirt on louvers and guards can reduce airflow by as much as 40 percent. Fan louvers and guards should be cleaned regularly and lubricated (using graphite to prevent dirt accumulation) to prevent large airflow reductions in fans. Louvers should be removed where fans are running continuously to avoid the potential for restricting airflow, but guards should remain in place to prevent personal or animal injury.
Not only is the environment in a livestock or poultry barn dirty, but there are corrosive gases, especially in facilities with manure pits inside the facility. Continuously running fans or fans that exhaust from pit areas need a non-corrosive housing, such as fiberglass, to prevent deterioration. Whether the fan is controlled by a simple switch or a microprocessor, those controls should also be protected from the environment by a watertight and dust-tight electrical box to eliminate electrical contacts and switches from being exposed to corrosive gases. More information on electrical wiring in corrosive environments is given in the MidWest Plan Service "Farm Buildings Wiring Handbook" (MWPS-28), available from the University of Minnesota Agricultural Engineering Extension office.
Size of openings
Although fans are a necessary part of a ventilation system, they are not the only thing to consider. Also needed is some way to bring air into a negative pressure system (inlets), or to allow its escape in a positive pressure system (outlets). In both instances, a standard rule for sizing air openings is to provide one square foot of area for every 800 cfm of fan capacity. For more detailed information on designing a mechanical livestock ventilation system, please refer to the MidWest Plan Service handbook entitled "Mechanical Ventilating Systems for Livestock Housing" (MWPS-32).
Good quality fans are essential for proper performance of any mechanical ventilation system. Fans should be selected on the basis of air delivery in cfm against a 1/8-inch or 1/10-inch static pressure, and on their rated energy efficiencies. Energy efficient fans will pay for themselves from the electrical energy savings within a few years. The cfm requirements or recommended ventilation rates for livestock and poultry facilities can be found in the MidWest Plan Service (MWPS) handbooks available from county extension offices or Extension Agricultural Engineering, University of Minnesota, St. Paul, MN 55108. Look for the AMCA certification or other reliable testing label when purchasing fans to assure valid ratings. Make sure that louvers, shutters, and other restrictions are included when selecting fans for the required ventilation rate in animal confinement facilities.
This publication was prepared with financial support from Minnesota Power.