Tighten Your Fan Belts
Larry D. Jacobson, Minnesota
Extension Engineer
Richard E. Nicolai, Agricultural and Biosystems Engineering, South Dakota
State University
David R. Schmidt, Minnesota Extension Engineer
Verlyn J. Johnson, Minnesota Information Technology Specialist
Exhaust fans in mechanically ventilated livestock buildings are selected based on either the manufacturer’s or an independent laboratory’s airflow ratings. The airflow rates listed in these tables are generated under ideal conditions (no wind, dirt or dust on blades or shutters, or slippage of fan belts). However, under actual use, exhaust fans must operate under many conditions that may not be reflected in the ratings tables for that particular fan. Although most livestock engineers assume that an exhaust fan in a barn will deliver less air than the ratings indicates, there has been no easy or effective way to measure airflow rates of fans once they are placed in the livestock facilities. However, an airflow assessment unit was recently developed by USDA-ARS to do just that. It is called the FANS unit.
48”
portable shop fan (back) with FANS unit mounted on front.
The FANS unit has 5 free wheeling anemometers that were calibrated for airflow at the Bioenvironmental and Structural Systems (BESS) laboratory at the University of Illinois. The unit was constructed to measure airflows from the large diameter blade fans found in poultry, pig, and dairy barns, but can also be used for smaller diameter fans. Tests at the BESS lab have shown the FANS unit to be a very accurate means (within 1 to 2 %) of measuring the airflow capabilities of exhaust fans that are in a building.
The University of Minnesota is using a FANS unit in a six-state air emissions project to measure actual airflow rates of all exhaust fans from two sow gestation/breeding barns. The two facilities are tunnel-ventilated and have one 36-inch diameter blade direct-drive exhaust fan and five large belt-driven 48-inch diameter blade fans. Measurements were recently made from one of the 48-inch fans at various RPM (revolutions per minute) values as determined by the tightness of the driving belt. Figure 1 shows the variation in airflow rate as a function of static pressure and RPMs. Airflow rates are significantly affected by RPM variations that are primarily caused by belt tightness. Airflow reductions of roughly 30% occur at a typical operating static pressure range of 0.15 inches of water gauge due to shutters, dirt on blades, and other fan assembly components. Up to a 50% reduction occurs at static pressures of 0.25 inches of water gauge. Additional reductions of 30% or more will occur when the fan’s RPM is further reduced due to slippage of the fan’s driving belt.
Thus it is very important to maintain fan belt tightness and condition to make sure fans are operating at design RPMs. Otherwise barns that have belt-driven exhaust fans will be significantly under-ventilated. Although this is a concern at any time of the year, it can be especially a problem during the summer when more fans operating creates a higher static pressure in the barn leading to further loss in the barn’s ventilation capacity.