Do you know how much airflow your hot weather fans move? Independently rated 48 inch diameter fans can move between 14,600 and 26,800 cubic feet per minute (CFM) at 0.10 inches of water static pressure (in SP). Fan diameter does not determine airflow rate. If you have a tunnel ventilated barn with 50 dry cows that needs 52,800 CFM of airflow (30 ft wide x 8 ft tall x 220 ft per minute = 52,800 CFM), you need four of the fans that move 14,600 CFM. You need only two of the fans if they move 26,800 CFM.
When selecting agricultural fans for use in dairy barns there are several factors that you and your equipment supplier need to consider including: the airflow rate between 0.10 and 0.15 in SP, ventilation efficiency ratio (VER), dealer service and support, fan reliability, useful life, purchase price and operating cost.
The University of Illinois BESS lab conducts independent third party testing of agricultural fans. Test results are available online at http://bess.illinois.edu. Reports give the make, model and manufacturer as well as a complete description of guards, shutters and discharge cones used during the fan test. Reports also give airflow rates in cubic feet per minute (CFM), motor speed in revolutions per minute (RPM), electricity use in watts (W), and VER in cubic feet per minute per watt (CFM/W) at different static pressures.
Static pressure describes the resistance to airflow that a fan must overcome. Things that produce resistance to airflow include wind blowing at the fan outlet, inlets, baffles in tunnel or cross ventilated barns, fan shutters and guards. The goal is to manage the airflow resistances and select fans that can overcome the expected resistances and provide the airflow needed for the animals in the barn. The general recommendation is to select fans, tested with shutters, guards and discharge cones in place, based on their airflow at 0.125 in SP. That static pressure allows the barn to have between 0.05 and 0.10 in SP between inside and outside the barn and to overcome the resistance to a 10 mph wind blowing against the fan. A 10 mph wind can produce about a 0.05 in SP resistance. Higher winds blowing against a fan can produce greater flow resistance and reduce airflow.
I was at a mechanically ventilated barn on a very windy day and saw the wind close the shutters while the fan was running. The fan was running and using electricity but the fan was not providing any ventilation. On that particular day, that barn was ventilated by fans on the other side but the barn was not performing as desired. Deflectors can be built to reduce the wind effect if wind blowing against exhaust fans is a major problem.