Chapter 7 - Fan Accessories

Accessories are necessary for proper functioning of the fan as part of a ventilation system, even though they often reduce airflow and efficiency. Typical equipment installed on a fan includes a guard, which prevents animals, people, or objects from contacting the blades; and shutters, which prevent airflow when the fan is not in use.

Guards always should be installed for the safety of people and animals near the fan, but they also protect the fan from damage. Guards generally disrupt airflow and efficiency by less than 5%. Round ring guards with concentric circles of wire disrupt airflow less than wire mesh guards. A guard should be installed on any side not protected with fan shutters.

Shutters can be used on either the inside or outside of most fans. Interior shutters are preferred over exterior shutters because they are easier to clean and provide about half the resistance to airflow. Shutters placed on the discharge side of fans are particularly detrimental to airflow. Air exiting the fan blades circulates forcefully in a spiral pattern, and this fast-moving, circulating air will be disrupted by horizontal shutters. Expect a 10-15% airflow reduction using inlet-side shutters and a 15-25% reduction using discharge-side shutters. For stage-one fans that operate continuously year-round, shutters can be removed for improved airflow. Fans staged to operate in only warm or hot weather need shutters or they will act as an inlet and hence disrupt the ventilation system airflow and static pressure when not in use.

Fan performance can be improved with well-designed fan housing and with inlet or discharge cones. A well-designed fan has a tight clearance between fan blade tips and its housing. This discourages air from coming off the blade tips and flowing backwards through the housing. Streamlined airflow improves fan capacity and is particularly effective with inlet cones. Discharge cones offer some airflow improvement and will provide the fan some protection from weather.

One inherent, but often overlooked, fan characteristic is aerodynamic stall, which is characterized by a dramatic fall in airflow rate when static pressure is only slightly increased above the stall-pressure. Stalling severity depends on fan design and how guards, shutters, cooling pads and other airflow obstructions affect static pressure against which the fan has to operate. The best way to evaluate fan performance is to obtain rated fan data showing that specific fan’s performance at various static pressures. Look for the fans where stalling occurs above 0.25 IWG static pressure.