What Every Industrial Hygienist Should Know about Fan Selection
Exhaust Ventilation
As a rule, industrial hygienists don’t design exhaust ventilation systems. In most industrial settings a mechanical engineer will design the system and specify its requirements. But this fact does not ensure that all exhaust and ventilation systems are designed, applied, installed, and operated correctly. The more intricate the system, the more susceptible it is to misunderstanding and improper operation.
Often, a well-intentioned system operator or maintenance technician will take the most well-designed system and “fix it” to match his or her level of understanding. Also, once a system leaves the drawing board, the design professional has seen it for the last time. Even systems that have been properly designed, installed, and commissioned often lose their effectiveness over time due to various reasons, not the least of which may be a well-intentioned “fix.”
Enter the industrial hygienist. In keeping with his or her charge to anticipate, recognize, evaluate, and control occupational exposures, the industrial hygienist is often the most capable set of eyes on an operating exhaust ventilation system. The tenets of recognition, evaluation, and control apply to mechanical systems as well as to occupational exposures, so the industrial hygienist should have a basic understanding of the engineering concepts of exhaust ventilation. It all starts with the fan or blower.
This article is not intended to be a ventilation design class. Instead, it focuses on the basics of exhaust ventilation equipment (fans and blowers) so the practicing industrial hygienist can recognize their appropriate application to the reduction of occupational exposures. The sidebar below lists some typical considerations related to hazard recognition and evaluation as they apply to fan or blower systems. MATCHING CONTROL TO CONTAMINANT The first step in evaluating an exhaust ventilation system is to identify the type of contaminant that needs to be controlled. It might be heavy particulate matter such as in crushing or grinding applications, or light particulate as in smoke or fume. Other types of materials handling systems, such as paper trimming or sawdust control, might present special capture and transport requirements. Gas- or vapor-phase contaminants—including combustible vapors or solvents, or heavy gases such as in dry ice sublimation—have their own requirements. Each contaminant presents demands to the overall design of the system, especially in the selection of the appropriate equipment or hardware.
The second step is to match the appropriate control to the application. At the heart of every ventilation system is the air mover, which may be a fan or a blower. The American Society of Mechanical Engineers differentiates these devices according to the outlet-to-inlet pressure ratio: fans have pressure ratios of up to 1.11, and blowers have ratios of 1.11 to 1.20. (Ratios of 1.20 and greater are defined by ASME as compressors.) However, many practitioners apply their own definitions. Some identify fans as propeller-style devices and blowers as centrifugal, wheel-and-scroll devices. For others, the difference depends upon the primary application: fans draw air from a location while blowers deliver air to a location. Still others use the terms interchangeably. For this discussion, fans will be designated for low-pressure applications and blowers for higher-pressure systems. Regardless of which name is applied, the air mover must be appropriately selected for the contaminant being controlled, and a practicing industrial hygienist should be able to recognize whether a system requires a fan or a blower.
Exhaust ventilation systems are normally defined by the terms “general exhaust ventilation” (GEV) and “local exhaust ventilation” (LEV). GEV systems typically ventilate large open areas such as production or manufacturing floors with wall-mounted panel fans or roof-mounted axial or centrifugal power roof ventilators (PRVs). LEV systems typically capture contaminants at their point of generation and employ centrifugal blowers often remotely located and ducted from the source to the outdoors. Any exhaust ventilation system will use either axial fans or centrifugal fans. AXIAL FANS Axial fans (see Figures A through D below) use a propeller with two or more blades that move air in a direction parallel to the propeller shaft. They may have an open design as in wall-mounted panel fans, or they may be enclosed in a housing or tube for ducted arrangements. The propeller may be directly mounted to the motor shaft or belt-driven with a motor and fan pulleys. Some rules of thumb for axial fans include:
  • Enclosed designs generate higher pressures than open designs.
  • Larger hubs with shorter, more numerous blade designs generate higher pressures than small hubs with longer and fewer blades.
  • The tighter the tip clearance between the blade end and the ring, the higher the fan efficiency.
In the industrial hygiene arena, axial fans are best suited for high-volume, low-pressure air circulation of clean or relatively clean air. Examples of these applications include spray booth exhaust; process cooling and exhaust of machinery or systems; personnel cooling in hot work areas; forced cooling and exhausting of heat-producing areas; mist, smoke, and vapor exhaust in mills or parts-washing areas prior to painting parts; supply air and make-up air general ventilation in factories, foundries, and warehouses; and parking garage exhaust.