Flat-blade BI wheels usually contain 10 to 16 blades angled backward away from the direction of the wheel rotation. They are typically applied when higher pressure and volume is necessary or in light dusting applications, and especially on the clean-air side of dust collection systems. Flat-blade BI wheels are more robust than the forward-curved blade and cost less than the airfoil shape. Like the other BI-style fans, the flat blade is a non-overloading wheel, which allows wide capacity variation from cutoff to free air delivery.
Backward-curved (BC) wheels have similar performance characteristics and application to BI wheels. They may be slightly more efficient than flat BI blades and are generally quieter due to the curved construction of the blade.
Airfoil (AF) wheels (Figure I) are also similar in design and application to BI and BC wheels, with one significant difference: the shape of the blade. AF blades are hollow and, like airplane wings, are thicker on the leading side of the blade than the trailing side. The airfoil shape combines the highest efficiency of the centrifugal fan wheel designs with the lowest noise levels. Their disadvantages include higher costs of production and susceptibility to erosion of their thin, hollow steel, which can lead to wheel damage. The applications of AF wheels are similar to those of the BI and BC wheels.
Radial blade blowers (Figure J) have efficiencies in the 60–75 percent range, lower than the BI fans. Nevertheless, radial blade blowers may be the most commonly applied of all the fans in industrial hygiene applications because of their robust construction, high-pressure characteristics, and ability to handle heavy dust and material loads as well as their resistance to erosion and corrosive contaminants. The straight blade design lends itself to self-cleaning, minimizing blade imbalance, and maximizing bearing life. Their heavy construction and higher horsepower per cubic feet per minute (cfm) ratio makes them more costly than other centrifugal fans. They are overloading fans in that their horsepower rises continuously from cutoff to free delivery.
Radial blade wheels come in a number of designs and shapes characterized by the enclosure of the wheel. The blades are attached at the hub and are directed outward in a straight spoke-like arrangement, neither aiming forward nor backward from the hub. Shrouded radial blades have a back plate and an outer ring to which the blades are attached; these blades are typically applied with long duct runs and high-pressure blow-off systems. Open-back plate radial blades (Figure K) are attached to the back and to the hub but have no outer ring and are the most versatile wheels designed for conveying powdery or granular material and long stringy materials such as paper trimmings or fibers. Open radial blades, also referred to as paddle wheels (Figure L), have no back plate or outer ring but are attached only at the hub. They are used in conveying powdery and granular materials. TEAM APPROACH Designing and implementing a functional engineering control solution can seem daunting given the wide range of contaminants and the number and types of fans available to control them. To design the best solution, the engineer needs to have the best information regarding the potential exposures and contaminants, and thus relies on the industrial hygienist. And to best control the contaminants identified, the industrial hygienist relies on the engineer’s knowledge of fans and systems. The teaming of the engineering and industrial hygiene disciplines allows a more complete understanding of employees’ exposures and potential controls. This team approach results in the best possible solution for contaminant control and employee protection, and should be applied from the initial determination of the need to control contaminants, through the implementation of the engineering control solution. THE AUTHORS are members of the staff of Chemistry & Industrial Hygiene, Inc., a consultancy based in Wheat Ridge, Colo. They can be reached through Bill Mele at (303) 420-8242 or bmele@c-ih.com.
Read more about engineering controls from The Synergist: • “Embracing Engineering,” September 2015 • “Troubleshooting Industrial Ventilation,” June/July 2016
Axial fans: Figures A and B courtesy of Cincinnati Fan. Axial flow fan blades: Figure C courtesy of Air Control Industries; Figure D courtesy of Twin City Fan & Blower. Centrifugal fan: Figure E courtesy of Chemistry & Industrial Hygiene, Inc. Centrifugal forward-curved fan: Figure F courtesy of Twin City Fan & Blower. Forward-curved fan wheels: Figure G courtesy of Hi-Tech Blowers. Centrifugal backward-inclined flat blade: Figure H courtesy of Greenheck. Centrifugal airfoil wheel: Figure I courtesy of Greenheck. Radial blade blower: Figure J courtesy of Greenheck. Open radial blade: Figure K courtesy of Greenheck. Open paddle radial blade: Figure L courtesy of Greenheck. The Synergist thanks the listed companies for permission to republish the images used in this article.
Fans and Blowers: What to Look For
The first steps in recognition and evaluation of fan or blower systems include answering these questions: 1. What is the application? a. Is the system a high-pressure or high-volume application? b. Is the system conveying materials? c. Is the exhaust air stream corrosive? 2. How does the fan look? a. What kind of fan is being utilized? b. What kind of wheel is present, and is it appropriate for the application? c. Is the fan wheel corroded or fouled? 3. Is the system performing up to expectations? a. Are contaminants being captured or contained? b. Are materials properly transported? c. If discharged to the outdoors, is the exhaust air stream adequately dispersed?

CENTRIFUGAL FANS Centrifugal fans (Figure E) or blowers use one of a number of fan wheels enclosed in an expanding scroll-shaped housing. Air enters one or both sides of the wheel and turns 90 degrees where it is radially accelerated and discharged through the opening in the housing. The fan wheel may be belt-driven or directly coupled to the motor shaft. Fan wheels, typically constructed of steel, may be vinyl-, epoxy-, resin-, or phenolic-coated for corrosion resistance. Fiberglass-reinforced plastic is also applied to some highly corrosive exhaust air streams. The air- and material-handling characteristics of the fan are determined by the type of wheel and the following rules of thumb:
  • Larger wheel diameters and shorter wheel lengths generate higher pressures than smaller diameters and longer lengths.
  • The tighter the clearance between the wheel rim and the inlet ring, the higher the fan efficiency.
  • The type of wheel may be the most critical aspect of the fan selection.
Typical applications include capturing and conveying material in dust collector systems or material handling systems; ventilating or pressurizing hoods, cabinets, booths, or enclosures; cooling production equipment, motors, or processes; removing dust or moisture; drying applications for water, ink, or paint; and circulating air in ovens and dryers.
TYPES OF FAN WHEELS Axial fan systems may require specially formulated or designed fan blades. These designs include non-ferrous or non-sparking construction, or blades that reverse direction for both supply and exhaust applications. Once specified and applied, however, axial fan systems typically don’t require design modifications or capacity changes. A single fan might serve a single process such as a paint spray booth, or a bank of wall fans might ventilate a large production area with capacity control a simple matter of turning one or more fans on or off. This article focuses instead on centrifugal fans and fan wheels. These are considered more frequently for system design modifications and in industrial hygiene evaluations.
Of the centrifugal fan types, the forward-curved (FC) fan wheel (Figures F and G) may have the least application in industrial hygiene settings. Sometimes referred to as a squirrel cage fan, this type of wheel is generally wider than backward-inclined or radial blade wheels. It typically contains 24 to 64 shallow-scooped blades cupped in the direction of rotation, making it well suited to general ventilation and cooling applications such as make-up air systems where higher volumes and lower pressures may be acceptable. The horsepower curve rises continuously from cut-off (restricted airflow) to free delivery (unrestricted airflow), allowing the fan to overload the motor in certain speed ranges.
FC fan efficiencies vary from design to design but generally fall in the 60–65 percent range. They are not recommended for contaminant-laden air because the fan blades are easily fouled, resulting in unbalanced wheel conditions, which could subject the lightly constructed wheels to catastrophic failure. They are most often applied to clean air delivery in general ventilation and for supply air or tempered make-up air but may also be found in fume or vapor-hood applications where dust-laden air is not normally encountered.
Backward-inclined (BI) wheels (Figure H) are more commonly employed in IH applications than forward- curved wheels and may be seen in three designs: backward flat blade, backward curved blade, and airfoil blade. These three styles are the most efficient fans (79–83 percent) with the airfoil shape having the greatest efficiency.
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.