Basic guide to fans and blowers
Fans are used throughout the industrial plant. They are found in an infinite number of components and used in a variety of systems from heating, ventilating, and air conditioning to industrial processes (Fig. 1).
General ventilation typically involves the movement of uncontained air. Applications include dilution ventilation, makeup air, and cooling of personnel and equipment. Process ventilation typically involves controlled or contained movement of air from one point to another. In these situations, air is often contaminated in some way.
Understanding the principles of fan system operation helps the user in determining performance, selecting the optimum device, and keeping the fan system well maintained. Basic guidelines are provided here. More detailed information on fans and blowers is available from any of a number of sources including manufacturers and trade associations.
Fans are turbomachines that provide the means to move air through a system. A fan that produces pressures above 30-in. w.g. at a relatively low airflow is referred to as a blower or a pressure blower . Turbomachines producing pressures greater than 15-psig (416-in. w.g.) are referred to as compressors.
Fans are classified into two major design categories: centrifugal and axial. Centrifugal fans have an intake at the center and force the air radially outward, perpendicular to the axis of rotation. They are used to generate higher pressures at lower airflow rates. Centrifugal fans are further classified on the basis of impeller design geometry: backward curved, airfoil, radial, and forward curved. The backward curved and the airfoil fans have the highest efficiencies among centrifugal fans.
Axial fans move the air parallel to the axis of rotation. Axial fans are used to generate lower pressures (less than 12-in. w.g.) while handling large airflow rates. When space is a concern, axial fans are often preferred. Axial fans are further classified as vaneaxial, tubeaxial, and propeller.
Figure 2 shows exploded views of each of the two major fan types. Detailed data are found in the various textbooks, application guides, handbooks, manuals, and reports available from manufacturers and associations. For more information, see the resource guide in the expanded version of this article on our web site: www.plantengineering.com.
Fan affinity laws
The performance characteristics of fans follow relationships known as the affinity laws. These three laws are used to correlate the flow ( Q ), pressure developed ( P ), and power required ( H ) with the rotational speed ( N ), fan impeller diameter ( D ), and density ( å ) of the gas. These fan laws hold true only when all fan component dimensions are scaled by the impeller diameter ratio.
Volume: Q (sub 1)/ Q (sub 2) = ( N (sub 1)/ N (sub 2)) X ( D (sub 1)/ D (sub 2))(super 3)
Pressure: P (sub 1)/ P (sub 2) = ( N (sub 1)/ N (sub 2) )(super 2) X ( D(sub 1) / D (sub 2) )(super 2) X ( P (sub 1)/ P (sub 2) )
Horsepower: H (sub 1)/ H (sub 2) = ( N (sub 1)/ N (sub 2) )(super 3) X ( D (sub 1)/ D (sub 2) )(super 5) X ( P (sub 1)/ P (sub 2) )
Each fan belongs to a family and each family has a set of performance curves. For a particular fan, the performance curves provide a graphical relationship between the pressure developed and the flow rate for different fan speeds. In addition to the pressure /flow relationship, the required power and the fan efficiency are also provided. Figure 3 shows a typical fan performance curve. Every fan manufacturer provides this performance information in a catalog for fan selection. Many also provide performance curves in a software package to assist distributors and fan end-users in the selection process.
Fans may be tested in accordance with ANSI/ AMCA or ISO standards. Specifically applicable are Standard 210 or ISO Standard 5801 (AMCA Standard 210 is contained in ISO Standard 5801). Catalogued fans tested to these standards may have their performance ratings certified according to AMCA publication 211, Certified Ratings Program — Air Performance . Fans are tested with clean and unobstructed airflow into and out of the fan. Good system designs allow for the same smooth airflow conditions. Systems that introduce obstructions to the airflow, abrupt turns, combinations of turns in rapid succession, or that contain improperly located dampers and other air system components are likely to suffer in performance if the system effect of these detriments are not eliminated or taken into account.
Plant Engineering magazine acknowledges with appreciation the contributions made to this article by the following companies and organizations: Air Movement and Control Association Intl., Arlington Heights, IL; American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA; Greenheck, Schofield, WI; Hartzell Fan, Inc., Piqua, OH; and New York Blower Co., Willowbrook, IL. Special thanks to Paul Saxon of AMCA and Riyaz Papar, PE, Lawrence Berkeley National Laboratory, Washington, DC, for their contributions to the “Operating fundamentals” section.