How to find out if you're getting the best performance from your fans

Do you know what to look for to ensure the best performance from your air handling equipment?

By Paul Saxon, Air Movement and Control Association International, Inc.

What can the fan specifier or user do to feel assured that the choice of fan equipment is an optimum one? For the record, the word optimum does not necessarily mean "least expensive," nor does it mean "most expensive." An optimum choice is a fan that delivers the aerodynamic performance stated in the manufacturer's catalog.

This implies fan testing. The fan testing facility must be qualified. Performing the original testing in either the AMCA Testing Laboratory or having this lab verify the results of an AMCA Registered Laboratory ensures reliability and accuracy. The AMCA Testing Laboratory checks the references of each applicant laboratory to ensure that the facility, including hardware, procedures, and personnel produces test results of high and consistent accuracy. Laboratory registration is carried out under the provisions of AMCA 111--Laboratory Registration Program. It should be noted that while the laboratory registration fee is modest, fewer than 45 laboratories in the world meet the strict criteria of this laboratory program.

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Having established the basis for reliable accuracy, the second step is conducting a test according to ANSI/AMCA Standard 210. The means of ensuring that a manufacturer satisfies steps one and two is to look for fans that bear the AMCA Seal for "Certified Ratings - Performance." Only those fans with ratings thoroughly checked for accuracy are allowed to bear this seal. In addition, the manufacturer must periodically submit a fan, selected at random, for follow-up verification testing of a production unit. AMCA Publication 211--Certified Ratings - Air Performance--gives the requirements of the AMCA Certified Ratings Program for fans. There are ways to ensure that custom-engineered or unique fan designs that do not "fit" under the umbrella of AMCA Publication 211 are "optimum" fans.

Installation geometry

Connection between a fan and its duct system are important. Fan rating tests are conducted in standardized configurations which are rarely exactly reproduced in the field. In most real world installations, the connections between fan and duct system have a disruptive effect on the flow conditions at the fan inlet and outlet. The effect of these flow disturbances sometimes exceed the pressure losses from friction.

Space requirements often result in a system with elbows at, or near, the fan outlet. Published values for pressure losses in elbows are based on the assumption of a uniform velocity profile entering the elbow. The velocity profile at the outlet of a fan is not uniform and an elbow located at, or near, the outlet develops a pressure loss that is significantly larger than the published handbook value.

The fact that AMCA data includes more than one hundred combinations of blast area/outlet area ratio, outlet duct lengths, and elbow positions demonstrates the complexity of developing appropriate System Effect Factors. Certain elbow positions on double inlet fans require an additional multiplier. This results in 224 factors that can be applied to a simple duct elbow in the outlet duct.

Pay attention to the path that the air stream follows in approaching the fan inlets. An air stream entering the inlet unevenly or generating spin results in loss of performance. If it is not practical to alter the flow pattern ahead of the inlet, inlet baffles usually can improve performance.

An obstruction to air flow in the plane of the fan inlet also reduces fan performance. Structural members, columns, butterfly values, blast gates, and pipes are examples of inlet obstructions.

Time spent in an analysis of System Effects is a rewarding investment. AMCA Publication 201--Fans and Systems--presents an analysis of the System Effects to take into account in the design of a system. An optimum system design provides long-range energy savings while helping to maintain the profit in a job.

Sound and movement

There are two more important aspects of selecting and using a fan. The first is the relationship between air performance and sound and the second is the operating point.

When fan specifiers and buyers look for an air performance rating, they seek information on airflow, pressure, speed, and power. These quantities are conveniently represented in multi-rating tables or on a fan performance curve. It should come as no surprise that each air performance point has a corresponding sound performance point, given in decibels of sound power. But where the several values associated with air performance may be easily represented in tables or on performance curves, each corresponding sound performance point requires values for the eight octave bands necessary to describe the distribution of sound energy over the frequency range. This allows any necessary attenuation to be designed into the silencer in the event that a suitable one is not available off-the-shelf. But eight octave bands present the user and manufacturer with a problem: how does one handle eight times the data (for sound) as is needed for air performance? Having posed that question, let's leave it for a moment and consider the origin of sound performance data.

Air performance tests, to be considered reliable and accurate, require the tests be performed according to the appropriate test standard in an appropriate laboratory. There are parallel requirements for sound performance testing but with one major extra requirement: before a fan product line can be proposed as a candidate for the AMCA Certified Ratings Program for Sound, that fan product line must have first met the requirements for the Air Performance CRP Seal.

The AMCA standards and publications covering the testing and certification of sound performance ratings are AMCA Publication 111--Laboratory Registration Program--under which a laboratory achieves registration by demonstrating that the facility, personnel, and procedures produce consistent and accurate test results. This process is similar to that followed in registering a laboratory for air performance testing. Because of the nature of sound, this registration is a more difficult process. There are fewer than a dozen laboratories in the world that are AMCA registered to conduct sound tests on fans.

The sound performance test standard used throughout the fan industry in the United States and most of the world is AMCA Standard 300--Reverberant Room Method for Sound Testing of Fans. This standard helps determines the sound power levels produced by a fan by comparing sound pressure levels from the fan with those produced by a calibrated Reference Sound Source. Another test standard, AMCA 330--Laboratory Method of Testing: In-Duct Sound Power Measurement Procedure for Fans--may also be used to determine the sound power of a fan.

The next step is for the fan manufacturer to rate fans of other sizes and speeds on the basis of the lab test data following the requirements of AMCA Standard 301--Methods of Calculating Fan Sound Ratings from Laboratory Test Results. This standard ensures that fan manufacturers that certify sound ratings consistently convert lab sound data to ratings.

Now let's return to the question of what to do with the data. Clearly, there is a limit to the amount of data that is of genuine, usable interest to the specifier or customer. This data is generally given at, or around, the range of the highest fan efficiency. Therefore, the sound performance data determined during the lab testing and converted during the rating process comes from four, or more, test points covering the application range of the fan design because sound performance varies with the efficiency of the fan.

An fan produces the least amount of sound when it operates at its point of highest efficiency. Therefore, if a fan is to operate quietly, select it so the designed operating point coincides with the fan's highest efficiency.

Having made the selection, it is then necessary to actually operate the fan at the desired operating point/highest fan efficiency point. A large centrifugal fan that is "turned-down" to near shutoff conditions produces low-frequency rumble. A variable-pitch axial flow fan running wide open produces high-frequency components. In either case, the fan is operating at a performance point where fan efficiency is poor and this necessarily produces more sound than indicated for the original rated performance point.

There are several simple rules to follow if a fan installation is to be successful:

  • select a fan from among those listed in AMCA Publication 261--Directory of Products Licensed to Bear the Certified Ratings Seal,
  • match the fan with the system so that fan efficiency peaks at the desired system point,
  • provide aerodynamically clean connections to the fan, and
  • operate the fan and system at the originally selected design point.

Following these few guidelines should easily make a project the happy and satisfying experience it should be.

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