AMCA standardizes induced flow fan rating

Air Movement and Control Assn. International Inc. (AMCA) Standard 260-07 “Laboratory Methods of Testing Induced Flow Fans for Rating” establishes a uniform method of laboratory testing for fans. The standard determines the aerodynamic performance in terms of inlet and outlet airflow rate, pressure developed, power consumption, air density, speed of rotation, and efficiency. This standard supports ANSI/AMCA Standard 210-99, “Laboratory Methods of Testing Fans for Aerodynamic Performance Rating” to accommodate the induced flow fan's unique characteristics. To ensure that a specified induced flow fan will have the expected performance, engineers should include language that the fan is tested in accordance with AMCA 260-07.

An induced flow fan is a housed fan whose outlet airflow is greater than its inlet airflow due to an induced airflow. An induced flow fan uses a nozzle and windband on the outlet to enhance induction (see Figure 1). Typically, this type of fan is installed as an alternative to a traditional centrifugal or axial fan that exhausts into a stack. Induced flow fans do not exhaust into a stack, but may or may not have ducted inlets. These fans generally are used in laboratory or hazardous atmosphere exhaust applications.

Standard 260 and its corresponding Certified Rating Program were developed to answer questions such as, “What is an induced flow fan, how is its performance quantified, why does it need AMCA certified ratings, and what does the AMCA certified ratings program accomplish?” Recently, at the ASHRAE technical meeting, “Should Induced Flow Fans be Certified?” these questions were discussed. During the seminar, wind engineering and airflow analysis expert Ron Petersen, vice president and principal-in-charge of air quality services at CPP Inc., Fort Collins, Colo., presented examples of what products were marketed along with manufacturers' performance claims. Specific needs of professionals in the laboratory or hazardous exhaust fields were presented in comparison to the performance specification made available by manufacturers. In some cases, the promotion of inadequate ratings occurred. The summary included a recommendation that said, “Vendors supply certified flows and exhaust diameter (or area) in their specifications for use in numerical or wind tunnel air quality assessments.” Accurate ratings are needed to enhance the health and safety of building occupants and for energy minimization assessments.

Because the majority of manufacturers of these fans are AMCA members, and because AMCA has a great amount of certification experience, it was only natural for AMCA to lead the development of a new Certified Ratings Program.

AMCA previously certified the inlet air performance of induced flow fans using an AMCA 210/ASHRAE 51 airflow measurement setup on the inlet that measures the airflow through the fan only (see Figure 2). However, this did not totally satisfy the needs of the user, as Petersen's seminar showed.

Consequently, AMCA members that manufacture these products formed a committee that developed an initial certification program. The program's goals were to meet the needs of the user, provide the information for input into numerical models, wind tunnel modeling for air quality assessment, and also provide a level competitive playing field for manufacturers' products. This committee included manufacturers, engineers, and induced flow fan experts who discussed particular problems in the industry.

The first key to developing a certified rating for induced flow fan performance is the determination of specific customer and industry needs. Petersen's seminar and the expertise of the committee members were critical important parameters had to be resolved, including:

• Outlet airflow

• Outlet area

• Thrust

• Crosswind effects

• Outlet velocity.

The committee struggled to select performance parameters that adequately supply needed information for certified ratings. Without question, exhaust velocity or exhaust diameter is needed in addition to outlet airflow. Some committee members thought that thrust performance would be the only parameter that was needed, while others thought the influence of crosswinds was important.

The committee could not conceive a test that could be performed under current lab conditions that is easily reproducible and practical. Crosswind performance, although not currently included in a standardized test—and not in a certified ratings program—was not completely abandoned by the committee. Testing and ratings for crosswind effects is a potential development in the future.

The committee considered developing a method to quantify the exhaust velocity profile. The velocity profile would be useful to determine an effective area or diameter at the outlet, which is what was needed in the plume equations. The suggested velocity measuring device was a 16-hole probe. Manufacturers noted that the velocity profile would change for each operating point of the fan. Further investigation showed that the increase in velocity with an effective area, compared to the results from using the average velocity (gross airflow divided by outlet area) was less than 10%. The committee consensus was to use inlet airflow, outlet airflow (i.e., the induced airflow), and outlet area as the parameters of interest in the certified rating program.

Inlet airflow already was being certified by AMCA using a multi-nozzle inlet chamber for measuring airflow. Measuring the outlet-induced airflow required a new test method. Several committee members operated laboratories from their own facilities and conducted various testing schemes to determine the outlet airflow. It already had been determined that the pressure at the outlet should be at atmospheric conditions, with the inlet at varying pressures. Several methods were discussed and researched. As a result, the committee finalized a method where an inlet chamber is attached to the fan and an outlet multi nozzle chamber is attached to the outlet (see Figure 3). The outlet chamber is kept at 0 in. water gauge pressure for each inlet pressure over the operating envelope of the fan. At least two labs performed the test using this method prior to completing the test procedure, and all questions were answered. In the end, this test method proved successful.

Fan performance is measured using several setups per the AMCA 210 standard. Two common methods include a multiple nozzle inlet chamber and a multiple nozzle outlet chamber. The multiple nozzle inlet chamber (see Figure 2) uses a variable supply fan or system to regulate the operating point of the test fan. The test fan's airflow characteristics are determined by measuring the pressure drop across a nozzle wall, which correlates with the airflow. The fan's pressure characteristics are determined by reading static and dynamic pressures in the appropriate location. In order to enhance the accuracy of the test, precise air density is required at several locations in the setup. A multiple nozzle outlet chamber is similar, but connected to the outlet of the test fan.

To determine induced flow fan characteristics, a multiple nozzle inlet chamber determines a traditional fan curve for the device. To measure the induced flow found at the outlet of an induced flow fan, a multiple nozzle outlet chamber is used and kept at 0 atm at varying inlet pressure. The varying inlet pressures are obtained by using the damper on the inlet box shown in Figure 3. In many laboratory test setups, the airflow profile often is conditioned using settling means (see Figures 2 and 3). The conditioning is used to help obtain a more fully developed profile to ensure greater accuracy of the pressure readings.

To date, the test standard and the certified ratings program were published in April 2007 as AMCA 260-07 and an inlet chamber was commissioned and completed in October 2007 at the AMCA Headquarters Lab. The new test facility allows AMCA to certify induced flow fans as well as accredit authorized laboratories for the testing of induced flow fans.