An Analysis of Induced Flow Laboratory Exhaust Fan Systems and the Benefit of AMCA 260 Certified Performance FA/128-12

April 6th, 2012

Independent third party verification of ventilation system components is valuable in ensuring performance and safety. This is especially true for critical lab exhaust systems. Air Movement and Control Association (AMCA) International Inc. certifies induced-flow fan air and sound performance based on two established testing standards:

  • AMCA Standard 260-7, Laboratory Methods of Testing Induced Flow Fans for Rating.
  • ANSI/AMCA Standard 300-08, Reverberant Room Method for Sound Testing of Fans.

The AMCA 260 and 300 certified rating seals ensure a laboratory fan system will perform as stated by the manufacturer and as required for a project.

Comparing Lab Exhaust System Efficiencies

Fan efficiencies are evaluated and compared by calculating the static efficiency of a fan at an operating point. Fan static efficiency relates the cfm moved by the fan at a given static pressure versus the energy required to do so.

The calculation for fan static efficiency is:

Figure 1
In fan application engineering, it is generally accepted that airfoil blade housed centrifugal blowers are the most efficient fan designs available with maximum attainable static efficiencies of approximately 80%. For inline mixed flow fans the peak static efficiency is about 75%.

When evaluating and comparing lab exhaust fan efficiencies, other exhaust system components (such as discharge stacks and nozzles) need to be considered, whether the systems being evaluated are field built-up or factory provided. Typically, all lab exhaust systems utilize a discharge stack and a high velocity discharge nozzle which increases the momentum of the exhaust air, dispersing contaminants high above the roof line. This high velocity discharge benefit is accompanied by the cost of increased horsepower.

The pressure loss associated with any high velocity discharge is equal to the velocity pressure at the discharge:

Figure 2
For a field built-up lab exhaust system, the stack and nozzle static pressure loss is not included in the fan manufacturer's performance data. It must be added to the fan inlet static pressure in order to appropriately size the fan. For a factory provided system (induced high plume flow fan), this loss is typically included in the manufacturer's performance data and only the inlet static pressure is used to size the fan.

Accurate analysis comparing fan system static efficiencies requires the inclusion of the high velocity nozzle pressure loss. To calculate the static efficiency of induced flow fans, the nozzle pressure loss must be added to the inlet static pressure.

Induced flow lab exhaust fan systems can be categorized into two groups:

  • Induced flow inline mixed flow fan systems. (Figure 1)
  • Induced flow housed centrifugal fan systems. (Figure 2)

Comparing Induced Flow Fan Efficiencies

Due to the rising concern for sustainability and energy consumption issues, it is important to evaluate several lab exhaust system fan types and their performance with respect to fan static efficiency. The examples will also demonstrate the benefit of AMCA Certified performance.

Fan types for comparison:
Figure 3

The following induced flow lab exhaust system performance requirements used for this analysis are from actual project documents.
Figure 3

Figure 4

In order to calculate the static efficiencies of the three fan systems, we must calculate the static pressure loss of the high velocity nozzle per Equation 2. This pressure loss is added to the system static pressure to calculate the fan static efficiency (Equation 1).

Table 1 shows the selection of fans. It includes each fan's performance, based on the manufacturer's data, framed in black.

Based on the manufacturer's data, Fan 3 has the high entrained flow and lowest brake horsepower.

Table 2 provides additional information about the selected fans. Calculating the static efficiencies for each fan reveals the following:

Fan 1 and 2 (Greenheck models) have efficiencies that are commonly accepted and fall within usual application acceptance. These efficiencies fall below the maximum of 80% for housed airfoil centrifugal fans and 75% for inline mixed flow fans respectively.

Fan 3, Competitor A, however, has a static efficiency that exceeds the possible maximum of 75% for inline mixed flow fans. How can this be?

Assuming Fan 3 can exhaust the required 35,524 cfm at six inches static pressure from the building, the stated performance in question is the entrained airflow and nozzle velocity. Further investigation reveals Fan 1 and 2 have AMCA 260 certified performance. Fan 3 is not AMCA 260 performance certified and therefore has NO independent third-party performance verification.

This example demonstrates and reinforces the benefit of AMCA Standard 260 independent, third-party testing and certification.

Figure 5

Conclusion

To be assured that fan performance is factual for critical lab exhaust applications utilizing induced flow fans, be sure to specify fans and fan systems that have AMCA (third-party) certified performance ratings. The ratings are based on testing in accordance with AMCA 210 for standard fans and AMCA 260 for induced flow fans. AMCA certified ratings ensure that the product performs as tested and documented by the manufacturer.

Caution must be taken when using performance data that is not verified by an independent third-party (AMCA).

References:

2004 ASHRAE Handbook, HVAC Systems and Equipment, Chapter 18; American Society of Heating, Refrigeration and Air Conditioning Engineers

Fan Engineering, 3rd Edition, Section 4; Buffalo Forge Company

ANSI/AMCA Standard 210-07, ANSI/ASHRAE 51-07, Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating.

AMCA Standard 260-07, Laboratory Methods of Testing Induced Flow Fans for Rating, Air Movement and Control Association International, Inc.

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