Understanding the limitations of fan efficiency grades

Fan efficiency grades (FEG) are a poor metric in determining the most efficient fan (in terms of actual power consumption) for a given airflow and pressure operating point. The best simple metric to ensure the lowest power consumption is the operating brake horsepower at the specified design point.

10/29/2013


Learning objectives:

  • Understand the fan efficiency grade (FEG) metric.
  • Learn about the relationship between peak fan efficiency and actual operating efficiency.
  • Understand the difference between static efficiency and total efficiency.
  • Know that, depending on the actual fan application, the highest efficiency may not yield the greatest energy savings.

Government agencies and regulatory bodies in the U.S. and around the world are working on regulations to help reduce power consumed by fans in commercial and industrial ventilation. As a part of this effort, AMCA International (in support of a request from ASHRAE Standard 90.1) developed an efficiency metric known as fan efficiency grade (FEG) that could be used to establish minimum acceptable fan efficiency.

FEG definition and rating

FEGs, as defined in AMCA 205, are designed to be a simple system to indicate the aerodynamic quality of the fan and are based on the fan’s peak total efficiency. The total efficiency is calculated using the traditional airflow, pressure, and input power as measured per AMCA Standard 210. Fan efficiency does not take into effect the efficiency of the drive (belt drive) or the motor. Efficiency is defined as the air power divided by the fan input power. Both static and total efficiency can be calculated from fan performance data as follows:

Figure 1: Fan efficiency is plotted along with a fan curve. Fan performance (measured in cfm) can be plotted as a function of static pressure or total pressure. Respectively, fan efficiency can be plotted on the fan curve for either static pressure or tot

Where:

CFM = Fan flow rate, ft3/min
Ps    = Static pressure, in. wg
Pt    = Total pressure, in. wg
BHP = Fan power input, hp

Fan static and total efficiency (Figure 1) can be plotted along with the fan pressure curves. The peak total efficiency occurs at the top of the “bell” shaped efficiency curve. This peak total efficiency is used to determine the FEG value.

Figure 2: In this AMCA FEG curve, a 24.5-in. diameter fan with a peak total efficiency of 69% is classified as FEG71. Courtesy: AMCA InternationalNote that the peak efficiency occurs at just one point on the curve and all other points on the curve have a lower efficiency. It is important to understand, as the efficiency curves illustrate, that each fan has a large range of efficiencies depending on the airflow and pressure at the operating point. For example, a fan with a peak efficiency of 70% easily can be selected to operate at a point of only 50% efficiency.

Another aspect of AMCA FEGs is that their value depends on the fan size. Smaller fans are inherently less efficient than larger fans. This is because the smallest dimensions—material thicknesses and running clearances between parts—cannot be held as tightly in proportion to other dimensions as they can on larger fans. The AMCA FEG curves have been established such that fans of a given model that are geometrically similar will each have the same, or nearly the same, grade.

Figure 3: This fan curve shows how the selection range is reduced when following AMCA Standard 205, which recommends all selections be made within 15 percentage points of the peak total efficiency and FEG values. Courtesy: GreenheckOnce the peak TE is known, the FEG value can be determined from AMCA Standard 205 (see Figure 2). For example, a 24.5-in. diameter fan with a peak TE of 69% would be classified as an FEG71. Note that a 12-in. diameter fan with a peak TE of 60% is also FEG71.

Selection range

Because the efficiency curve of a fan is bell shaped, specifying a relatively high FEG by itself will not necessarily result in high fan efficiency. To realize the potential efficiency of a fan, the fan must operate near its peak efficiency. AMCA Standard 205 recommends that all selections be made within 15 percentage points of the peak TE. This requirement effectively reduces the allowable selection range (see Figure 3).

Limitations of FEG

A significant shortcoming of the FEG metric is that the highest FEG fan does not necessarily result in the lowest energy consumption. Table 1 illustrates this point. Notice that the 72-in. fan requires the least energy (lowest BHP). Yet, the 48-in. fan has a greater total efficiency (66% vs. 60%) and a higher FEG (71 vs. 63).

Table 1: The highest fan efficiency grade (FEG) fan does not necessarily result in the lowest energy consumption. Courtesy: Greenheck 

Additionally, Table 2 relates the first cost of each fan size, along with the 5-year total cost of ownership (TCO), excluding maintenance. The size 27 blower could be selected for this application, having the lowest first cost but not the lowest power consumption. The largest blower, size 36, has the lowest power consumption, but at a 27% premium cost over the size 27. If the application can accommodate the dimensionally larger blower, the energy savings will pay back the additional cost of the larger blower in 1.4 years. ($0.10/kWh and 2400 hours/year operation) The size 33 blower, however, has the lowest 5-ear TCO.

So how can the fan with a higher efficiency consume more than twice the power?

How can the same type of fan, selected for the duty point of operation, have the same FEG value yet consume twice the power of another (see Table 2)?

Table 2: Various size fans of the same type, selected for the same duty point of operation, have the same high fan efficiency grade (FEG) value (85), but significantly different power consumptions (ranging from 12 to 24.5 BHP). Courtesy: Greenheck

First, the FEG metric is based on fan total efficiency and fan total pressure. Total pressure is used because it is a measure of the total energy imparted to the air. However, the velocity pressure exiting a fan can only be used when it is contained in a duct—and is lost on nonducted fans. This makes FEG an inappropriate and often misleading metric for many fan applications, such as sidewall propeller fans, powered roof ventilators (PRVs), and plenum fans. For fans without a discharge duct, static efficiency will correlate to power consumption.

This is why the fan industry has standardized on selecting fans using static pressure and not total pressure: both ducted and nonducted fans use static pressure, whereas only ducted fans use total pressure.

In the example in Table 1, the 48-in. fan has a much greater discharge velocity than the 72-in. fan. This contributes to the high TE and FEG values, but since this is a nonducted application, the velocity pressure is lost. Notice that the 72-in. fan has the highest static efficiency, which is the proper metric for nonducted applications.

Second, as communicated earlier, the FEG value is based on the peak efficiency of the fan. For a given point of operation (CFM and pressure) an FEG63 fan could consume less power than an FEG75 fan simply because it is selected closer to its peak efficiency point. Fans with higher peak efficiencies do have a greater potential to operate more efficiently. However, the actual fan efficiency as selected is the correct measure of actual energy consumed.

Another limitation is that while specifying a single FEG value for all fan applications would be desirable, it is just not that simple. ASHRAE Standard 90.1-2013 requires a minimum FEG67 for all fans. This is also the direction being taken by the International Energy Conservation Code (IECC). Additionally, it is well known that weather guarding of PRVs inherently impacts fan efficiency negatively. And as discussed above, PRVs and other fans with a nonducted discharge should not be held to the same metric that is based on ducted total efficiency. To accommodate these realities, several exemptions have been added to the proposed language so that the industry doesn’t unwittingly eliminate economical and efficient fans from existence. Meanwhile, ducted housed airfoil centrifugal fans and ducted vane axial fans already greatly exceed the proposed minimum value of FEG67, so this won’t drive greater efficiency for these fans. The end result is that the single FEG value approach as adopted in ASHRAE Standard 90.1 has little ability to actually save energy.

Code language

FEGs are a simple measure of the peak total efficiency of a fan. Although other alternatives are being considered for code regulation and energy savings, FEG was initially incorporated into proposed code language and has not been replaced (as of the time of this writing). Because of the current state of events on this front, the final code language may use FEGs to establish minimum aerodynamic efficiency levels. If this occurs, fans below the mandated FEG value will not be allowed.

Regardless of the outcome of code language, FEG is a poor metric in determining the most efficient fan (in terms of actual power consumption) for a given airflow and pressure operating point. Clearly the best simple metric to ensure the lowest power consumption is the operating BHP at the specified design point. From a specifying engineer’s perspective, there are three key takeaways regarding fan efficiency:

  1. Specify the specific operating BHP in your fan schedule and specify that fans are licensed to bear the AMCA seal for air performance. This ensures that your fan application performance/energy intent is met.
  2. Consider total cost of ownership as well as first cost to economically justify fans that use lower brake horsepower.
  3. Reputable manufacturers will provide information and tools to help you comply with the minimum code requirements. And, in many cases, economical products will be available that exceed the code minimum.

Tim Mathson, principal engineer, has been with Greenheck for 25 years. He is a member of ASHRAE TC 5.1, AMCA Fan Committee, AMCA Air Movement Engineering Standards Committee, and is chairman of the standards writing committees for AMCA 210/ASHRAE 51 and AMCA 301. Anthony (Tony) Rossi, vice president of marketing, has been at Greenheck for 10 years. Rossi is a member of the AMCA Marketing Board and ASHRAE TC 9.10 Laboratory Systems, and has served on both ASHRAE and ARI standards development committees as well the Education Committee Chairman for ASHRAE Central Indiana.



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