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Here is what the PBER equation looks like.

The only input variables are CFM and pressure – the two variables that engineers start with

when they select a fan.  

It calculates a required minimum efficiency by multiplying three factors:

‐ One, a Target efficiency, determined by a regulatory body

‐ Two, a CFM factor, which reduces the efficiency required by smaller fans, exactly

the way that FEG does with the banana shaped curves

‐ and Three ‐ a Pressure factor, which reduces the efficiency required for low

pressure fans.

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Now here is a an example of an inline fan with a high fan efficiency – a mixed flow fan.

Note that the red balloon of allowable fan selections extends all the way to the maximum

fan RPM and down to about the system curve that is 15 points off the peak efficiency of

67%.  If you wanted to select this inline fan for 4000 CFM at one inch of total pressure,  you

are outside of the red balloon and you would have to pick the next larger size.

As a fan designer, your goal would be to create as large of an area as possible within this

red PBER balloon.  This would give you an advantage over your competitors by being able

to meet the maximum allowable horsepower with a smaller fan, and hence a lower price.

When we consider a metric based only on peak total efficiency, fan manufacturers could

game the system by maximizing peak total efficiency at the expense of decreasing fan

efficiency at selections points to the right of peak.  The PBER equation closes this loop hole.

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In this slide, I want to demonstrate the impact of applying the PBER equation onto a

product.

Here is a fan map of a typical square inline fan, which is a very popular low cost fan in the

marketplace that unfortunately has a low fan efficiency.  The solid black lines represent

pressure versus CFM from the lowest cataloged fan RPM to the maximum cataloged fan

RPM.  

The black parabolic curve is the surge line (no fan selections are allowed to the left of that

curve), the red parabolic curve is the peak total efficiency (54%) and the green parabolic

curve is 15 points off peak total efficiency (39%).  

Note that a regulation that required fan selections within 15 points of peak total efficiency,

you could select anywhere on this fan map from the surge line to the green 39% efficiency

line, bounded of course by the minimum and maximum fan RPMs.

The solid red balloon represents only that part of the fan map that selections would meet

the PBER requirement.  For this inefficient fan, this is a very small portion of the overall fan

map.  For fans over about 3600 CFM, you would be forced to a larger fan size.  For fans over

about an inch and three‐quarters of total pressure, you would need a more efficient fan

model.

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Here again is a fan map of a relatively inefficient fan similar to the one that I showed earlier.  

Through an iterative process traveling along the Best Efficiency Point system curve of 52%,

you would have to reduce the maximum allowable fan RPM until you just satisfied the

minimum required efficiency as required by the PBER equation.

Now, instead of the maximum allowable fan RPM being controlled by the fan manufacturer,

it would now be controlled and labeled on the product and in the catalog by the DOE.  The

DOE could not stop a customer from selecting this fan at 5000 CFM at a half an inch, but it

would eliminate all fan selections on the fan map between the heavy black line labelled

“Max Allowable Fan RPM” and the original maximum fan RPM.

Because our fans have efficiency curves that vary widely over their allowable selection

range, any independent metric enforced by the DOE will have a small impact on energy

savings.  But the DOE’s acceptance of the PBER metric would set the stage for the

application dependent regulations – energy codes, building codes and utility rebates – and

they could save significant amounts of fan energy.

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So, to summarize, there will be two important drivers for fan manufacturers to develop

more efficient fans if the PBER methodology were imposed through regulation:

Number one ‐ Natural market pressure.  The more efficient of a fan you make, the smaller

the size, and hence the lower first cost you will have in the market place.

Number two – over time, regulators will be raising the bar on efficiency and we will have to

increase fan efficiency towards “max tech”.

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Now here is how the Fan Efficiency Ratio could be used by various regulators.

The DOE could require the FER be greater than or equal to one at the Best Efficiency Point

at the Maximum Fan RPM.  This would be labeled on the product with an FER number

greater than or equal to one on the nameplate or on a separate label.  A purchaser could

compare different fans – the one with the higher FER would be more efficient and most

likely use less energy when applied.

ASHRAE 90.1  could demand that FER be greater or equal to one at the actual design point

of operation.  In this case, the fan with the higher FER would definitely use less energy.  The

ratio of FERs between two different fan selections would be inversely proportional to the

amount of energy actually consumed.  

ASRHAE 189.1, the green energy code, could demand an FER greater than or equal to 1.1,

which would save exactly 10% more energy than the base code – ASHRAE 90.1.

Utility rebates could specify any level of FER and could actually calculate kw‐hours per year

savings over baseline based on the actual FER, actual input kW and an assumed or

measured duty cycle.  

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