Fan Efficiency and Fan Types

 
In recent months, there has been an increased curiosity about fan efficiency.  This is a normal reaction to word that the DOE is going to start regulating fan efficiency.  We have also discussed the work that is being done to get fan efficiency levels specified in the ASHRAE codes, the IGCC codes, and other places.  The specifications and regulations for fan efficiency will eventually include not only the peak efficiencies available, but also a requirement to select the fan within a certain range from the peak efficiencies.

Ultimately, the goal is to reduce the energy consumption from fan usage, whether that comes from proper fan design, proper fan selection, proper system design and installation, or proper maintenance.  In order to understand what is being regulated, we must first start with a basic understanding of what fan efficiency is, how it gets used, and how different fan types will result in different fan efficiency levels.


Let’s start with a couple definitions – static efficiency and total efficiency.  These definitions are unique to the fan industry.  Fan static pressure is best described as a calculated value, obtained by the following formula:

Fan Static Pressure = Fan Discharge Static Pressure – Fan Inlet Total Pressure

There is no universally accepted explanation of why the inlet pressure in the equation is based on total pressure.  A couple of the most common explanations are – in the ‘early days’, fans were tested with a discharge duct, so the inlet velocity was small compared to the discharge; and in recent decades, many tests are run on test chambers where the velocity in the test chamber is low (say, 300 fpm), so the inlet velocity pressure is also very low (0.0056”), and can be ignored for all practical purposes.

In any case, the formula for efficiency is:

Efficiency = Flow * Pressure Rise / Power / 6356     (in I-P units)

You can see that the more efficient the fan is (lower power at the same flow and pressure), the higher the static efficiency (SE).  The same may not be true for total efficiency (TE).

Total Efficiency = Flow * Total Pressure Rise / Power / 6356

When you look at a list of possible fan selections for a given flow and pressure, the total pressure rise can vary widely.  The discharge velocity pressure gets added to the discharge static pressure to create the discharge total pressure.  So, fans with high discharge velocity frequently have a higher total efficiency.

Here is an example for a simple wall fan:

10,000 cfm / 0.5” static pressure required
28” size – 42% SE – 1.89 bhp – 70% TE
36” size – 55% SE – 1.44 bhp – 68% TE

This example shows that selections based on total efficiency alone may not be the best solution if the goal is to save energy.  There may be many legitimate reasons to select the smaller fan (lower cost, lower weight, size restrictions, etc.).

Another common discussion relates to the different types of efficiency curves inherent in different fan types.  For the sake of our discussion, we will limit it to three types – backward curved centrifugal fan, tubeaxial fan, and vaneaxial fan.

Typical fan curves for each type as shown below.  Curve 1 is a backward curved centrifugal fan.  You can see that the static efficiency curve peaks at about 80% SE at 13,500 cfm.  The corresponding peak TE is just over 83%.  If this fan is used at 16,500 cfm, the static efficiency drops to 70%, and the total efficiency drops to 77%.  The high efficiency band is quite wide.


A tubeaxial fan (Curve 2) is normally designed for lower pressure applications, and has a completely different efficiency curve.  At 13,500 cfm on this fan, the efficiencies are 58% SE and 73% TE, respectively.  At 16,500 cfm, those values change to 48% SE and 74% SE, respectively.


The vaneaxial fan (Curve 3) has a peak at just over 71% SE (81% TE), but the static efficiency curve drops quickly as you go further out on the fan curve.  The peak SE is at 13,500 cfm, but if this fan is used at 16,500 cfm, the SE has already dropped to 46%.  At the same time, the TE is still at 73%.  Depending on what type of efficiency is being used while using this fan, it can be thought of as a wide or a narrow efficiency band.


In summary, moving the selection point from 13,500 cfm to 16,500 cfm changes the efficiency values by the following table:

     Fan Type                                    Power Change               SE Change          TE Change
     Backward Curved Centrifugal       11.2% increase             -10 points            -6 points
     Tubeaxial                                   6.0% decrease              -10 points            +1 point
     Vaneaxial                                   21.3% decrease            -25 points            -8 points

If you are looking at just SE, the backward curved fan looks very good at 13,500 cfm.  But if your system curve moves to the right, the required power actually increases.  The tubeaxial and vaneaxial both decrease in this scenario.

The moral to the story is that specifying either SE or TE, and expecting to get the best fan for the application, is a good way to potentially increase your customer’s power bill.  Only an analysis of all the options available will provide the best overall value.