Fan Efficiency Explained

As costs continue to increase for operations, maintenance, and equipment replacement, the importance of efficiency in industrial processes cannot be overstated. The cost per unit of production continues to be the measure of decision makers to assess their best capital investment opportunities in maximizing returns. In this blog, we’ll also explore why fans may not be operating at peak efficiency.

It is with this in mind that we turn to the analysis of fan efficiency; its calculation, sources of error, and ultimately its use as a parameter for making decisions.

Calculating Total Fan Efficiency

Generally, the efficiency calculation takes the form:

 

Fan Efficiency

Sources of Error

We use the term “total fan efficiency” to differentiate our parameter from the pseudo-efficiency values of “static fan efficiency” and “fan static efficiency”. Of these terms, only total fan efficiency accounts for the total mechanical energy that the fan imparts to the gas stream.

Frequently fan curves will be published based on laboratory testing under ideal conditions (e.g. low pressure, fully developed flow). However, it is rarely the case that ideal conditions are found in the field. We employ field testing to account for conditions on site, including system characteristics that directly affect the efficiency of a given fan. Note that the total efficiency in the above equation takes into account the compressibility of the gas under the conditions of actual fan operation, which can affect the calculated efficiency value by up to 10%.

A Decision-Making Parameter

When evaluating your fan, the fan curve can be a useful tool to understand why the efficiency may be low at the given operating point.

fan efficiency

Figure 1 shows a typical fan curve (green) with two numerated system lines (dashed), and the rated efficiency curve.  The “S” denotes the selection point for the desired performance.  In this example, the fan design was developed for system line 1, while testing reveals that the actual system resistance curve is represented by line 2.  The selection point is at the peak efficiency of the design, however, under normal operating conditions, the fan efficiency is approximately 10-20% lower than design.

When deciding on a fan upgrade to improve efficiency, performance, or wear life, we  design the impeller to the actual system and system characteristics to maximize these benefits and will restore or exceed the original design fan efficiency.

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