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Understanding Mechanical Draft Fan Efficiency Ratings

While there is a lot of talk about draft fan efficiency improvements, we often lose sight of how efficiency is derived and defined. Moreover, RFPs often call for efficiency ratings without stipulating qualifications and constraints. As a result, competing fans are evaluated according to incongruous efficiency ratings and projects are awarded based on what appear to be superior efficiency ratings without fully understanding how the ratings were derived.

Types of Draft Fan Efficiency

Mechanical draft fan efficiency can be thought of in a number of different ways, including total efficiency, hydraulic efficiency, volume efficiency, mechanical efficiency, and static efficiency, all of which are calculated values.

A fan’s total efficiency is defined as the ratio of theoretical air horsepower (AHP) to the actual brake-horsepower (BHP) input to the fan shaft. The equation that describes fan total efficiency can be expressed as:

Ƞt = (AHP/BHP) X 100

Losses between AHP and BHP can be attributed to skin friction, turbulence, leakage, and mechanical friction. Total efficiency can therefore also be expressed as a culmination of several other types of efficiency — hydraulic, volumetric, and mechanical efficiencies.

Ƞt = Ƞh x Ƞv x Ƞm

Ƞh = Hydraulic efficiency

Ƞv = Volume efficiency

Ƞm = Mechanical efficiency

Hydraulic efficiency accounts for the imperfection of the flow path. Volumetric efficiency takes into account leakage through shaft seals and recirculation around the inlet cones and fan casing. Mechanical efficiency represents mechanical losses in the bearing, coupling, and seals in a fan system.

Total efficiency can also be used to calculate another important rating, a fan’s static efficiency, which is defined as the ratio of fan static pressure (FSP) to fan total pressure (FTP), multiplied by the fan total efficiency.

Ƞs = Ƞt (FSP/FTP)

FSP = Fan static pressure

FTP = Fan total pressure

Misperceiving Draft Fan Efficiency

With so many different efficiency ratings possible for mechanical draft fans, it’s easy to end up with a series of proposals that use disparate calculated values when an RFP doesn’t clearly stipulate which efficiency ratings matter most.

For example, say an RFP requests an efficiency rating but didn’t specify which type of efficiency rating will be used to assess proposals. Two proposals come back, one of which delivers a total efficiency rating; the other a static efficiency rating. Otherwise, the plans are identical.

Inevitably, the one that calculates for total efficiency has a higher number while the one that documents static efficiency records a lower efficiency rating. Although the proposal with the higher efficiency rating may appear better, the higher-rated fan will require the exact same horsepower as the lower-rated one since they are essentially the same fan. In other words, both fans would be just as efficient as one another despite the disparate efficiency ratings.

What really matters is how much horsepower is required to drive the fans in question because that directly correlates to power consumption, which, in turn, affects operating costs and the bottom line.

Determining Which Efficiency Ratings Really Matter

Understanding the efficiency ratings used to evaluate mechanical draft fans will undoubtedly help plants and engineers better define their requirements. This will help them receive better proposals from manufacturers — ones that are truly efficient.

For a more detailed and comprehensive analysis of common draft fan efficiency ratings, please download the article “Decoding ‘Efficiency’ for Mechanical Draft Fans” from our Learning Center. The article defines each of the prevalent efficiency ratings and offers some thoughts about which ones really matter. Determining which are the most important, though, will ultimately be the outcome of a larger industry-wide dialogue. We hope you’ll join us in the conversation!