How Do You Evaluate Efficiency for Mechanical Draft Fans?

Efficiency for Mechanical Draft Fans

Efficiency is the name of the game for mechanical draft fans. An efficient fan is a productive one, and a productive fan is one that doesn’t increase costs and hurt a plant’s bottom line.

There are several types of fan efficiency ratings. Not all are created equal.  It helps to understand how efficiency ratings are derived and how you can evaluate your plant’s mechanical fan systems to determine how the system is doing its job.

Defining Efficiency

Efficiency is the buzzword in industry today. We want to be more efficient in every way, such as with our air handling systems. But what does efficiency mean?

Efficiency is a calculated value that is the ratio of theoretical air horsepower (AHP) to the actual brake horsepower (BHP) input to the shaft of the fan. In an ideal world, you could divide AHP by BHP and multiply by 100 to get 100%. But no draft fan system is ever 100% efficient. There are losses caused by a variety of factors, such as leakage, friction (skin and mechanical), and turbulence.

Because of these factors, we can calculate total efficiency by multiplying several other efficiency ratings, such as:

  • Hydraulic efficiency
  • Volume efficiency
  • Mechanical efficiency

Multiplying these factors together creates a total efficiency rating. Once you have total efficiency, you can calculate the static efficiency, which is a ratio of fan static pressure (FSP) to fan total pressure (FTP), multiplied by the fan total efficiency.

It is important to note the difference between these two efficiencies. Fan total efficiency gives a higher number while static efficiency calculates a lower number. Paradoxically, a calculated higher efficiency does not necessarily demand a lower horsepower motor. Motor horsepower requirement for a given fan, at a given condition, stays the same.

What Are Acceptable Efficiency Ranges?

Acceptable efficiency ranges vary by the type of fan system you have in your plant.

For radial fans, you have straight radial blades and radial tip blades. Straight radial blades have the lowest efficiency ratings, from 65-72%, thus they require the highest horsepower demand. Radial tip blades have 72-78% efficiency and have lower horsepower demand than straight radial blades but higher demand than some other types.

Forward curved fans have 72-76% efficiency ratings and moderate horsepower demand.

The third type, backward inclined fans, can be divided up into three sub-types: single thickness backward inclined flat (79-81% efficiency), single-thickness backward curved (80-82% efficiency), and dual thickness airfoil (83-88%). Dual thickness airfoil backward inclined blades have the highest efficiency ratings of all mechanical draft fan types and therefore have the lowest horsepower demand.

Is Efficiency the Be-All Metric for Projects?

The truth is, evaluation of mechanical draft fans based on “efficiency” may not offer the desired benefit for a project. There are other factors that come into play, and after all, an apparent higher efficiency ratings may not contribute to lower input power to the fan.

That’s why we consider efficiency in the context of total input power to the fan. This puts the focus where it should be – the fan’s bottom-line impact on a plant’s resources – so that plant managers can make informed decisions. Whenever we custom engineer a mechanical draft fan system, we strive to maximize a plant’s resources and minimize power costs, and we don’t just focus on efficiency for efficiency’s sake. At the end of the day, lower input power will result in better economic benefits for the plant.

ProcessBarron designs and constructs custom mechanical draft fan systems for plant air handling needs. Contact the team to discuss your project or facility.

Article written with specific contributions from Nurul “Moni” Talukder, Mechanical Draft Fan Lead for ProcessBarron.