To achieve optimum efficiency, an industrial fan manufacturer needs to size and design a fan so that its performance curve and system resistance curve intersect at an efficient point. If the system resistance curve is not accurately predicted, fan performance and operating efficiency are likely to suffer.
Unfortunately, not all industrial draft fans were manufactured using calculations that accurately reflect operating conditions. Also, many older systems have been subsequently modified, often in ways that adversely affect fan efficiency. Analyzing fan performance and taking steps to “right size” a system’s fan can correct these shortcomings and save hundreds of thousands of dollars, often delivering a complete ROI within a year.
Know Your Volumetric Flow Rate
When working on an industrial fan retrofit, it is imperative that the design engineer on the project has a good understanding of the volumetric flow rate needed for the process and the pressure requirements of the system.
If a fan’s volumetric flow rate is overstated, the fan can be made too wide so that it operates close to the peak of the pressure–volume curve, running the risk of unstable operation. And if pressure is overstated, a fan can require excessive dampering to reach its point of operation.
On the other hand, designing for too little flow rate or pressure will leave a fan short and unable to meet the draft requirements at process peak loads.
When to Tip or De-Tip
Often, the fan impeller can be tipped or de-tipped to enhance its performance characteristics. Tipping and de-tipping alter the effective diameter of the fan impeller without changing its effective width.
Tips, which add to a fan’s effective diameter, increase the overall pressure-generating capability of the impeller, change the point of rating on the fan curve, and generally increase the volumetric flow rate. However, it should be noted that since an impeller altered in this fashion will require more horsepower since it will perform more work.
If a fan is being dampered by 40 percent or more at maximum or peak loads, it is an excellent candidate for de-tipping (removing some of the blade tip to decrease the effective diameter). A smaller diameter will decrease the pressure-generating capability of the impeller so the damper to be opened more, lowering the required horsepower.
Changing Fan Blade Designs
A very effective method for altering a fan’s performance, as well as improving its efficiency, is to change the blade design of the impeller. The payback can be rather immediate thanks to the savings in power consumption.
Typical blade designs include radial, radial tip, forward curved, flat backward inclined, backward curved, and airfoil. The static efficiency is different for each of these designs, with radial blades being the least efficient and airfoil blades being the most efficient. Radial fans typically require 15 percent more power than airfoil fans to do the same amount of work.
Often, a more efficient wheel can be designed to fit into an existing housing with little to no modifications to the surrounding system. Or, if the housing is not suitable, it may be possible to retrofit a new housing onto the existing foundation and connect it to the existing ductwork.
These are some basic strategies for right-sizing industrial fans. To learn more, download the article “Right-Sizing Mechanical Draft Fans” from our Learning Center. Or, if you have a specific project in mind, please call our air handling group at 1-888-663-2028.