The two most important criteria when specifying a custom heavy duty industrial fan are efficiency and reliability. Air handling systems are increasing in complexity with the addition of more restrictive emission controls and air cleaning devices. While the results have been a cleaner environment, system pressure requirements have risen significantly. As these requirements increase, greater levels of energy are needed to force the air or gas through the system. Operating efficiency of the fan has become much more critical. Every dollar saved on the purchase of a less efficient fan will be consumed in additional operating costs. Maximum fan efficiency will not only have a positive impact on the bottom line, it will have a positive impact on the environment. Greater fan efficiency results in lower power requirements and fewer natural resources consumed. Efficiency also influences sound levels. The more efficient fan will run quieter than the less efficient fan. The quieter fan will need less sound insulating materials, or none at all.