Fans and Blowers: Getting the Right Fit

Fans and blowers are at the heart of a facility's mechanical systems. Their commercial, institutional and industrial uses are many, including exhaust, supply, recirculation, filter, process and gravity applications.

Specifying the right fan depends on a careful consideration of three key factors: speed, airflow and static pressure. There are, of course, other issues, such as the need to weigh cost against efficiency, and also, concerns about noise levels. But above all, fan specification is about determining the appropriate fan model for the unique requirements of each project.

There are many ways to classify fans by type, but for all applications, whether located on roofs, in walls or in ducts, there are two basic choices: belt-driven and direct-drive.

A major advantage of belt-driven exhaust and supply fans is that the airflow can be more easily modified for system balancing. Belt drives operate with pulleys that can be changed to adjust the fan speed.

According to fan manufacturer Greenheck, Schofield, Wis., belt-drive fans are better suited for air volumes above 2,000 cubic feet per minute (cfm) or static pressures above 0.50 in. water gage (wg). Adjustable pulleys allow fan speed and cfm to be adjusted by about 25%. Also, fans that operate under high temperature—above 120ºF—are almost always belt-driven.

The advantages of direct-drive fans is that they don't use belts, which can break. Consequently, they are appropriate for areas where maintenance is difficult. And while these fans do not allow the flexibility of changing fan speed by adjusting pulleys, they can be fitted with inexpensive speed controllers, which operate like a dimmer lighting switch and are normally available for models up to 1/3 hp at 208 volts. Direct-drive fans are economical for low volume (2,000 cfm or less) and low static pressure (0.50 in. wg or less).

In choosing a fan model for a particular application, the specifying engineer has another choice to make: propeller vs. centrifugal.

Propeller fans provide an economical method to move large air volumes—greater than 5,000 cfm—at low static pressures of 0.50 in. wg or less. Motors for propeller fans are normally mounted in the airstream, which limits applications to relatively clean air and a maximum temperature of 110ºF.

Centrifugal fans are more efficient at higher static pressures and are quieter than propeller fans. Many centrifugal fan models are designed with motors mounted out of the airstream to ventilate contaminated and high-temperature air.

Specifying the appropriate equipment is only part of successful incorporation of fan technology into a mechanical design. Noise control is always a major issue with any HVAC system. Oftentimes, the key to controlling fan noise levels in a building is the use of non-critical spaces such as halls and corridors as a buffer between fan and blower locations and areas where controlling noise level is crucial—auditoriums, conference rooms and the like.

The best way to control duct-borne noise and vibration from fans and blowers is to avoid it in the first place. This requires specifying quiet fans, designing proper outlet conditions and choosing appropriate isolators for fan and blower motors.

When considering the load on a fan motor, keep in mind that motor load does not tell an engineer anything about the air volume being handled. Any new installation should have the motor horsepower, air volume and static pressure checked at several places.

The Air Movement and Control Association International Inc. is the primary organization for testing and rating air moving equipment and systems. For more information, visit the AMCA International web site at: www.amca.org