Selecting a fan can be fairly complex. This article outlines the basic process for matching a fan to a system:

1. Define the application.
2. Select the basic fan.
3. Determine fan size and operating-point location.
4. Identify method of fan control.

Define the Application

The first step in selecting a fan is to define the application and system operating requirements in detail. Consider:

• What the fan is supposed to do.

• The location of the fan equipment. If the equipment is to be located outside, what are the ambient conditions (wind, temperature, moisture)?

• The adequateness of space for maintenance and repair.

• Facility limitations concerning weight, electrical capability, noise, and vibration.

• Whether the fan will be ducted on the inlet, the outlet, or both.

• Whether the fan will need to be controlled to meet changing system requirements.

• Sound limitations, including inlet, outlet, and casing-radiated noise.

• The number of fans in the system and whether redundancy is required.

Select the Basic Fan

Housing, impeller, and drive type are the most important considerations in basic fan selection.

Housing — inline vs. scrolled centrifugal

A fan's housing usually is dependent on the design of the system and whether or not the inlet and/or outlet is/are ducted. For a ducted, ceiling-hung fan, for instance, an inline housing (Photo A) is optimal because of its straight-through airflow profile and ability to be ducted on either end. In a high-temperature application, such as 750°F continuous duty, a scrolled-centrifugal housing (Photo B) is the better choice because the motor, bearings, belts, and sheaves are located outside of the air stream. With an inline fan, drive components usually are located at least partially in the air stream.

Impeller — axial, centrifugal, or mixed flow

Once a fan's housing is determined, its impeller can be chosen. Two basic types of impellers are axial (propeller) and centrifugal (wheel). Axial impellers typically are used in high-volume, low-pressure systems, in which they are more efficient than centrifugal impellers. Centrifugal impellers typically are used in high-pressure applications, as well as applications involving high-temperature or contaminated air; they are less affected by poor inlet conditions and unclean air than are axial impellers.

A third type of impeller — mixed flow — is a hybrid of centrifugal and axial impellers. In most inline-fan applications, the mixed flow impeller is the best option. Mixed flow impellers are capable of higher pressures than axial impellers and greater airflow than centrifugal impellers. Also, at pressures in excess of 1 in., they have lower sound levels, better sound quality, and — typically — higher efficiencies than axial and centrifugal impellers.

Drive type — direct vs. belt

Once impeller type is determined, drive type can be selected. Although the industry trend is toward direct-drive fans (Photo C), belt-drive fans (Photo D) are not without their benefits.

Direct-drive fans require less maintenance because of fewer moving parts. Also, they are easier to control with variable-frequency drives (VFDs) and can be more efficient, as they do not experience frictional belt loss.

Belt-drive fans typically have more performance flexibility because they can be selected at any speed. They can be used in high-temperature- or contaminated-air applications, as the motor usually is located outside of the air stream.

With larger fans, motor cost can be a crucial factor in deciding drive type. Larger fans typically run at lower speeds. With belt-drive fans, one can accomplish those speeds with a standard 1,800-rpm motor by adjusting the size of the sheaves. With direct-drive fans, 1,200- and 900-rpm motors can be cost-prohibitive.

For low-horsepower applications, direct-drive fans with electronically commutated motors (ECMs) often are a much better choice than belt-drive fans. ECMs can be selected at a wide range of speeds and are significantly more efficient than other single-phase motors.

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