Fan Selection & Application Guide

The purpose of this post is to provide an explanation of the various parametric attributes used to describe the AC and DC fans sold by Digi-Key, explain distinctions among different attribute values of a qualitative nature (such as bearing type) and offer guidance on common considerations involved in choosing a fan to meet a given need.

The topic encompasses a fair amount of information, and becomes unwieldy to navigate as a single page. The remainder of this one is dedicated to basic descriptions of the parametric attributes. Where discussion at greater depth is warranted, a post doing so is linked from the relevant attribute description. These related/subsidiary posts are also linked at bottom.

Fan Attributes

Voltage-Rated

This attribute reflects the nominal input voltage for a device; it describes both an intended operating condition for which a device was designed, and typically also a test condition used in the process of measuring other listed performance characteristics. Unlike similarly-named attributes in other contexts, it does not represent a maximum permissible value. Instead, it more commonly represents a midpoint of a voltage range within which a device has been qualified for operation.


Above: Excerpt from 259-1567-ND datasheet with rated and operating voltage ranges highlighted.

Size/Dimension

As might be expected, the Size/Dimension attribute for fans communicates shape and dimensional information about a device. It describes the overall outline of a device in two dimensions, typically from a perspective looking into a device’s air intake or exhaust depending on the fan type. A number of preferred device shapes/sizes have emerged; devices in these preferred sizes are interchangeable or nearly so with regard to their mounting provisions, allowing different performance characteristics to be obtained through substitution with minimal mechanical changes to a system. Width (some might call it depth) information is treated separately, as the two information sets are often items of separate or distinct interest.

Above: Excerpt from 259-1567-ND datasheet with size/dimension information highlighted.

Width

The width attribute among fans describes a device’s overall size in the 3rd dimension that is not encompassed by the Size/Dimension attribute. It is treated separately as it is often an item of distinct or separate interest, and one which may vary as a result of differing performance characteristics among devices having compatible or nearly-compatible mounting features.


Above: Excerpt from 259-1567-ND datasheet with width information highlighted.

Air Flow

The air flow attribute for a fan describes the volume of air moved by a device under zero-pressure (sometimes called free-delivery) conditions when the rated voltage is applied. Typical values are shown, and actual results will vary based on various factors, particularly the applied voltage. It should be noted that this value represents an endpoint of an operating curve, listed static pressure values represent the other endpoint. The two values do not apply concurrently , and are usually not directly applicable in practice; most applications operate somewhere in between the extremes represented by these two values. Exact test methods may vary among manufacturers, and since the quantity is not an easy one to measure with high accuracy in the first place, it’s advisable to treat data shown for this attribute as approximate. See the Fan & System Curves post for additional related information.

Static Pressure

The static pressure attribute for a fan describes the maximum amount of air pressure that a device can produce under zero-flow, or plugged-outlet conditions. It represents the extreme of a device’s operating curve opposite to what the air flow attribute represents, and the two values do not apply concurrently ; neither is usually directly applicable in practice, since most applications operate somewhere in between the extremes represented by these two values. See the Fan & System Curves post for additional related information.


Bearing Type

The bearing type attribute for a fan indicates the means by which the rotating portion of a fan is mechanically supported. It has significant implications for operating life, noise levels, permissible operating positions, and other application concerns. A more detailed discussion of this topic can be found in the post on Fan Bearings & Longevity.

Fan Type

The Fan Type Attribute indicates a device’s general form factor or design concept; there are several basic patterns with different characteristic directions of intake and output flow, and different characteristic curve shapes. A more detailed treatment of this topic can be found here.


Above (L-R): Examples of a tubeaxial fan, centrifugal blower, and motorized impeller.

Features

The Features attribute enumerates auxiliary functions or equipment incorporated into a fan, with speed control and feedback mechanisms being the most common. More detailed descriptions of available fan features and their functions can be found in the post on Fan Features.

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Noise

The noise attribute for a fan describes the amount of sound produced by a device in operation, as characterized by the manufacturer. Though test methods may vary, common practice is to measure fan noise in an anechoic chamber under free-delivery conditions with rated voltage applied and the test microphone placed at a specified distance from the fan intake. Such an approach serves to characterize noise produced by a fan itself, in isolation from application variables; measured noise levels in practice will be influenced by the combination of a fan and the equipment in which it is installed. A diagram excerpted from the 259-1567-ND datasheet describing the manufacturer’s test setup is at below, and a more detailed discussion of noise-related considerations can be found here.

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Power (Watts)

The power attribute listed for a fan characterizes device power consumption under a set of manufacturer-defined test conditions, which commonly involve application of rated voltage and a free-delivery (zero static pressure) state. It may differ from a similar value printed directly on a device itself, and observed values will vary with actual application conditions. This variation results from the fact that not all points on a fan’s operating curve require equal amounts of mechanical input power, and that the amount of current drawn by a fan in normal operation may differ from the amount drawn under a locked-rotor (fault) condition. It is recommended that listed values be understood as approximations only, and that users characterize power consumption of a given device under their specific application conditions if power consumption is a matter of concern.

RPM

The RPM attribute for fans characterizes rotor rotational speed under a set of manufacturer-defined test conditions, which typically include application of rated voltage and a free-air (zero static pressure) delivery condition. Actual values will obviously vary depending on any speed control input applied, as well as with the actual supply voltage and the static pressure against which a device is delivering airflow. Broadly speaking, lower operating speeds tend to correlate with reduced noise levels and longer operating lives, as well as need of a physically larger device to provide comparable airflow to a device operating at a higher rotational speed. Note that in context of AC-input fans, this figure is typically specified for a 60 Hz input, and operation from a 50Hz source will likely result in a 15-20% reduction.

Termination

A fan’s termination attribute describes the manner in which power and any applicable signal connections are made, usually in terms of a number of conductors present and/or the basic style of any connector used. A few examples of common termination styles are pictured at right, and a more thorough discussion of the available options can be found here.


Ingress Protection

A fan’s ingress protection rating characterizes it’s resistance to environmental factors such as intrusion of solid bodies and fluids, usually according to the criteria established under IEC standard 60529. It should be understood that these ratings apply to the electronic/motor structure portions of the device only and with regard to entry into the device itself; they to not transfer to the system or environment in which a fan is used.

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Above: Example of a fan lacking an IP rating. Note the exposed solder connections and circuit board in the motor structure.

Operating Temperature

The operating temperature attribute for a fan indicates the range of ambient temperatures over which a device has been characterized to be operable without risk of near-term malfunction or damage. Occasionally, a wider storage temperature range will be listed by the manufacturer also, describing the maximum temperatures to which the device should be exposed while in a non-operative state. It should be noted that the expected service life of a fan will be strongly affected by the temperature of a fan’s operating environment, which is essentially the air that is being moved.

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Approvals

The approvals attribute for a fan enumerates the entities from which a device has received recognition, certification, acceptance, or bureaucratic blessing by some other name in accordance with the schemes set forth by the enumerated entities.

The image at right is of Digi-Key P/N 1053-1205-ND, with the approval marks this product has obtained highlighted.

Related Posts

Fan Types
Fan Features
Fan Bearings & Longevity
Fan Terminations
Electrical Interfacing, Monitoring, and Control
Fan & System Curves
Suggested Resources for Fan Selection and Appliation