The Effect Of System On Fan Performance

By Author: Oleg Tchetchel
Total Articles: 50
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Fans are typically tested and rated in prescribed test configurations defined by the Air Movement and Control Association. This is done to ensure standardized procedures and ratings so that system designers can make realistic choices among various manufacturers. Beyond the routine system resistance calculations, the location of some common components and their proximity to he fan inlet or outlet can create additional immeasurable losses commonly called System Efect. These losses, if not eliminated or minimized, will necessitate fan speed and horsepower increases to compensate for the performance deficiencies.

The term system refers to the path through which air is pushed or pulled. Since it can be any combination of ducts, heat exchangers, filters, etc., through which air flows, a system can range in complexity. The system can be as simple as exhausting air through an opening in the wall of a building, or as involved as a multi-zoned system with varying flows and densities. The effects of the system design on the actual performance capability of a fan represent separate and equally important considerations.

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... the typical process of system design, the performance requirements are calculated and then used to select the appropriate fan. However, in many cases the effects of the relationship between the system components and the fan are not considered in the calculation or selection process. For instance, the resistance of a given size elbow at a given flow can be easily determined using the equivalent length calculation method. However, if that elbow is located at the fan inlet or outlet, further immeasurable losses will be imposed in addition to the simple loss through the elbow itself. Most importantly, these losses cannot be measured or even detected with field instruments because they are, in fact, a destruction of the fan performance characteristics. Standardized testing and rating methods for fans have been established by the Air Movement and Control Association, (AMCA). The test methods are described in AMCA Standard 210, titled Test Code for Air Moving Devices. Specifying fan equipment tested and rated in strict accordance with AMCA Standard 210 is the best way to ensure accurate fan performance. However, the system effects that alter or limit the ultimate performance remain the most frequent causes of field performance problems. The four most common causes of system-induced performance deficiencies:

- Eccentric flow into the fan inlet.
- Spinning flow into the fan inlet.
- Improper ductwork at the fan outlet.
- Obstructions at the fan inlet or outlet.

Fans perform correctly when air flows straight into the inlet. Air should be drawn into the fan inlet with an evenly distributed velocity profile. If the air is not drawn into the fan inlet evenly, performance deficiencies will result from the combined effects of turbulence and uneven air distribution. When the system attempts to change the direction of flow, the air hugs the outside of the inlet elbow entering the fan. This causes uneven, turbulent airflow into the fan.

may also increase slightly, but far less than indicated by the increased power consumption. The evaluation and control of pre-spinning flow is more difficult than eccentric flow because of the variety of system connections or components that can contribute to pre-spin. Pre-spinning flow can result from any number of common situations. Two elbows in close proximity to one another can force the air to make consecutive turns in perpendicular planes to form a corkscrew effect.

The ideal fan inlet connection creates neither eccentric nor spinning flow. Where an inlet duct is required, the best connection is a long straight duct with straightening vanes. However, it is usually necessary to adapt the system to the available space. When space becomes the limiting factor, two choices are available:

- Install corrective devices in the duct.
- Increase fan speed to compensate.

The first choice is preferable, but the second is often necessary. In many cases, the corrective devices themselves will represent some resistance to flow. A combination of both choices could be necessary to correct extreme field performance problems. If the available space dictates the need for a turn into the fan inlet, a standardized inlet-box design, with predictable losses, should be used whenever possible.

The connection made to a fan outlet can affect fan performance. An outlet duct ranging in length from 21/2 to 6 fan wheel diameters, depending on velocity, is necessary to allow the fan to develop its full rated pressure. If the outlet duct is omitted completely, a static pressure loss equal to one half the outlet velocity pressure will result. The system resistance calculation should include this loss as additional required static pressure. Air is not discharged from a fan with a uniform velocity profile. The main reason for this is the fact that air has weight and is thrown to the outside of the scroll. In a duct with a uniform cross-section, the average velocity will be the same at all points along the duct. However, where velocity distribution changes (such as the duct adjacent to the fan outlet) the velocities are not typically the same. Since velocity pressure is proportional to velocity squared, the average velocity pressure at the fan outlet will be higher than the average downstream. Since total pressure will be virtually the same, the static pressure cannot be fully developed until some point 21/2 to 6 duct diameters downstream. Although duct turns directly at the fan outlet should be avoided, there are times when they cannot. In such cases, the turns should follow the same direction as the wheel rotation. Turns made in the opposite direction of wheel rotation can have a pressure drop beyond normal system calculations. Usually the drop is between .5 to 1.5 fan outlet velocity pressures.

AMCA Publication 201 - Fans and Systems, presents an in-depth discussion of system effect and provides methods for estimating losses associated with many common situations. If system effect situations cannot be avoided, their impact on performance should be estimated and added to the calculated system resistance prior to sizing or selecting the fan. Ignoring the system effect could lead to difficult field performance problems later. It could be that the installed fan does not have the necessary speed reserve, or the motor is not of sufficient brake horsepower. The cost of correcting such a field performance problem could escalate rapidly. System designers need to carefully consider the system effect values presented in AMCA Publication 201. By accurately defining the true performance requirements of fans in installed systems, field performance problems can be reduced significantly.

Oleg Chechel
Industrial Air Systems Engineer
CB Blower Co.
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