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EFI - System Effects

System Effects

How Do They Impact The Performance Of A Fan?

Fan performance curves and data show how much pressure and airflow a given fan will produce at a given speed.  This data is based on the system resistance (pressure) occurring away from the fan (i.e. the air entering and leaving the fan has a uniform velocity profile).

Most installations, however, have less than ideal inlet and/or outlet conditions, which create local airflow dynamics that are not predicted by the usual system resistance summation.  These local disturbances are extremely difficult to measure, and can change the whole relationship between the predicted fan curves and the predicted resistance curves.  These disturbances are erratic velocity profiles caused by the orientation and arrangement of the ductwork adjacent to the fan, by the fan accessories, and sometimes the fan itself.  They can all cause system effect error.

Fan performance curves are developed by fan manufacturers based on the testing of a prototype fan built to a theoretical design.  A model of the new fan is put through a full range performance test in a room set up as a laboratory built according to the method of test standard used by the fan industry.  This standard contains several test methods and is published by the Air Movement and Control Association International, Inc. (AMCA) as ANSI/AMCA Standard 210.  For many years, these have formed the basis of most fan performance certifications and performance warranties in the U.S, Canada and now internationally.  (AMCA is located in Arlington Heights, IL, Tel: (847) 394-0150; Fax: (847) 253-0088; e-mail: amca@amca.org; URL: http://www.amca.org).

The fan manufacturer will use the test data to plot the prototype performance curve, and then apply the fan laws to calculate the performance table for the complete line of fans to be built to that design.  Computer-generated selection data for that model of fan, in its various styles and at different speeds, will be published and the manufacturer will guarantee that data.

Real systems, as installed, vary somewhat from the theoretical system design.   Some variation is to be expected, but every effort should be made to ensure that airflow into and out of the fan is smooth and unobstructed.  The fan manufacturer cannot be expected to guarantee the performance of a poorly installed fan.

Systems seldom perform at the predicted design, as the remote resistances will often be somewhat different from what was predicted or calculated.  This variation, however, is usually expected and corrected by balancing the system following installation.   This is a normal design and startup procedure.

The real difficulties arise when the airflow patterns right at the fan inlet or outlet are irregular or obstructed.  When these local airflow dynamics are different from the standard test setup, then the installed fan performance will be deficient.   This is commonly referred to as "System Effect".  Unfortunately, awareness of such a problem usually occurs only after the fan is put into operation.

Knowledge of what can contribute to deficiencies in fan performance due to System Effect is invaluable in avoiding potential problems with an installation.

Two typical AMCA laboratory fan test arrangements are shown in Figure 1.   These are the typical test chambers where fan performance is certified.

System Effects resulting from inlet or outlet conditions different from the laboratory test arrangements have been recognised by AMCA and they have provided guidelines to show how to compensate for System Effect in AMCA Publication 201 (1990), "Fans and Systems".  There are correction factors that are based on the shape of a duct fitting, on its relative position to the fan inlet or outlet, and on the air velocity.  Some fitting arrangements are worse than others and the higher the velocity, the worse the effect.  Besides corrections for ductwork, there are also corrections for accessories and even for the fan itself.

The technique given in AMCA Publication 201 is to identify each type of arrangement needing correction with one of a set of System Effect curves on a nomograph as shown in Figure 2.

Figure 3 presents the System Effect curves assigned to various arrangements of fan enclosures or inlet obstructions.  An inlet positioned closed to a wall will result in a higher correction factor from the curves.

Figure 4 presents the correction curves for the various shapes and relative positions for single elbows located at fan inlets.  At the top is shown a compound elbow arrangement for which there is no published correction.  This arrangement causes a spinning swirl entering the fan and must be corrected with straightening vanes.

Figure 5 represents typical fan outlet elbow arrangements; it shows that some are preferred and others are discouraged.  Fan accessories can also be a source of predictable and unpredictable problems.

Generally speaking, fan inlets are more sensitive to System Effect error than are discharges.  Most problems arise because of space limitations, which deny a straight run of duct into or out of the fan.

Following the procedures given in AMCA 201, a total correction factor, shown as pressure, is added to the initial system curve just as if it were another resistance.   The fan speed is normally increased to provide the required additional pressure to compensate for the added dynamic resistance.

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Last modified: July 21, 2003