When selecting a fan for an application, it is common practice to use the manufacturer’s catalogue data.

Catalogued air flow performance data is created from test results conducted to standards such as ISO5801. Testing to a standard ensures accurate and consistent results between different laboratories. However a test standard specifies certain duct configurations that must be used in order to achieve this consistency. These standardised duct configurations ensure even air flow distribution onto the inlet of a fan, and a straightening of the air on the fan outlet.

When the air flow or velocity distribution is modified on either the inlet or the outlet of a fan, the performance of the fan can be affected. The effect of the inlet or outlet conditions on the fan’s performance is known as a system effect. It should also be noted that as air density affects fan performance (which is affected by temperature and altitude), the test results are always corrected to a standard density of 1.2 kg/m³ to ensure results from different laboratories and different atmospheric conditions give consistent results.

Non-standard conditions

When the fan inlet and outlet conditions differ from the standard test conditions, the fan performance curve should be modified to account for the system effect.

Publications such as AMCA 201-02 “Fans and Systems” or AIRAH DA3 “Duct Design” give estimates of “system effect factors” that may be appropriate for various conditions. While the list of system effect factors in these publications is limited, they do give an indication of expected performance reductions.

The system effect factor is effectively a pressure drop that is subtracted from the manufacturer’s fan performance curve to create the fan system curve. Where more than one system effect is present, the system effects should be added together.

While the fan performance curve is reduced, the power curve remains the same. Therefore if the fan performance is reduced due to a system effect, yet consumes the same amount of power, the fan efficiency also reduces.

Example

An AP0804CP6/25 fan is installed with a flexible connection immediately on the fan inlet. The design duty point is 8,000 L/s @ 200 Pa. It is assumed that the flexible connection bunches up when the fan is running, creating a 30mm air flow obstruction around the circumference of the fan. So how do we calculate the fan system curve caused by this bunched up inlet flexible connection?

AMCA 201-02 provides a table of system effect factors for inlet obstructions. This figure requires the percentage of unobstructed area and the distance of the obstruction from the inlet plane.

The fan area in this example is 0.502 m².

If the flexible connection bunches up by 30mm, the obstructed area is 0.43 m².

The ratio of these areas is 85.5%.

As the obstruction is at the inlet plane, the system effect curve is S-T. We will use curve “S” for this example.

System effect curve “S” is equivalent to a Dynamic Pressure Loss Coefficient C of 0.75.

To calculate the system effect factor, the formula below is used.

Where V is the obstructed velocity.

Obstructed velocity = Airflow / Obstructed Area = 8/0.43 = 18.6 m/s

Density (ρ) = 1.2 kg/m³

SEF = 0.75 × (18.6/1.414)² × 1.2

SEF = 156Pa

The above calculation has been performed at all points of the complete fan performance curve and the resultant fan system curve is shown in red in the below graph.

Table 2 - Dynamic Pressure Loss Coefficient C

Dynamic Pressure Loss Coefficient C
F	16.00
G	14.20
H	12.70
I	11.40
J	9.50
K	7.90
L	6.40
M	4.50
N	3.20
O	2.50
P	1.90
Q	1.50
R	1.20
S	0.75
T	0.50
U	0.40
V	0.25
W	0.17
X	0.10

Following the duct system curve until it intersects with the fan system curve provides the expected fan performance with a bunched up inlet flexible connection.

Fan manufacturers commonly encounter questions as to why site air flow and pressure measurements do not match the catalogued performance curve. One reason for this discrepancy is that the fan’s performance curve should always be modified to include all system effect factors.

Conclusion

Fantech publishes a “Do’s and Don’ts” in Section O of the Fans by Fantech catalogue. These “Do’s and Don’ts” are designed to minimise the effect of system effect factors and therefore allow the engineer to obtain as close to the catalogued fan performance as possible.

However, it must be acknowledged that installations are frequently tight for space and therefore the ideal installation conditions are often not possible. In these cases, an appropriate system effect factor should be determined so that the corrected fan system curve can be estimated.