Greenheck Fan 240XP-CUb Combining Fan and System Dynamics, System Resistance Curve Sample problem

System Resistance Curve

Sample problem:

If a system is designed to move 1000 cfm at a resistance of 0.25 in. Ps, what static pressure would the fan have to overcome to produce 2000 cfm of airflow?

Solution:

Since static pressure varies as the square of cfm, we can solve for the new Ps (Ps2 ) with the following equation:

Ps2 = Ps1 x ( cfmcfm12 )2 = 0.25 in. x ( 10002000cfmcfm )2 = 1.0 in.

Referring to the figure above, this results in sliding up the system resistance curve from Point A to Point B.

For this system, it is impossible to move 2000 cfm at only 0.25 in. Ps. For any given system, every cfm requires a unique Ps. This series of cfm/Ps points forms a system resistance curve such as the one above. Once the system resistance curve is defined, changing the fan rpm will change the cfm and Ps simultaneously, which results in sliding along the system resistance curve.

Varying System Resistance Curve

Note: Physically changing the system will alter the system resistance. For example, closing a damper from 100% open to only 50% open will add resistance and increase the “steepness” of the system resistance curve. The same effect occurs as filters become dirty. The figure above illustrates this point.

Curve A defines a system that requires 0.5 in. Ps to move 1000 cfm. Curve B requires 0.75 in. Ps to move the same amount of air. This is typical of how a system reacts to increased resistance.

In this section, there are three key points to emphasize:

1.As airflow through a system changes, so does the static pressure.

2.For a steady-state system, operating points must lie on the curve defining that system’s cfm/Ps characteristics.

3.As the system’s resistive elements change, the steepness of the system resistance curve changes.

Combining Fan and System Dynamics

The previous two sections introduced fan curves and system resistance curves. This section will show how these relate to each other to provide an understanding of the way the fan-system operates as a complete entity.

Remember that a fan curve is the series of points at which the fan can operate at a constant rpm. Likewise, a system resistance curve is the series of points at which the system can operate. The operating point (cfm, Ps) for the fan-system combination is where these these two curves intersect.

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