AMCA Supplement: Understanding Fan Curves
Pressure Volume CurveThe most important characteristic of a fan or a system is the relationship that links the primary variables associated with its operation. The most commonly used fan characteristic is the relationship between pressure rise and volume flow rate for a constant impeller speed (rpm). Similarly the relationship between pressure loss and volume flow rate is the most commonly used system characteristic.
Fan pressure rise characteristics are normally expressed in either total pressure (TP) or static pressure (SP), with static pressure being the unit most commonly used in the United States. The fan volume flow rate (airflow) is commonly expressed in cubic feet per minute, or cfm. Therefore, the system pressure loss and volume flow rate requirements are typically expressed as a certain value of static pressure (SP) at some cfm.
The fan “pressure-volume” curve is generated by connecting the fan to a laboratory test chamber. By following very specific test procedures as outlined in the Air Movement and Control Association (AMCA) Standard 210, data points are collected and plotted graphically for a constant rpm, from a “no flow” block off condition to a “full flow” or wide condition. Figure 1 represents such a curve that is typical for a vaneaxial fan, and is commonly referred to as a “static pressure” curve.
Point A represents the point of zero airflow on the static pressure curve. It is frequently referred to as “block off,” “shut off,” “no flow,” and “static no delivery.”
Brake Horsepower CurveHaving established a SP and an airflow (cfm), an operating brake horsepower (bhp) can be established (Figure 4).
By adding the bhp curve to the static pressure curve from Figure 3 we complete the fan performance curve. To determine bhp simply extend vertically the cfm point established in Figure 3 until it intersects the bhp curve. Draw a horizontal line from this point of intersection to the right to the bhp scale to establish a bhp of 7.27 units, which corresponds to the previously established performance of 4.71 cfm units at 3.0 SP units.
Similarly, we can add the bhp curve to the static pressure curve of the backward inclined centrifugal fan from Figure 2 to complete that fan performance curve (Figure 5).
Even though the performance curves for the vaneaxial fan and the centrifugal fan have completely different shapes, the curves are read in the same way. Locate some unit of pressure on the left hand SP scale (4 units) and project a horizontal line to the point of intersection with the SP curve. Projecting downward from this point of intersection to the cfm scale, we establish an airflow of 6.55 units. Now project vertically upward to intersect the bhp curve. Project a horizontal line from this point to the bhp scale and read a bhp of 6.86 units.
Operating PointThe operating point (point of operation or design point) is defined as the fan pressure rise (SP)/volumetric flow rate (cfm) condition where the fan and system are in a stable equilibrium. This corresponds to the condition at which the fan SP/cfm characteristic intersects the system pressure loss/flow rate characteristics.
Figure 6 illustrates this fan/system operating point using the centrifugal fan performance curve from Figure 5.
The system line is simply a parabolic curve made up of all possible SP and cfm combinations within a given system and is determined from the fan law that SP varies as rpm2. Another fan law states that cfm varies as the rpm. Therefore, we can also say that SP varies as cfm2. Note: Some systems have modulating dampers that will not follow this parabolic curve.
Sometimes a fan system does not operate properly according to the design conditions. The measured airflow in the fan system may be deficient or it may be delivering too much cfm. In either case, it is necessary to either speed the fan up or slow it down to attain design conditions.
Knowing that the fan must operate somewhere along the system curve, and knowing that it is possible to predict the fan performance at other speeds by applying the following fan laws:
- Cfm varies as rpm;
- SP varies as rpm2; and
- Bhp varies as rpm3.
These speed changes represent an example of fan control that can be accomplished through drive changes or a variable speed motor.
Another way to present an “operating line” is to add a damper, making the system the variable characteristic. By modulating the damper blades, new system lines are created, resulting in an operating line along the fan curve. This can be seen graphically in Figure 8.
Combining the fan control curve (Figure 7) with the system controlled curve (Figure 8) results in a fan/system controlled curve having an “operating region” as shown in Figure 9.