This is a subsidiary page to the Fan Selection & Application Guide, discussing the concept of characteristic fan & system curves; the relationship between pressure and flow generated by a fan, or the amount of pressure required to move air through a system at a given flow rate.
Air movers and the systems that they move air through both exhibit characteristic curves of static pressure versus airflow. The concept is similar to that of electrical resistance; as the amount of current flow through a resistor increases, so does the voltage drop appearing across it. Likewise, as the amount of airflow through a system increases, the pressure drop across the system increases also.
Unlike most resistors however, air movers and the systems that they move air through tend to have distinctly non-linear relationships between pressure and flow. As an example, the characteristic curve for a sample fan (FFB0912SHE) is shown in figure 1, with some annotations. The maximum pressure and flow points are the extreme left and right points on the plot respectively, and are commonly reflected in parametric data for a given fan. Note that the maximum for either pressure or flow occurs when the other is zero; listed pressure and flow figures for a fan describe opposite extremes of a device’s operating curve, and do not apply simultaneously.
A frequently-encountered feature in many fan curves is a flattened, level or positive-sloped region near the middle of the plot, often referred to as the “knee” in the curve. It corresponds to the operational region where airflow becomes detached from a fan’s blades rather than flowing smoothly around them. Such detachment results in increased noise levels, and because small pressure changes in this region can cause large changes in flow, the actual operating point of the system can be both difficult to predict and unstable, resulting in increased wear, mechanical vibration, and other undesirable effects. For these reasons, the preferred operating range for most fans is toward the right (free-flow) side of the knee in the curve.
Figure 1. A sample fan curve with annotations
In electronics applications, the relationship between the pressure P across a system and the volume rate of air flow Q through it tends to have a relationship of the form P=xQy , where x and y are characteristic “constants” of the system through which a fan is forcing air, with the exponent y typically having a value somewhere between 1 and 2, and usually closer to 2. For any given combination of fan and system through which air is being moved, the operating point is where the system and fan curves intersect–that rate of flow where the static pressure developed by the fan is balanced by the pressure required to move that amount of flow through the attached system.
The non-linear character of fan & system curves can have effects that are not very intuitive. A hypothetical system curve is shown in figure 2, along with the same example fan curve as used in figure 1 and curves for two of the same fan in series, two in parallel, and the same single fan operated at 80% speed, with operating points for each scenario circled. One might expect things to act in a roughly proportional way–that using two fans would give a person roughly twice the airflow, or that reducing fan speed would reduce airflow proportionally. While it does work out here that dialing back the speed of a single fan by 20% results in a roughly proportional reduction in airflow, doubling the number of fans used would only be expected to increase flow by 10 to 17%, as tabulated in figure 3.
Figure 2. Hypothetical system curve, and curves for a single fan, single fan at reduced speed, and two fans in both series and parallel configurations.
Figure 3. Expected changes in airflow through a hypothetical system in response to reduced fan speed and use of multiple fans.
This result comes as a consequence of the wide, flat knee region in the curve of the fan being used, together with a system pressure curve that rises relatively quickly and already falls close to the knee in the single-fan case. While use of a single fan would be viable here, there’s little margin for error or changes in the system, such as from filter loading. Using multiple fans in parallel simply extends the overall fan curve to the right, pushing the knee in the fan curve through the system curve; this would not be a good design choice. Use of two of these fans in series greatly improves the amount of margin between the system curve and the combined fan curve and might be a viable solution if the doubling of parts count and space requirements over a single-fan solution are acceptable, or the element of redundancy is desirable. A better solution however, might be to look for a different fan with a characteristic curve that’s better suited to the application.
Air movers can be mounted on the intake or exhaust side of a system, or both. As with most such design choices, there are advantages and disadvantages to each.
Figure 4: Fan pushing air into enclosure
(+) Fan operates in cooler air, extending its service life
(-) Close proximity of fan to filter reduces filter effectiveness and flow efficiency
(-) Less uniform airflow within application enclosure
Figure 5. Fan drawing air out of enclosure
(+) Improved filtration and flow through filter due to increased fan-filter distance
(+) Convenient for closed-loop temperature control based on exhaust air temperature.
(+) More uniform airflow within enclosure
(-) Fan is operating in a warmer environment: fan lifetime is decreased
Figure 6. Two fans, one pushing, one pulling.
(+) Redundancy: air flow doesn’t cease if one fan fails
(+) Fan speeds can be reduced while maintaining cooling effect (beneficial for fan life)
(-) Double the price and space required compared to a single fan