Selecting fans to run in parallel

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Parallel fans are often used where space restrictions occur.

When two identical fans are run in parallel, double the airflow of one fan is achieved at the same pressure. However, this article explains how this is not necessarily the case for fans in ventilation systems. Two or more fans in parallel are commonly used when a larger diameter fan is not possible due to space restrictions. A typical application being a car park ventilation system where a larger fan would intrude in to the car park, such that some of the potential parking space has insufficient head room to accommodate a car.

For fans that have a non-stall characteristic, parallel operation is simple and predictable. Such fans would include backward inclined centrifugal and axial fans with a low blade pitch angle.

The fan curve for two identical fans in parallel is shown in Figure 1. You should note that even though two fans give double the airflow at the same pressure, the system curve (where pressure alters proportionally to a change in air-volume squared) will mean that in practice two fans will deliver considerably less than double the flow of one fan. In Figure 1, a single fan will deliver approximately 1.9 units of flow and two fans approx. 2.5 units of flow – that is, two fans in parallel will actually deliver approx 32% more flow than a single fan due to the increased system pressure.

Figure 1. Curve for two fans in parallel

The performance as shown in Figure 1 is considered satisfactory, as the fans are operating at a stable point. As previously stated, parallel fans give twice the flow of a single fan at the same pressure. If the single fan has a stall characteristic, as shown in Figure 2, then the single fan can deliver more than one air-volume at a given pressure - and two fans will deliver either twice of any of the volumes or any combination of one volume and another at that pressure. Here we see the situation becomes interesting and somewhat complicated. Figure 3 shows the performance of a fan with stall characteristic (not unlike an inversed J curve) for a single fan and two fans in parallel.

System curve ‘S’ has stable operation, with two fans in parallel delivering approximately 14.2 m3/s at approximately 112 pa and a single fan approximately 8.9 m³/s at approximately 43 pa. This is a satisfactory outcome as the fan is stable. In the case of system curve “U”, the situation is unsatisfactory as the system curve intersects with the parallel curve at three points - Q1, Q2 and Q3.

By definition, a fan will always perform at the intersection of the system curve and fan curve. If there is more than one intersection, the fan will operate at one or more of these and will usually “hunt” from one to another, giving unstable operation with random changes in performance and noise level. So, the question arises, what is the best way to select axial flow fans for parallel operation? It is not possible to predict with certainty if fans are suitable for parallel operation but the following points may assist:

For axial fans, low blade pitch angles, around 15º to 20º (depends on where the blade is set), are nearly always suitable.

For higher pitch angles, selecting a fan at 50% of its peak pressure should be safe. Centrifugal fans are generally not a problem. To be absolutely certain for any fan type, drawing the curves as shown here will remove doubts.

Figure 2. Single-stage fan with stall type curve.

Figure 3. Curves of two fans, with stalling performance curve, running in parallel

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