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Fan Curve 101

Updated: Mar 8, 2022

Fans simply spin, but there's a lot more to it than what meets the eye.

The purpose of the fan is to accomplish dust/mist/fume collection out of all your workstations, otherwise known as the total system design air volume. To do this, it must generate enough pressure to overcome all pressure losses of the dust/mist/fume collecting system.

Total System Design Air Volume

Total System Design Air Volume is the sum of the air volume of all workstations connected to the dust/fume/mist collection system. It is measured in Cubic Feet per Minute (CFM). Typically, the manufacturer of the workstation will define what drop air velocity should be achieved for proper dust (mist, fume) collection and what the diameter of drop should be. The Air Volume (CFM) can be calculated as the Drop Area in square feet multiplied by Air Velocity (in Feet per Minute, FPM).

Total System Pressure Losses

Total system pressure losses can be calculated as the sum of all losses including workstation hoods, duct system, abort gate, dust collector filter, and return air (if used).

Typical hood losses are 4”…11” wc (the lower number for smaller, simple machines, the higher number for larger complex machines). Duct losses depend on duct diameter, length, number of elbows, and air velocity. They can be in the range of 4”…14” wc (the lower number for small, short systems, the higher number for larger and longer duct systems). Safety abort gate losses are published by manufacturers for a certain air velocity and they are generally in the range of 0.5”...2” wc (lower number for low speed and simple design, higher number for higher air velocity, and more complex design).

Dust collector filter losses are in the range of 1.5”…5 “ wc (the smaller number for new filters and well-functioning bag cleaning system, the higher number for dirty filters, and less effective filter cleaning. If the losses are higher than 5”, the filter bags should be changed).

The range for return air losses is 0.5”…1.5” (the lower value for properly sized short clean air return, the higher number for a longer duct, smaller diameter duct, and with add-on filter).

Fan Curve

The fan curve defines the relationship between the two most important fan values – Air Volume and Pressure. Additional information typically includes fan power (in BHP) – i.e. the necessary fan motor mechanical power to drive the fan -- and fan efficiency.

The fan can be operated only within a certain combination of air volume and pressure – i.e. it has to be on the fan curve. See Figure 1.

Situated completely on the left side of the fan curve is the operating point where air volume is zero (i.e. no airflow) and pressure is high. This is not a point for practical use – you can imagine this operating point as if you would cover the fan inlet with sheet metal and measure what pressure the fan produces).

Another impractical point is on the right side of the curve: the fan pressure is zero, the air volume is maximal. This situation can be only measured in a lab without the fan connected to the duct system (because the air would not move in the duct if the pressure is zero).

There are two additional limitations where the fan cannot be operated: the unstable fan area and the area under total system pressure losses.

Unstable Fan Area

Should you try to operate the fan at the left side of the pressure peak (i.e. low air volume and high pressure), you will discover that the fan is mechanically unstable, noisy, and vibrating. The fan could mechanically disintegrate if operated like this, or at a minimum, it will wear quickly. You also risk voiding the manufacturer’s warranty.

If you check the fan curve, you will discover that for certain pressure values (in the unstable area) there are two air volume values – meaning the air volume is not stable; instead, it is oscillating in a range of air volumes, and causing vibration of the whole fan.

The Area Under Total Pressure Losses

As explained at the beginning of this article, the fan must overcome the total pressure losses of a given dust (mist/fume) collecting system to achieve sufficient airflow. Therefore, a fan generating pressure that is lower than the total losses cannot be used for that particular dust collecting system.

Figure 1: The fan cannot be operated in the unstable area on left (A), or at the bottom of the chart where pressure is under total pressure losses for a given system.