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 a sum of 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 manufacturer 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).
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 air flow) 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 total pressure losses of a given dust (mist/fume) collecting system to achieve sufficient air flow. 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 at grayed unstable area on left, and in grayed are in the bottom of the chart where pressure is under total pressure losses for a given system.
What is the System Curve?
The System Curve is the air volume that is pulled through the particular duct system at a certain pressure. Or what pressure must be achieved to pull a certain air volume.
The fan cannot be operated where you see pattern fill. The remaining portion of the fan curve represents practical combinations of the air volume and fan pressure that can be used for a particular fan.
Fan Curve for Fan as Installed (System Effect)
The fan curve is generated by the manufacturer’s software or in a better case measured in the lab. Will the fan behave in exactly the same way in real world installation? The fan curve is measured under ideal laboratory conditions, but when the fan is connected to the duct system efficiency will drop. Mainly it is caused by non-ideal distribution of the air at fan inlet. Non-ideal air distribution can be typically caused by inlet and outlet duct elbows too close to the fan inlet and outlet (the effect is pronounceable if elbows are closer than three duct diameters from the fan inlet and the outlet).
The fan curve for the fan as installed is below the lab measured fan curve, resulting in decreased air volume and decreased pressure compared to lab measured values. See Figure 2.
Figure 2: the red fan curve "as installed" represents combinations of fan pressure and air volume for given fan that can be used.
Fan Inlet Box (Engineered Inlet)
A fan inlet box is a space saving option – it allows connecting the vertical duct directly to the fan inlet at the cost of decreased fan efficiency. If the fan is ordered from the manufacturer with a so called Fan Inlet Box (sometimes called an Engineered Inlet) the fan curve includes the system effect of the Fan Inlet Box, i.e. the fan curve with an inlet box is below the standard fan curve. The inlet box is saving space, but increasing pressure losses by 1”...2” w.c.- be sure that these losses are included in calculations.
Fan Curve Summary
Now we understand where on the fan curve the fan can be operated: on the fan curve corrected for system effect or inlet box and above the line that defines total system pressure losses and at the same time air volume cannot be in an unstable area for the fan.
The fan curve dictates what combinations of fan pressure and fan air volume are allowed for a particular fan. Generally, we can say that at lower fan pressure the air volume is higher. Or we can say that the fan motor power is divided between fan pressure and air volume. If for example we can by better design decrease duct losses, the same fan and motor can move a higher air volume (i.e. the same fan can be used for dust collection from more workstations). Better duct design means hoods with lower losses, shortest possible duct system, minimized number of elbows etc.
You can learn more about the fan curve and change of the fan operating point in this article.
Author: Ales Litomisky