It’s a question we get fairly regularly.
All you need to slow down the the fan on your dust collection system is a VFD, right? So simply installing the VFD will slow down the fan and therefore save electricity, right?
Yup, that’s right – installing the VFD will slow down the fan, which will reduce its energy use. However, this will also reduce the air volume moving through the system, and thus decrease the amount of suction at all drops. Unless your existing system is dramatically oversized – more air volume than necessary, and higher air velocities than needed – this will create a health and safety hazard.
So what can you do to save money on dust collection without compromising health and safety?
High Level View of a Dust Collection System
Let’s take a quick step back to so that we can build up a more complete picture. Here’s an idealized version of a dust collection system:
Simple Dust Collection System Overview Diagram
Let’s assume that this system is designed well, and that if the fan is running at full speed all workstations have adequate suction. Of course, this is not always the case in practice – even in systems that are well-designed the day they are installed, machines are moved and additions over time will generally degrade performance. But for the rest of this article, let us assume that we are dealing with a well-designed, well-performing system.
The relationship between the fan power and air volume is governed by the Affinity Laws (also known as the “Fan Laws”):
The Fan Law describes the relationship between the fan power and the air volume moved by the fan.
When running the system at full speed, we are all the way in the top right corner of the graph: 100% fan power, and 100% air volume.
Slowing the fan down using a VFD will move our operating point down down and to the left along the curve. This is intuitive – there is no way to reduce the fan power without also reducing the air volume.
So Why Can’t I Just Add a VFD?
Now it’s obvious why just adding a VFD alone won’t be able to save any energy – it would reduce the system air volume, and thus decrease the air volume and suction at all drops. Unless your system is dramatically oversized for your machines’ requirements, this will compromise the function of the dust collection system.
OK, so in order to reduce electricity use we must also reduce the total air volume. Can we somehow reduce the total air volume without compromising system performance?
The good news is that in the vast majority of factories, not all machines actually need suction at the same time. Here’s a representative example (showing real data from a US-based furniture manufacturer) of how much different workstations are used:
Workstation utilization at a US furniture manufacturer.
This means that the vast majority of the time, the air volume that is actually required by the entire system is significantly less than the design air volume – the air volume needed when all workstations need suction at the same time.
So, to answer our previous question: YES, we can reduce the total system air volume without compromising system performance – if we can somehow direct air flow exactly to the machines that need it.
How could we do that?