Question
I recently read an article about a shop with a 75 hp variable speed blower on its dust collector. As more blast gates were opened or closed, the motor rpm varied accordingly. This was stated to be a great energy saver since most of the time the motor ran at about 25% capacity.
Our collector will be 60 hp, which is more than we need most of the time. Is anyone familiar with such a system? I saw one at IWF a while back. It was on a new Dantherm unit. I am looking to put one on my used Torrit which will be installed this summer.
Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor J:
I thought about doing something like this with my smaller collector. You should be able to get a VFD that uses a static pressure sensor as an input device. Then program the drive to maintain a constant negative static pressure in the duct. With few gates open, the SP will be high, so the motor will be run slower. As more gates open the SP drops and the drive would speed the motor up. Is that similar to what is on the market? Or is anything (non-proprietary) on the market at all?
Setra makes a duct pressure sensor with variable output (either voltage or current) that can be used as an input for a VFD. They said that controlling fan speed is a typical application, as is measuring pressure differential across filters. Price ranges from about $320 (no display) to $470 (with display).
A VFD for my 15 HP blower would be in the $500 - $1,500 range.
I am no engineer, so if anyone has an idea about how much can be saved by going this route, I would appreciate hearing your thoughts. My napkin calcs show that If I average 30A @ 208V for 6 hours/day that is 37.5 KWH, or 750 KWH per month. At my $.086/KWH that is $65/month. If this could save 20%, the ROI would be about 5 - 7 years.
On the dust system: The manufacturer recommended a 10 HP motor. I forced them to drop it to 7.5 HP. Of course, that caused a huge inrush current (150 amp). We fixed that with a VFD and a SP analog input to help it stay in bounds. We set min and max power manually and allowed the VFD to move things around within that range as SP demands changed.
Now the bad part - cost.
ABB VFD $3,500
NEMA 12A enclosure with cooling and filter system $1,900
Sparky's time $12,000
Analog input $500
Endless time lost with the inspectors, insurance company, fire department and UL (you got it - don't forget them) $Unknown
Big joke was on me. The manufacturer tried to get me to buy their VFD model for 15% of my actual cost. I am okay with that because I learned a huge amount about all this and have applied it with amazing luck to things like a custom made variable speed 24" planer built around a Northfield monster. Great fun, high cost.
We calibrated the fan using three bits of data - CFM at the end of an 11' spiral duct 20' long; SP at a point 6 inches above the fan inlet; and amp draw on one leg of the 3 phase feed to the fan (not to the VFD). All that data convinced me that SP at the fan inlet is a good approximation of fan performance.
The manufacturer thought all we needed to do was use their fan table but I wanted some kind of field data for the insurance company. There was never more than 10% difference between the fan table, a "ductulator gizmo" (that round thing the installers carry around) or the field test.
In my practical experience, if the fan is operating at its designed SP, and you have used correct duct size calculation from any fan vendor, I have never seen a problem.
Make sure the SP is at or above your wide belt sander specs and that the CFM is at or above your planer specs. Both are measured right at the machine connection. I know the engineers will say you are measuring two aspects of the same thing, but I find low SP kills sanders and low CFM kills planers. I don't know why.
That is all the info I have by way of testing. The whole thing works great and is so quiet I forget to turn it off at the end of the day. For the eco-nerds, we cut power use by 15% at maxload (wider belt or planer) and 65% at low loads (two 4" pipes running together).
I talked to the Belfab engineers last week at the show. They run into this occasionally and suggested thicker gaskets on the doors or a partition in the middle of the unit to divert airflow from the opposing fans. I think weighting the dampers may help also. The last resort would be automatic gates on each 12” inlet. (Expensive.)
Other than this, the system has worked out good and saves electricity. We have high demand rates and are still considering a VFD system with auto gates. The DC is the only equipment in my shop without soft starts. The utility recently recorded my meter and determined the starting of the DC was pushing demand up more than the larger motors with soft start.
VFDs are an option, but I think I will try the system without them at first. They can always be added later. I wish I could get straight answers from my power company, but maybe I will have to push them harder.
Honestly the most successful systems at the least cost that we have out there for the past 46 years are simple shaker systems using Beane material and one fan, picking up several hoods using blast gates for on/off or balancing. We just had a customer replace their original bags after 26 years, on a shaker! You won't get that out of a pulse or cartridge system. Unbelievable. And the unit still performs very well in suction and clean modes. I think you should consider two 30hp shaker systems.
If your shop allows the cleaning breaks required at coffee and lunch, the timer system will allow you to maintain the lowest operating cost. Running the two smaller 30hp systems will probably in all cases be less hassle than a VFD unit.
I was planning on ducting the two fans into the 4' square inlet and having one larger return air duct back into the shop. This would make them dirty air fans. Is there a better configuration that would make the fans clean air?
This Carter Day is an RJ model and has an externally mounted fan type blower mounted at the top of the unit blowing through a rotating arm that back blows into the bags to clean them.
