Not just another dust collection system question....... VFD/flow related

[Larry Edgerton]

I picked up a 10 hp 3 phase cyclone/baghouse system that I am installing in my new shop this summer , but.......

I am most often working alone and also often on machines that do not need as much flow as this system will provide. So at first I thought about using a smaller motor but decided that that would not be a good solution for saving power. Then I read this when reading about VFD's.

"Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable-speed, by means of VFD. Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the loads' torque and power vary with the square and cube, respectively, of the speed. This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed. For example, at 63% speed a motor load consumes only 25% of its full speed power. This is in accordance with affinity laws that define the relationship between various centrifugal load variables."

So, according to theory I should be able to run the dust collector at 63% of its rated speed and save 75% of the power usage. So my questions are two fold if there is anyone on here that knows much about VFDs and internal cyclone speeds and the effect this would have on the heavies dropping out. I am not worried about lines plugging, just the effectiveness of the cyclone at the reduced RPM. For reference this fan wheel is 26" in diameter.

My other question would be the effect on the motor running at 63%. Mt thoughts are that the reduced load would cancel out some of the additional heat from using the VFD to reduce speed, but I really don't know Jack about these kinds of things.

Yes, I would love to have someone design a system for me but the reality is that I am a junkyard dog, these days by necessity, so I will just make due with what I have.

What say you? In case you are wondering why I am asking so many questions, I have a dead spot in my schedule and not much money, so I pester every one here to keep me busy.

Larry

[Ryan Mooney]

If it were me and I had a lot of spare time (its not, I don't ) I'd try to figure out how to marry the VFD output to the airflow. Basically as long as you can maintain sufficient airspeed I think it should be ok. You can also save a fair bit of juice by slowing the motor ramp up a bit (I'm sure there's a curve but I don't know how to calculate it) at the very least you could avoid nuisance breaker trips.

[Larry Edgerton]

Hey Ryan

Ya, there was something in the same article on how much you save in power and motor life by using the VFD to ramp up power slowly. I'm not going to get too carried away on the math, then I would have to buy the right stuff instead of stuff at the right price.

Larry

[John McClanahan]

I think you will find that the impeller's efficiency will drop rather quickly as you start slowing the motor down.

John

[Jim Andrew]

When you move into the new shop, will you still be a one man shop? With a 26" impeller, that fan should not have to run at huge speed to supply you enough cfm's to run any one piece of equipment. If you can measure your cfm's, you should be able to set the speed to match your needs.

[Ryan Mooney]

Yeah I think most of the VFD's have a programmable ramp up curve so pick one and it should be mostly ok. If its to slow go faster

I was thinking about it and I reckon you could probably rig together some sort of a pitot tube on the outlet and use that to drive the VFD speed. There are enough small/cheap embedded computers (arduino or raspberry pi these are both in the sub $50 per range for models that ought to do what you want + a pressure or airflow sensor of some sort - which is likely the pricier bit) that you ought to be able to have something pretty cheap that measured the loss and fed the data into the VFDs speed controller without a whole lot of moving parts.

Here's a short list of pressure sensors for the arduino:
http://playground.arduino.cc/Main/In...ware#presssens
my guess is that the honeywell series is probably in the right capability set.. but others might be cheaper.
The arduino forums have some folks doing something not altogether unlike what you're talking about
for example: http://forum dot arduino dot cc/index.php/topic,17209.0.html
if you search google for "arduino vfd control" you end up with a bewilderingly large set of hit.

The part I'm sort of struggling with in my head is figuring the difference in cases where you don't have many gates open vs having them all closed - I think? the pressure sensor will allow you to make that work cleanly.

Sounds like a fun project!

Assuming that that gets to geeky I'd bet a fairly goodly amount that you can just run it slower just fine. The nice thing about the VFD is if you see to much bypass you can just tweak it up. You might poke David Kumm assuming that he doesn't chime in as he has some sort of (slightly smaller) franken collector he did something not altogether different on.

[Christopher Clark]

Hi Larry,
I would price out your 10hp VFD 1st. They are not cheap!
Try contacting the collector manufacturer to get the fan curve data. Ask for their opinion on your project. That would save you a lot of time.
Many VFD have an analog input for FM /static DP pressure transducers. They have a simple PID type controller built in to control fan speed via this analog pressure input.
Otherwise I would set the fan speed to a set HZ and adjust until it works.
Good luck,
Chris

[Michael W. Clark]

Hi Larry,
Generally speaking, the fan flow is directly related to fan speed. If you run the fan at 63%, the flow will be approximately 63%. The power varies with the cube of the volume (or speed) change as long as the system doesn't physically change. If you cut the flow through the cyclone this much, cyclone efficiency will drop off. You will see it more on fine dust than planer shavings, but likely some of both. The bag filter will not care, lower flow actually helps it. However, it will continue to build pressure drop until cleaned and it may see more dust due to cyclone carryover. Is it a pulse jet collector with demand/timed cleaning? Whose system did you get? Typically VFDs are avoided on larger cyclone systems unless used on large direct drive fans for startup and balance.

Mike

[Phil Thien]

Hi Larry,
Generally speaking, the fan flow is directly related to fan speed. If you run the fan at 63%, the flow will be approximately 63%. The power varies with the cube of the volume (or speed) change as long as the system doesn't physically change. If you cut the flow through the cyclone this much, cyclone efficiency will drop off. You will see it more on fine dust than planer shavings, but likely some of both. The bag filter will not care, lower flow actually helps it. However, it will continue to build pressure drop until cleaned and it may see more dust due to cyclone carryover. Is it a pulse jet collector with demand/timed cleaning? Whose system did you get? Typically VFDs are avoided on larger cyclone systems unless used on large direct drive fans for startup and balance.

Mike

Yeah but the resistance of the cyclone is proportional to the airspeed squared. So his drop in CFM may not be all of the 37%.

It is a single-user shop, with a 10-HP cyclone feeding a bag house. Even at reduced RPM this is probably going to work quite satisfactorily, IMHO.

[Jim Neeley]

Hi Larry,

As you slow down the fan you do significantly dramatically decrease the power consumption but you also dramatically change the fan curve in terms of volume vs. suction pressure. This would show up in the drop of velocity of the air in the piping system if you have a sizeable run or small ducts that would incur pressure drop.

the reason the flow remains proportional but the power goes up by the cube is in part the amount of suction it is capable of drawing. While running the fan at 63% speed may give you 63% flow under very low head loss, as the head loss goes up the flow drops off much more dramatically. Think about it this way: if you were to slow a 3600 rpm (60 revsa/sec) down to 1% (1.5 seconds/revolution), you may still get 1% flow if it is just moving air assuming no pressure drop but it won't be able to overcome any piping pressure loss.

Jim

[Larry Edgerton]

Yeah I think most of the VFD's have a programmable ramp up curve so pick one and it should be mostly ok. If its to slow go faster

.
Assuming that that gets to geeky I'd bet a fairly goodly amount that you can just run it slower just fine. The nice thing about the VFD is if you see to much bypass you can just tweak it up.. .

This is what I figured, its a win/win for me. I don't have to run it on that annoying 50Hp converter so if I am using single phase tools I have quiet and don't have to turn on the converter first. I can slow it down till it doesn't work and then turn it back up, no science involved.

I didn't really need one [dust collector]quite this big, but I bought this one for $250 complete with new bags. The motor is old school so I may put a temp gauge on it to see if it likes being underdriven. I have a new large frame Baldor 5hp three phase on the shelf and thought about that briefly as a way to cut power but did some reading and gave that up.

Well, now I can scrounge around for a VFD. Question on that, can a larger than needed VFD work? Say I find a used one for a 20 Hp max, can I run the 10 Hp on that with no ill effects. I know I can't go small, but I was wondering if I could go big?

Thanks, Larry

[Larry Edgerton]

How does this look?

http://www.factorymation.com/Product...0-2010-N1.html

I have been thinking about buying a Phase Perfect but I am going to look at running VFD's on all of my machines. I already have a new VFD for my drill press, and some day I will figure out how to wire it up. So if I put a VFD on my collector I will have my:

9 hp planer,
5 hp bandsaw,
5 hp radial arm saw,
5 hp table saw
7 1/2hpX2 shaper. The shaper has two 7 1/2 hp motors, so I wonder if there is a way to share the VFD as only one motor will be running at a time.
3hp Lathe, not a priority.
I have a MiniMax shaper that I converted to single phase but I still have the original motor if I ever choose to use it that way.

So...... If you add up the cost of those VFD's they still cost less than a Phase Perfect. 20 hp Phase Perfect is $4985, total of VFD's $2186.

How dependable are VFD's? Best brand at a reasonable price?

Another thought... Will I be able to use a remote control on the dust collector with the VFD? I've kind of go used to that and really like it.

Larry

[David Kumm]

I run my 7.5 hp system on a vfd. With a 10 hp system I am assuming you will run smaller diameter pipe than the 10-12" it is designed for. The type of impeller will also influence how well the system copes with the reduced speed and flow. I suspect you will vary the speed over a more narrow range but what you are proposing does work. I vary my impeller between 55 and 65 hz depending on what machine and the noise does decrease greatly. The vfd does cost but I've found 50 amp input vfds on ebay for < $500 used. You can also rig up a mag starter on a plug to serve as a backup if needed. Dave

[Michael W. Clark]

Yeah but the resistance of the cyclone is proportional to the airspeed squared. So his drop in CFM may not be all of the 37%.

It is a single-user shop, with a 10-HP cyclone feeding a bag house. Even at reduced RPM this is probably going to work quite satisfactorily, IMHO.

Hi Phil,
Duct losses vary by the same square relationship. The system curve is essentially the same, the fan curve shifts down but keeps the same shape with a VFD on the fan. The baghouse varies with a more linear relationship, but its DP varies with loading anyway. The cyclone efficiency drops very quickly with a reduction in flow. Since it is before a baghouse, I doubt it is a high efficiency cyclone to start with (no need for it). If you drop the flow ~40% (assuming design flow was 60Hz on the fan), it may only be slightly better than a drop-out box.

I didn't say it wouldn't work. It depends on a lot of variables and, ultimately, Larry's definition of "it works". Larry asked if the cyclone efficiency would be affected if he slowed the fan to 63%. The answer is yes, cyclone collection efficiency will definitely go down.

Mike

[Phil Thien]

If you drop the flow ~40% (assuming design flow was 60Hz on the fan), it may only be slightly better than a drop-out box.

I'm thinking the separation rate will drop at a fairly linear rate, and resistance will drop at an exponential rate, related to reductions in CFM.

Not trying to argue with you, I just don't think separation will suffer as badly as I think you might.