Blower Curve

Greetings,

I have a blower used for vacuum on a set of conveyors. The blower is currently on a starter and running at a fixed RPM. It was asked if I could put the blower on a VFD to run it a little faster if necessary. I wanted to find the performance curve of the blower to see if running it beyond 60HZ (3470RPM in this case) would give more vacuum, or possibly even less before adding the VFD. I don't work with blowers often.

I went to the manufacturer's website and found the information on the blower. It is a Cimme blower model GCH005020. I found the performance chart for the blower, but I cannot tell by looking at the chart what the effect RPM has on it's performance. The chart appears to be all about pressure, velocity, and horsepower. I have included some links below. I appreciate any input, since I don't work with blowers and I'm not sure I'm looking at the chart correctly.

PDF page 5:
http://www.cimme.it/pdf/cataloghi/CatGch0906-lk.pdf

Technical information on the blower:
http://www.cimme.it/pdf/cataloghi/InfTec0906-lk.pdf

Thanks in advance,

Greg

I'm by far, no blower/fan expert.
But, years ago, I went through similar discussions.
Blower/fan guys are really big on 'BHP', or Blower Horsepower...the amount of energy it takes to make the blower do something.
More specifically, if you could, in fact, measure the horsepower the blower was 'drawing' from the motor, and make some additional measurements (like delta-p across the blower) you would come up with all kinds of really neat information...flow, etc.
Dwyer Instruments used to have really good tech info pages in their catalogs, which might help.

Your answer is in the manual, look under rotation speed.

The Power required from the motor will increase with the cube of the speed change. So a 10% speed increase requires 1.1^3 = 1.3 times more motor power.

is it a positive displacement blower (lobe type like a car supercharger) or a multistage centrifugal one with a 3600 rpm motor.

The PD model is easy to put on vfd, you just has to check for maximum temperature as temperature increase with pressure (or vaccuum depend on the use). The blower manufacturer will be able to tell you

For a multistage centrifugal one is another story. This machine are very sensitive to surge condition (too much restriction or not enought resulting in not enought flow) This condition destroy the machine like an harmonique sinewave but in the blower shaft going back and forth destroyig stages until parts comme out of the machine.
In a fix speed frequency, it's safe to just read the current and stop the machine in lower amps portion of the curves..Temperature, pressure and other patern react on the same way lowering the amperage and still protect the machine....
In variable speed, the curve become a live one that change each time: inlet temperature change, Inlet pressure or outlet pressure change or speed and power on the machine. And usually most of them change toghether.....
If you wanna go this way with a multistage, i suggest you get a special PLC panel from the blower manufacturer with a corresponding programing.

The blower is a single impeller, PD.....


Greg

forget my answer, it's none of these, i should have look to it before answering. Its a centrifugal simple stage, you have to check with maximum speed of the squirel cage itself...

Single impeller, cent....

and motor power also by the way as power curve from the motor stop to increase after 60hz and torque tend to decrease.

Greg,

I'm not a blower expert either. But a couple of questions come to mind.

1. The PDF's you linked to seem to indicate that the model you listed has an RPM of 2900 for a 5.5kw unit. This sounds like a full load speed for a 50 hz motor in that KW range. I'm presuming that you are in the US so your Hz is probably 60 Hz. If this is true and your blower has no mods done to it for US voltage. I'm surprised it works at all. See my pic.

That said if there is some severe vacuum on the inlet the motor may not be overloaded at all. Is your goal a more aggressive vacuum pressure or are you wanting more flow?

Is your motor setup for dual voltage (240/480)? If it is, are you running it on 480? The reason for these questions is that the torque of a motor when VFD driven higher than 60 Hz drops off. This can somewhat be overcome by connecting the motor leads in the pecker head for 240V but running the VFD on 480 volts. A custom V/Hz needs to be setup in the VFD. The VFD needs to deliver 240 volts at 60 Hz and 480 volts at 120 Hz. The VFD also needs to be able to deliver twice the full load motor amps. Chances are you will still run out of torque before you get to 120 Hz. Remember in my pic of the manual you linked to just going from 50 Hz to 60 Hz should have nearly doubled the absorbed HP.

I'm presuming that the conveyors are either slat chain or perforated belt, and the vacuum is to hold the product against the conveyor. To test the amp reading you need to test it under the worst case scenario. Believe it or not, this would be when the most amount of air is flowing through the blower. So have you taken a amp clamp reading?

I would classify this as a fan, not a blower.

The fan laws apply. Take a given point on the curve for a specified speed. When you change the speed the curve flow changes proportional to speed, the pressure changes proportional to speed squared, and the power changes proportional to speed cubed. If you repeat this for several points on the curve you can create a plot of flo0w vs pressure and flow vs hp at a new speed.

Once you have the new curve you still don't know the operating point of the fan. That is a function of the system curve - a plot of the pressure drop through the ducting etc. vs flow. Plot it over the fan curve and the intersection is the operating point.

You have several problems. First, the 3470 rpm you cite would be at 50 Hz, not 60. Your operating speed at 60 Hz will be 3570 or so.

Second, there are several limits on speed change. Will the motor handle the higher rpm? Maybe, maybe not for a 2-pole motor. Then as Vaughn points out you need to make sure the operation at the new speed won't overload the motor. Then you need to make sure the fan impeller can handle the higher speed without centrifugal force making it fly apart.

Confused? Good - I've been doing this for 30+ years and I still have to think about it. If you could do it in your first sitting I'd be REALLY embarrassed!

I suggest you get your fan supplier to stop in and review the application. He may be confused to, but he should be able to get you in touch with the engineers that should have the answers.