VFD alternatives 3

[BruceTheEngineer]

Hello all,
Let me start out by saying I'm a chemical engineer by training, so please don't laugh at my lack of electrical engineering knowledge too hard.
The problem I'm encountering at work is that our positive displacement Roots blowers are blowing too much air into our pneumatic conveying lines. In my research and experiments, I've found that decreasing the amount of air (in CFM) by bleeding out excess air has dramatically improved our ability to convey product and reduce defects such as degradation, streamers, and pipe wear because our solids/air ratio has increased. The thing is, I really don't want to bleed off air because that's just a waste of 40 and 50 hp motor.
I have thought about installing VFD's, but they are expensive to install. I have about 6 motors (all between 40-100 hp) that are single speed AC motors. Are there any alternatives to VFD's I should consider? Like I said, I'm not that familiar with electrical products on the market today, so I don't know what's out there.
For four of the applications, I'm thinking about just changing out the sheaves because the product flow rate stays the same, but for two of the motors the product flow rate changes so I would need to vary the amount of air I use to convey product depending on how much product I have going into my pipe. The manufacturer of the blower said "NO ATTEMPT SHOULD EVER BE MADE TO THROTTLE INLET OR OUTLET AIR FLOW VIA A VALVE" so installing a valve is out of the picture.

 

[cavbilly]

I would like to weight in here, since at least I have some experience with roots blower driving a chemical process (oxidation).
1. We had no VFD at all, and the blower was belt-driven, but as I recall sheave ratio was in the form of a speed up ratio.
2. Out objective was to push as much air as possible through a molten wax for the auto-oxidation process, at a controlled elevated temperature, unfortunately at near atmospheric pressure (more efficient reactor designs are pressurized, but slightly more dangerous).
3. It is very scary when you switch to a paraffinic oil (from Pennsylvania crude) that is very low in aromatics and naphthenes, as the auto-oxidation (with catalyst) is quite vigorous, and the off-gases are highly combustible with a high flame velocity. On one occasion with such a truckload batch, I witnessed the fire bricks in the off-gas incinerator were beginning to melt at the surfaces in contact with flames. Even a "hiccup" on that root blower would have resulted in a plant explosion of the reactor, and certainly my death.

Your situation is less critical. Personally, there are many choices in very good VFD's these days to vary motor speed, and these make economic sense.
However, if only applying this to one processing line, you may choose a slow-down sheave ratio, as this is by far the cheapest, and it will lessen motor load.

 

[titip]

Nyawade (Instrumentation)
VFD is the best nevertheless can considered running these pumps using a logic controller. Few pump can run when demand for air is low When more is required logic controller calls in additional pumps sequentially.

 

[BruceTheEngineer]

Sorry for my late reply. We found out that bleeding the air did save us some energy. We went from 55.6 Amp draw with no bleeding off to 45 amps when bleeding off some of the air. I need to get an anemometer to figure out exactly how much air is being bled off, but it's significant (went from blowing air at 13 psi to 6 psi on a 4" line).

 

[cavbilly]

OK, you lowered the power requirement, but does this not in some way reduce process efficiency, since you are wasting a large portion of the blower energy input?

 

[BrianPetersen]

For a ruff estimate, the back pressure in a turbulent flow situation is the square of the flow rate, so going from 13 psi back pressure to 6 psi probably involves about 70% of the flow rate (so 30% is being bled off). Doing this in an efficient manner (by slowing down the pumps to 70% of current speed and eliminating the air bleed) would be expected to use (roughly) 70% as much power as the already-reduced power achieved by doing it with an air bleed. The amp draw on the motor is an indication of the load but isn't in proportion to the load because of the magnetizing current (which would show up as the power factor) so you've probably saved more real power than the amp draw indicates.

This will only be an approximation because we haven't considered compressibility effects of air, nor any influence of the material being conveyed in the pneumatic conveying system.

Is it worth the cost of a VFD? If you can save a further 10 kW per motor (seems plausible) multiply that by hours per year of running and multiply that by your power cost per kWh ... if it's running most of the time during work hours, it's potentially a few thousand bucks per year.

 

[itsmoked]

Thanks Brian.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lvsenthil]

Dear Bruce,

It's obvious the current (power) will increase when you arrest the bleeding in a positive displacement blower. Instead you should reduce the flow by reducing the speed.

Apart from VFD you may reduce the speed of blower by changing the Drive pulleys . It is cost effective one.