Remember the ABB invertors story?

[Steve Bailey]

jpd24,

I think you've rephrased Goody's original question. Did anyone from either the engineering staff at Goody's customer or the applications people at the local ABB sales office ever perform an analysis of the process to see how much energy could be saved by installing these VFDs?

Now that the VFDs have been purchased, consider the motivations of the parties involved. The ABB salesman pocketed a nice hefty commission for selling those drives. If the customer decides they don't live up to the promises the salesman made, he'll ship them back to ABB and the salesman will lose the commission (which he has undoubtedly already spent). The person at Goody's customer who approved the purchase in the first place may be worried about keeping his job, because it appears that he didn't do any calculations of his own to verify the claims that the ABB salesman was making about the potential energy savings.

This is getting really interesting now, because we're likely to see the technical aspects of the system get shoved to the background as politics and arse covering move to center stage.

 

[seppoalanen]

Control: (these are guessings)
How contor of Burning? Maybe we measure O2 content and how the control works -by fresh air fan- ? For instance if oil is burning, there are more cas going out than fresh air are going in, so there have to be some exhaust gas fan ? Can we control both fan by oil flow content ? How much fress air we need and how much we have exhaust cas, process engineer can tell them to me becourse I have not competence. What kind of fan I need, 'fan-specialist' engineer tell it and what size motors we need for them and what are the nominal values. Some times calculations have too have or too light. Fresh air humity, pressure, temperature, smoke pipe high etc. could be different than have calculated etc..

Energy saving:
1'st check fans with nominal loads, have we flow as much as have calculated. Flow depends of the fan speed, not the motor power.

What is motor power at nominal load, if we have 90kW motor and it takes 15kW in nominal (max usable) speed, 1'st we have to change smaller motor maybe 7.5 or 11kW is enough observing efficiency of 90kW motor.
Last part is change suitable VFD for final motor.

My point was that process must research 1'st, where and how we can save energy.

 

[PLucas]

This is getting really interesting now, because we're likely to see the technical aspects of the system get shoved to the background as politics and arse covering move to center stage.

This is so true, especially as it now seems that the only person to express concern over these energy saving figures is being pushed out of the equation.

That in itself seems like arse covering to me.

I would be very interested to find out the end result of this little 'saga'. Keep us informed Goody, please.

Paul.

 

[DickDV]

Ahhhh! jdp24. You're making way too much sense here!

You are exactly right, of course. But, we live in a broken world and reasonableness, as you suggest, is more an exception than the rule.

It follows then, that for each of us, we need to determine what level of judgement and ethics we are going to practice and then live by that at work and elsewhere.

The good news is that, if you choose a relatively high level of performance for yourself, there always seems to be a portion of the marketplace that will value it.

My recommendation is to choose excellence and good ethics and let the part of the working world that doesn't appreciate it go by the wayside. It seems that you have done this and I commend you. At the end of the day, it will probably have cost you a few dollars (or pounds or marks or whatever) but you will be able to sleep a bit better at night.

 

[Tom Jenkins]

You are correct, Dick, that the operating characteristics of these blowers are a little different than a typical fan's. These are multistage machines, with a series of impellers arranged on a single shaft. The discharge of the first impeller is routed to the inlet of the second, etc. The impellers are a mix of radial and backward curved types. I usually think of fans as having discharge pressures measured in a few inches of water. These blowers are operating at discharge pressures of a few psi - typically 6 psi to 10 psi.

In addition, because these blowers are discharging air into the bottom of a large basin of sewage, there is a high static pressure associated with the system curve. Most fan applications are primarily friction loss systems, with little or no static head. Just as with pumping, a high static "lift" system limits speed range and turndown on the fan, pump, or blower.

Finally, this is actual field test data. The kW was read from analog meters, and so there is undoubtedly some slight error in readings.

The purpose of displaying these graphs was two-fold. The first was to verify that there is indeed a lot of energy savings available from properly using VFDs on air moving and pumping equipment. I was afraid that some of the participants in this thread would get the impression that it is all snake oil. I wanted to dispell the idea that you couldn't really save energy with VFDs - you certainly can!

The second purpose was to illustrate the proper way to analyze the power requirements of a system and determine the useable range of the VFDs. This is a fundamental step, and it was missing in the application that Goody is working on. You have to take the actual range of air requirements, the actual fan performance characteristics, and the actual system curve. Then, with proper application of the fan laws you can calcuate with confidence the power required for constant speed and variable speed operation. Had Goody's customer done this basic engineering analysis they would have known ahead of time what was gong to happen, or not happen, to the system power useage.

And, after all, that is what engineering is all about - using the laws of physics to predict ahead of time how things are going to work!

 

[DickDV]

I agree with you, Tom Jenkins, entirely. Especially when good process data exists. How utterly ridiculous that variable speed motor controllers be purchased when no speed change is possible.

There is another situation, however, that isn't quite so easy to pass judgement on. That is where good process data cannot easily be obtained. Those of us that work in the field (you may too, I can't tell) face this situation occasionally and then, the only practical recourse is to rely on your own judgement after studying the process.

After some years of doing this kind of work, you develop a bit of confidence doing this judgement-type analysis but, unless you are totally foolish, you should temper the confidence with a large dose of humility and risk sensitivity. It's alway easier to do it wrong that to do it right.

In Goody's case, it seems to that a major lapse of good judgement occurred when all 30 systems were modified at once. If only one or two had been done for analysis, there would be a lot less discomfort to be managing about now. Of course, the commission check would have been a lot smaller but that's where personal ethics come in.

Regarding your turbo fans, as you can tell, I've never encountered one of them. I've worked with Roots type lobe blowers as well as axial blade fans but never one like you've described. I've stored away the curves you posted for future reference. Thanks.

 

[Tom Jenkins]

You're right, Dick. I also do a fair amount of field work, and you learn how and when to use "Kentucky windage". After enough years of correct SWAG (Scientific Wild A$$ Guess) you get some confidence. You also learn how to minimize everybody's risk by testing one of 30 instead of the bunch at once! Again, that is all part of good engineering.

I've also done Roots type blower systems. The main difference is that they are a positive displacement blower, and are constant torque load.

 

[Guest]

Greetings from a newbie / lurker on this forum.

I’ve read the thread, not only do I see a puzzle to solve, I see an opportunity to add to my meager knowledge. Would the experts please check me on my assumptions (yes, I know what assume really means).

Using the VFD to slow down the fan has an adverse effect on drying, less air flow, lower mass of air being heated, causing the burners to be on high burn for a shorter period of time, and low burn longer, as the thermostat cycles. Thus less actual heat is applied to the product for drying.

From what I’ve seen on VFD applications, they are best used to replace high/low or switched applications with a continuously variable control. In the drying process described, the only high/low switching is the burn rate; with a constant fan speed this provides warm - warmer - warm - etc. air to the process.

IMHO, the best application of a VFD to the scenario would be to set the burners to one fire rate, perhaps a medium-high, and vary the speed of the fan to maintain a constant temperature at the product being dryed. With experience an ideal drying/curing temperature could be determined, quite possibly improving drying times for increased production.

Please let me know if I, like the improperly dried product, am all wet

ThankYou

 

[kamenges]

I've been known to be wrong...

I might be wrong on this but I thing the raw airflow is as important as the temperature. A clothes dryer wouldn't work very well if it just applied the heat to the enclosure but didn't provide any airflow. So regardless of temperature the pressure/mass flow of air is important as a separate variable.

Keith

 

[DickDV]

Guest, I don't think there is enough info out here to answer your question. It seems that there might be multiple products made by this line further complicating any analysis.

I suppose, ideally, you would requlate the flow of constant temperature air over the various prducts with a VFD on the fan and then regulate the burner to maintain a constant air temperature. I don't know to what extent that would be practical in this application.

Nothing beats being on-site and asking the necessary questions face to face.