Problem with a fan controling by the inverter ;/

Hi!     I've got, I believe, an easy problem that I can't solve ;/ My situation look like this: an air flow through the fan is controlled now by the
guide ring. A fan works in its normal rotation, pressure and flow. I need to change control by the guide ring on inverter connected with the fan. And here appear my first problem. When I use an inverter, I'll be able to change a rotation then I'll decrease pressure to the level that I have already (when I have a guide ring open to the 20-30%). And then I'll decrease also a flow - and this is my unfavorable situation, because I can't change an air flow. It must be constant.     So - how to solve it? How to control (by the inverter) a fan to get a stable flow and less than normal pressure? It is possibile?
Best regards, Michal
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Micha wrote:

I once lent a colleague a power supply that had both an adjustable output voltage and an adjustable current limit. He connected it to a fixed resister, and concluded from his inability to adjust voltage and current independently that the power supply was defective.
He was a physicist, not an engineer, but it I was nevertheless surprised that it took 15 minutes of equations and graphs on the blackboard to show him why the supply necessarily behaved as it did.
Where is the pressure measured? Flow? Once you leave the fan chamber, the relation between pressure and flow depends only on down-stream conditions.
Jerry
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Engineering is the art of making what you want from things you can get.
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Jerry Avins wrote:

Perhaps he was a _theoretical_ physicist?
--

Tim Wescott
Wescott Design Services
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Osoba nazywajaca sie Jerry Avins w liscie z dnia 2006-06-26 21:04 napisala nastepujace slowa:
(...)

I measure only pressure after the fan...
Hmm.. I read your text about a physicist.. And.. And I see some connection to my problem but.. but it's very far :P
I need to keep constant flow.. I control a fan speed.. And I need to keep a pressure about 20-30% of the nominal pressure fan. I know from the characteristic of the fan that it isn't possible. But.. Maybe you have an idea how to change some part of the system to reach this parameters?
--
Best regards,
Michal
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Micha wrote:

The flow you want may not be possible with the pressure you need to keep. Think of the outlet duct as a resistor, with pressure analogous to voltage and flow analogous to current. Although the flow through it is not linearly proportional to pressure, they rise and fall together. Maybe if you tell us more about your constraints and the reasons for them, we'll be better able to offer specific suggestions.
I congratulate you on your command of English. It certainly is up to the task.
Jerry
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Engineering is the art of making what you want from things you can get.

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Osoba nazywajaca sie Jerry Avins w liscie z dnia 2006-06-26 23:21 napisala nastepujace slowa:

Yes. I know that. But if we make an assumption that it is possible, what I need to control my pressure with keeping stable air flow? Maybe some chamber before the fan (in place where is already a guide ring)? Then I can try to keep constant air flow.

Ok. I'll try to describe my problem more precisely :)
Already my system looks like this:
--- oven -----> guide ring ---> fan --- | | ------<-----------burner---<-----------
I measure a pressure drop and a temperature (but it's not the case in this situation) in the oven. To keep needful parameters in oven I must control an air flow in a fan. Nowadays I control it by opening or closing a guide ring. It's not ease to control and very not optimized system.
Future: I need to dismount a guide ring and replace it by the inverter connected to the fan. But I know if I do it I won't be able to keep constant air flow, because if I decrease a fan speed to some level of pressure (pressure necessary to continue all technological process in the oven) then my air flow also decrease.

Pardon? I think that I don't understand you.. Well my English -- I still learning that language and I know that I make many mistakes but I believe that you'll have understanding for me and nevertheless you won't leave me alone with my question :)
--
Michal


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Micha wrote:

...

It seems that the air is circulating in a loop.

I don't know what you mean by "inverter". Is the pressure in the oven too high, or too low when the flow is right? If it is too low, throttle the oven's outlet. If it is too high, throttle the oven's inlet. Either way, adjust the fan speed to maintain the needed flow.

I mean that you write English better than merely well enough to communicate your problem. I salute you!
Jerry
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Osoba nazywajaca sie Jerry Avins w liscie z dnia 2006-06-27 15:18 napisala nastepujace slowa:

Right. About 80-90% of the air is still the same. 10-20% is a 'fresh' air, which contain oxygen (burner needs it).
[...]

Well... Word 'inverter' I found in a dictionary. In polish this element is called as 'falownik'. In english, inverter is a electric device that can control for an example a rotary speed of the engiine, by the controlling frequency of the current - ehh.. it's difficult to explain that - but I believe that you will understand me ;)
Right. I must adjust a pressure in the oven when the flow is still constant. So - solve of the problem could be mounting a throttle after and before the oven? Throttles can be connected with the inverter (which control speed of the fan) by the feedback loop. Am I right?
[...]

Thanks you ;)
--
Michal

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Micha wrote:

It's a good word, but it has too many meanings. I understand now what your device is.

At any given flow and pressure combination, one of the throttles can be wide open. (You may never need it at all.) The relation between throttle setting and fan speed won't be simple. You may need to experiment and create a table. Moving one of the controls (throttle, speed) slowly and the other more rapidly is a way likely to succeed.
I hope that someone who has actually built a system very much like yours can offer you his/her experience.
Jerry
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Osoba nazywajaca sie Jerry Avins w liscie z dnia 2006-06-27 21:13 napisala nastepujace slowa:
[...]

Well, maybe this is a solve of my problem, but - it's similar to the situation that I've already: a manual guide ring (it may be called as throttle) installed befor a fan. So, here an inverter isn't as much needful ;/

Anyway - thanks you for your time and help ;)
--
Michal



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Micha wrote:

...
Assuming that the flow is held constant by varying the speed of the blower -- that's what the inverter is for -- the pressure in the furnace can be raised by throttling its outlet, or lowered by throttling its inlet. There's not much else available to be controlled.
Jerry
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Micha wrote:

I'm not sure what you mean here by 'guide ring'.
If you have a fixed mechanical arrangement with a fan blowing into a plenum, and your only degree of freedom is to adjust the fan speed, then you will, of necessity, adjust the pressure inside the plenum and the flow simultaneously -- there's no other way.
Is the 'guide ring' you're talking about some movable device that changes the airflow somehow? If so, and if you want to adjust things for an airflow and pressure, then you'll need to adjust the fan and ring independently, and have some fun making a responsive, stable system.
--

Tim Wescott
Wescott Design Services
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Osoba nazywajaca sie Tim Wescott w liscie z dnia 2006-06-27 00:00 napisala nastepujace slowa:

Guide ring is a fixed mechanical element that can control an airflow.

Ok. I understand.

Yes. You right. But my boss want's to dismount a guide ring, and control a fan only through a inverter.
If we mount a big chamber, before a fan, which can keep constant flow, a decrease a pressure, what will be?
--
Michal



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Micha wrote:

I assume that by "inverter" you mean drive? "Guide ring" sounds like something that, well, guides, where you must be talking about a variable orifice.
Whether you have a fan running at fixed drive with the airflow controlled by an orifice or a fixed orifice with the airflow controlled by the fan drive you're still controlling pressure and flow simultaneously. It seems that what your concern should be (and perhaps is) is whether you can adequately control your airflow by controlling drive to the fan.
Ultimately you'll have to answer this question on your own, because it depends a lot on the type of fan, the type of motor, the type of drive you select ('inverter' covers a lot of territory) and your performance requirements.
How are you controlling your orifice now? Does your temperature control loop generate a position command for a position loop around the orifice, or does the temperature control generate a speed command for the orifice, which then runs open loop? How much fluctuation can you stand in your flow?
Assuming that your fan will have a well behaved flow vs. drive characteristic, it seems like you'd have a pretty good chance of putting it in and having it work. So the real question you need to ask is what fan and motor combination do you need to achieve a flow vs. drive characteristic that's as good as the fan, motor and variable orifice combination you have now.
If it were _me_, I'd be saying how big the motor is, what kind of fan it is (squirrel cage, axial, waving palm leaves, whatever), and what kind of motor/drive technology (brushed DC with amplifier, brushless DC with brushless amplifier, induction motor with variable frequency drive). Then I'd sit back and hope some of the folks on this group who know more about this sort of thing than I do tell me what's right or wrong about the proposed choices.
--

Tim Wescott
Wescott Design Services
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Osoba nazywajaca sie Tim Wescott w liscie z dnia 2006-06-27 16:37 napisala nastepujace slowa:

Inverter - electrical element which can control a drive by changing current frequency :)
Guide ring - let's tell that's someting similar to orifice.

Yes. And this situation was. Now, I'll control a fan speed (by changing his current frequency) and then I'll be able to control only flow. But I need to control also a pressure. But how? It this is the main point of the problem.

Correct. That's right.

Manualy. And I need to change it. I need to control flow and pressure automaticaly - so I want to dismount an orifice and mount an 'inverter' (connected to the drive of the fan).

Already I only see temperature on the digital indicator. When the temperature is too high, worker needs to go down and more turn an orifice.

I determin that flow must be about 2 m^3/s, and pressure drop in the oven form about 800 Pa to 1500 Pa.
Parameters of the fan: V = 10 m^3/s, p = 5kPa.
Now a fan works in his 100% of power. And my parameters (V=2m^3/s and p0-1500Pa) are setting up by the orifice. Changing this all I want to save an electric power (decreasing supply of power to the motor of the fan) and controll my parametres directly from the fan.

Fan parameters: n60 rpm V,8 m^3/s PS00 Pa
type: radial
control: now: motor connected directly to the fan; future: motor controled by the inverter (electric element which is able to change the frequency of the current :-) connected to the fan.
I know that motor and fan are redimensioned. As I sad earlier, I only about 20-30% power of the fan. I can't replace fan to the new - so I must apply some way to decrease fan power keeping air flow at the level from the 100% fan power.
--
Michal



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Micha wrote:

Doesnt seem to be that you are too concerned with pressure, just temperature control via air flow regulation?
A good inverter (VF drive), eg Invertek, Hitachi, will offer the option of PID control to a given set point - usually motor speed fed back from a shaft encoder
But if the drive were fed a set point in a form it understands - perhaps via a temperature-frequency converter or something - surely this would act to control temperature via fan speed
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Just how many groups are you posting this to? Typically one active group will get you an answer.
Michael
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Osoba nazywajaca sie Herman Family w liscie z dnia 2006-06-27 06:05 napisala nastepujace slowa:

I'm looking for the many points of view at my problem. I'm very sorry if I made something wrong ;/
--
Michal

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Micha wrote:

Generally if you're going to post a question to multiple groups you should cross-post it -- that is, post it all in one message so that replies go to all the groups. This lets everyone see all the answers, getting you a better quality of answer and keeping me from giving you duplicate answers. Even so, you should limit yourself to just a few newsgroups.
--

Tim Wescott
Wescott Design Services
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Osoba nazywajaca sie Tim Wescott w liscie z dnia 2006-06-27 16:23 napisala nastepujace slowa:

Thanks you for information. Next time I will do that.
> Even so, you should limit yourself to just a few

I send my message to one polish group and 4 english language.. So.. - in my opinion - I send it 'to just a few newsgroups' :)))
--
Michal

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