Exhaust Fans 4

[Ripcord13]

In our plant we installed a Regenerative Thermal Oxidizer to "cook off" hazardous fumes from the dozen or so large kettle mixers. Previously each mixer's exhaust fan exhausted individually into the atmosphere. Now all of the exhaust fans are ducted into a main 4' round duct which has a 50 hp booster fan 480 volts connected to a VFD which exhausts the fumes into the RTO. The factroy installers set the speed of the booster fan to almost 90% which we believe has a total CFM draw greater than the total of all of the individual fans and is "spinning" the indivual fans faster than their rated speed (eg 1725,1140 rpms etc) are the small er fans being harmed by being spun faster than they would be by being energized? All of the smaller fans 1 to 5 hp 3 phase are turned on. I believe that we don't need the smaller fans any more but I have to convince "management" Thanks.

 

[waross]

Start the small fans. put an ammeter on one of them.
Start the big fan. You should see the current drop as the large fan assists the smaller fan. If the current starts to rise again as the air velocity increases you have started to overspeed the fan. It is not a problem as long as the rated current of the small fans is not exceeded.
The small fans, at this point, will be putting energy back into the power grid.
If the small fans are de-energized, they should be able to take 3600RPM safely. Manufacturers use the same frame size and bearings for 3600 RPM as for 1800 RPM.
Most squirrel cage motors will withstand considerable faster speeds.
Just remember to check and/or change the grease more often. Like every 3 years instead of every 5 years.
yours

 

[jraef]

Manufacturers use the same frame size and bearings for 3600 RPM as for 1800 RPM.

I am compelled to point out that while extremely common, that is not a universal truth. Best to check with the factory on that.


All that said though, if the 50HP booster is now doing all the work, the old exhaust fans now just represent a drag on the air flow, i.e. additional losses. Even if they are energized and regenerate as waross said, they will never regenerate as much as the extra energy they cause the 50HP fan to consume by their flow restriction.

The only valid reason to keep them there would be as a redundant backup in case the 50HP fan dies. In that case, leave them energized and recover as much of the losses as possible through regen.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hi jraef;
You're probably right on that. I did some research on one manufacturer web site and found that they used the same bearings for all speeds. Some other manufacturers may do it differently.
The manufacturers recommended more frequent bearing service at higher speed roughly proportional to 1/speed.
I agree with the rest of your post as long as the small fans are not producing more than rated current.
Respectfully

 

[jraef]

I have experienced several low cost (politically correct term for el-cheapo) motor mfrs in 3rd world countries put in only exactly what you ask for, no more. That extra .1% for higher rated bearings may represent 1/2 of their profits on that motor.

 

[itsmoked]

Also some fans may pitch a blade at twice the expected RPMs.

You could also leave the fans in place but block them from rotating. This would probably be the best energy situation and would avoid the entire over speed question. Of course removing the blades would be better followed by, better yet, removing everything.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

I agree with the above comments. I propose it might be valuable to look at speed to help determine your condition since you apparently have the ability to monitor fan speed (with a handheld strobe?).

I'm also assuming the smaller fans I assume are not on a vfd.

If a small fans rated 1725 operate between 1725 and 1800rpm, it is operating between full load (1725rpm) and no-load (1800 rpm). As waross says, with small fan running try starting large fan. If speed increases within this range (1725-1800), the large fan is helping reduce the workload on the small fan. Generally not a big problem (although even in this case there might be efficiency and vibration issues to think about)

If small fans rated 1725 operate above approx 1800 rpm, then they are being turned from a motor into a generator and are starting to really hurt efficiency. For the 1725rpm motor I would think you would be in danger of electrical overload as speed approaches 1875rpm. I would also think overload protection will remain effective and motor is not in big thermal jeoparedy as long as it doesn't trip. In this range you may want to consider whether overspeed capabilities are being exceeded. If <30hp according to NEMA MG1 TABLE 12-5, a general purpose s.c.i.m will have at least 25% overspeed capability (continuous).

Also in general I believe looking at vibration magnitudes and trends (does it increase when you start the large fan) will help you determine whether you may be affecting the life of the motors in these abnormal conditions.

If you can clarify which of these two ranges you are in (1725-1800 or 1800+), it would help to focus the discussion further.

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[electricpete]

... and if the small motors are on vfd most of my comments go out the window... please clarify this point.

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[Ripcord13]

Thanks all for your input, I will check the small motor(s) amperage (they are not on VFDs) This is going to be interesting Thanks again.

 

[electricpete]

Personally I think you get more direct info from speed. Current won't tell you whether it's acting like a motor or a generator (unless you have voltage probes for a phase reference).

Since you measured speed to begin with I assume you have a strobe available.

There is one factor I forgot to mention which will complicate the analysis whichever method you choose. The behavior of the mechanical loading produced by the fan (bhp) varies widely between radial flow designs and axial flow designs and is quite non-intuitive for axial flow fans. (At 5hp I would say axial flow is more likely although you can tell by looking at the blades. Axial flow looks like a houshold fan. Radial flow looks like a squirrel cage).

See the BHP vs flow curve for an "axial" pump here (3rd graph down)

http://www.gouldspumps.com/cpf_0009.html

Axial flow fan act roughly the same as axial flow pump.

When you choke off an axial flow fan, the mechanical load on the motor goes down! When you "help" by reducing backpressure, the mechanical load on the motor goes up!

Some consequences of this surprising behavior:
1- If you take a vane axial designed to run in ducting out of the system and run it without backpressure in the shop you can trip or damage the motor.
2 - we prefer to start radial-flow centrifugal pumps with the discharrge valve closed (and small recirc path) to minimize torque during startup so the motor can start faster (assuming there are not other fluid system considerations). For axial flow pumps, you would in fact prefer to start with discharge wide open to get the motor up to speed faster (assuming there are not other fluid concerns such as water hammer).

This is a little bit counterintuitive but that's the way it is.

So... with the extra confusion that an axial flow fan works MORE when you "help" it by reducing the backpressure, I will summarize some main points:
1 - Speed rather than voltage will give you a less confusing picture of where the motor is operating (will tell you generator or motor and what is the load....current only tells you load but not generator or motor).
2 - Vibration will give you some indication if the fan is at an operating point that it doesn't like.

Since you have a non-vfd motor <30hp, I'm pretty sure that overloading the machine in generator mode would be a concern long before you have to worry about overspeed (doesn't occur until at least 125% for these motors).

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[DickDV]

Bottom line---get the old fans out of the system. They are doing nothing to move air and are wasting energy.

Keep a spare fan and motor in stock for the main fan if redundancy is important. Surely cheaper than wasting energy every minute of every day!

 

[electricpete]

Dick - all we really know is that the motors are faster than there "rated" (nameplate) speed. The same applies to pretty much any non-vfd squirrel-cage induction motor at normal voltage that is not overloaded. How do you have enough information to recommend that a system mofication required? Am I missing something?

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[DickDV]

I see your point, electricpete, since it is unclear whether the motors are being pulled over base speed or not.

I was adding a little judgement into the equation and concluded that since 1) it is clear that the fans are being unloaded to some extent, and 2) the new fan is running at 90% speed, it would be reasonable to remove the old fans. If the old fans are not being completely unloaded at 90% speed it is almost certain that they would be at 100% speed.

It still seems like a reasonable conclusion to me even tho, I must admit, there is a small chance that the new fan couldn't pick up the whole load.

 

[electricpete]

No offense to Ripcord, but I think if he understood the significance of the difference between "operating above rated/nameplate speed" and "operating above syncronous speed", he would have used the latter.

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[electricpete]

Sorry, I didn't mean it that way.

The original post as it stands says the motors are operating above rated speed (example 1725rpm or 1140rpm).

Ripcord, do you know whether the motors are also operating above syncronous speed (1800rpm and 1200rpm)?

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[electricpete]

Going back to Dick's comment - beyond the complexities of unknown variation bhp vs flow (don't even know whether we have radial or axial fans), we certainly don't know how many little fans feed to the 1 big fan. Ripcord - how many fans and what type?

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[electricpete]

Honestly I did not see that the original post asked about removing the small fans, so it's good that the discussion should turn to that aspect.

A few other objections which need to be looked at before getting rid of those small fans:
1 - Is there flexible ducting downstream of the small fans currently under positive pressure which would change to negative pressure upon removal of the small fans and implode?
2 - As Dick mentioned is there adequate capacity of the big fan without the little fans?
3 - Does the process benefit in any way from more flow and reduced exhaust pressure, even if it costs a little more electricity?

On the flip side, we still haven't established the motivation for even wanting to change yet.

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