Impeller Trimming to Reduce Power Consumtion

IMHO, it will be futile to entertain such train of thought. The present solution in vogue is the best under the existing circumstances. Let sleeping dogs lie!

Well, the best way to reduce power consumption would be to switch it off when it isn't needed.

purvesh. you have made three comments here, and none yet are good.

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You should gain some experience before the next time you attempt to instruct someone on proper ise of CR4 who has made 18,000+ comments on CR4 with 400+ good answers.

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You look ridiculous.

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By all means, mangle your fan. send us pics please.

touche

It isn't the input that is worrying, though the sentiment is appreciated. It's the output that supplied it.

The important thing here is to throw the forum some information so that the full range of solutions can come out.

• Mangling the blades is one way.
• Altering their pitch is another.
• Removing some of them is a third.
• Changing the belts and pulleys is a fourth.

And so-on.

Simple answer No. If the flow remains the same, then the pressure will remain the same, and hence the power will remain the same.

Trim the fan in 10 mm increments, until desired results are obtained.

See #8↑.

It is nonsense looking at the fan characteristic without looking at the ducting characteristic too. The ducting consists of a pressure/flowrate function that the fan is supplying. One way to reduce power consumption is to make the duct bigger, for example.

One of the things that is missing is the process requirements. A Process Engineer has determined that the process need will be satisfied by this particular fan. Now, it is no good reducing the power consumption of the fan if fewer widgets or even a load of off-spec widgets are produced by the fan as a result, is it? So the process requirements come first, about which the forum is told nothing. Get the drift?

The inlet side is restricted by 50%.

It seems to me that trimming might work.

Good point. One wonders what sort of widgets this thing is supplying, and why it is currently oversized by a factor of around two. Perhaps the original poster will advise. Perhaps not.

You're assuming the OP knows.

Dear Mr.purvesh2u,

The answer is NO. Trimming means to what extent.? is not specified by you. Trimming - as I understand from your statement, REDUCING THE IMPELLER Dia. If so, the Draught or PRESSURE DEVELOPED, by the FAN WILL FALL or will be REDUCED.

Best option will be to put a variable speed drive, of course this is an additional expenditure.

DHAYANANDHAN.S

Fan curves will indicate the operating point and any change of Fan impeller diameter changes both flow rate and the Total Pressure. Reducing the fan rpm would also have a reducing effect on the Fan flow rate , Total Pressure and Power absorbed.

50% suction throttle for an FD Fan is to suit the negative pressure within the Boiler Furnace . This is a requirement since the FD Fan needs to balance against the operation of the ID Fan ( Induced Draft Fan). Hence the requirement for flexibility of operation.

So why are you reducing this flexibility ? Just leave it as it is has been designed.

I like these easy questions, because they only require a VERY short answer!

The question was.. "Can any one tell me that trimming of existing installed impeller of same fan will reduce the power consumption provided fan flow should be same as mentioned above?"

The answer is.. No!

IF the fan is presently throttled back 50%, the answers is one letter longer and is yes.

Trim impeller until you can run at 100% open valves and get the performance desired.

For that rotational speed, this fan must be belt driven. I think you would maintain better efficiency by changing sheave sizes to slow the fan. This might seem more expensive than trimming the impeller, but you won't need to disassemble the fan and re-balance the impeller after trimming. And, it's reversible if you over-do it (as long as you don't discard the old sheaves). Two other things: 1) if this boiler/furnace/? is running at part load, you must leave sufficient capacity for full load operation, and 2) if this fan has pre-rotation vanes as opposed to a throttling damper, you may not see much savings.

You can try a Variable Speed Drive. I know it can be very expensive but in this case it is a temporary alteration of the fan parameters and it will also help with reducing overall energy consumption since it acts on both voltage and frequency.

In case you want the fan to be permanently altered you can replace it with one that has about half the power requirements. In this way the motor will work at almost rated power (close to the maximum efficiency). Otherwise you'll have a 50% loaded 400 kW electric motor and this sounds like a lot of waste.

Purvesh

Some data are missing: In order to compute the power reduction, please also provide pressure profile (inches of H2O) at inlet, discharge of vane and discharge of fan. Also the existing diameter of the fan blade. Knowing these data, then use fan-affinity law to determine the power saving.

Hope the above helps.

Agreed on the concept, but please note that as mentioned earlier, the operation in the boiler draft control requires both modulation of FD ( Forced Draft) and ID ( Induced Draft) Fans. In addition there is also the Boiler Pressure auto control which includes the FD Fan.

The FD Fan and Fuel supply is regulated in auto mode based on the Boiler Pressure achieved to prevent over pressure. This will of cause create a higher draft within the furnace ; to offset this, normally there is a control system interlinking the FD Fan and ID Fan functions, so that with every movement of the FD Fan damper, there is a corresponding adjustment of the ID Fan damper to sustain a negative pressure within the furnace.

If your set up is not in this auto mode, then you better be very careful what you do with the speed setting. Should the furnace acquire a POSITIVE draft, you can have a serious blow-back of the furnace out through the front fire doors.

Suggest that you consult with your boiler manufacturers before you try any modifications. Good luck !

CR4 ADMIN: Deleted Post Spam: This post was deleted because it contained advertising outside the Commercial Space forum. Please review Section 14 of the CR4 Site FAQ about advertising.

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This is such a pitiful attempt at spam, that it's funny.

"In our normal operation (@full load operation) we used to throttle suction IGV of fan (Opening :50%), discharge damper condition:100% open, motor amperage in this condition is 42.0 Amp, flow rate : 60000 m3/hr"

It is not clear whether the normal operation is full load operation or throttled operation. There must be reduction in pressure rise when throttled, is it OK. What are the pressures anyway?

GA to comments # 21 & 22. OP is missing in the discussion. Trimming may be feasible for impeller with backward curved blades due to longer passage length, but speed control with pulley drive or with VFD is much better for power saving. Depending on requirement of flow and pressure rise you can use these fan laws to decide:

Flow = Ka. d**3. N (d= impeller outer dia, N= rpm)

Pressure = Kb. d**2. N**2

Power = Kc. d**5. N**3 (product of flow & pressure)

Theoretically the correct answer depends upon the construction of the fan and its characteristics and the goal desired for the overall operation. Once the fan characteristics are changed by the trimming, the fan curve will shift, though most likely in a non-advantageous direction since as we increase the space between the impeller and the casing, the efficiency, at that operating point, should go down.

There are no free lunches, if you want to move a given quantity of fluid at a specified condition, then the power required will not change. What will change are the losses due to changing the resistance to flow through the dampers.

In real life changing the physical characteristics of the fan can be foolish, after all it was sized that way for a reason. If this is a multifuel boiler then when you switch fuels you will probably need to change the balancing act between the FD and ID fans. With your trimmed fan you may find that you can't supply enough combustion air for a different fuel.

If this is an older plant then the dampers are there as a cheap way to control the flow back when electricity costs were much lower. VFDs might be a better choice if the plant will be in operation long enough to recover their cost.