saving power by using VFD instead of throttle valve

[usamaegypt]

Hi;
We have a fan drived by a motor 200 KW and has a throttle valve to control the flow. Most of the time the throttle valve opening is 30% . We would like to know how to calculate the power saving in case of replacing the throttle valve by frequency drive.
Best regards.

 

[cbarn24050]

Hi, hard to say from the details provided. I would expect your power consumption to halve, yout motor should need less current and so should run cooler but remember that the motor fan will run slower so you would have to check during operation. Some contries allow tax breaks for investment in energy saving schemes so don't forget to check.

 

[DickDV]

You state that the existing system has "throttle valves". It makes a lot of difference from an energy usage standpoint whether these are on the suction or discharge side of the fan.

Please confirm where these are positioned in your system.

 

[electricpete]

Let's say you can reduce speed of to put the motor to establish flow rate same as before with the valve wide open.

Let's say valve wide open is approx DPvalve=0.
The savings in power loss in the air system is approximately
Pairsavings = DP30%open x VolumeFlowRate
Where DP30%open is DP before when valve was 30% open

But you likely have some increased losses on the motor and drive system which will reduce your net savings.

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[electricpete]

I think my analysis was too simplistic. I assumed the losses would be described by VolumeFlowRate*DP. This does give units of power. However real throttling processes are more complex than this.

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[Warpspeed]

It would probably depend rather a lot on the characteristics (type) of fan. When you throttle it, does the pressure differential of the fan increase, or does it run at a similar pressure ratio with just reduced flow ?

Throttling flow may increase or decrease drive power.

If you plot motor amps at a range of throttled positions, it will give a much better picture of what is actually happening. If motor amps fall as it is throttled more, a VSD will probably not make a huge difference. If motor amps rise sharply with throttled flow, go for the VSD.

 

[digitrex]

If your throttle valve is having problem and you are comparing the price of replacing the throttle valve with a VFD, then it is worth investigating.

If you consider to abandon the good throttle valve with a VFD, I don't think you will receive a good payback.

When the throttle valve at 30% opening, the fixed speed motor should be at lower load compared to 100% opening. Therefore changing the fixed speed motor to VFD driven will only reduce the loss of the motor at low load.

In my opinion, it would be a good comparison only when a new project is being planned. If you select a VFD, you would save the cost of the throttle valve and some saving in electricity cost due to better efficiency.

 

[quark]

What type of a fan it is and what is your system configuration? Generally when you reduce the speed of a fan, its pressure developing capacity also decreases by 1/2nd power of the speed ratio. So you can't simply use the Q1/Q2 = N1/N2 and P1/P2 = (N1/N2)³ formulae to assess the power savings.

Check the total pressure developed by the fan(with valve wide open) and pressure drop across the fan. The difference of these two readings will give you the required pressure to pump air into the system. Now calculate the required RPM from (N1/N2)² = SP1/SP2. Based on this RPM you can be able to calculate the final power consumption.

As Warpspeed already noted, horsepower for vane axial fans and industrial panel fans increases when you throttle the valve. But speed reduction does help you. Have you thought of cheaper alternatives like belt drives?

For a good understanding on fans and systems, go to

http://www.tcf.com/TCFBlower/literature.htm#bul

Regards,

 

[DickDV]

Just a reminder here, folks--the performance rules for centrifugal fans in a relatively free-flowing system (air duct with open vanes) are well pubicized and understood.

They are: Flow is proportional to speed
Pressure and shaft torque is proportional to
the speed squared
Horsepower or kw is proportional to speed cubed

In view of these, we can safely state that the fan, at 30% of full speed will flow 30% of full speed volume, torque and delta p will be .3 x .3 = .09 or 9% of full speed and fan horsepower will be .3 x .3 x .3 = .027 or 2.7% of full speed.

The problem here is that the original poster is asking for a comparison of energy usage with a vane system but doesn't say whether the vanes are on the suction or discharge side of the fan. It makes a huge difference!!

Hopefully, this info will be forthcoming so the comparison can be made.

 

[electricuwe]

Replacing flow control by a throttle valve with a flow control by using a variable frequency drive will save a lot
of power.

If this will lead to a cost saving justifying the cost of VFD mainly depends on the time the device is operated at reduced flow. If you operate at reduce flow for one day every year stay with the throttle valve but if you operate with reduced flow for several 1000h a year you will the cost of the VFD will be saved within very short time.

Contrary to several "Energy Saving Devices" pushed on the market very agressively a VFD in a flow control application is a real "Energy Saving Device".

 

[Warpspeed]

Maybe not if the required fan shaft horsepower actually decreases as it is throttled.

This can happen if the fan provides an almost constant pressure differential right down to zero fully throttled flow.

Not all fans have this characteristic, but it does occur.

 

[usamaegypt]

DICKDV
the throttle valve is installed in the suction side
Regards

 

[quark]

Inlet guide vanes are definitely not an option at the reduced flowrates we are speaking about. They are generally economic if the minimum flowrate is around 80% of maximum flow, below that efficiency drops to a great extent. The new curve due to inlet control starts from the same shutoff head but with more slope towards maximum flow. So there is no significant difference in performance at lower flowrates.

Moreover, it is always possible to run a fan on the locus of BEP by using VSDs. Life of the rotating equipment improves due to low speed but care should be taken to avoid vibration.

This link gives a calculator to assess approximate savings when the fan is running at 60% of maximum flow.

http://www.hayesmechanical.com/news/articles/estimator.html

This link gives a pictorial view of fan performance with various flow reducing techniques.

http://oee.nrcan.gc.ca/Publications/infosource/Pub/ici/eii/M92-242-2002-11E.pdf

Usamaegypt,

Get the data of fan performance vs inlet guide opening from the manufacturer before attempting energy savings calculations.

Regards,

 

[DickDV]

ABB and many other drive manufacturers have energy-saving software available to find savings between inlet vanes and using variable fan speed.

But, without using the software or determining the exact numbers, I can say with confidence that there is significant energy savings by going to variable speed especially when operating at 30% flow.

So, for decision-making purposes, the variable speed choice is clear. If someone if pressing you for exact savings, then you will need a lot of specific data from the fan manufacturer and some measurements of your present system using inlet vanes.

There are also maintenance issues related to the inlet vanes that are gone when using variable speed as well as noise issues. These may or may not be important in your case.

 

[raisinbran]

I have an interest in the outcome of this discussion as our company is in the same situation, but with a larger motor on a centrifugal fan. The way I look at it is if the valve (damper) is on the inlet side, the work being done to move the air will be approximately the same regardless of whether the fan is controlled by an inlet damper or a VFD. If that is true, the VFD would have to be justified based on things like no load loss savings and less mechanical upkeep. It would be a tough sell and introduce reliability concerns that are not there now. VFDs are much more attractive if the "valve" is on the discharge side, especially on a liquid pumping application. Am I missing something with this summation?
Regards,
Raisinbran

 

[jraef]

Raisinbran,
Yes you are missing something big. There is no point in using the VFD at all unless you ELIMINATE the damper control. It makes little difference if the damper is on the inlet or the outlet. Power consupmtion relates to total flow, be it air or liquid, inflow or outflow. That's the point of it, the VFD modulates power put into the system, the damper modulates power used by the system. The difference between the two is the energy savings from the VFD, and the more time spent at low flow, the greater the savings. But if you use the VFD AND the damper, the difference then becomes negligible enough that the reliability of the VFD does in fact become an issue.

If for some reason you can't remove the damper control, some of the mechanical upkeep issues can be addressed by using a soft starter.

"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

Mmmm, jraef, wait just a minute please. It does make a huge difference where the dampers are located. Inlet dampers cause the fan to operate in a partial vacuum (less dense air) so the fan hp does go down---not nearly as much as when a vfd slows the fan but still there is a significant reduction.

On the other hand, a discharge side damper raises the pressure in the fan housing (air that is more dense) and the fan hp actually goes up as the flow goes down.

You can see this principle at work in an ordinary vacuum cleaner. If you block of the suction hose, the motor increases speed and the amp draw falls. This is similar to the inlet damper closing and since the fan is in a vacuum, it turns easier.

It takes a responsibly designed computer program to predict the savings in energy between inlet vanes and vfds. From experience, I know there is significant savings and vfds are replacing inlet vane systems all the time but, if you want real numbers, you have to run the program.

 

[jraef]

DickDV,
Point taken. Had I thought about it in more detail I may have remembered that, but I'm an electrical guy. My thought process sometimes stops at the motor shaft. I actually was not focussing on the two types of damper systems, just that if it remained in the system there would be no significant benefit to adding a VFD. I bow to those such as yourself better informed as to the mechanical issues.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Warpspeed]

The simplest way to explore this problem is to just fit an amp meter to the motor and exercise the dampers through the full range of adjustment. See what actually happens in practice.

 

[NI008]

I am sorry to jump in this discussion but could not resist.

I am also working on a proposal for installation of VFD on centrifugal pump with throtlling valve. The application is a standard boiler feed water pump 13 stages providing water to steam drum through a control valve.

Motor is 2600 HP. Valve at minimum steam flow condition provides 120 t/hr water as against 260 t/hr rated flow of pump so throtlling and recirculation take place.

At max steam flow conditions, 2 such pumps run but control valve opens only 60 %. water flow at these conditions is around 430 t/hr but both pumps have a capacity of of total 520 t/hr.

The problem here is that motor is not suitable for inverter duty so might have to be changed as suggested by GE and ABB. But still they think that payback period will be within 3-4 years.

I will appreciate if someone can give a clue as how to proceed on payback calculations.

Thanks,
Victor.