saving power by using VFD instead of throttle valve
saving power by using VFD instead of throttle valve
(OP)
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.
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.





RE: saving power by using VFD instead of throttle valve
RE: saving power by using VFD instead of throttle valve
Please confirm where these are positioned in your system.
RE: saving power by using VFD instead of throttle valve
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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RE: saving power by using VFD instead of throttle valve
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Eng-tips forums: The best place on the web for engineering discussions.
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
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,
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
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".
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
the throttle valve is installed in the suction side
Regards
RE: saving power by using VFD instead of throttle valve
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://ww
This link gives a pictorial view of fan performance with various flow reducing techniques.
http://oe
Usamaegypt,
Get the data of fan performance vs inlet guide opening from the manufacturer before attempting energy savings calculations.
Regards,
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
Regards,
Raisinbran
RE: saving power by using VFD instead of throttle valve
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"
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
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"
RE: saving power by using VFD instead of throttle valve
RE: saving power by using VFD instead of throttle valve
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.
RE: saving power by using VFD instead of throttle valve
You have a constant volume flowrate application. Why can't you change the pumps? Is new pump+vsd cost less than new pump cost? BTW, what is the discharge pressure of the pumps?
Jraef,
It is a common practice to control fans with IGVs and VSDs simultaneously for better capacity control(pariticularly with centrifugal chillers). One advantage (worst case) with having dampers though we use VSDs is that when the VSD fails we don't risk our system to overload.
Regards,
RE: saving power by using VFD instead of throttle valve
The factory will have such software and it is remarkably accurate. (It is not designed as a sales tool) ABB's system engineers use the same program for their own calculations.
You will find immense savings, I assure you.
This sounds like a Medium Voltage job. Ask your rep about running these pumps sequentially with a single drive and PFC software (that's Pump/Fan Control). I don't know if it can be done in Medium Voltage but, below 600volts it works very well. Basically, as flow demand increases, the first pump speeds up until it reaches line frequency. At that point, the drive transfers this first pump to across-the-line operation and begins accellerating the second pump. At maximum flow demand, both pumps are at full speed. At flow demand falls, the drive unwinds the whole process down to the first pump running at minimum speed. Each time the system is restarted, the lead pump is changed to balance pump wear.
It sounds like the perfect setup for your system if it can be done at MV.