VSD energy savings 18

I have been studying this as well. I found these quotes on this forum and others.

"Saving energy means reducing the energy cost per gallon of water pumped, not reducing the cost per hour to run the motor turning the pump."

"If a 10 HP, 100 GPM pump could be slowed by 15% and still deliver the head required, it would be producing 10 GPM, and drawing 6.1 horse power. This would only be 1.66 GPM per horse power, compared to 10 GPM per horse power at BEP, which means a VFD system is using 6 times or 600% more energy per gallon produced."

"Also when running the pump at full speed, maximum flow, or BEP, the energy used by the VFD itself, along with motor losses from the VFD’s sub-standard voltage wave form, causes extra energy to be used per the gallons produced."

"High flow or low, a VFD always causes more energy to be used per gallon produced."

jonr12

I don't know for shure if i agreed with that...sorry.
I can't consider that all people around the world that applies this equipments and measures clearly the energy cost savings are mistaken...
I didn't understand well those affirmations and i challange those people to prove it by means of calculations, as i am trying to prove the energy savings...

Thanks anyway for the answer!

It took some time before it started making sense to me as well. Even when reducing the speed of a pump with a VSD, the reduction in power used is not linear to the drop in gallons produced. The reduction in energy when using a VSD is figured by the cube of the speed, and is no where close to a linear reduction in flow. I have found that most of the claims of energy savings are being made by people who sell VSD's, and I have not found it to be true in real life. A VSD cannot make a pump more efficient, and therefore cannot save any energy.

Q2=Q1 x n2/n1
Q2=100 x 0.85 x n1/n1
Q2=85gpm


control valve + pump replaced by VFD controlled pump:
installation cost:
power supply cable section =1.5 nominal (reduced section through skin effect),VFD
added losses:
-heat from switching electronics
-increased heat loss in electric motor (skin effect)
savings:
pump always operates on BEP:
kW ofset BEP (currently)-kW BEP =savings
soft start/stops inncreases relyability

Saving energy with a VSD requires that you must operate a lot of the time within 50% and 85% of the pump's BEP flowrate and that the head delivered when at reduced speed is sufficient to circulate the reduced flow.

Below 50% speed, motor and VFD efficiencies at reduced load starts to hurt the economics considerably (I have the typical tables), which is why I don't recommend that, not to mention that you are only producing a head of 0.5^2 * BEP_H = 0.25 At 33% speed, that's 10% of BEP_H. Those two factors combined will kill VSD applications right there.

Its true that control valves waste energy, but at reduced flows, they don't waste a lot of energy and what they waste is usually less than the energy used with a VFD, due to the above inefficiencies, unless you are in a range of 50% to 85% of BEP_Q.

Above the range of 85%, the energy saved by a VFD OVER that of a properly sized pump and control valve is simply not worth (IMO) the extra maintenance headaches and sometimes the power quality problems that VFDs often bring along with them.

Once the energy use at various flowrates is established, to know if you will save energy or not requires that you know what flowrates you will have to run in the system over a given time period. Then you can determine the energy you will save over that time period. As I say, typically between 50% and 85% BEP_Q is where there is a potential to save energy, so you have to be operating inside that range to do it. Look at your system history and see if and how much time you operate there. Calculate the energy savings over those times.

Since variations in flowrate are needed in order for a VFD to produce savings over a properly sized pump, there are a few other possibilities you can consider. Look at how you can eliminate flowrate variations. Can you provide a tank and let the level in the tank fluctuate to smooth down those peaks and valleys in the flowrate? Can you adjust your process so that there is less variations in the batch sizes? Can you operate the system for longer time periods and pump at a constant average flowrate over, say 24 hours rather than just pumping at a big flowrate for only 8 hours per day and a small flowrate during the night? Fill a tank at the average flowrate then have the process draw from the tank at high demand periods. There may be many possibilities you could draw upon to eliminate any need for a VSD at all. I simply recommend that you consider those too.

VSDs do serve very well in certain situations. I find them most useful for,

Adjusting flowrates in a process where the output required today is typically less than the original design parameters of the system.

Adjusting the head in a system that has an unusual fluid, such as a heavy nonNewtonian hot crude oil that requires very high pressures to get it moving, but as the pipeline fills with hot oil and forces out the cold heavy oil, less and less pressure is needed to hold the design flowrate. Having said that, it is often just as economical to use two pumps arranged in series, operating 2, then 1. Which one is better? That depends on the specific fluid properties, the pipeline thermodynamics and the flowrates you need to run during startup and shutdown.

VSDs may also serve a purpose where you have many pipeline branches that are operated individually and each branch has a widely varying head requirement, such as in an irrigation distribution system, but then you must be very carefull to get the head you need at each flowrate. Too much head needed and a VSD won't deliver it at reduced flow. For that reason, in irrigation systems I usually prefer to open each branch with a valve, flow a certain time to deliver the needed water and close, then move to another branch. That way a pump can be sized for a constant flowrate. If head varies considerably a VSD may help, but again subject to its ability to deliver that head at a different flowrate. Because when a VSD changes speed to adjust head, it also changes flowrate, so be carefull if it can deliver the water needed during the operating time available for any given branch.

In general, I think I can agree with jonr, that VSDs are not the energy savings panicea that many manufacturers claim, but its not only their fault. Some guy (hopefully not an engineer) still has to install them in a system where they do not belong. If flow varies between 50-85% of BEP_Q, and you have to operate there, and at the same time head varies from 0.25-0.72 of BEP_H, you stand a chance to save some bucks.





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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Jonr12 - how do you get that a 100 GPM pump with a speed reduction of 15% would only deliver 10 GPM?

The laws affinity are well proven and states that

F1/F2=n1/n2, and H1/H2=(n1/n2)^2 and HP1/HP2=(n1/n2)^3

So assuming that the system you had initially would throttle about 30% of the head - then the 15% reduction in speed would mean a 15% reduction in flow - or 85 GPM.

And your power would drop by 39% (as you also wrote) - but not with 10 GPM but with 85 GPM. IMO you would expect a lower efficiency and the actual HP would be higer e.g. 7 HP. So not you get 12GPM/HP - better than originally.

Best regards

Morten

I let someone convince me that a 10 HP pump on a VSD would drop to a 1.25 HP load, when my demand dropped from 100 GPM to 10 GPM. The pump could only be reduced to 85% of speed, because I could not reduce the head requirement, only the flow. Figuring a reduction in load by the cube of the speed, this leaves it drawing 6.1 HP, even when the demand is only 10 GPM. I was told the BEP would follow the speed but, if it does, then the best efficiency is not as good at reduced speed as it is at full speed. Also the RPM can't be reduced as much as I first thought. The owner wants to go back to filling a big tank at full speed to save energy. This got me in trouble with the owner and I have been studying several things that claim to save energy. What I am finding out is the only way to reduce energy consumption is to increase the efficiency of the pump. A VSD did not make the pump more efficient.

You won't save energy if the head cannot be reduced.

The operating point occurs where the VSD adjusted curve intersects the system curve, then look down and get the flow.

The pump efficiency tracks the flow adjusted for rpm. Example: Say a flow of 80 gpm on the pump's 100% speed curve has an efficiency of 66%. When the pump's speed is reduced to some percentage N of rated rpm, say N=60%, the Pump flow will tend to be 60% * 80 = 48 gpm and the efficiency at that flow of 48 gpm will be (more or less) the same 66%.

But, if the system curve and the pump curve intersect at some other flow, not the 48 gpm, the efficiency will be thrown off that track. Say the pump curve and the system curve intersected above a flow of 55 gpm. Then you have to regress the 55 gpm efficency from that 60% speed back up to 100% speed and then get the efficiency off the 100% speed curve at that new flowrate. 55 gpm/60 * 100 = 92 gpm. Maybe that efficency is around 69%. A slight improvement over the 66%, so you'd gain a little pump efficiency with that one.

If the flow at the curve intersection point was less than the 48 gpm, Say instead of the 55 gpm we had before, this time its 43 gpm, you regress that back up to 100% speed, 43/60 * 100 = 72 gpm, so look at the efficiency on the 100% curve for 72 gpm and maybe that's 62%, so its less than 66 and you lose some pump efficiency with that flowrate.

If you can only reduce speed to 85% because of head, then that fits you into my general rules for NOT using a VFD. I agree you will not save money with a VSD on this system, or if you do, it will be such a very small amount it won't be worth the headache of installing the thing.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Another thing I've noticed lately, after having done quite a lot of hard calcs on the subject, is that it usually isn't cost effective to design a new system with a VSD. If you take the usual rules and method for defining a flowrate and making an economic decision on pipe diameter verses energy consumed and then sizing the pump for the head required by that diameter, you have to operate very close to that flowrate in order for the pipe diameter solution to be valid. Just a small drop in average flowrate is often enough to affect the economic selection of the pipe diameter to such an extent that the pipe diameter needs to be made smaller. Since the need for a VSD is usually based on the necessity to run smaller flowrates, a VSD can hardly ever be an economic decision for a new design, since reducing the pipe diameter also tends to decrease the flowrate. The net result is that you wind up pegging the absolute minimum flow required for your process with the pipe diameter and pump at BEP, without the need for a VSD at all. That reinforces my opinion that you must have a defined and very necessary flow variation in order to ever consider using a VSD to begin with.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Dear BigInch,

First of all, that you very much for your extended and careful answer!

1) I told that you have “typical tables”. Can you tell me where I can found them?
2) You mentioned that, for example, we can apply a buffer tank, that in my opinion is very correct from the point of view of the energy losses. But from the financial point of view, it might not be so advantageous in comparison with the VSD…
3) When we determine that a pump is functioning at a lower point that it ment to, isn’t it better to consider the motor substitution? And what about PWM controllers?...
4) It was a good example the one that you gave about crude oil, because I have many situations of installations with fuel oil that have the same specifications. In this kind of applications, usually there are positive displacement pumps. Is there any special problem in VSD appliances?

Thank you very much for your kindly answer. It was very clear for me!

Thank you Big Inch. That is one of the best ways I have ever heard it explained. I wish I had understood this earlier. I am going to figure out how to give you a star. So in my case with the pump at 10 Gpm and speed of 85%, 10/85 * 100 = 11.7 GPM at 100% speed. The VSD makes this pump only about 15% efficient. But I guess you can still say that is the Best Efficiency Point at this speed and flow. And you can still say BEP follows the speed, it just changed the Best Efficiency from 66% to 15%. Now I understand why the VSD actually increased the electric bill. Even with a system that can live with a 75% reduction in head, so the efficiency stays up at low RPM, it is at best just a linear reduction in flow rate and energy use, not energy savings. Researching energy saving devices the VSD or VFD comes up a lot. These calculations and my own experience prove this is not true. So are all the people around the world that applies this equipments and measures clearly the energy cost savings mistaken?

MortenA

Correct. That was what made me a bit confused

Dear Bing Inch

Thanks for that last explanation. In fact it is very complex to determine effectively what will be the benefits.
My only experience (and that is why i posted this topic) is that the only way that we can make this is to install one VSD in a pump that fits those conditions that you mentioned and see how it works for a month and determine the potencial annual savings.

Thanks! I think i understood correctly.
1) the system must fit in the 50%-85% flowrate and have a variable behaviour;
2) consider first other options: analyse the whole system, consider some changes in the process or even to install a buffer tank
3) make the calculations to detect the potencial savings with the installatio of a VSD

In fact, i have another question that i would like to have a opinion. I will give my point of view and you can tell me if is a correct approach.
To make those calculations in the specific case that i may detect that a VSD is possible to apply, i must make some measurements in loco. So, my problems relates with this. The only thing that i can measure is the power consumption of the pump, and maybe the differential pressure across the pump if there is installed some instrumentation. With the power consumption i can calculate the flowrate through the affinity laws, but my dilema is:

1) In systems with control valves near each consumer but with no bypass line witch can balace the circuit, the power consumption oscilates, even if this oscilation is little, and i can measure it during time and so on...

2)The power consumption should be the same in a system where are control valves in each consumer and with a bypass line also with a control valve that balances the circuit. This is a BIG problem, because in this kind of systems i can't calculate the aproximate flowrate during time.

What should i do in this cases?
Thanks

You have little hope of making a decision for a complex system and supporting it by calculations unless you can model the system operation quite accurately and you know the flow you must have going through the pump at all times. Your only alternative may be to make that decision by direct testing.

The only alternative to having proper head and flow measurements is to assume some typical operating scenarios, calculating the flows and heads and any resulting energy loss or savings for each scenario. Then you will have to guess how much time you spend in each scenario. It may be able to get an idea about if you should proceed with conducting actual tests, or if there appears to be such little chance to save energy that you should abandon the tests entirely. It should be possible to at least estimate some basic scenarios based on the power you are consuming now. The disadvantage in that method is you will know the power consumed, but not the actual combination of head and flow that resulted in that power consumption. Since flow varies directly with speed and head with the square, its slightly more important to know head required and guess the flow. Can you install suction and discharge pressure gages?

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

A good break point for the decision could be my 85% flow rule. If you can't reduce speed below that, and assuming that you do in fact still spend the most time nearer to 100% flow than 85%, the money you save will be very small indeed. If less than half of your operating time is below 92% speed, don't do it. If more than half your operating time is between 85% and 92%, go ahead and test. Don't have very high expectations about big percentage changes in the bill.

If you do the testing, please let us know how it works out.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Dear BigInch

thanks once more.
I suspected that...So what you say (and its logic) is that in most cases, does not worth it to perform so many calculations do obtain few savings. Ok.
About the pressure gauges i have mentioned that before. That is one thing that i migh be able to obtain if that is already installed, and i think so... What you are saying is that i can use those values and with the affinity laws obtain the flowrates? This means that i have to register all values for some hour or twon in site, correct?

Another thing, previously, you mentioned the buffer tanks. How can i estimate the energy savings in this case? May i consider that those points that the power consumed should be the same as in normal conditions and the the excess is eliminated by the buffer tank?
I ask this because i am studying the same thing for compressed air: the installation of buffer tanks near big consumers. Here i also am needing to calculate those savings, witch is quite dificult...

In relation to testing VSD, as soon as i have that situation, i will make some kind of report and post it here, because i am very interested in this subject.

thanks!

Use the discharge press - suction press gages to estimate the head that the pump is adding. With the power reading and the head from the pump, you may be able to estimate flow in the pump by looking at the pump curve.

Tanks can save energy by moderating the maximum flow in certain cases. Consider that in most systems you have a range of flowrates that a process can run. Let's consider a process that flowrates vary from a low of 50 gpm for 30 minutes, with a time adjusted daily average of 60 gpm (for 24 hours) and a maximum flowrate, needed for only emergency conditions of 100 gpm only for about 10 minutes. When designing this system with a pump only, no tank, you must be able to reach all flowrates over the range of 50 to 100 gpm and the pump and piping would have to be sized to deliver the full 100 gpm. But there may be a more optimum solution if you consider providing a tank. With a tank of the proper size, you could design the pump and pipe for any flowrate you choose between the average flowrate and the maximum flowrate. As system flow capacity increases from the average flowrate, the cost of pipe and pump increase, but cost of tank volume goes down. At 100 gpm, tank cost is zero. The optimum tank size is the tank volume that balances the cost of providing the tank against the cost of increasing the flowrate of the system and operating the system at the higher flowrate too. As you can see, providing a tank reduces flowrate and reduces operating cost, so very large savings are sometimes possible if a tank is provided and the system can be designed to run at a lower flowrate.

The amount you can save on the pipe size varies with the location of the tank in the system, because the pipe coming from the tank still has to be able to reach the maximum flowrates. If its a pipeline feeding a delivery tank, you can feed the tank at average rates and draw from the tank at maximum rates, so the long pipeline could be designed for average flow with only the final delivery piping from the tank sized for the maximum flow. If its a supply tank, the saving from reducing pipe size will not be possible.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Allrigth. I understood. Its logic that this kind of tanks should be installed near the consumers.
I will spend some hours to perform some calculations, because my intention is to obtain energy savings to include in energy audits. So i need to make that.
As soon as i do that i will try to share it.

Thank you!!!
I think its all for now in this topic.
Thank you all, specially to BigInch!

Next time i will post some results regarding my VSD experiences!

These are the figures I have for VFD Efficiencies at partial load. There is virtually no public information, but after a lot of searching, I managed to find these in a report done for the US DOE. The original table had some missing values in it that I had to estimate. If anyone has more or better info on this, please post it in this forum and/or e-mail me a copy and I'll do it.

Percent of full load
25 33 50 66 75 100

VFD HP Eff_25 Eff_33 Eff_50 Eff_66 Eff_75 Eff_100
1 0.09 0.21 0.44 0.63 0.71 0.83
2.5 0.20 0.34 0.62 0.78 0.82 0.90
5 0.30 0.44 0.75 0.86 0.88 0.92
10 0.35 0.50 0.79 0.88 0.90 0.94
25 0.36 0.51 0.79 0.89 0.91 0.94
50 0.43 0.58 0.84 0.91 0.92 0.94
100 0.55 0.66 0.89 0.94 0.95 0.97
250 0.61 0.72 0.91 0.95 0.96 0.97

I have attached a typical chart for motor efficiencies at partial loads. I would also appreciate any better info someone might have about these values too.
See attachment.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Something to be taken into account is that the VFD takes up room in the switchroom. The VFD loses 3-5% in heat. This heat has to be disposed of by airconditioning , at least in Australia. So there is added energy in the extra heat load.

The increased size of switch room needs to be taken into the Cpaex and Opex calculations.

Motor efficiency data is predicatd on a sine wave form AC supply. By having a chopped square wave or PWM the motor efficiency is affected. teh motor manufacturers do not publish this data even if they have tested the motor.

If you intend overspeeding your pump to get more out of it you will not doubt need a special motro that may not work with a DOL supply. this increases maintentance costs. If the motor fails and there is no stock to hand you have the spectre of loss of availability of plant. FMECA is required .

Biggest challenge is the inability of engineers to select pump correctly. They add factor to factor and drive the efficiency backewaards. Then they add a VFD to be sure, to be sure. This all spurred on by the VFD salesfolks.

How many engineers allow for the future without doing a NPV evaluation of replacing the pump "when the future comes" rather than the cost of the VFD and its ineffiiencies.As we cannot see the future it may never come or it may sneak up on your and bite you on the @rse.

Have you considered that many authorities require the string testing of pump, motor and the duty VFD. What a nightmare getting VFDs from Sweden to a pump manufactuer in Brazil with a motor from Taiwan. The who is to bame if it does not work when you get it to site, unpack it install it and try to make it work??? Without a VFD this becomes a whole lot simpler.

Go to a refinery where the competence of engineers is recognised as the highest. You dont see VFDs. Why? They can size a pump and do the costings and risk assessments. the have a better take on the future and they are not bnervous nellies. They do not rely onthe supply to select the pump/control valve/VFD as do the "catalogue" engineers commonly found in the water and mechaanical services industries.