VSD energy savings 18

[mfqd13]

Hi,

I would like to perform some detailed calculations to determine the energy saving in the appliance of a VSD (variable speed drive) in electric motors mainly for pumps.
I searched in many references and there are some explanations, but i didn't find yet one that suits for my intentios. Complete enough...
So, my base point is that i can only measure in site the power consumption of the motor and with this i would like to perform some calculations to estimate the energy saving after apply a VSD.

Can anyone help me?

 

[ScottyUK]

"Go to a refinery...You dont see VFDs. ...they do not rely on the supply to select the pump/control valve/VFD as do the "catalogue" engineers commonly found in the water and mechaanical services industries."

Total bull. They are applied all over refineries, but in applications where they can make a useful difference. Same applies in power plants, where the engineers are of at least the same calibre, if not higher, than the refineries.

In the right application they can work, in the wrong application they are a nightmare. Anyway, who are the real guilty ones - the over-zealous salesmen pushing product where it isn't suited, or the lazy engineers who don't bother to do the calcs before buying?




----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[jonr12]

I don’t think the engineers are lazy. I think they have been convinced by the manufacturers of VSD’s, as well as many utilities and government entities, that a VSD will make any pump match any condition, and still save energy. Why bother doing the calculations when we can just match the pump to the maximum demand, then use a VSD to “save energy”, while making the pump match any demand from 100% flow to 0% flow?

I think a lot of facts like the loss of efficiency at partial load, and that head is reduced by the square of the speed, are being purposely omitted or ignorantly neglected. Purposely omitted by the manufacturers, government entities, and utilities, that want us to believe that a VSD always saves energy. The manufacturers benefit from tremendous sales, by being able to claim they produce a product that will help the world save energy. Government entities and utilities benefit from being able to claim, that they are helping the world by promoting or even mandating the use of products that “save energy”. Facts are being ignorantly neglected by engineers, plant operators, and the general public, because they believe that manufacturers, government entities, and utilities would not purposely deceive, to justify their existence or simple greed.

The fact that input power is reduced as the speed is reduced, makes it easy to dupe us into believing that a VSD saves energy. The average person sees the amps or watts being reduced as the speed is reduced, and thinks energy is being saved. Sadly, the energy use or electric bill actually increases because a VSD always increases the energy use per gallon produced. The owner in my case is still so convinced that a VSD will save energy, that I as the engineer am taking the blame for applying it incorrectly. There is so much propaganda on this subject that I am having a hard time convincing the owner, that a VSD will always increase the energy use per gallon produced, no matter how it is applied. My suggestion to go back to DOL controls and use a big tank which will save energy, is being met with resistence. Now a couple of other engineers have been brought in to reveal the error of my ways.

These other engineers are also of the mind that a VSD always saves energy, and are convincing the owner that I do not know what I am doing. I am sure that their attempt to “save energy” by “correctly” applying a VSD will also fail. By the time the owner is convinced that a VSD will not save energy, he will be too embarrassed to admit that I was correct, and I will have lost one of my best customers forever.

I believe that there are applications where VSD’s can “make a useful difference”. I do not believe that a VSD can save energy. What I have learned from this is, “all the people around the world that applies VSD equipment and measures clearly the energy cost savings are either mistaken” or outright lying.

 

[BigInch]

Simple. In fact I've been looking for a client like that. Tell him you're so sure that he won't save energy with this VFD that you will make a special contract to pay him equally for any savings he makes over the entire year, if he agrees to pay you equally the amount he loses. Settle up each month.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[mauricestoker]

BigInch,

Always enjoy the VFD/VSD conversations. Don't overlook the fact that is easier to put a VFD on a pump than it is to select the proper pump curve. If the customer is sold on VSD's, I have a site where the customer is convinced his inlet guide vanes are controlling air flow to 17% turndown. I have a special hat made out of tin foil just for meeting with that customer.

Jonr12,

How is this part of gub'mint blame? Being a gub'mint energy engineer, I assure you I find it all very funny excpet that tax money is wasted. Gub'mint has enough ignorance to dance around without having private sector nonsense added. VSD's and VFD's clearly can save money, being stupid can clearly waste money. Gub'mint clearly does not have a monopoly on stupid, or pet rocks would never have been sold.

 

[BigInch]

Right Maurice. Its a band aid for tax credits. Just like wind turbines, IMO.

The Fed. government has made it too easy to get tax credits
for any system that installs energy savings equipment. I would imagine the proof requirements are nonexistant, or at best quite lax. From what I understand there are also state aid programs that fund utility rebate plans. They would pay a good portion of cost and installation and I'm told that many VFDs went on irrigation or wate supply systems with water wells with high static heads, etc. The latest news is that a lot of the utilities wised up last year and the practice has slowed down somewhat.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[MisterDonut]

VSD wizards, what about VSDs on boiler feed pumps? I suspect this is not an application in which energy will be saved. Specifically, putting VSDs on boiler feed pumps in a small power plant with multiple pumps serving multiple boilers, each boiler having a modulating drum level control valve.
In this application, as I understand it, the speed of the pumps is modulated to meet a pressure setpoint in the pump outlet header. The flow rate will vary, since the power demand of the plant varies.
I appreciate any insights…

 

[jonr12]

Maurice, I meant no disrespect. I believe the government is being duped by the manufacturers, like everyone else. A VSD/VFD will show to save energy when it is compared to an extremely inefficient system. But compared to a pump running DOL at it's Best Efficiency Point, a VSD/VFD is wasting energy.

My simple math tells me that the most efficient system would be to run the pump DOL and always be filling a tank at BEP. Then I can draw from the tank at any flow rate, and every gallon I draw from the tank was produced at the lowest cost per gallon possible.

I can get fairly close to the same efficiency with a two pump system, using a 3 HP and a 7.5 HP pump, instead of a single 10 HP.

At any flow rate, 10%, 50% to 85%, or even 100%, a single pump controlled by a VSD/VFD will use more energy per gallon than either of the above systems. IMO a VSD/VFD causes more energy use per gallon. So I don't see how they can be promoted as energy saving devices.

I am beginning to believe that the only reason to use a VSD/VFD is that, "it is easier to put a VFD on a pump than it is to select the proper pump curve". That would be the mark of a lazy engineer, government or not. I have a special hat made out of tin foil just for anyone who thinks a VSD/VFD can efficiently turn down a pump to 17%, or really any flow.

 

[BigInch]

Boilers usually have a narrow range of high operating pressures which have to be maintained for most all flowrates, therefore VSDs may lose too much head when rpm is reduced to match lower flowrates. At 80% of rated speed, you produce 0.80^2, or 64% of rated head, at 50% speed only 25% head. Check your operating pressure variation with flowrates. If your required head doesn't follow those same head variations with flow, its probably not a VSD project.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[stanier]

There was a major VFD manufacture promoting the use of VFDs to avoid waterhammer. When I explained to them that VFD s do not function when there is a loss of power and thus adds nothing to this form of mitigation they were not the slightest embarassed.

If a pump is being stop/started and needs a VFD for this purpose then a larger suction tank, flywheel on the pump, three smaller pumps instead of two larger pumps may be the way to go. Or perhaps the pump is too large.

If a large pump is needed for flood conditions then have a small pump for routine duties and the large pump for the flood conditions.

My work with engineering students and trying to recruit them to the Institution of Mechanical Engineers has left me dismayed as to what is being taught at university. Students seem to be right up to speed with management topics, the environment, risk analysis, industrial relations, computer graphics etc etc. However they cannot size a pump or control valve. When they get into the market place they aim straight for management as there lies the money.

 

[ScottyUK]

BFW pumps are usually high speed, high power machines. Once you cross the line from LV to MV drives the costs jump significantly, and the payback is much longer if ever achieved. MV drives appear to have a higher failure rate and repairs take longer and cost more. For large utility-class power plants VFDs are unlikely to be a good retrofit. If the BFW pump was selected from the outset for variable speed operation and the system designed around a VFD-controlled pump then they're likely to be more successful, but VFD control of BFW pumps is a bad example for the reasons BigInch has clearly noted.


Stanier,

The IMechE is not alone, the IEE-accredited courses are no different in their bias toward the soft skils and in the neglect of principles and the 'old' subjects. And I know it is now 'IET' but I opposed the creation of the IET when it happened and, having seen what a disaster the idiots at Savoy Place have masterminded, I oppose it even more strongly now.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[jonr12]

I found this in one of my searches.

"Even if a VSD/VFD is programmed to operate very fast, it is still a programmed amount of time, instead of a direct and instant reaction to a change in pressure. Because the reaction time of a VSD/VFD is just a bit off, it is like landing on a trampoline a split second after someone else. It causes a catastrophic collision and shoots the person (pressure) extremely high. Under normal operating conditions, VSD/VFD controls can accentuate or even perpetuate transient pressure waves, which can actually cause water hammer, not prevent it."

On top of all that as Stainer said, "a VSD/VFD can do nothing to mitigate water hammer problems when the power goes off". Surge tanks, and/or surge anticipator/pressure relief valves are still needed to help with water hammer during a power outage, and apparently during normal operation of the VSD/VFD system as well.

As BigInch said, "Boilers usually have a narrow range of high operating pressures which have to be maintained for most all flow rates, therefore VSDs may lose too much head when rpm is reduced to match lower flow rates." Also hot water has no lubricating or cooling value for the pump. The minimum flow required for a pump moving hot water is fairly high. The pump will still probably require a re-circulation or by-pass line, which limits the minimum flow possible, and further limits the usefulness of a VSD/VFD.

As ScottyUK said, "The IMechE is not alone, the IEE-accredited courses are no different in their bias toward the soft skills and in the neglect of principles and the 'old' subjects". The manufactures have convinced most people that a VSD/VFD will make any pump match any job, so we no longer have to waste our time selecting a proper pump curve. They also claim a VSD/VFD makes any pump, doing any job, save energy, and eliminate water hammer. I think these are just more instances where, "all the people around the world that applies VSD equipment, measuring clearly the energy savings and water hammer reduction, are either mistaken" or outright lying.

I fell for these things just like everybody else. One of the largest manufacturers of VSD/VFD controls had very little problem convincing me these things were true. Now I am having to educate myself about VSD/VFD controls, and work my way through all the problems these lies created. Isn't there an old saying about "the bigger the lie, the easier it is to get people to believe it". Aren't there laws against such mis-representation?

 

[BigInch]

The more reputable manufacturer's say this in the fine print, so its all legal... kind of.


Now. I'd like to talk to you about this bridge right in the middle of NYC.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[MortenA]

Another possible conclusion could be that energy is too cheap (at least in the US).

A 15% saving on power can buy you some hardware here in Denmark where energy taxes are higher.

There was a link to some very interesting articles on the subject:

http://canterburyengineeringassocia...ion=com_docman&task=cat_view&gid=13&Itemid=33

They tell why you often dont see the saving that the affinity equations promise on first glance. Read them!

Best regards

Morten

 

[BigInch]

Or on the second and third glances either.

Yes, nobody said that VSD don't have their place and, if certain conditions are meet, they will save more than 15% too. Its just that those systems where VSDs will save money I think are more common in industrial fabrication or food processing plants where static heads are not high, fluids may be thixotrophic and flows vary anywhere from 0 to 100%, with all flowrates of more or less equal probability. I just think that those conditions are not nearly so prevalent in continuous fluid transfer processes at industrial sites, product pipelines, irrigation systems and water supply. And when they are its often cheaper to stop during the day and pump all night when power rates are even cheaper. Even in product pipelines where specific gravities vary a lot and pressures change, delivery rates vary with viscosity, etc. you could control a lot of things with a VSD, but over the long term, its still better to just load the product as fast as you can and run at 100% with all products, because that maximizes volume delivered ... and sold ... and, after all, that is what pays the bills. VSDs need to fit the system and the objective too. I've been doing product handling for quite a while now and I can count the times I needed a VSD on one hand in a new system.

Take away lesson. You just can't ever beat the energy efficiency of running a properly sized pump at BEP 100% rated flow, or the volume delivered.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[jonr12]

Thanks MortenA for that link. Mr. Vaillencourt does an excellent job describing how the second Affinity law prevents the VSD/VFD from being able to reduce the pump's RPM by as much as the salespeople will describe. However, even Mr. Vaillencourt says that the .9 reduction in horse power in his first example, and the 1.7 HP reduction in his second example are savings in energy, when this is absolutely not true.

In his Example 1, the second Affinity law shows that when the flow is reduced by 50%, the speed of the pump can only be reduced by 10% and still produce the head required. This reduces the horse power from 3.3 to only 2.4 HP. Although Mr. Vaillencourt says this is "not sufficient to economically justify the use of a VFD", he still claims the .9 reduction in horse power is a savings. In reality the BHP equation and even pump curves I checked show that this flow and pressure can be accomplished with only 1.65 HP. So a VSD/VFD reducing a 3.3 HP load to 2.4 HP is still wasting .75 of a horse power. This means the VSD/VFD system is wasting 32% more energy than a smaller pump running at BEP, or the larger pump filling a tank at BEP.

In his Example 2, Mr. Vaillencourt shows the speed can be reduced by 21.5%, and that reduces the horse power from 3.3 to 1.6 HP. He says that "clearly the 0.413 bhp calculation would result in a gross overstatement of the savings". But he also says that the actual drop from 3.3 to 1.6 HP "is a significant amount of savings from the application of a VFD in this example". Again this is not true. The BHP equation shows that 1.6 HP is the correct amount of power for this flow and pressure so, the VSD gives only a linear reduction in HP to flow, not energy savings. To top it off, neither of these examples takes into account the loss of motor efficiency at partial load as described by BigInch, which would limit the reduction of speed, horse power, and energy even more.

MortenA says, "A 15% saving on power can buy you some hardware here in Denmark where energy taxes are higher". If this is true then there should be tax credits or incentives paid to anyone who removes a VFD from a system. There are considerable savings to be had for correctly sizing a pump to run at BEP, or for using a big tank where the pump always runs at BEP regardless of the flow rate, compared to using a VSD/VFD.

BigInch says, "The more reputable manufacturer's say this in the fine print, so its all legal... kind of". You have to study the fine print to see that VSD/VFD's actually waste energy. Saying that a VSD/VFD saves energy is absolutely not true, so I don't see how it could be legal. The best thing I got from Mr. Vaillencourt's paper was his use of the word "Myth-Applied". I now believe almost every VSD/VFD is "Myth-Applied".

 

[btrueblood]

"I now believe almost every VSD/VFD is "Myth-Applied". "

Dunno about "almost every", but then I deal with people whose systems cry out for VFDs and other technologies. Your post is true and correct when the design point is just that - a single operational condition (or, as in BigInch's examples, the maximum possible flow, e.g. pipelines delivering commodities). But, if the design condition is a range of flow rates, or if rates vary unpredictably over time throughout a system (variable heating loads in chilled water piping systems, e.g.), and TDH is not a concern, then a VFD can save significant power.

 

[jonr12]

I feel your pain. Even with all I have learned recently, I have heard it so many times that I still want to believe a VFD can save energy. I now understand that a VSD/VFD cannot reduce the energy used per gallon under any condition. It is like a Tar Baby. Once people see the amps drop and have it in their head that a VFD/VSD will save energy, it is hard to shake it off. You are the third person just in this discussion that still holds to this myth, even after several reputable people have carefully explained why a VSD/VFD cannot save energy.

TDH is always a concern, otherwise there would be no need for a pump. A VFD cannot magically cause the pump to produce more flow with less horse power. I have been talking about a system with flow rates that vary unpredictably over time, and the installation of a VFD increased the electric bill by 300%.

My calculations show that there are several ways to maintain the best efficiency as flow rates vary but, VFD control is not one of them. I can see where with things such as a chiller, it would be "easier" to just add a VFD to vary the flow, but it would also be less efficient than a multiple pump set up. A VFD may even be a good way to control a multiple pump system and round the edges between the pumps but, it is still the multiple pumps that would save energy, not the VFD.

I have been reading many articles where a VSD/VFD was praised for "saving energy". In every case I find the VSD/VFD is not the reason for the energy savings. Usually the system was altered to reduce the head required, or a smaller pump was installed. Yet just like this discussion, somebody always tries to end the article with, "the VFD is saving $90,000 a year", which is total BS.

Once I finally understood that a VFD waste energy, and I got the VFD idea out of my head, I came across several other good ideas and ways of controlling pump systems. I wonder how many real energy saving ideas are being neglected because everybody already mistakenly thinks the VFD is the end all, be all, of energy savings. I even wonder how much energy is being wasted using motors that were designed to withstand the abuse of VFD controls, instead of designing motors and pumps to be more efficient.

As BigInch said, there is nothing more efficient than a properly sized pump running at BEP. The soft skill of using a VFD is no substitute for the "old school skill" of choosing the right pump curve(s).

My take away lesson from all this is; Any VSD/VFD that was specified to supposedly "save energy" has been "myth-applied".

 

[BigInch]

Now wait a minute. Even I still believe in the "myth".

While kW/gal is more with a VFD, that statistic alone does not tell the whole story.

Say you have to pump less than BEP for some reason (whatever it might be), say 80% BEP, for a specific time interval. With a VFD the energy bill will be less, because WITHOUT a VFD and a control valve you are effectively pumping 100% BEP at 100% head INSIDE THE PUMP, as 20% of the BEP flow is being pumped up to the top of the pump curve head is actually recirculating within the pump. That is an energy loss too, which is bigger than the energy used by the VFD.

You have to compare kW/gallon in the discharge pipe, not just kW/gallon, as a lot of that might just stay inside the pump.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[btrueblood]

"You have to compare kW/gallon in the discharge pipe"

Right. Or better, $/gallon. Or euros/liter.

"Any VSD/VFD that was specified to supposedly "save energy" has been "myth-applied""

Um, change "Any" to "Many", and "save energy" to "pay for itself" and I think we all will mostly agree.

 

[BigInch]

Yes, its the cost of USEFUL work that we need to minimize. That's delivered flow, not flow circulating around in the pump. Of course the cost of mostly useLESS work per hour would, and should, be low.

**********************
"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)

http://virtualpipeline.spaces.live.com/