PDA

View Full Version : VFD's on screw compressors







keepitcool
02-07-2008, 06:48 PM
Hello Everyone,
I am looking for a little feedback about the subject of VFD's on screw compressors. We have done a couple of payback studies for retrofitting VFD's to existing machines and it does not seem practical to me to do this. I have seen some benefits to it in a brand new facility but from what I can tell it seems as though it is hard to do better than just installing a high quality controls system. Most plants I have seen have many more efficiency problems than how fast the compressor motor is turning. I look forward to everyones thoughts on this.

Grizzly
02-07-2008, 06:57 PM
If the load is constant and the Screw has been sized then you are probably correct.
But the huge benefit of using a VSD.
I assume you mean V.. variable,
S..speed, D.. Drive.
Is when the load is variable particularly when loads fluctuate wildly.

If used correctly huge energy savings can be realised.
But to be fair this is just a general view.
And to truly evaluate the savings, you need a professional survey carried out!
Cheers Grizzly

US Iceman
02-07-2008, 08:28 PM
If the compressor operates at full load all of the time, inverters are waste of money. You might get some operational benefits due to the controlable ramp up for in-rush current though o medium voltage systems.

If the compressor operates with wildly variable loads 24/7 then the inverter is worth looking at.

Control systems only turn things on & off basically. If you want to save a lot of money look at how the system is designed...

Sergei
02-07-2008, 11:06 PM
I think that this is very interesting issue. Many people believe that every compressor VFD saves energy. It depends of operating conditions.
Why do we need VFD? To recover losses related to part load operation of screw compressor. What is the magnitude of these losses? Four factors are important.
1. Operating time. Usually, screw compressor operate as trimming one or 8760 Hrs annually.
2. Suction pressure. Higher suction pressure will reduce losses.
3. Discharge pressure. Lower discharge pressure will reduce losses.
4. Average part load. This is the most difficult factor to determine. Losses will be higher at 30% of average part load than at 70%.
If your plant operates at high suction pressure and low condensing pressure at 70% part load , the losses will be minimal.
VFD isn't 100% efficient. Typical compressor VFD losses is 2-3%.

Grizzly
03-07-2008, 07:20 AM
VFD?
OK Guys I am showing my ignorance now!

Do you call, what we in Europe call VSD.
VFD in the Americas?
Sorry to keep on, but I am intrigued to know if we use different terminologies?
Cheers Grizzly

PaulZ
03-07-2008, 07:57 AM
Hi Grizzly
It's the same. Some people in Aust call them a VFD (Variable frequency Drive) some call it a VSD (Variable speed drive).
I think it depends on what side of the track you grew up on.
Paul

keepitcool
03-07-2008, 12:13 PM
Thank you for the feedback. Sergei, I agree. To me this is very interesting. We have done some research to find where the real value is. I have been told that two compressors at 75% FLA are better than one on a slid valve at 100% and another at 50% FLA. I disagree. It may help to have one high side and one low side compressor on a VFD but putting them on all the compressors seems like a waste of money.

US Iceman
03-07-2008, 02:10 PM
It may help to have one high side and one low side compressor on a VFD but putting them on all the compressors seems like a waste of money.


This gets back into how you stage your compressors with the control system. If you try to run some logic that says equal run-time is what we want, then you might need more than one VFD. A lot of this depends on the number of compressors, sizes of compressors, etc.

I certainly don't think you need a VFD on each compressor, but several might be a good idea. A lot of this depends on your control logic and load profile.



I have been told that two compressors at 75% FLA are better than one on a slid valve at 100% and another at 50% FLA.


It depends on the total power costs, which is actually better. Power is what you pay for so you are looking for the best combination of options that minimize the total power costs. In some cases you might find the system operates slightly less efficient at some period, but overall the cost of the power might be less.

Sergei
03-07-2008, 02:24 PM
Thank you for the feedback. Sergei, I agree. To me this is very interesting. We have done some research to find where the real value is. I have been told that two compressors at 75% FLA are better than one on a slid valve at 100% and another at 50% FLA. I disagree. It may help to have one high side and one low side compressor on a VFD but putting them on all the compressors seems like a waste of money.
Usually, screw compressor efficiency will decrease linearly(gradually) from 100% to 50% capacity(not FLA). So, no difference between 2 compressor on 75% or one 100% and one 50%.
Efficiency will decrease exponentially(fall) from 50% to 10%(min). It means that better 2 compressors on 60% than one 100% and one 20%.
Certainly, VFDs(VSDs) on all compressors is a waste of money. Usually, one compressor with VFD per every suction pressure. Sometimes one VFD is a waste of money as well.
Why did you invest in VFDs? Probably, because everybody around invest in VFDs. I don't think that it is good investment. Assume that $100 investment in VFD will save $20. Optimization of condensing pressure, very often, will save $1000 per $100 of investment. Why do invest in VFDs first? I think that it should be the last step in energy saving process.

US Iceman
03-07-2008, 02:44 PM
Hi Sergei,



Why do invest in VFDs first? I think that it should be the last step in energy saving process.


Because these are what the salesman sell!

I agree with you on this point entirely. VFD's are the last thing you should investigate. There are so many more opportunities to save energy in a refrigeration, unfortunately though, not very many people know how to find them.

Minimizing the discharge pressure is a good place to start looking.

keepitcool
03-07-2008, 03:00 PM
Your right. Salesmen are pushing them. Lowering head pressure, proper compressor sequencing and cycling evaporator fan are all good. That is why I think a proper control system is a far better investment than VFD's. (No, I am not a controls contractor).

SteinarN
03-07-2008, 03:05 PM
Hi Sergei,



Because these are what the salesman sell!

I agree with you on this point entirely. VFD's are the last thing you should investigate. There are so many more opportunities to save energy in a refrigeration, unfortunately though, not very many people know how to find them.

Minimizing the discharge pressure is a good place to start looking.

This is to a large extent true. And if someone know where to save energy, there is not enough understanding of this amongst those paying for the investment. Those paying for the investment is in many cases not the same as those paying for the operating expences.

What I've found regarding comercial systems is large condenser, low condensing, floating condensing, low RPM low wattage condenser fans, large evaporators, low pressure drop in the suction line especially, high suction pressure, SG/LL heat excanger when possible, defrost on demand and efficient compressors.

It is easy to find two different semihermetic resips with 10% difference in power consumption/COP at same operating conditions. 1C excessive pressure drop in the suction line and 2C lower evaporating due to low capasity evaporator is 3% to 4% increase in power consumption. 5% difference between low RPM and "normal" rpm fans. And the list goes on......

It's not hard to design a system semingly professional with twice the pover consumption of a system designed with power consumption as the first priority.

But, when this is said, a VFD can also contribute to large savings on some systems.

Sergei
03-07-2008, 03:53 PM
Recently, I heard presentations of 2 compressor manufacturers about compressor VFDs. Surprisingly(or not surprisingly), all estimations of energy savings were done for 180 psig of condensing pressure. Personally, I don't know any refrigeration plant(ammonia) operating at 180 psig all year around. May be they know. However, reducing pressure to 160 psig will save more energy than all VFDs.

US Iceman
03-07-2008, 06:41 PM
That is why I think a proper control system is a far better investment than VFD's.


I think a better investment is to put the money into the refrigeration system to improve it's capability to operate during all 4 seasons under any operating condition.

Then after doing this, put in a control system to manage the operation. Sergei and I have had several discussions about this before.



It's not hard to design a system seemingly professional with twice the power consumption of a system designed with power consumption as the first priority.


That's a fact. Usually when someone says they have designed an energy efficient refrigeration system what they mean is; they have used high efficiency motors.:(



Surprisingly(or not surprisingly), all estimations of energy savings were done for 180 psig of condensing pressure.


Probably because this where the greatest benefits are found. When you start to decrease the discharge pressure the benefits start to disappear, which would not help them in their sales presentation.:D

Mike W
12-07-2008, 12:44 AM
I agree with Iceman, if the compressor runs near or at full load most of the time then a VSD will not have a reasonable payback.
Don't forget the fact that you can overspeed compressors to get additional capacity. When you are designing a plant with wide load changes, by using a VSD or 2 you can reduce the number compressors required while still maintaining overall plant efficiency.
We had a plant with two compressors, the smaller compressor (265kW motor)had 50% of the larger compressor (450kW motor). By fittings a VSD to the smaller compressor and over speeding the compressor from 50 to 60Hz (some compressor can go further upto 4550rpm but motors need to be checked to see if they are suitable) we where able to get an additional 20% capacity(motor and oil sep and cooler could handle this). This meant that when could delay starting the larger compressor and when the larger compressor ran, it ran with the load at a higher precentage.


I have done a couple of VSD installs and conversions and they certainly start alot easier. We run the compressor at min speed(1450rpm) and have a min slide of 50%. Once the slide is increase to 100% we then increase the speed.
Another advantage is by running the compressor with the load slide fully loaded the compressor dosen't wear out as much as a compressor with the load slide constantly adjusting. On compressor that load/unload alot we see a lot of wear on the slide valve.

Sergei
12-07-2008, 10:03 PM
I agree with Iceman, if the compressor runs near or at full load most of the time then a VSD will not have a reasonable payback.
Don't forget the fact that you can overspeed compressors to get additional capacity. When you are designing a plant with wide load changes, by using a VSD or 2 you can reduce the number compressors required while still maintaining overall plant efficiency.
We had a plant with two compressors, the smaller compressor (265kW motor)had 50% of the larger compressor (450kW motor). By fittings a VSD to the smaller compressor and over speeding the compressor from 50 to 60Hz (some compressor can go further upto 4550rpm but motors need to be checked to see if they are suitable) we where able to get an additional 20% capacity(motor and oil sep and cooler could handle this). This meant that when could delay starting the larger compressor and when the larger compressor ran, it ran with the load at a higher precentage.


I have done a couple of VSD installs and conversions and they certainly start alot easier. We run the compressor at min speed(1450rpm) and have a min slide of 50%. Once the slide is increase to 100% we then increase the speed.
Another advantage is by running the compressor with the load slide fully loaded the compressor dosen't wear out as much as a compressor with the load slide constantly adjusting. On compressor that load/unload alot we see a lot of wear on the slide valve.What is the reasonable payback? Who did estimation of this payback? What is the operating conditions of these compressors?
I found that very often slide valve moving back and forth because of wrong settings. Sometimes people don't realise that pressure can be change very quickly but temperature change takes time.