Energy Efficiency Screw Compressor with VFD

[georgedvf]

Hello everyone!!

I've been reading about using VFD to control capacity on Screw Compressors, but I can't understand yet if using the VFD improves the Energy efficiency when partial load scenario. As far as I understand when unloading on normal capacity control (100%, 75%, 50%) energy efficiency drops a lot, so it is not a very common practice to use Screw Compressors on partial loads. Now using VFD is there such an energy consumption so it is acceptable to use Screw Compressors on partial loads??????????

Thanks!!

[georgedvf]

Hello!!

I've read that Screw compressors a most efficient when Big Capacities requeried, and that Reciprocating are most efficient on Low Capacities, Is that true??? What's the reason???

Thanks!

[Josip]

Hi, georgedvf

Hello everyone!!

I've been reading about using VFD to control capacity on Screw Compressors, but I can't understand yet if using the VFD improves the Energy efficiency when partial load scenario. As far as I understand when unloading on normal capacity control (100%, 75%, 50%) energy efficiency drops a lot, so it is not a very common practice to use Screw Compressors on partial loads. Now using VFD is there such an energy consumption so it is acceptable to use Screw Compressors on partial loads??????????

Thanks!!

it is a little strange but very simple

.... usually when we use VFD to control screw compressor capacity, that compressor is without slide valve and we control compressor capacity only with speed 1500 (or even less) up to 6000 rpm and changing Vi (volume ratio) automatic or manually ...

.... if we have an old compressor (with slide valve and many times without changeable Vi) then we adjust capacity change in the way like this .... at small speed first we push slide valve to maximum position to achieve the best Vi and then increase capacity with higher speed and v.v. when we need to reduce capacity ...

.... for both above examples we have to choose very good Vi (volume ratio) which depends on suction pressure/temperature and discharge pressure/temperature (also is a big difference if we use or not Economizer) and of course Vi is not the same for each refrigerant gas .... in case when we have manual adjusting of Vi we can change it usually 2 times per year and compressor must be stopped ... with automatic change of Vi all is under control of PLC changing during normal run of compressor ...

... one more thing ... used power is not proportional to work for screw compressors and for that reason we want to come a.s.a.p. and run our compressor at 100% to achieve the best COP thus engage the minimum power to obtain maximum work ....

... with reciprocating compressors we do not have changeable Vi i.e. it is always the same Vi determined with design of compressor ... our capacity is always determined with capacity of one single piston .... so we engage and disengage pistons and work is directly proportional to power at any capacity ....

all about efficiency depends on good design ..... sometimes on common sense too

hope this is of some help ....

Best regards, Josip

[georgedvf]

... one more thing ... used power is not proportional to work for screw compressors and for that reason we want to come a.s.a.p. and run our compressor at 100% to achieve the best COP thus engage the minimum power to obtain maximum work ....

I though that was only when normal capacity control was used , so It is the same using VFD?????

Thanks!

[Josip]

Hi, georgedvf

I though that was only when normal capacity control was used , so It is the same using VFD?????

Thanks!

no it is not the same

... that valid only for older compressors when we have first to come to 100% of mechanical capacity i.e. slide valve position at max cap. ...

.... with VFD our compressor (new models) are always at 100% of mechanical capacity then furthermore improved with changeable Vi ... + economizer .... thus, always working at maximum COP ..... we are changing capacity only with rotation for example .... from 1000 up to 6000 rpm ...

Best regards, Josip

[georgedvf]

I think I'm a little bit lost.

So, New Compressors with VFD with changeable Vi + economizer are designed to work good at low partial loads???

How much is the energy efficiency improved in this new models?? Can we say that at 60% of capacity it's power consumption is about 60% as well????? Or how is this relation of power consumption behavior???

[Josip]

Hi, georgedvf

Hello!!

I've read that Screw compressors a most efficient when Big Capacities requeried, and that Reciprocating are most efficient on Low Capacities, Is that true??? What's the reason???

Thanks!

partially answer you have in above posts .... regarding big capacities required this is also simple ... with one relatively small screw with VFD it is possible to do the work of couple of reciprocating compressors with much less kg of installed material, less services and many other advantages ....

that does not mean piston compressors are bad just opposite .... use of one or another type depends on many other things .... it is not that simple ....

Best regards, Josip

[georgedvf]

THanks Josip, that was supposed to go on another Thread but I can't tell why I post it here.

Lets retake this:

I think I'm a little bit lost.

So, New Compressors with VFD with changeable Vi + economizer are designed to work good at low partial loads???

How much is the energy efficiency improved in this new models?? Can we say that at 60% of capacity it's power consumption is about 60% as well????? Or how is this relation of power consumption behavior???

[Segei]

VFDs can be helpful to save energy. However, it use additional energy(3- 3.5%).
Reducing compressor capacity by slide valve is not efficient especially at capacities below 50%. The lower capacity, the greater losses of compressor efficiency. Some modern compressors with variable Vi can keep required Vi until 50%.

[georgedvf]

So can we say that using VFD on Screw Compressor working at 60% of capacity it has 63.5% power consumption????

[Segei]

You are right. However, when you use slide valve at 60% capacity energy use can be 70-80%. It may looks that difference is not significant, but at capacity below 50% energy use will have significant difference.

[georgedvf]

Thanks Josip and Segei.
One last question, is there problems with overheating of screw compressor when working on low capacitiy using VFD????

[Josip]

Hi, georgevfd

THanks Josip, that was supposed to go on another Thread but I can't tell why I post it here.

Lets retake this:
Originally Posted by georgedvf
I think I'm a little bit lost.

So, New Compressors with VFD with changeable Vi + economizer are designed to work good at low partial loads???

How much is the energy efficiency improved in this new models?? Can we say that at 60% of capacity it's power consumption is about 60% as well????? Or how is this relation of power consumption behavior???

I merge your threads because in both we are speaking about similar things .... I was thinking it is best to discuss all that in one thread .... it is written between posts ...


Going back to power consumption for screws equipped with VFD .... we can say that compressor is always at the max capacity ... we are changing its capacity only with rpm... thus, screw with VFD is very similar to piston compressor .... i.e. at small capacity VFD is rotating with small speed - piston compressor is running with two pistons engaged .... now we need more capacity and we change speed for some percentage .... piston compressor is running with 4 pistons engaged .... then we need some more capacity and VFD is increasing speed for some percentage .... piston compressor is running with 6 piston engaged .... and so on ... until we come to max capacity .... piston compressor with all pistons engaged ... and v.v. when we reduced capacity ...

Unfortunately, I am not sure in percentage how much energy we need for that much capacity ... for example what are the numbers at 50% of capacity, probably we need some more power, but what I want to say we cannot speak about partial load because with VFD we do not have screws partially engaged .... there is not a slide valve ... so the screws are engaged within complete length and capacity change is only with change of rpm ... compressor with VFD is always rotating with that speed which is giving 100% of capacity for given load in that moment, and that load is changing constantly so does the speed ...

I'm sorry my English is not that good to explain that in better way

One last question, is there problems with overheating of screw compressor when working on low capacitiy using VFD????

Cooling of compressor is better with max flow of refrigerant, but here we have oil system capable to keep screw temperature within working range or we have some other possibilities to maintain working temperature, for example ... liquid injection .... but, don't forget always is much bigger problem with compressor overheating when we have high suction superheat and high condensing temperature and probably very poor oil cooler ... but that is another story


Best regards, Josip

[georgedvf]

I merge your threads because in both we are speaking about similar things .... I was thinking it is best to discuss all that in one thread .... it is written between posts ...

OK, got it!

Thanks a lot for the explanation, it's been very very helpful!

[Segei]

Thanks Josip and Segei.
One last question, is there problems with overheating of screw compressor when working on low capacitiy using VFD????

Compressor will not overheat if it run within limits of operating conditions. However, el. motor require additional cooling. Usually, small blower can do the job.