VFD on Compressor

[mechantaeus]

I have a 60 HP compressor, I don't need such a large compressor, will a VFD reduce the HP? Can I fit a VFD to this thing?

Thanks

-mech

Work Hard and Work Smart

 

[aolalde]

The VFD will reduce the motor speed. Reducing the speed you could adjust the compressor to the desired flow and proportionally the power requirement will be reduced. Check the motor input current which follows approximately the power consumption.

 

[rconnett]

I assume the compressor is a reciprocating (positive displacement) type.

If it a screw operated in parallel with other machines, you may need to look more closely.

 

[mechantaeus]

It is a screw type compressor..does that make a difference for the VFD add-on?

-mech

Work Hard and Work Smart

 

[rconnett]

Check with the compressor vendor. I can't recall if a screw is a positive displacement or not.

The concern is whether it would surge - but you would know more than I might about the characteristics.

 

[mechantaeus]

Screw is positive displacement (medium is trapped and pushed forward).

mech

Work Hard and Work Smart

 

[rconnett]

sounds like it's OK.

I've spent 2 days in meetings dealing with a large (30,000 hp) centrifugal natural gas compressor on VFD's & my brain is a little tilted -

Thanks

;-)

 

[rbulsara]

VFD on centrifugal compressors for chillers are routinely used....not sure of what happens when speed is reduced for a screw type. It will reduce HP not sure what happens to the compression. May be some mechnaical guys can respond. Try posting this in mechanical forum too.

 

[mechantaeus]

Reducing the HP will reduce CFM of the compressor

Work Hard and Work Smart

 

[pennpoint]

mechantaeus

In reality its the reduction in the amount of work required (cfm, pressure, etc.) that results in the reduced horsepower.

Regards

pennpoint

 

[awil3]

That is what VFDs are for! I use them for my compressors and NO more blown fuses, triped breakers, ect. If properly
programed you cannot kill your motor no matter what happens to your power or compressor. Your motor will run much cool-
er and use less power even at 60 hz. The compressor will have to be positively unloaded on start-up with silenoid
valves on both stages. They can be controlled with any vfd. I currently use around 40 hitachi vfds now and they will not die, even after 11 years!

 

[quark]

I would prefer VSDs for screws rather than recips. I have been discussing with manufacturers about variable speed recips but no concrete decision so far.

Some darwbacks I presume are,

1. Ineffective lubrication, if the oil pump is connected to crankshaft.
2. Increased vibration due to imbalance at other speeds than designed.

I saw and used good many refrigeration chillers(screw and centrifugals) with variable speed application. I have reservations for using VSDs with screw air compressors. Proper receiver sizing, minimization of leakage and correct usage pressure with compressor on/off(or load/unload atleast) will give better results and at reduced investment.

PS: Power consumption reduces if we reduce mass flowrate through a compressor.

Regards,

 

[Farkel]

another thought is that screw compressors do not load up until at least 1/3 speed, so the area which you can control speed may be greatly reduced.

 

[jraef]

I'll just add a reiteration of quark's comment on lubrication. I have found several screw compressors where the mechanically coupled lube pump performance dropped off at a faster rate than the equivalent speed reduction in the compressor, resulting in rapid compressor failure running at less than 70% speed. That reduced the effective speed control range even further, and thus the energy savings argument. In some cases we added positive pressure lube systems, but the added cost again affected the payback period adversely.

"Venditori de oleum-vipera non vigere excordis populi"