VFD for reciprocating compressor

[ukram]

Dear all,

Greeting from me to all of you.

I quite confused to using VFD for reciprocating compressor to save energy. Reciprocating compressor has itself loading-unloading system. it already saving energy.
First, is it realy saving energy in recip. compressor. Beside this if i plan to used VFD for the same compressor, what will be the payback period for 90kw motor.
compressor is using for -40*C,ammonia rerigeration system. compressor having 4 solenoid valve to control the loading unloading system.

Your feed back is very much wel-come.

regards,

[D.D.KORANNE]

Vfd for recip compressors can be used . But need reliable pressure transducer ( feedback system for vfd) . There is little info available in india on such application since a wrong notion is afloat in indian market that it is good only for screw comps !
It is
however, for ammonia taking signal for vfd to maintain a constant suction presure need to be identified. Or an alternate signal through the cold storage thermostat / chiller thermosta (indirect system ) need to be applied. It would be a lot easier for nh3 application since speed modulation does not pose oil return issues unlike other refrigerants wherein suction piping at a lower speed is an issue ; piping design require critical consideration.

With mechanical unloaders usually power saving is non-linear . In fact power saving ratio is always adverse since 50% unloading power is saving is in the range between 20 to 30 % . Besides, current drawn on a partially loaded motor is high .

All in all vfd can typically return the investment in about 18-24 months . Moreover, system balancing is also must .

[D.D.KORANNE]

FOR FURTHER DISCUSSION ON VFD , SEND A PM ON :ddk0123@gmail.com

[sterl]

I note you said (4) solenoids: I'm presuming that means an 8-cyl all the way to 0-flow.

The major drawback to operating a VFD on a single open drive compressor is the control loop: A recip "loses" compression just about instantly 2-cylinders at a time with a normal unloading arrangement and those that lift the suction valve mechanism are not looping gas through small passages over and over, so their efficiency in doing this is near-perfect, the motor and the (mechanical) drive become the energy hogs.

You are still spinning a flywheel and crankshaft at normal design speed so lower end wear and oil requirements remain just about the same. By modulating speed: friction, lower and upper end wear and drive losses all reduce but with a recip you can only do this to a limit established on (2) major factors:

The motor cooling will at some stage stop distributing, or the compressor's oil pump will simply not develop enough flow to make an oil pressure...Or at a very low speed: Sleeve type bearings stop shearing the oil, you loose the lubrication in its entirety...

And a VFD itself is an energy drain to some extent; at high turndowns its conversion efficiency and the motor's low-freqeuncy characteristics start crawling on top of each other.

A screw with external oil pump doesn't have some of these limitations; and a slide valve cannot make the geometry "right" for a fixed Vi machine at high turndowns....But those using VFD drives to something like 25% speed add extra fans to the motor and drive the oil pump independently....at even lower capacity requirements, they re-employ the slide valve.

What is the naure of your process? A VFD arrangement on a sizeable motor is never going to load & unload as fast as those solenoids will unload a recip; but at that suction temperature, I'm guessing your load only changes slowly as well...

[RANGER1]

VFD on recip is still handy for low turndown as well as max speed .

If you want very constant suction pressure speed changes could be made between each step on mech load /unload .
Some machines like mycom you can get a low speed ( high volume) oil pump to accomodate lower speeds .

Also if you run min speed compressor will last much longer .

On small liqiud overfeed systems it can also reduce cavitation of liquid pump between steps of unloading , due to small pressure change .

[Sridhar1312]

The reciprocating compressors capacity varies on the speed and many times minimum torque will be needed for the reciprocating compressors even when it is operating on one cylinder.
The above needs to be taken care.Nowadays Reciprocating compressors are limited special applications where load is mostly constant and hence the use of VFD may not be that user friendly

[Yuri B.]

Also, not any motor can work with a VFD. Motor's bearings will suffer if the motor is not specially designed to work with a VFD as there in rotor are induced currents circulating rotor-one end bearing-ground (motor's body) - other end bearing-rotor harmful for the bearings' surfaces.

[RANGER1]

On motors 150 kw and less it may not be nacassary for the special insulated bearings or rotor earthing .
We have done plenty with no problems .