Booster compressor?

[zolimer]

We have a small booster compressor that we use to run a post pasteurizer @ 15 degrees F and a small cooler @0 degrees. Our high stage runs @ 28 psig and our booster is running @ 15 psig, Both evaporators are flooded! Our high stage compressors work off of a lead/lag-step up/down system via supervisory computer control, our booster is variable frequency drive controlled!

Here is the issue that i am trying to figure out. Instead of the booster compressor adjusting its slide valve in response to load or lack of load, it runs @ 100 percent at all times until it reaches the low suction cut out, then cycle's on and off! This does not seem like an ideal set up to me.

Here is the question. Where would you start? With the VFD or compressor set-points?

This is an FES gs series compressor with a micro 3 panel. Thanks, Tim

[keepitcool]

You said the slide valve is at 100% but the motor is turning down correct? If so Is it possible to use the economizer port to add load to the compressor? It sounds like it is too big.

[US Iceman]

Is the slide valve at 100% most of the time with the speed varying?

[zolimer]

Yes the motor is ramping up and down with the load! The slide valve is always @ 100%! My train of thought is that if there is no load on the compressor the slide would ramp down too, reducing capacity, if this where the case it would take longer to reach our low suction cut out set-point @ 6 psig! If the slide valve was playing a bigger role, we could raise the low suction cut out and not over compress!

I need to trend this to see if the slide valve does unload before shutdown! If this is the case, i will look into the economizer port suggestion. Is the economizer port fed by the high stage suction to add a load when load is not present? And if this compressor is oversized for this application, would using the economizer just be a work around to keep it running, but not increasing effiency?

Also could this be done using a bypass @ the header using a pressure regulator i.e. LSS set-point is 18 psig @ 15 psig a solenoid in bypass energizes allowing HSS in @ a regulated volume?

[keepitcool]

Don't think of the economizer as adding false load to the booster. Think of it as taking load from the high side. It all has to be compressed. We often can run an entire dock of the economizers. You could also install a liquid sub cooler on it if you had a good application for it.

[US Iceman]

Don't think of the economizer as adding false load to the booster.


True, this would be useful for subcooling however... at full load the increase in mass flow can raise the motor power input slightly. This could cause some problems if the motor is sized to small (which does happen).

[keepitcool]

That is true. If he wanted to add a sub cooler it would have to be the right size. It sounds like the compressor may be a bit too big. If that is the case he could add some economizer load to help keep the machine running. If the system has a decent control system it could be set up to only use the economizer during low load conditions.

[Mongrel]

Hi guys, This forum is new to me, but I've been around Screw Compressors extensively. May I add my comments. It is ususal for a screw compressor operating on a VSD / VFD to start operation with the slide valve fully unloaded at the compressor @ minimum speed. As capacity is required, the slide valve should then load to 100% before any increase in speed. Any load fluctuations can then be accommodated by the VSD up to the maximum freq. If the load decreases to a minimum speed situation. The Slide Valve should then provide additional load control down to a minimum. Be careful not to operate the compressor @ minimum load slide valve position for extended periods as this could have a detrimental effect on the compressor. It seems very strange to me that with so much available capacity range, the compressor could be so heavily oversized so as to cause such operational headaches. Is the facility (load) fluctuating excessively?? That is to say, Max load to Min load constantly?? I'd be interested to hear. My apologies to all if I have stated the obvious. Mongrel.

[RANGER1]

Agree with mongrel and would check actual minimum speed is approx 1440 rpm for screw ( see manufacturer min speed for this machine ) . Screw could then unload and if reaches say 50 % or below , have timer to cut it out or if LP switch cuts it out prior to timer. If load picks up in this time it avoids short cycling and frequent starts.
If this is only load on low side ,economizer will have no impact on low side load or create any more efficiency (in my opinion). Unless directed into its suction line as a false load to keep it machine running.
It can also depend on conditions required for product as the best way to elimnate problem .

[US Iceman]

It seems very strange to me that with so much available capacity range, the compressor could be so heavily oversized so as to cause such operational headaches.


My suspicion is the load causing this issue is so low the compressor cannot turn down far enough. So to prevent the short-cycling the suction pressure setpoint has been set too low in an attempt to prevent the short-cycling.

I think the load is either too low at minimum capacity/speed or there is something causing a situation like off-gasing that generates just enough of a gas load to cause this.

Good post BTW.;)

[zolimer]

You guys are good! Thank you for your comments! Keep em coming! I did trend it and it cuts out @ 100% slide!

[keepitcool]

Who controls the VFD? FES or is it a stand alone?

[RANGER1]

You could also limit VFD max speed and slow loading of compressor to try to match load consistently.
If you overshoot to quickly comp wants to short cycle if it is to big.

[US Iceman]

I did trend it and it cuts out @ 100% slide!


100% unloaded or 100% loaded?

If the compressor is shutting off with the slide valve at 100% loaded, someone in the controls dept. should get their bum spanked. You can still unload the compressor. Although... When you get down to minimum speed, you really need a smaller compressor (like a recip) to handle this small load you are experiencing.

[zolimer]

It is controlled by an Allen Bradley VFD via PLC!, It was not packaged together by FES. I will gather up VFD settings and report back in a few days! Thanks, Tim

[zolimer]

I would also like to take some screenshots of this compressor and would like to post them! Can i post them to this thread or do i have make an album or somthing?

[US Iceman]

Go ahead and post them in this thread if you can. Ensure they are under 100 kb, though.

[keepitcool]

It sounds like your Micro III and your PLC are not communicating. Is the Micro set to remote and if so does the PLC handle the capacity controls for the compressor? Was the VFD added after the compressor was installed? When the micro III's came out the were not set up to communicate with VFD's. Please let us know more about how the machine is controlled.

[martinw58]

unloading is controled by oil pressure
is there a small hand valve to control speed of unloading my need to be opend

[Iceehot]

Check speed of Unload. pres "Manual Unload" and see if it works and how long it takes. Speed can be adjusted with the small hand valve mentioned above. I had one that unloaded too slow and compressor died on low press. cutout before it could unload itself.

[zolimer]

Hello, I have been busy, i have the pictures, but need to edit my company logo out before i post them! US Iceman, It is 100% loaded when it shuts down, and there are times that there is no load to speak of. It is a 40 hp compressor and i dont think that it is oversized it runs consistent at 100% slide when under a load.

Icee and martin, Oil differential pressure is running 40 psi. I need to look at the FES documents and see what the recommended setting is! I will try checking the manual unload, you might be on to something here!

Keepitcool, It is set to remote, and it is communicating fine, i just dont think it was ever set up correctly! I am sure that with the help of the fine folks on this board that we can do something about that!

Ranger and Mongrel, I have ruled out the VFD for now, it seems to ramp up and down pretty consistently and when under fluctuating loads stays consistent. When i lose the load, i reach low suction cutout much too quickly. Welcome to the board Mongrel and thanks for your input!

[Mongrel]

Looks like you almost have it under control. If you are checking the controller, and if it is a rather modern program, I suggest that you might look at possible settings of Integral Band, Derivative Band and Proportional Band. I would strongly suggest that the Propotional Band be checked first as this is most often misunderstood. This being that the larger the Propotional Band figure input, the slower the response time. Under this realm, the panel looks at the Target Suction pressure and Actual Suction pressure. The greater the difference the quicker the response. As the Target and Actual approach each other, the slower the response. Try not to operate the Screw compressor at minimum load for extended periods as this may be detrimental to the compressor. Some manufacturers recommend not less than 30%. I suggest that you check with the manufacturer.

[McFranklin]

On the older FES Micro II panels during the start up sequence the oil pump starts and builds pressure, the panel would unload the slide valve until it was at 5% or less prior to the compressor advancing to the next step in the sequence. I doubt if that feature would be removed in the Micro III, thunderstorms would often shut down a plant where I worked. On the packages we had the oil flow to load and unloaded the slide valve was adjusted by means of two needle valves located by the directional valve (Vickers, Continential e.g.)

[mcalko]

Check your load valve the orings will bypass and not allow it to unload properly. Is it trying to unload during low demand. The panel will blink UL by thee %