Vilter Unloading

[NH3LVR]

I am not directly involved with this compressor now, but was asked by my former boss about it.

The machine is a Vilter 440, 12 cylinder, two stage. 8 cylinders low stage, 4 cylinders high stage. It is not internally compounded, but has a external intercooler.
The refrigerant is NH3. Settings were at: Suction 2lbs (.1BAR) Discharge 135 lbs (9BAR) the last time I saw this machine. I do not recall where the intermediate operated.
It services freezer and cooler loads in Alaska. On occasion the high stage is not big enough and a high stage screw is used to maintain suction pressure. Condenser is adequate year around.
I did some work on it years ago.I set it up to unload 1/2 the low stage on a pressure switch. The high side remained loaded at all times.
This machine was inspected and the valves replaced a few months ago. The mechanic is a competent service person. (Yes, I said something nice about you Danny)
This machine has ran for years under these conditions.
One thing changed. The operator felt that the unloader was not working. He apparently replaced the unloader control with a manual switch and would leave it the cylinders not pumping overnight.
Now I am aware that some manufacturers do not like machines to run unloaded for long periods of time. This has never been a problem for me with machines running for hours at light loads. I do remember that Vilter will allow a compressor to run fully unloaded for up to 20 minutes in some applications.
A few days ago the compressor was heard to be making noises. A serviceman flew up to determine the problem. The four cylinders that were capable of unloading were found to have gotten hot enough to deform. The cylinders that were loaded are fine. The crankshaft was also damaged because of a failed rod.
My former employer talked to Vilter. The Rep works mostly with screws, but said you could unload 4 cylinders for up to six hours.
This seems strange, as I feel to see why 6 hours would be a magic number. If it was going to get hot I think it would do so sooner than that.

Comments or suggestions?

[RANGER1]

NH3LVR ,
I agree with all you say & sympathise with repair that now have to be done .

- To my knowledge I thought 10 minutes fully unloaded but thats on single stage carbo-cooler compressor .

- Were the unloaded cylinders at one end of machine possibly limiting suction gas cooling & flow throughout low side of compressor ?

Normally I thought you would have 1 pair of cylinders each end of machine & probably the lower banks so cool suction gas travels over unloaded cylinders for mixing of warm gas.

When you did it using pressure switch at least it had the chance of loading occationally .
Could it be that compressor was not unloading as you thought from years ago & now that it can be unloaded permanently problem has reared its head !


The only problem I see with Vilter compounds compared to other brands is that it does not have water cooling on low side heads ( except Grasso ).

Without knowing what safety controls are on machine & realizing it has run for years I would still want HDT cutout controls on both high & low side discharges each end of machine as well as HOT.

Out of interest also what is loading low side heads high side or interstage gas?

[NH3LVR]

I misspoke. It is the pistons not the liners that deformed.

The machine was unloading fine then, I set it up using a ammeter on the motor.

As I say I have not seen the machine since this happened, but the low side is loaded by oil pressure on the two stage Vilter.

I will be able to get more details when the mechanic comes back.

[NH3LVR]

I can't believe a group of Industrial guys would not want to argue over this. (Humor)

[RANGER1]

NH3LVR ,
Deformed as in melted?
That would be phenominal heat if thats the case !

Maybe you will have to re-visit this site .

Have only seen something similar with a machine with equalization on start up solonoid between suction/discharge failing to close, but this destroyed all pistons etc

[RANGER1]

NH3LVR ,
Any news on problem ?

[NH3LVR]

Nothing much. I saw Danny when he came back and it is still a mystery.
I asked him to contribute his first hand knowledge of the problem, but he has not done so yet.

[PaulZ]

Hi NH3LVR
Just read this post and I don't know whether there is a lot to argue about.
I would think that 6 hours is way too long for any compressor to remain unloaded. The problem is that there is a certain amount of heat generated in recompressing the gas plus friction heat in an unloaded cylinder and most compressors I know of rely on the cool suction gas as well as jacket or head water cooling to keep the cyl / piston temp down. In the case of cyls with no cooling the suction gas is what will do the cooling.
I agree with Ranger about 10 mins unloaded or even up to 20 mins, were possible we try to limit the time a bank of cylinders is unloaded to a max of 20 mins.
I would suggest as Ranger said that they get the pressure switch back on.
Paul