Interesting analysis of screw compressors

[RANGER1]

http://www.fjb.co.uk/wp-content/themes/fjb/publications/large screw compressors.pdf - Google search

Go to publications Large screw compressors

Thought this was interesting to say the least.

[HVACRsaurus]

Interesting eh, I think that they might be drawing a bit of a a long bow from an isolated incident.

Here is an idea.*

Install a mass flow meter in the main liquid line (& possibly in the interstage discharge) to monitor compressor actual displacement versus previous recordings & manufacturer data. Provided mass flow is within reasonable tolerance just stick with a traditional overhaul schedule.*

I am familiar with at *least one instance of a (large) screw compressor running 120K hours before major service & internally when it was stripped it was in mint condition. Whilst this example of longevity is by no means expected, it is testament to screw compressor capability.

[RANGER1]

I think this uy is from another planet saying overhaul every 10 000hrs.
But it seems like a very proffesional show by the website, so interesting if anyone would backup the statements!

Why would main bearings wear out in a screw compared to a recip, its beyond me.

[Grizzly]

Brilliant read Ranger.
Yes you can argue that things have moved on. Many issues have / are being resolved, however
The main arguments still hold true.
No-one is going to install large numbers of recips instead of Screws.
However you will notice most have gone down the route of VSD screw as lead and software to bring in the lag, lag screws at higher loads.
Also the use of side load ports / economisers has widely increased however most still only operate at the higher (More favourable) rev range.
Which have introduced another set of issues that can plague the Fridge Engineer.

What oil you use still has huge implications as we all know.
So in fairness to the Author it's a brilliant read for 2003.
It would be an interesting read as to how he now saw the situation nowadays?


Why would main bearings wear out in a screw compared to a recip, its beyond me



2 perfectly balanced cylinders spinning would indeed not cause bearing wear.
However add side forces between the working faces of the Males / Female lobes.
Would create forces that can contribute to bearing wear.
Also ask any Toolmaker about up climbing when milling and you can begin to understand that undesirable lateral forces.

I have seen excessive wear on a screw when the drive motor bearings are wearing.
The resultant droop in the drive coupling and driven rotor caused huge loads on the bearings.

I suppose what I am trying to say is that "side load" and movement along the centreline of the rotors will cause bearing wear.

10.000 hr services may seem unusual now but 9yrs ago maybe not?
Back then most oils used were mineral but more importantly we were all a lot cheaper to employ!

How many go for a rebuild replacement nowadays?

Sorry but cannot give you rep points, got to spread the love more. You deserve some though for the consistent quality of your posts.

Grizzly

PS referance to the comps used 100yrs ago check my avatar, it's a Linde from a cold store plant room circa 1923.
I actually have the original photo.

[al]

I remember Gram screws being rebuilt where i worked previously at around the 10000 hours, but much smaller physically. Screws the likes of York have now are really hermetique, they realistically can't be rebuilt on site.

al

[Segei]

I'm just wandering about operating temperatures of these compressors. If you bypass compressed refrigerant from higher pressure to lower pressure, gas temperature and oil temperature will increase. Load of the el. motor should be lower as well. This is not 3-5% of lost capacity, this is 30-40%.

[RANGER1]

Grizzly thanks for the kind words & letting me down gently.

I would only believe the report, if it was in the 70's not 2003, which was not that long ago, to me anyway.

I would also say that main bearings worn at 10-20 000 hrs would have to be seriously abused, mineral or synthetic oil.
We still use mineral oil on new plants & no real problems unless lots of air entering system.

The biggest downfall with nearly all screws is the slide valve rotating & contacting rotors, whether it has a guide block/pin or discharge port guide in most latest compressors.

We still overhaul machines that were from mid 1960 ie S65, S67 & S56 Stals no complaints.

[NoNickName]

Why would main bearings wear out in a screw compared to a recip, its beyond me.

Maybe because recips do not have roller or ball bearings.

[Adam gao]

Interesting, and thanks

[glenn1340]

I think this uy is from another planet saying overhaul every 10 000hrs.
But it seems like a very proffesional show by the website, so interesting if anyone would backup the statements!

Why would main bearings wear out in a screw compared to a recip, its beyond me.


To right, every 10,000 hours! most places I know now run their screw compressors until they give out or at least until they can actually hear the bearings rumble.