Bitzer s6f30 2-stage compressors

[JONAH]

Hi guys,
im looking after a low temp pack,which is powered by 5x Bitzer s6f30 2 stage intermediate compressors.
Basically my intrest is this - what state is the ref 22 in as it passes thru intermidiate pipe, what direction,passage its taking and its principle for existing i.e is it increasing compressor capacity? etc.
rang bitzer tech a few times for enlightenment but i felt i knew more about product than them!
I should also point out that each comp has liquid subcoolers on it , which i believe not also helps increase capacity of system but also cools windings of each comp.As phial for liquid is mounted on comp intermediate pipe in a special pocket.
Marc if i remember did u work for nrc? as install is by them.
Ive downloaded regarding this comp off bitzer site but is all spec.

thanx jonah

[al]

I look after a site with 6 of these packs working at -80, have you experienced any oil trip problems or overfeeding of the subcool expansion valves?

[Frosty]

Hi Jonah

Normally, two stage compressors are utilised when the system pressure ratio is deemed excessive i.e. DX blast freezers etc.

We have two of these machines running in parallel at cosworth technology, on an engine test chamber. The chamber can be controlled down to -30 degrees C, so suction pressures can be in the order of -38 to -40 deg C. This plant is DX using R404A, as you can probably appreciate, the ratio across the system can be in excess of 17 bar during summertime ambients!

This ratio is considerd excessive for single stage operation due to the high discharge tempertures that would occur under these conditions...its particularly worse for R22! Also the capacity of the machines would be severely reduced due to the rarified nature of the refrigerant entering the cylinders.

An easy way to combat the above mentioned problems is to use two-stage compression. This works by compressing the 1st stage vapour and drawing it into the second stage, normally on a ratio of 3 or 4 to 1 depending on application.

This poses a problem, the first-stage compression process will add heat to the vapour (heat of compression). If this vapour is drawn straight into the second stage suction, then final stage discharge temperatures will be excessive, resulting in high oil and high motor winding temps!

Some form of second stage suction (or first stage discharge!) de-superheating must be deployed. The common way of doing this on Bitzer machines is to use Bitzers own CIC system (controlled refrigerant injection) This system uses two sensors, one in the second stage suction and one in the second stage discharge. The system then simply calculates the correct amount of liquid to inject!

However, injecting liquid to suction reduces the refrigerating effect (reduction of liquid at the evaporator) It is common practice to inject the liquid into some form of liquid line heat exchanger, normally a plate heat exchanger. This practice not only desuperheats the second stage suction vapour but provides a degree of liquid line sub-cooling, therefore increasing refrigerating effect!

Hope this info helps you!

Feel free to ask for further info if you require it.

Cheers

Frosty

[JONAH]

ORACLE!
Thanx frosty!

[Frosty]

My My Marc

Was that almost a compliment? You must realise Marc, that when you were based at Star Oxford....I had never heard of you...in fact, I never heard of you full stop, even while you were employed by Star.

And vice-versa, you'd never heard of me either, eh? Just goes to show you Marc...maybe you really shouldn't slag of Star in its entirety beacuse, yes, there are some good guys here that almost have....... half a brain!!!!!

Frosty:D

[frank]

Marc

I follow most of your post except where you say to introduce the tev? If we are talking about the first stage discharge vapour being de-superheated prior to entering the second stage compressor - how does the tev work on a vapour??:confused:

[Frosty]

Frank

What do you mean "how does the TEV work on vapour"? Are you presuming that the TEV is injecting vapour? The TEV injects saturated liquid into the first stage discharge stream to de-superheat the vapour...in fact, it doesn't actual totally de-superheat - that would be catastrophic! The valve injects liquid to produce vapout entering the 2nd stage suction in a slightly superheated state...4 - 6 deg C superheat being the norm.

Frosty

:)

[frank]

That was what I was thinking but it didn't read like that from Marc's post - maybe it's me - it's been a long day!!:cool:

[Frosty]

Frank

That is why its best to try and talk in laymans terms (but obviosly not in a condescending way!) It then avoids any confusion.

Frosty

[frank]

Marc - I follow now - thanks

[Frosty]

Marc

I do get a little lonely from time to time...LOL

Frosty

[frank]

Is there any possibility that by over injecting liquid into the first stage discharge that you could actually lower the discharge temp to below saturation point? - this would obviously ruin the second stage compressor??

[Andy]

Hi, I was talking to a fridge tech I used to work with yesterday and the subject of compound bitzers came up. There is a certain supermarket site in N.Ireland with these fitted on wall and well deep freeze cases on R22. Over the years one pack has went through numerous head gaskets on the high stage and a fair few compressors. All I would say due to poor system design. My thoughts on it was that liquid flood back was occuring, mainly after defrost when the second compressor kicked in. The obvious cure to I felt was to check and sett the expansion valves on the cases and to check and sett the injection valve and to fit a suction accumulator with the liquid line running through it on the low stage.
My question was would an accumulator with the liquid line passing through it superheat the low stage vapour too much leading to high discharge temperatures leaving the low stage cylinders and increase refrigerant volume entering the low stage reducing the compressor capacity?
Regards. Andy.:(

[Andy]

Marc, I think you have hit the nail on the head with both your sugestions regarding the de-superheat tev. The phail is poorly insulated in a warm plant-room and the valve starts injecting at compressor start. I did sugest fitting a Teat injection valve, this only starts injecting at between 55c and 70c, the only problem with that sugestion is getting one small enough, also they tend to have quite a large diff, from memory the minimum is about 15k(this type of valve measures the superheat, or the discharge temperature on the high stage outlet). I have read various of the posts above and they all tend to go for a low-ish superheat setting of 4-6k, I personally would go for a setting of 10-12k, this I feel is easier to control, with less likly-hood of of hunting and over injection, leading to liquid entering the high stage. I may be wrong and I stand to be corrected, but I think that the few degrees less superheat won't make much difference to the high-stage discharge temperature or the high stage volume displaced. Oh and from memory the bitzer compressors come fitted with an Alco expansion valve.
Anyone ever worked with these compressors ustilizing an open-flash intercooler?
Regards. Andy.

[al]

Andy

We work on systems utilising these compressors and they have the discharge running through the suction accumulator, the manufacturer give a procedure for setting up the interstage expansion valve(an alco) to guarantee no excessive liquid carry over to the final stage.This also ensures the oil temp remains fairly constant.

We replaced numerous head gaskets due to air infiltration, systems run in deep vacuum.

[Andy]

Hi, Al, Marc. I like the idea of the hot gas being used to boil of liquid, this would be a lot less hassel than diverting a portion of liquid from the main liquid line off the pack to the suction accumulator (the pack has high stage compressor and a common liquid line), what way is the hot gas controlled? On a thermostat attached to the low stage discharge?
Also the scoring of the bore on the HT linners without an carbon build-up is correct.
I remember being sent to site because head gaskets were being used at the rate of one per day, I condemed the compressor for bad scores on the HT linners. A rebuilt compressor was sent, this was fitted with a new Alco valve, the old one had been wound out until the retainer clip had fell off the superheat spring. I sent the engineer who had been working on the plant to sett the superheats on the low temp cases to 10K, whilst setting the valves he had to replace a number of valves and a good few fan motors. The valves on the cases had been replaced with MOP types, originally they had been an N range valve, I had N range tev's in my car so that is what was put back on.
The engineer responsible for this site had to then explain why he was fitting head gaskets while all along the problems were mostly at the cases, with the exception of the valve for injection which had been wound out.
After the above work was carried out the site ran well for about six months, with no head gaskets being fitted. The service managers comment to me after reading my report was " so the compressor was at fault then"
Well do you think he deserved his position of trust?
Regards. Andy.

[Andy]

Marc, with regards to the llsv, this is fitted quite close to the valve, well within 1m. There is the possibility of off cycle injection of the trapped liquid in the 3/8" line, with the heat of the compressor and the warm plant room, this liquid would I think boil off quite quickly. I suppose in the ideal world the llsv should have been closer, this compressor was a retro-fit to replace the existing single stage 502 compressor when the pack was changed to R22, each pack has two compressor for LT duty and four for HT duty, mainly the head gaskets go on the slave LT compressor, renforcing Marcs thought that the high stage of this compressor is blown by liquid being injected to cool a near saturated low stage discharge.
I also heard that the owner of this plant was thinking of changing the refrigerant in the pack once again, this time to R404a, leak city or what in a 10Year old fridge plant?
Regards. Andy.

[al]

Marc

What you said about the closeness of the llsv to the interstage T.E.V., is it possible that the vapour in the interstage line could condense and wash oil out of the sump at start up. The Bitzers we use utilise an A.E.V. to keep discharge temp below 130*c which we originally thought were overfeeding but your thought may explain the intermittant trips on oil pressure. Food for thought.....

Alec