Two Stage Cooling

[kev]

I came across a two stage cabinet at -80 Deg C and had a job working out the controls on the system, the 1st compressor had lost part of its charge and was short cylcling, how or what controls the two compressors, any help would be appreciated!

[Brian_UK]

Kev, at it's basic the high side compressor should run first to provide the correct condensing conditions for the low side system.

When the stat/pressure switch is satisfied then the low side should start up.

Sometimes however you might find timers as well just to confuse things further.

The other fun thing is getting the charge correct, but you'll find that no doubt.

Good luck

[Dan]

I speak from the grand experience of having observed a cascade box. I was toying with the idea of perhaps repairing these as a new business opportunity.

The box I worked on would have required an investment of more than $1,000 for a 20 lb tank of refrigerant. The other refrigerant was isobutane. The charges are so small, that even hooking up gauges affects the working charge.

They are charged with static charges, per the books recommendations. The one I worked on had a nice electronic controller which staged the compressors, controlled temperatures, etc. If I recall there were at least 6 sets of thermocouple leads factory installed at key postions, that you could hook your fluke up to and get important readings.

I read more than 100 deg F superheat on the colder operating compressor.

I do not recall any pressure controls for compressor cycling.... which is why I am surprised you saw short-cycling.

There was a backup system for injecting liquid nitrogen into the box in the event of failure. Also, the compressors were the same model in each stage.

I found the engineering and performance intriguing, but when I found the door switch not working, I decided to return the box repaired without investing the money in the refrigerant.

My thinking is that somebody who works on these boxes should be working in support or with the support of the manufacturer. Without the manufacturer's support, I would guess it would be difficult to make money working on cascade systems from a normal service perspective.

Not that I don't love the fridge aspects.

[Brian_UK]

I agree with you Dan, they are interesting but a real pain at times. I was involved with a compressor change, well both over a year on a -80°C cabinet for lab. samples.

We ended up with the last model Bristol compressor in the world for the box and even that arrived upside down (!!). Two months later they finally found the money to replace the whole thing.

Yes, charging a whiff of R23 can make a hell of a difference to the performance and the time waiting for the box to cool down generating quite a few coffee breaks

Getting back to the original problem, as you say, if there aren't any pressure switches then it could well be the compressor overload shutting it down on overheat if the charge is low (?)

[Dan]

My first thought too, but it just doesn't strike me as a short cycle when you cycle on the overload. That would the source of additional coffee breaks.

[kev]

Thanks for the comments guys,
The problem was indeed a shortage of fridge, when topped up of it went to -80, took a long time to do, didn't have time for a coffee break though!!

kev

[Gary]

Strictly speaking, this is not a two stage system. It is a cascade system.

A two stage system splits a single compressor into two stages. A cascade system has multiple compressors.

That said, this cascade system has a high stage system and a low stage system, connected by an interstage heat exchanger.

The interstage heat exchanger is the evaporator for the high stage system and the condenser for the low stage system.

It may be useful to think of the high stage system as the "condenser fan" for the low stage system, and the low stage system as the "evaporator fan" for the high stage system.

You wouldn't want to start the low stage system without it's "condenser fan", especially since it is using very high pressure refrigerant, so you want to make sure the high stage is running and cold before starting the low stage. Hence the high stage interlock.

Many of these systems run the high stage continuously, cycling the low stage compressor to control box temperature. As you might expect, the high stage runs close to floodback when it's "evaporator fan" is not running.

[cpt. cascade]

give me your make, model, and serial and i will tell you your problem. or at least give you tons and tons of options

[kev]

Thanks for the reply.
The 1st stage was short of gas, topped it up and it has run ever since!
If and when it stops I will post on the site again!!
Regards,
Kev

[cpt. cascade]

Depending on the make and year of production, there are many causes for the symptoms you gentlemen have been discussing. These thing are gettin moodier and moodier, they basicly have little angry computers in them, that hate bells and whistles. Unfortunatley the manufacturers like bells and whistles. Im glad it was taken care of. good job