Crushed evaporator tubes caused by pulling a vacuum.

[Grizzly]

I was advised last week that I could crush the evaporator tubes whilst pulling a deep vacuum!
Unless I ran the chiller water pump. (Evap was full of water although its circulating pump was off.)
This I disputed!
I know adding liquid refrigerant prior to the system being at it's relative pressure for 0c.
Can crush tubes, however I have never heard of a vacuum being powerful enough!

Has anyone else heard similar?
Grizzly

[Quality]

Its a possibility at the extreme! but its down to how quick the vacuum is created and the temp of the water surrounding the heat exchange surface

[Tayters]

Read about similar, only on this site though.

Old wives tales or not I'm not sure.
From the point of view of the tube I think it would have more inherent strength if it had higher pressure on the inside than the outside. Weather the pipe could actually deform I don't know, can't see why running the pump would make any difference as the pressure wouldn't change that much.

[chemi-cool]

Impossible!

Absolute vacuum is -1 bar,
A circle has the strongest geometrical structure.

[The Viking]

Old wife's tale or misconception?

It is quite common to burst "wet" evaporator pipes if there is no flow whilst the pressure is lowered but not when you are puling a vacuum...

This will happen when you reclaim the refrigerant, as the refrigerant boils off the water on the other side of the pipe walls might freeze and therefore expand, thereby bursting the pipe.

But during the vacuum process? Assuming good engineering practices has been followed there shouldn't be any refrigerant to boil off.



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[Grizzly]

Thanks for the input guys!
My answer at the time was that it was impossible to remove sufficient heat from a evap full of standing water by Vacuum alone.
Whereas when adding refrigerant was another story!
The theory may well exist but I am not going to freeze the sort of volume of water that I was talking about with 2x 5.5 cfm Robin vacuum pumps!

Worth asking all the same, I have been wrong in the past.
Grizzly
PS Viking. With 407c it seems to boil off nicely for ages and has a perpencity for sitting in the oil and playing havoc with my torr gauge.

I hate the stuff!

[The Viking]

PS Viking. With 407c it seems to boil off nicely for ages and has a perpencity for sitting in the oil and playing havoc with my torr gauge.

I hate the stuff!

Ahh, we are both getting too old my friend...



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[james10]

I have had this happen and posted on RE about it when I get on my pc ill post a link

[james10]

http://www.refrigeration-engineer.co...ht=#post244754

[Grizzly]

All well and good James.
But why advocate running the pump then?
Surely running the water pump would increase the actual forces acting inward on the outside of the tubes within the bundle.
When the gist of the argument was to prevent Ice crystals forming and thereby crushing the tube/s.
IE. whilst pulling a vacuum the moisture is boiling off, however you can theoretically cause ice crystals from this moisture.
So you could ultimately cause ice formation on the outside (Water side) of the tube, causing tube implosion. My argument is, that the amount of heat rejection required. Simply is not available by vacuuming alone.
Particularly if the evap is full of water.

I have no doubt that these forces could be reproduced in a lab.
But have yet to see this phenomenon in the field.

Therefore I think it is a case of theory blinding fact!
Even cases of tubes being crushed with the rapid introduction of liquid refrigerant, normally only happen when there is little or no water in the evap.

I am at a loss as to where this instruction to run the water circulation pump whilst pulling a vacuum. Has originated from?
Is it now being taught in college? If so why?
What are the trainers advising?
Yellow Jacket in their training videos don't mention it!
Grizzly

[james10]

I've no idea why the pumps should be ran whilst vaccing down, the only time i have done this when vaccing is when i have had a wet system and got site to run warm water through the tubes on the evap/cond to put heat in and try to speed up the vac process, other than that whilst degassing and gassing like you say to prevent thermal shock and tubes freezing.

[Quality]

simple theory is that running water will not freeze and its ice thats the culprit