R508 charging

[g.mccormick]

I was discussing a low temperature cascade system that we have with one of our techs. I have only seen the system one time before. He and I discussed that the charging of the system (low side) is to be done with static pressure. I have never dealt with a low temp cascade system before, I am use to seeing receivers/sight glasses/ etc.

I looked into R508 and see that the critical temperature is 57°F. It would seem to be then that the reason you have to charge it by static pressure is that you will not be able to get liquid out of the tank at a temp above 57°F. You have to charge by gas, and since it is an azeotrope then there is no need to charge liquid (like 410).
I am still not quite certain why you cant charge by mass even if charging with gas.
Anyone want to give a better explanation?


Thanks all.

[Rob White]

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It's critical temp might by 12 degsC what you say as 57°F but the pressure is what matters.
Critical pressure at 12 degsC is about 38 bar that's 550 of your American pounds. :)

As long as the pressure is high enough it will be in liquid state and therefore you can charge as a liquid.

Rob

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[g.mccormick]

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It's critical temp might by 12 degsC what you say as 57°F but the pressure is what matters.
Critical pressure at 12 degsC is about 38 bar that's 550 of your American pounds. :)

As long as the pressure is high enough it will be in liquid state and therefore you can charge as a liquid.

Rob

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No that's not correct. The critical temperature by definition is the temperature above which the substance cannot be in a liquid state no matter how high the pressure. The region above the critical temp is considered the supercritical region.

[Rob White]

No that's not correct. The critical temperature by definition is the temperature above which the substance cannot be in a liquid state no matter how high the pressure. The region above the critical temp is considered the supercritical region.

You are correct :) I'm not sure what I was thinking :D

So options available is to run primary system to bring the secondary down in temp. Cool the R508 cylinder below 0 degC your 32°F.

That way you bring the refrigerant back down to a saturated mixture and then can transfer it.
It would bring the pressure down as well.

Rob

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[Brian_UK]

Yes Rob, that would work but if the manufacturer recommends filling by pressure then surely that is the method to follow.

I always used the pressure method when I had to perform repairs on cascades and they worked for me.

Mind you, I haven't touched one for years now so things might have changed, as long as there is sufficient expansion room built into the system then maybe you can be a little freer with the charge.