2 questions, breaking vacuum and receiver subcooling

[WangFu]

I find myself repeating things that I was told or learned over the years. Unfortunately I can't explain these things when asked and can't find any info that spells it out for me.

I was taught that after evac/dehydrate (following a repair and leak check) to break vacuum with vapor, then continue charging with liquid. Anyone know why? I seem to remember it being related to freezing up or causing damage somehow but that explanation is flimsy.

I have read in tech manuals that a reciever will reduce or eliminate subcooling that was achieved in the condenser, but I cannot find an explanation. My is guess that the change in piping size causes a pressure drop and at that point the boiling temp drops wiping out the subcooling? I would love to have a better explanation here too.

Thank you

[Brian_UK]

I was taught that after evac/dehydrate (following a repair and leak check) to break vacuum with vapor, then continue charging with liquid. Anyone know why?
I seem to remember it being related to freezing up or causing damage somehow but that explanation is flimsy.Hi WangFu and welcome.

I was told that injecting liquid at that stage could cause thermal shock and possible component fracture due to the rapid temperature changes.

It has been known for the copper pipe or a braze near the access port to crack due to the rapid contraction.

Hope that helps, now all you want is someone to answer question two;)

[US Iceman]

Hi WangFu,

Welcome to RE.


I have read in tech manuals that a receiver will reduce or eliminate subcooling that was achieved in the condenser, but I cannot find an explanation.

#2

The condenser is exposed to warm ambient air. Therefore, any subcooling achieved in the condenser is lost in the receiver (a flow through receiver only... This is the type with a diptube supplying the liquid).

The surface area of the receiver can warm the liquid back up. For this reason, the subcooling (achieved in the condenser) provides essentially no benefit to the system.

On the other hand, if the receiver is a surge type (where the subcooled liquid flows directly to the TXV) then the subcooling is not lost IF THE LIQUID LINE IS INSULATED.

Any subcooled liquid line should be insulated if you want to maintain the subcooling to a greater degree.

[wambat]

There are certain places in a refrigeration system where the P-T relationship must exist such as the evaporator, condenser and the liquid receiver but will not exist in superheat and sub cooling, that said when a system employs the use of a liquid receiver, there can be no subcooling at the surface of the liquid in the receiver. The reason for this is that when liquid refrigerant and vapor exist together, they must obey the P-T relationship and the refrigerant must be saturated. In order the get the proper subcooling a heat exchanger must be employed or as iceman said you go straight to the expansion valve

[old gas bottle]

in most cases now you have to break the vacum with liquid just because its the proper way to charge with blends[most gases now] even if it possibly cause problems,if you can drop a magnet on the liquid line solinoid,that stops liquid down the suction.

[Peter_1]

On the other hand, if the receiver is a surge type (where the subcooled liquid flows directly to the TXV) then the subcooling is not lost IF THE LIQUID LINE IS INSULATED.

Any subcooled liquid line should be insulated if you want to maintain the subcooling to a greater degree.

I just want to add that you can indeed - like Wangfu described - loose subooling due to too small liquid lines.

But USIceman made a good point about the insulation, something I rarely see in long lines like in supermarkets.

BTW is it insulation or isolation and what's the difference?:confused:

We allways charge immediately liquid (in bigger installations even with 3/8 tubes connected on the preheated bottles) and never encountered problems with it.

[Argus]

in most cases now you have to break the vacum with liquid just because its the proper way to charge with blends[most gases now] even if it possibly cause problems,if you can drop a magnet on the liquid line solinoid,that stops liquid down the suction.


Absolutely.
Add liquid with all 400 series blends into the liquid line.

However, we have to go back to the good old days to answer the question why the initial vacuum was broken slowly with vapour.

This was particularly important with sub-atmospheric flooded evaporators. (typically R11 hermetic chillers and the like).

The rapid introduction of liquid into the unishell ran the danger of splitting tubes if there was residual water and no pumps running. It's essential to have water flow.

So, to add to that, if it's a water chiller, it's essential to have ALL the pumps running and proved.


If its an air to air system, it doesn't really matter, but liquid's quicker.

.
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mflb vaporizer (http://mflbvaporizer.com)

[US Iceman]

Hi Peter,


BTW is it insulation or isolation and what's the difference?

Insulation is the word for a layer of a material used to restrict heat flow.

Isolation is a word for limiting access to another area. You might say you and I are isolated by the ocean between the US and Belgium.

Does that help?

[US Iceman]

Hi Argus,

You went straight to the heart of the matter.


The rapid introduction of liquid into the unishell ran the danger of splitting tubes if there was residual water and no pumps running. It's essential to have water flow.

It has been awhile since I thought of that. This also serves as a reminder of where some of our procedures comes from.

[Argus]

It has been awhile since I thought of that.




Same here.

Deep in the memory banks is a recollection that Carrier, for example, stipulated a vapour-only vacuum break up to 18" or 20" minimum vacuum.

Are there any old-school Carrier engineers out there who recall that?

.
________
YZR-M1 (http://www.cyclechaos.com/wiki/Yamaha_YZR-M1)