valve plate failure

[Andy noblet]

taken over system with 2 semiherm units working freezer @-25 c one system works perfect, other system discharge valve fails or in the past compressor has broken a piston .
system on r404a tev s/heat 5c suction press 35psig disc press 190 any suggestions

[dai kin]

you say it has two units but does it have one or two evaps?

[chemi-cool]

superheat too high for a freezer, should be maximum 3C

at 5C superheat you are most likely to get liquid in your suction line,.

chemi

[Argus]

Is there any way that refrigerant can condense nearby in the discharge line or in the valve assembly, say on the off cycle, or syphon back when the unit is off? If this happens when the head is cold, a tiny amount of liquid can cauase a trnendous amout of damage.

This may happen if your unit is working as a lag system. Because the lead unit works most of the time, the conditions for liquid settlement don't exist.

It's not unknown in the UK. I have found it in the past where hydraulic damage is caused in the top end of the piston assembly and in the muffler that may be right in front of it.

If you have air cooled condensers sitting above the unit, have a look for check valves to prevent liquid settling at the lowest point by gravity.
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C77

[frank]

at 5C superheat you are most likely to get liquid in your suction line,.

How is that Chemi?

Any superheat above boiling point surely means that the liquid is now certainly a vapour? am I missing something?

[chemi-cool]

of course you right argus.

NRV after discharge valve will protect it from liquid migrating back.

also good design of discharge piping can do wonders.

chemi

[chemi-cool]

hi frank,

I was talking about a freezer. in AC its fine.

chemi