Sight Glass in Systems with Zeotropic blends

[lana]

Dear all,

For HCFCs and HFCs the sight glass must be full, i.e. solid liquid column must be supplied to TEv.
For Zeotropic blends (400 series) if the sight glass is full then the system is over charged.
Any comments ....?

Cheers

[AUcooler]

I think both comments are misleading, as the liquid line sight glass is often a poor indicator of refrigerant charge unless under ideal conditions.

If the refrigerant liquid line sight glass was made redundant, it would force more techs to meet up with their PT charts, and become more aquainted with superheats

ps- did you post this question because you wanted to know the answer, or did you post it as an aptitude test for readers?

[TRASH101]

Hello lana

For Zeotropic blends (400 series) if the sight glass is full then the system is over charged.
Any comments ....?

The statement is flawed.

What justifies correct charge?

If vapor exists in the liquid line then you have compromised the optimal mass flow rate for that liquid line regardless of anything else. So you would hope that the blends would be such that the liquid phase of the components would match the system dynamics of the application.

p.s. good to see your posts again lana

[lana]

Dear Aucooler, I want to know the answer.

I agree with you on correct charging procedure. Sight glass is a help but charging must be done by measuring other parameters.


I have read somewhere (some of the sight glass producers technical bulletins or a book , which I don't remember which one) this statement :
The pressure drop of sight glass makes the zeotropic blends to vaporize a little bit which makes the bubbles in the sight glass.

I am not completely sure on this.


Dear TRASH101 thanks.

Cheers

[AUcooler]

Heres an example of why a sight glass in a 400 series refrigerant is a redundant item.

I frequent a local supermarket which has a pair of Carrier semi hermetic compressors(06E from memory) powering its air conditioning plant on R407c.

The sight glasses of each system flash refrigerant vapour constantly regardless of the ambient temperatures varying from 18degC(Winter) to 35+degC(summer) or head pressure which ive set at around these days at 1600kpa(42). The suction pressure runs at around 350kpa(loaded) with a superheat which hunts a bit betwen 4-10k. You would think with a hunting TXV that the sight glass would clear during the closing period of the TXV hunt, but it does not. I cant recall the liquid line temp off hand other than its not a problem. The plant is indoor and their is quite a short pipe run to the evaporator.
Sight glasses in 400 series are a problem in my view and should not be fitted. Basically because young techs will almost certainly add refrigerant and cause an oil failure fault.

In answer to your question Lana regards the 407 A/C plant I used as an example, I think the oil would be flushed from the compressor before, if ever the sight glass was full.
I cant say your comment regards SG full equals over charge is correct, because as Trash said, its a flawed statement.

[lana]

AUcooler,

Thanks for the input.

Cheers