Charging a blended gas?

[Neddy]

Hi everyone,

What is the correct way to charge a Close Control unit that has a TEV, fan speed controller for head pressure on a blended gas like R407C ?

Some Guys at work charge to the site glass?

Some charge by Subcooling ? But if you charge by Subcooling the site glass flashes on R407c . So you don't have 100% subcooled liquid before the TEV?

Should you find out the charge for the Condensor/Evaporator and liquid line per metre from the manufacture ?

So many engineers have different views what are your views?

[james10]

if you can find out the correct charge then that is the best way otherwise charge to superheat in liquid form

[nike123]

Charge by weight stated in documentation, or charge by subcooling.
Flash gas in sight glass doesn't mean that liquid is not 100% subcooled.
It could flash because of sudden change in pressure drop which occur because of change of shape in line where is sight glass.
For calculation of subcooling, you need to use bubble point temperature of R407C for measured pressure at condenser outlet.

[nike123]

if you can find out the correct charge then that is the best way otherwise charge to superheat in liquid form

You cannot charge by superheat system with TXV, since TXV control flow to keep superheat constant.

[Neddy]

Thanks Nike 123. Is it acceptable to check Subcooling before the TXV as most close control units have the compressor inside with the evaporator so charging is done inside a room not outside?

[nike123]

Yes it is, if there is not significant pressure drop in lines (and parts like filter-drier) from condenser to TXV.
In fact, just before TXV is best place to check both pressure and temperature, since that is place where we want full liquid state.

[JoeAT50A]

Even though direct charging R407C is allow for some manufacturers those who have composition sensors for three different blends 32/125/134a. If there is gas leak, then their composition and working pressures are changed.
Due to different boiling points, lighter one's percentage reduce repidly. So that you need to drain all the R407C and recharged. And have to charge in liquid phase of course. here is the attached. Hope that will help.

6588