Charging a system with evaporator fan off.

[Burrah Boy]

Hi, I was chatting to a chap recently about charging a small freezer running on R404a with capillary expansion. He said he charges the system by putting the evaporator fan OFF and waiting for about an inch of frost past the evaporator outlet. The thermostat would obviously be on its lowest setting to put the unit on full load. I have heard of this before, but never tried it. Is it a trick of the trade? Can someone explain the theory about whats happening!
Another technician told me you can also use that method with an internally equalised txv. I pressume the bulb senses the S/H, lowers the pressure on the diaphram, thus the valve starts to throttle back, until all three pressures in the valve equalise. This frosting would then indicate that there is enough liquid in the evaporator. He said if no frost was present it would indicate that there is still too much vapour in the evaporator.
Thanks

[old gas bottle]

theres allsorts of little tricks to charge a system,probley the best is to look on the label for the correct charge and weigh it in !!:D

[Burrah Boy]

Yeah, thanks, I would also weigh it in, or charge on subcooling, or on motor current, but I am just curious as to this method!

[stuartwking]

i have tried this also and seemed to work. Not the best method of charging though.
I tried it on a water cooler with no water through evap. Charged it with liquid till the suction started frosting few inch before compressor. I wouldnt do it on kit that more expensive.

[Chef]

Hi Burrah Boy

Probably the main reason to turn off the fan is so that the evaporator does not have to cool down the whole box and its load. Once the evap plate reaches its design temperature then all the liquid will be in the evap plate and the charge can be adjusted to the desired frost line position.

If you have to wait for the whole box to be pulled down it could take several hours (or more) and it is just a quicker way to do it. The system balance will be the same either way.

Sometimes for systems without a fan the evap plate is cut of from the rest of the box with a bit foam so it cools down quicker - same idea.

Chef :D

[davidlleida]

I agree with you, Chef,
My father (1931), told that he used to charge this way.
He got sure that he feeded the evaporator with enough liquid, and that this would not arrive to compressor due to overheating.
He didn't had much problems proceeding that way.
I think it's not a bad procedure when you don't have all the concrete information mainly in old systems or renewed.