Quote Originally Posted by GXMPLX View Post
Good, Ok ... both! Can’t I make mistakes?

Subcooling is more important. Charge until you get enough to feed your expansion device correctly.

Then check superheat to verify that the compressor is working in safe conditions. If you have a suction accumulator you could really get by not measuring superheat (IM NOT SUGGESTING YOU DON'T, I’M JUST MAKING A POINT THAT SUBCOOLING IS MORE IMPORTANT... TO CHARGE).

Why both because the problem by charging with subcooling is that you have to do it at maximum thermal load and your system may be overcharged for minimal loads, so you need a balance or compromise between two opposites, complying at maximum load and operating safely at minimum load.

The correct thing would be check superheat at minimum load ... but as my wife says am at an age or repeating myself ... often... you really don't need superheat to charge.

Why you shouldn't charge with superheat is that you don't have an exact amount of superheat and it will constantly change in contrast you can calculate exactly the amount of subcooling you need and it changes less.

The correct amount of superheat is as low as possible without getting liquid in the compressor and not as high as to producer high discharge temperatures and that’s a wide range. Some manufactures say a minimum of 10c but it could really be as low as 0.1c as long as it is there!

The exact amount of subcooling is given by the loss of pressure and temperature gain in the liquid line. Convert the pressure drop to equivalent saturated temperature difference using the correct refrigerant tables and add the temperature gain.

In a working system you may measure both pressure drop and temperature gain.
Another thing about superheat is that if your expansion valve is sized correctly and the system operates in a narrow range of temperatures and pressures the TEV will take care of superheat for you.

So Subcooling! … no both! … or subcooling?
Logical statement！Why both? Subcooling is enough!

I appreciate the kind explanation by you. I found I become regular poster today. ！Next I will try to answer other’s questions, like you and Iceman. Then become senior poster….