superheat ahd subcooling

[jonc5553]

I need help.

I've read that cap tube or fixed orfices are charged by superheat and Txv units are to be charged by subcooling. I have purchased a Fieldpiece hand-held meter SSX34. OK heres my confusion. If I'm working on a cap-tube system, first I have to find my target super heat. Inside wet bulb and outside dry bulb to get this target superheat. Say it tells me 12 target super heat. Then I read other articles that say you need 20-30 degree super heat at unit. If my meter tells me 12 then where does this 20-30 come from. Fieldpiece shows hooking up hose on suction sevice valve at unit and taking temp reading on suction line at unit. Theres the conflict. My meter tells me I want a 12 degree superheat and the article says 20-30.

The article I have is Mike Taitano Air Conditioning and Refrigeration Systen Evaluation Guide. His book does not say if its captube or TXV as far as the metering device, like it doesn't matter. I thought that when you check the super heat on a TXV you wanted to check your pressure at the leaving line of the evap. and line temp also, to get superheat value, if there is a port on line, 6- 12 degrees super heat. So am I thinking right, that you can also check superheat at unit and thats where you should have 20-30 superheat. I'm confused. Sorry for making this so long, this is also my first time posting.

[frank]

To help you understand SH & SC, do a search on the forum where you will find lots of posts/threads on the subject.
It is something that has been discussed many times on here.
The search button is top left of the screen

[monkey spanners]

Hi Jonc,

We don't get those Fieldpiece meters in the UK which is a shame as they look like a good quality tool.
Also we don't see many of the type of system you are asking about, most of our ac tends to be mini splits which have a factory charge and then 'x' amount per meter over a set length, so to check the charge on these you have to recover and weight the charge.

Heres a vid i found on youtube of one of the meters being demonstrated, it would be worth asking Dr Z or following the link on the vid to Fieldpiece products channel for advice.

http://www.youtube.com/watch?v=vhrULNpdwd4

On the other vids i have seen there has been a chart that you look up your outdoor temp and this tells you your target superheat, but like i say i don't work on these style of systems :D
If you do find out please post back here, i'd be interested to learn more about these systems.

Jon :)

[cleverd]

i use 8-20 superheat on txv systems by checking at tail end of evap temp vs. the pressure on low side at svc port.