Someone with thier Masters Certification in HVAC should be able to easily describe this part - and provide lots of online links as well.
I said I had a NJ Master Plumber License. Yes I am a Union HVAC Pipefitter. I don't go online to purchase my supplies, I call them into the shop and they are delivered, or I stop at a supply house to pick them up on a service call. And I have no idea why you think being a journeyman in either trade would make me an internet search engine. I only come into this forum to help out the DIYers a little. I have no idea why you seem to have an attitude. So go screw yourself.