delta t for 404a in -21 upright freezer?

[b-cool]

Hello everyone, im new on here and would like to no the diff across the coil on R404a system trying to achieve -21. Im just getting back into fridge after 11yrs strictly wiv aircon my fridge exp is only the fundamentals from college and would like any advice or suggestions on where to get the best info (obviously thats by playing with systems!! but after that, cheers) is my question a bit too dependant on other factors? any comments will be appreiciated

[nando]

superheat with freezers 6K because to have not much ice in the evaporater.
but there's much change that the suction pipe get frozen to the compressor
We allways use 8K, and chose the evaporater bigger than the compressor capacity (30-50% bigger), never have problems with that
greetings

[nike123]

http://www.refrigeration-engineer.com/forums/picture.php?albumid=119&pictureid=745

[b-cool]

Thanks for the speedy replies guys! If i could ask another question and possibly make myself sound rather stupid, if for example the freezer had achieved -21 air temp, then odviously the refrigerant temp will be lower than this, is this where the 8k superheat comes in. Hopefully im right in thinking that the refrigerant temp will be more like -29. Please correct me if i am way off the mark. I may not sound like it but i am conversant with refrigeration systems and am qualified through city and guilds cheers guys

[nando]

indeed, take the warmth out with something colder than the request temp.
in the begin from the evaporater you have -29°, you take warmth out, at the last meeting at the valve-bulb you have -21°, your superheat is 8K.
"opening" the valve results a higher pression, lets say -27°, than you hav 6K.
The tabel from nike 123 is a good showing.

[b-cool]

thanks very much nando, you seem fairly interested in my subject. the system in question is capillery expantion so i have no easy means to change superheat as there is nothing to adjust but thanks for the help as i will teach myself to fully understand these rather basic systems compared to an electronically controlled air con system, so any advice or comment is welcomed cheers