Rims design

[bazbaby3]

Hi, After knocking out brew after brew, I plan to upgrade to a rims setup, sourcing cartridge elements is a tricky business as im finding out. I can get hold of 5/8"diameter x 12"long stainless steel standard density unit. It is 240v, 1200w, 53w per inch squared. If i put this in a 28mm copper tube would this be adaquate to maintain a 66 degree temp/raising temp 77 degrees to mash out? Ive looked at rims set ups on this site eg gary smiffs but i am worried about scorching the wort. Please can anyone shed some light on the subject/calcs, cheers!