Re: surface area, I get what you mean, but when I'm not aiming for boiling point I'm hoping the heat transfer to the surrounding material will be sufficient.yeah, it'd work.
You'd have to give some thought to the flow dynamics in there especially for the purposes of cleaning, given that you couldn't really open it up to inspect/physically clean the unit. Square things, 90 bends, dead corners etc.... chemical cleaning can be less effective of there are many of those things in a CIP situation.
You also have to think about whether it really is "high surface area" - most pf the electric frypans I've seen are just a round element fused to the back of an aluminium plate. So what you have is basically a heat sink that you hope will distribute the energy from a not very high surface area element, across a wider area. Having watched water boil in my electric frypan, all the action tends to be in a circle that is directly over the element, which leads me to think that perhaps in a liquid contact situation, the metal doesn't do that great a job of spreading the heat around.
Still, two frypans, two elements, two times what possibly limited heat sinking you might get from the metal.... its going to be lower dednsity than a lot of other rims solutions.
Steam through a coil... that'd work too, but I reckon you'd be making your life pretty hard for not a lot of benefit. If you are going to have coils and stuff flowing through them, and potentially other separate vessels for heating up the stuf...... might as well just go with a HERMS. If ou like your original idea better though... think creatively. Boiling water pumped through is also 100 and will work just fine, you just need to design it so the contact surface area will transfer enough heat. Steam is likely to be hotter than 100 anyway... so why not use a heat transfer fluid that also gets hotter - like oil. Then you dont have to worry about closed pressure systems and you can just use a pump (although I dont know which sort of pumps would be suitable) or set it up so it thermo syphons.
A rims can be complicated, or stupidly simple - up to you.
Think of an electric kettle from the shop.... your pump puts wort into the kettle, the wort runs out through the kettle spout back into your mash tun. The only other thing required to make that a "rims" is a way to control the temperature. Everything else is basically just window dressing.
Re: steam, well.... I have some "steam" producing equipment already as well as a nice bit of wound copper tube to fit inside a 50mm tube. But as you say, it's an indirect method of applying heat, with the associated lag and probably a challenge to get temp control working, more so than direct heating anyway.
Re:CIP I hadn't considered trying that. I was intending for a design that could be dismantled. But if I were to make it round, inlet in the edge flowing along the inner wall and takeoff in the centre that would make it CIP maybe.
Hehe funny you should mention kettles, I was just looking at SS kettles on the weekend for just this purpose. I don't like the base on the ones I could find. I don't want one that "sits" on the power socket, rather one that is hard wired or old style plugin.