Just my opinion but I kinda have to disagree here... as mentioned in another thread the enzymes are in the wort so you want to heat the wort to the desired temp as quickly as possible without going over that temp. With lower flow (and longer contact time you can achieve a specific temp point as the wort passes through the rims without a lot of element firing time... You dont want fast flow where SOME of the wort is super heated by a very hot element which is way over the desired temp mixing with other wort which is cooler and never actually touches the element...you want complete/even and stable heating in one pass without temp extremes. Ideally an element which never gets hotter (or barely) than the desired setpoint is the best approach.. plus with less flow you have MUCH less chance of stuck sparges and issues that come from varying flow rates.
5500w is a lot of heat packed into a small surface area... I step mash just fine with 1800w myself.
I do somewhat disagree with that - if, say, you want to switch from one enzymatic action regime to another by heating in a step mash, at a defined time, you need the wort throughout the entire mash/wort volume to be heated to the desired temperature. Otherwise the enzymes in the wort (in the mash tun) that hasn't been heated yet carries on working on the previous step. Enzyme denaturing also doesn't happen instantly, so wort that goes into the RIMS tube, gets heated to the next mash step and then cooled again as it goes into the mash tun doesn't fully transition from one mash step regime to the next*.
So I think you probably want to heat the entire bulk of the wort, and maintain that temperature, to go fully from one mash step to another, and that you probably want to do that in a fast** way. However, the point about the enzymes being in the wort is an important consideration for maintaining temperature within a mash step - if you have a large heat loss going from the RIMS tube to mash, and/or from the mash tun itself, and are trying to maintain temperature in the mash tun, then the wort temperature in the RIMS tube will be significantly higher than in the mash, and you may get significant enzyme denaturing and a drop in enzymatic action during a long hold at a particular temperature.
You can step the bulk mash temperature fastest when the element is on for as much as the time as possible, and that happens when the flow is fast enough that it doesn't need to switch off due to the wort in the RIMS tube overheating. It doesn't need to go any faster than that, as the heat transfer is then limited by the power of the element, but as the incoming wort temperature gets closer to the setpoint, then the heat transfer will slow again. This will happen at a higher input temperature at a faster flow rate. In the case of an element that is heated only to the set point temperature, you still get the fastest heat transfer to the bulk of the mash by faster flow rates, as the mean temperature difference along the contact area is larger - a HERMS will heat a mash faster with a faster flow rate.
The other point I'd make is that faster flow means more turbulence and mixing in the RIMS tube***, and thus more uniform temperatures in the wort around the element, and generally less chance of part of the wort overheating and scorching. Laminar flow is bad but is also more likely at lower flow rates for any given RIMS tube. And yes, I agree that varying flow is the worst scenario, so staying well away from stuck mashes is important.
Quite possibly I'm overthinking this****, and making too strong an assumption about what is desired from a step-mash...
*Assuming no significant overheating - you could use overheating of the wort to more rapidly denature one enzyme, but you'd be running the risk of denaturing the ones needed for the next step. I don't think people generally want to do that, but you could look at it as a continuous flow decoction mash. You'd want good mixing in the mash tun (and no scorching) if you were to attempt that, so that the entire wort volume didn't have to pass through the RIMS tube and get overheated.
**Consistency may be a lot more important if you are developing recipes that you repeat a lot or that are based off one another.
***Generally. Specific cases where the flow detaches in certains ways and leaves static eddies might differ, but I don't think they are likely in the typical RIMS tube scenario. Feel free to come back with CFD simulations that show otherwise.
****The number of footnotes might well be a sign, as would the loser length of this post...
