Yes people use RIMS successfully, but it requires a properly designed and mounted RIMS tube. Some design considerations:
- RIMS tubes must be mounted vertically. With horizontal mounting it's too easy to create conditions for a bubble in the tube. If the element gets exposed in a bubble, you will burn wort, and may destroy the element. Save yourself some headaches and figure out how to mount the tube vertically.
- The wort must enter the tube below the heated portion of the element at the bottom of the tube.
- The wort must exit the tube above the tip of the heating element.
- The temp probe must be placed close to the tip of the heating element at the top of the tube. It is critical to measure the wort temp at its hottest point.
- During mashing the max temp of the exiting wort should be kept below ~160°F (~71°C) to minimize denaturing of the wort during the mash time. If doing step mashes, with step temperatures above ~155°F (~68°C), then keep the exiting wort temp less than ~10°F (~5.5°C) over the target temp of the next rest. When heating to mash out, keep the exiting wort temp less than ~200°F (~93°C.) If the wort gets to close too boiling temps, you will likely get pump cavitation.
- Use the fastest cycle time possible with your temp controller. Most of the PID's used with home systems use pulse width power modulation (PWM), and have min cycle times of 1 - 2 seconds, which means the element "on" time at 50% power is 0.5 - 1.0 second, at 75% power, on time is 0.75 - 1.5 seconds. The longer the element is on, the hotter the surface of the element will get. The hotter the surface of the element, the more likely you are to scorch the wort. A better choice for a RIMS controller is one of the EZBoils (DSPRxxx) from Auber Instruments. They use pulse count power modulation rather than pulse width modulation. The EZBoil pulse width is a constant 0.0167 seconds (16.7 msec.) At 50% power or below, there is at least 1 off period between each on period, so the maximum time the element can be on continuously is 16.7msec. Above 50% power, you start getting consecutive on periods. At 75% power the max on time becomes 50 msec.
- The control system should have an interlock so that the RIMS element cannot be turned on unless the RIMS pump is turned on. Even better is a flow switch interlock on the RIMS element.
Now we will do some calculations on required flow rates vs. element power. 1000W (1kW) equals 56.87 BTU/minute, 1 BTU will raise 1 lb of water 1°F, and water has a density of 8.18 lb/gal at 150°F. 2000W (2kW) is 113.74 BTU/min, so 2000W would raise 113.74 lb of water 1°F/min, or 11.374 lb of water 10°F/min. Thus to keep the outlet temp under 160°F with 150°F input, you would have to put 11.374 lb/min or 11.374 lb/min / 8.18 lb/gal = 1.4 gal/min thru the RIMS tube.
The above calculation ignores things like heat loss to the environment from the surface of the tube and plumbing, and the fact that wort has a higher density than water. These effects would cause the temp rise to be lower at the same power and flow, so the calculated flow rate has a little safety margin built in.
A bigger factor that is ignored is the surface temp of the element (which is what determines scorching.) The rate of heat transfer from the surface of the element to the wort is determined by the temp difference between the element surface and the wort, and the surface area of the element. So, to minimize the element surface temp, you need to maximize the temp delta between the wort and element, and maximize the surface area of the element (minimize the watt density/in^2 of the element.)
If the wort has laminar (smooth) flow across the element, then the wort at the surface of the element will be significantly hotter than the wort farther away from the element surface (maximum wort temp is significantly hotter than the average wort temp), and the heat flow into the wort will be slowed down. To compensate for the hotter wort at the surface of the element, the element will get hotter in order to keep the heat flux in balance (power leaving the element must equal the power being input to the element, or the element gets hotter.)
Thus you want to insure turbulent flow thru the RIMS tube, which will keep the max wort temp at the element surface closer to the average wort temp. One thing to encourage turbulent flow is to have the smallest tube diameter that will allow the element to be inserted in the tube. Another thing you can do is put flow disrupters in the tube. One easy way to do this is wrap a stainless steel wire around the element tube (including only one leg of the element within the wrapping.)
Brew on
