Sizing RIMS heating element for mashing scenarios

[EscapeArtistBrewing]

Could RIMS tube users weigh in on the sizing of their heating element?
I use a 1500W (120V) element with a PID controller, with 120V input (mainly for maintaining temperature when it's cold).

I acknowledge that the element size probably depends on what one is doing with the heater. Some scenarios might be:

1. Simply maintaining temperature during recirculation of a single-infusion mash
2. Step mashing

What seems to be the minimal effective size used? For instance, I could add a 3500W/240V element running at 120V for an effective power output of 875W. Other elements can be used to adjust power to other values. Probably not enough oomph for a step mash, but would it be enough to maintain a temperature of a fixed-temperature single infusion mash?

Alternatively, what is the minimum rating for elements for step-mashing (just asking about user stories; there is no "spec")? Scorching is always a concern, of course.

Do folks rely more on the element rating for anti-scorch wort safety, or is the controller the more important system component?

It would be great to hear from more seasoned RIMS users on these issues.

 

[HighVoltageMan!]

The size of the element depends on batch size and how you want it to perform.

I have a RIMS system and brew 5-10 gallon batches. I use a 208Vac 5500 watt element and run it at 120Vac, it gives me @ 1800 watts. This is about the maximum that can be put on a 20 amp circuit.

I find this size it nearly as good as I can get. The temperature rise is perfect for conversion when step mashing. The temp rise is 1.5-2F per minute. As far as scorching, the biggest thing that I do to avoid it completely is to keep the flow as high as possible. If you want to down size the heater size, you could just reduce the power output from the PID. Depending on the PID, most can set to a maximum power output from 1% to 100%. If you have a 1500 watt element and only want 1000 watt output, you could program the PID to 66%. This will have several benefits, first your 1500 watt element will work for anything at 1500 watts and below. Second, the surface area is larger on the on the higher output and the watts per square inch can be less at lower power.

I run full power all the time with a flow as high as I can get and let the PID modulate the power as needed. I have never had any problems with scorching.

Good luck!

 

[quantumguy]

Well, if you already have a 5500W RIMS element and don't mind throwing another couple hundred dollars to never worry about scorching or flow rates again, use a CFC coil (I use the SS one from Stout) for your wort recirculation instead of directly through the RIMS tube. Use the RIMS and a cheap mag pump (got mine from Amazon) to run a closed, hot water loop through the outer coil of the CFC. Presto, an external HERMS setup with every advantage of RIMS as well as every advantage of HERMS (with none of the disadvantages, perceived or otherwise, of either).

 

[EscapeArtistBrewing]

Well, if you already have a 3500W RIMS element and don't mind throwing another couple hundred dollars to never worry about scorching or flow rates again, use a CFC coil (I use the SS one from Stout) for your wort recirculation instead of directly through the RIMS tube. Use the RIMS and a cheap mag drive (got mine from Amazon) to run a closed, hot water loop through the outer coil of the CFC. Presto, an external HERMS setup with every advantage of RIMS as well as every advantage of HERMS (with none of the disadvantages, perceived or otherwise, of either).

That's brilliant, and avoids the 3-vessel stainless-palooza. I really like the worry reduction!

I take it one would put the controller temperature sensor in the tun, and let the mash tun temperature itself control the temperature in the RIMS?
OTOH, I could see controlling the heating water temperature directly (sensor at the RIMS output), which would assume the mash temperature would take on the RIMS temperature in a "reasonable" amount of time with a "reasonable" hysteresis--due to the proximity of the CFC's RIMS flow and mash flow. Eh?
Or, would the sensor placement need some experimentation?
(Or maybe you already know )

 

[quantumguy]

You put the sensor in the same relative place you would in a regular RIMS setup, right at the wort output of the CFC (or alternatively right at the wort inlet to your MT). This avoids any overshoots (just like the normal RIMS case). I have a dial thermometer on the hot water input to the CFC just to satisfy my curiosity for how hard the hot water loop is being driven by the RIMS.

Also, note that due to the inherent efficiency of a CFC, the temperature of the exiting wort very quickly reaches the set point even for very large steps.

 

[EscapeArtistBrewing]

You put the sensor in the same relative place you would in a regular RIMS setup, right at the wort output of the CFC (or alternatively right at the wort inlet to your MT). This avoids any overshoots (just like the normal RIMS case). I have a dial thermometer on the hot water input to the CFC just to satisfy my curiosity for how hard the hot water loop is being driven by the RIMS.

Also, note that due to the inherent efficiency of a CFC, the temperature of the exiting wort very quickly reaches the set point even for very large steps.

Buddy, you get today's gold star. Thanks, @quantumguy.

 

[quantumguy]

Buddy, you get today's gold star. Thanks, @quantumguy.

I wish I could say I came up with this idea on my own, because I know there are others doing similar things. It's not clear why others aren't doing something similar since the extra cost isn't a big deal after you've already invested in a RIMS, but you do have to run a 240V RIMS in order to drive the CFC so that there is no lag time in heating the wort. I only have one 240V controller, so I have to switch my electrical cables between the RIMS and my boil kettle once it's time to heat the wort to a boil. Not a big deal though

 

[doug293cz]

Let's throw a little math at the question:

1 BTU is the amount of heat required to raise 1 pound of water 1°F. If we want to raise 1 lb of water 1°F in one minute, the heat input rate needs to be 1 BTU/min. 1 BTU/min = 17.584 W/lb-°F. 1 gal of water weighs 8.33 lb, so to heat 1 gal of water at 1°F/min requires 17.584 W/lb * 8.33 lb/gal = 146.5 W/gal-°F. Or, to raise 1 gal of water 1°F independent of time to heat requires 146.5 W-min/gal-°F.

So, we can write the following formula for how much power is required to raise a given number of gallons at a specified heating rate in °F/min:

Power Required [W] = Volume of Wort [gal] * Heating Rate [°F/min] * 146.5 [W-min/gal-°F]​

​

Now the above formula ignores heat losses to the environment, and the specific heat of the grain and vessel. These can be adjusted for by using a "fudge factor" that we will call "Efficiency." If 25% (0.25) of the input heat is lost to the environment and heating of the grain + vessel, then the Efficiency is 75%. The formula then becomes:

Power Req [W] = Volume [gal] * Rate [°F/min] * 146.5 [W-min/gal-°F] / (Efficiency [%] / 100%)​

To determine the heating rate for a given temp step-up you divide the delta Temp (Ending Temp - Starting Temp) by the desired step transition time in minutes.

Let's do a specific example, assuming 12 gal of liquid, a step from 147°F to 162°F, want the step time to be 10 minutes, and have an efficiency of 75%:

Power Req [W] = 12 [gal] * (162°F - 147°F) * 146.5 [W-min/gal-°F] / (10 min * 0.75) = 3516 [W]​

​

If you want to get an estimate of your actual heating efficiency, you can put your typical volume of water in your vessel, heat it to about 160°F, turn on your recirculation, run for 20 minutes, and then measure the temp of the water. The calculation then looks like this:

Loss Rate [W] = Vol [gal] * (160°F - End Temp) * 146.5 [W-min/gal-°F] / 20 [min]​

Efficiency [%] = 100% * 146.5 [W] / (146.5 [W] + Loss Rate [W])​

​

Efficiency will vary with ambient temp and wind velocity, since they affect the rate of heat loss from the system. Lower ambient temps, and higher wind velocity, both decrease the efficiency.

If you aren't interesting in step mashing, and only want to hold mash temps constant, then the power required is just the loss rate calculated with the above formula. If you are designing a system, I recommend spec'ing a heating element with a little more power than the above calculations suggest, if you want to be sure to be able to achieve your desired performance.

Some recommendations on heating elements and control units (PIDs, etc.):

For conventional RIMS: chose an element with the largest surface area that will work in your system. Use an Auber Instruments EZBoil (DSPRxxx) controller vs. a simple PID controller. A typical PID uses Pulse Width Modulation (PWM) with a fixed minimum cycle time of 2 seconds, which means at 50% power the element will receive full power for 1 second and be off for 1 second for each cycle. The DSPRs use a different power modulation method, and for 50% power the element is only at full power for 16.7 msec (0.0167 sec) and then off for the next 16.7 msec. The max element surface temp with the PWM controller will be higher than with a DSPR, and the chances of scorching will be greater.

If you use a water to wort heat exhanger (e.g. a counter flow "chiller") with the water heated by the RIMS tube, then scorching is not possible, so the longer power pulses of a typical PID will not be an issue. However, none of the cheap, commodity PIDs have programmable mash stepping capability. If you want automatic stepping, then go with one of the DPSR3xx models.

Brew on

 

[quantumguy]

@doug293cz this is awesome, but I don't think this accounts for the situation in a RIMS or HERMS recirculation where the heating of the mash and its liquid is accomplished entirely by the introduction of hot wort from the recirculation port onto the top of the mash. The temperature of this wort in the best case scenario (+ some offset depending on the brewer) is just the temperature of the final step. The rate of raising the contents of the whole mash vessel to the final step temperature is then dictated by how much hot wort you are flowing through the mash tun.

In my RIMS-driven CFC system, the power needed by the RIMS is dictated by the need to provide enough heat from the hot water loop to increase the temperature of the wort in the inner loop to raise it from the input cold temp to the desired set point output temp (final temp of step). The flow rates of both the hot water/RIMS loop and the wort loop impact the efficiency of the heat transfer in the CFC. My pumps limit both to about 1 gal/min. Initially I was working this with an 1800W RIMS but this gave poor results, i.e., the best change in temperature of the wort from input to output was only about 5 deg if I remember correctly. Moving to a 5500W element completely removed this problem, but is maybe an overkill.

 

[doug293cz]

@doug293cz this is awesome, but I don't think this accounts for the situation in a RIMS or HERMS recirculation where the heating of the mash and its liquid is accomplished entirely by the introduction of hot wort from the recirculation port onto the top of the mash. The temperature of this wort in the best case scenario (+ some offset depending on the brewer) is just the temperature of the final step. The rate of raising the contents of the whole mash vessel to the final step temperature is then dictated by how much hot wort you are flowing through the mash tun.

In my RIMS-driven CFC system, the power needed by the RIMS is dictated by the need to provide enough heat from the hot water loop to increase the temperature of the wort in the inner loop to raise it from the input cold temp to the desired set point output temp (final temp of step). The flow rates of both the hot water/RIMS loop and the wort loop impact the efficiency of the heat transfer in the CFC. My pumps limit both to about 1 gal/min. Initially I was working this with an 1800W RIMS but this gave poor results, i.e., the best change in temperature of the wort from input to output was only about 5 deg if I remember correctly. Moving to a 5500W element completely removed this problem, but is maybe an overkill.

I believe it does take into account everything that has a major effect. It is simply an energy balance:

Energy Input = Energy Used to Increase Fluid Temp + Energy Lost to the Environment​

​

When using a heat exchanger, the total fluid volume must be used, i.e. mash volume + HEX water volume. I should have stated that explicitly in my earlier post. It's also probably better to use total mash volume (water + grain) rather than just strike volume. The specific heat of grain is less than water, but treating it as just more water will give you some more margin on the calculations. Grain volume is about 0.08 gal/lb, so mash volume = strike water volume [gal] + 0.08 [gal/lb] * grain weight [lb].

The one effect that isn't accounted for is that at slower heating rates, the efficiency will be lower, as more heat will be lost to the environment if the temp transition takes longer. I believe this effect will be small enough to be safely ignored at realistic temp loss rates.

The analysis doesn't even attempt to address temperature gradients in the system.

As far as temp rise of the wort passing thru the RIMS or HEX, you don't want the temp rise to be enough so that the exit temp of the wort from the RIMS/HEX is more than a few degrees above your target mash temp, otherwise you risk denaturing a large fraction of the enzymes that are important for the next mash step (not a concern when heating to mash-out.) So, there will be a natural limit to the rate at which you can raise the temp of the mash without denaturing too many enzymes. A higher wort flow rate will reduce the temp rise going thru the RIMS/HEX - doubling the flow rate at a constant power input will cut the heat rise in half. So, at higher flow rates you can safely use higher power input to the element.

Brew on

 

[quantumguy]

I certainly agree it is an energy balance. I guess where I was coming from is that if the hot wort entering the mash tun in the recirculation is always the final temperature of the desired step (assuming this power is available), the time for the water+grain in the MT to reach the final temperature only depends on the flow rate through the MT since that dictates the heat transfer rate (minus heat losses from the MT). Nothing is in conflict with what you derive above, it's just that the devil is in the "efficiency factor".

 

[doug293cz]

the time for the water+grain in the MT to reach the final temperature only depends on the flow rate through the MT...

Very little dependence on the flow rate. The only thing that matters is the Net Heat Input Rate, which equals the Total Heat Input Rate (i.e. Input Power) minus the Heat Loss Rate. The flow rate would only be controlling if it forced you to lower the Total Heat Input Rate in order to control scorching or the outlet temp of the RIMS/HEX. Energy used to heat the equipment is included in the Heat Loss Rate, since it doesn't contribute to raising wort temp.

...it's just that the devil is in the "efficiency factor".

Which is why I gave a procedure for determining the Heat Loss Rate, which allows determining the efficiency via the formula:

Efficiency = (Total Heat Input Rate - Heat Loss Rate) / Total Heat Input Rate​

​

Brew on

 

[quantumguy]

umm, I guess I'm missing something here or were just talking about the same thing. Over a given time interval, the flow rate dictates the total heat input since the heat input in that time interval is the mass of hot wort that is introduced to the MT via the recirc multiplied by its specific heat capacity. The hot water entering the MT from the recirc will in practice never be hotter than the target temp of the step, so it has a fixed heat content per volume.

 

[doug293cz]

The total heat input rate is equal to the power input to the element. Heat = energy, and power = energy/time, so heat input rate = power input. Whatever energy is input has to go somewhere. It either heats the fluids, or is lost to the environment (heats the equipment or heats the air around the equipment.) There is nowhere else the energy can go.

For a given energy input rate (input power) and loss rate, the flow rate determines the temperature rise going thru the RIMS/HEX. More temperature rise will increase the loss rate somewhat, but that is a fraction of the total power. If we assume for simplicity that the loss doesn't change with temp rise, then the energy delivered to the fluid in the mash vessel is independent of the flow rate. If you decrease the flow rate, then the exit temperature from the RIMS/HEX rises in such a way that the rate of energy delivery to the mash remains constant.

Brew on

 

[quantumguy]

I'm not trying to break the 1st law of thermodynamics, but for the sake of argument, and I hope this discussion is not being taken as an argument, what if the setup was such that the exit temp as controlled by the PID from the RIMS/HERMS/CFC was independent of the flow rate up to say 2 gal/min. If in case #1 the flow rate was 1 gal/min, the energy/time being delivered to the liquid in the MT is related to the volume of hot liquid transferred in that unit time. In case #2 if the flow rate was 2 gal/min, the amount of energy transferred to the MT in the same amount of time is double that of case #1. Obviously in case #2 one would have to introduce more than twice the power to hold the exit temp to be the same as in case #1, but if this is feasible the flow rate would play the limiting role in how much energy could be delivered to the MT. Of course if the power input to the RIMS/HERMS/CFC can't keep up with the flow rate, i.e., keep the exit temp constant, then my argument is mute.

 

[doug293cz]

More temperature rise will increase the loss rate somewhat, but that is a fraction of the total power.

To get some idea of just how big an effect the larger temp delta across the RIMS tube will have on loss rate, we can do some simple modeling. The rate of temperature loss to the environment per unit of vessel surface area is proportional to the difference between the vessel temp and ambient temp. We'll treat the RIMS tube as one vessel and the mash vessel separately, as they often will have slightly different average temps. Assume room temp of 70°F, the mash and mash vessel are at 150°F, and the RIMS has an input temp of 150°F and an output temp of 156°F, for an average temp of 153°F. So, the temp delta for the mash vessel to ambient is 150°F - 70°F = 80°F, and the temp delta for the RIMS tube is 153°F - 70°F = 83°F. So, the heat loss rate from the two vessels per unit area will be quite similar, but the mash vessel will lose much more heat since it has a much larger total surface area.

Now, if we increase the output temp of the RIMS tube to 162°F, the temp rise thru the tube has doubled from 6°F to 12°F, so the rate of heat input to the mash vessel will double (at a constant flow rate.) But, the temp delta between the RIMS tube and ambient will increase from 83°F to 86°F, so the rate of heat loss from the RIMS tube will increase by (86°F - 83°F) / 83°F = 0.036 => 3.6%. If the heat loss from the RIMS tube is 10% of the total system loss, then the total system loss will increase by 0.36% with the doubling of the entry/exit delta of the RIMS tube. So, we can safely ignore the increased system loss due to increasing the delta across the RIMS tube, since it is less than other inaccuracies of the power required estimate.

Brew on

 

[doug293cz]

umm, I guess I'm missing something here or were just talking about the same thing. Over a given time interval, the flow rate dictates the total heat input since the heat input in that time interval is the mass of hot wort that is introduced to the MT via the recirc multiplied by its specific heat capacity. The hot water entering the MT from the recirc will in practice never be hotter than the target temp of the step, so it has a fixed heat content per volume.

The heat input rate to the mash vessel is equal to the heat capacity of the fluid times the mass flow rate of the fluid thru the RIMS times the temperature delta between the RIMS input and output. So, doubling the mass flow rate would double the heat input rate to the mash vessel, if and only if the temperature delta across the RIMS remains constant. But keeping the temp delta across the RIMS constant when doubling the flow rate would require doubling the power into the heating element. You can't get more energy coming out of the RIMS than you put into the heating element. What will happen at constant power input when doubling the flow rate is the temperature delta across the RIMS tube will be cut in half, and the energy delivery rate to the mash vessel will remain constant. I have already shown that increasing the temp delta across the RIMS tube makes an insignificant difference it the heat loss rate of the system. So on the system level, net heat input rate = total heat input rate - heat loss rate, and heat loss rate is essentially constant, thus net heat input rate also remains constant unless the total heat input rate (element power input) increases.

Brew on

 

[quantumguy]

You can't get more energy coming out of the RIMS than you put into the heating element. What will happen at constant power input when doubling the flow rate is the temperature delta across the RIMS tube will be cut in half, and the energy delivery rate to the mash vessel will remain constant.

ah, I think we're converging on something. For the same temperature delta over the RIMS tube, I believe the power input required by the heating element will be lower for a 1 gal/min flow, than it would be for a 2 gal/min flow. This is what I was referring to before in regards to the heating element not being able to "keep up" with the flow rate. I need to swap out my low power March pump for a Chugger pump I have lying around to test this, but I believe that my 5500W RIMS element does not run anywhere near full power to maintain a 10-20º delta with a 1 gal/min flow rate.

 

[doug293cz]

For the same temperature delta over the RIMS tube, I believe the power input required by the heating element will be lower for a 1 gal/min flow, than it would be for a 2 gal/min flow.

This is correct.

This is what I was referring to before in regards to the heating element not being able to "keep up" with the flow rate.

When designing, I think it's more important that the flow rate (whether limited by pump capacity, or flow thru the grain bed) be able to keep up with the element, so that the temp delta across the RIMS tube does not get too high, in order to avoid denaturing too many enzymes as they pass thru the RIMS, or in the worst case scorching the wort. Ramp rate of the mash temp is determined by the power going to the element, as all power going into the element get transferred to the wort. Most of the power makes it back into the mash, and some gets lost to the environment, but flow rate has almost no effect on the resulting temp ramp rate. Flow rate just affects the temp delta of the wort while passing thru the RIMS tube for any given power input.

As implied in my first post in this thread, you chose the element power based on your system volume, desired temp ramp rates, and the system losses. You then need to choose a pump that will maintain a flow rate that keeps your RIMS transit temp delta within whatever limits you are comfortable with. If the required flow rate cannot be maintained, then you have to reduce power to the element to keep the RIMS delta within bounds, and your ramp rates will be lower than desired.

Is there any interest some math to determine required flow rates to maintain a RIMS transit temp delta, for a given input power?

Brew on

 

[ITV]

Could RIMS tube users weigh in on the sizing of their heating element?
I use a 1500W (120V) element with a PID controller, with 120V input (mainly for maintaining temperature when it's cold).

I acknowledge that the element size probably depends on what one is doing with the heater. Some scenarios might be:

1. Simply maintaining temperature during recirculation of a single-infusion mash
2. Step mashing

What seems to be the minimal effective size used? For instance, I could add a 3500W/240V element running at 120V for an effective power output of 875W. Other elements can be used to adjust power to other values. Probably not enough oomph for a step mash, but would it be enough to maintain a temperature of a fixed-temperature single infusion mash?

Alternatively, what is the minimum rating for elements for step-mashing (just asking about user stories; there is no "spec")? Scorching is always a concern, of course.

Do folks rely more on the element rating for anti-scorch wort safety, or is the controller the more important system component?

It would be great to hear from more seasoned RIMS users on these issues.

I use a 1650W element @120V in my RIMs that I can obtain 1 Deg F/min temp rise which is sufficient for step mashing. For referance I use a gas fired boil kettle for heating strike water and boiling with a RIMs to maintain mash temps/or step mashing.

To minimize heat loss I wrapped my mash tun and rims tube in reflex insulation.
I down-sized my recirc pump from my March pump (7.2 GPM) to a Mark II pump (5 GPM) which is much quieter and still provides adequate flow.

To reduce the risk of scorching I run my recirc pump before energizing the heating element (approximately 5 minutes) which I compare the temperature measured in my mash tun and the temperature of the outlet of the RIMs tube to stabilize. This helps prevent the RIMs from "overshooting" which could cause scorching. On the flip side I turn off the heating element (2-3 minutes) before shutting off the pump to aid in cooling down the heating element. In reality this most likely does nothing but I did not like stopping flow through the RIMs tube with it full of sugary wort and a hot heating element.

 

[quantumguy]

On thing in regards to flow rate, is I purposely keep the flow rate in the 1 gal/min range (or even somewhat less) since I worry about compacting my mash bed and/or channeling if I try to crank the flow. With a 5500W RIMS element, I have no doubt it will keep up with a 2 gal/min flow or maybe more, but I think I would be asking for trouble at that point.

 

[Bobby_M]

RIMS is a tricky endeavor to avoid scorching. When you move into the higher wattage, I'd much rather use a large element and lower the effective wattage. I have a 3500 watt heater that just fits into my 18" tubes and even then, I'd want to run it on a controller that is really snappy and/or can throttle the wattage like the Brew Commander. Back up a bit. The reason why it's so tricky is that you need the false bottom/tun to keep flowing nicely to get the wort to constantly move heat away from the element that is already crammed into a tiny space. It's kind of like keeping a rocket engine from burning as hot as it possibly can but just below the energy required to blow the entire thing apart.

That's why I prefer single vessel step mashing. My RIMS "tube" is a 3 gallon cylinder of liquid in direct contact with the underside of my mash. I can dump a ton of heat into the liquid because it doesn't all need to immediately come from the grain bed. Even though it's only a 5/6 gallon batch, I can ramp with a full 5500 watts with no scorching. Moving the mash liquor to another location to get some heat has its limits.

 

[quantumguy]

For the most part, this why I moved to an external HERMS system with a high wattage RIMS running a closed hot water loop in a CFC. Zero chance now of scorching and it's as responsive as standard RIMS. I can also run pretty much any flow rate I want. If one is determined to use a 2- or 3-vessel system, I think one can't do better.

 

[Bobby_M]

For the most part, this why I moved to an external HERMS system with a high wattage RIMS running a closed hot water loop in a CFC. Zero chance now of scorching and it's as responsive as standard RIMS. I can also run pretty much any flow rate I want. If one is determined to use a 2- or 3-vessel system, I think one can't do better.

Any reason you don't use the HLT to push your hot water into your external HERMS?

 

[quantumguy]

Any reason you don't use the HLT to push your hot water into your external HERMS?

Much, much faster response times. With this RIMS setup, the closed loop only has about a 1/2 gallon of water to heat. From a brew day point of view, it's just like switching on a RIMS tube.

 

[doug293cz]

Is there any interest some math to determine required flow rates to maintain a RIMS transit temp delta, for a given input power?

RIMS is a tricky endeavor to avoid scorching. When you move into the higher wattage, I'd much rather use a large element and lower the effective wattage. I have a 3500 watt heater that just fits into my 18" tubes and even then, I'd want to run it on a controller that is really snappy and/or can throttle the wattage like the Brew Commander. Back up a bit. The reason why it's so tricky is that you need the false bottom/tun to keep flowing nicely to get the wort to constantly move heat away from the element that is already crammed into a tiny space. It's kind of like keeping a rocket engine from burning as hot as it possibly can but just below the energy required to blow the entire thing apart.

Now seems like an appropriate time to look at what kind of flow rate is required to keep temp rise in the RIMS to a reasonable limit as a function of power to the element. We can rework the following equation:

Power Required [W] = Volume of Wort [gal] * Heating Rate [°F/min] * 146.5 [W-min/gal-°F]​

​

As:

Power [W] = Flow Rate [gal/min] * Temp Delta [°F] * 146.5 [W-min/gal-°F], or​

​

Flow Rate [gal/min] = Power [W] / (Temp Delta [°F] * 146.5 [W-min/gal-°F]), or​

​

Flow Rate [qt/min] = 4 [qt/gal] * Power [W] / (Temp Delta [°F] * 146.5 [W-min/gal-°F])​

​

This can be put into a spreadsheet to create the following chart (chart is in quarts/min rather than gal/min, as flow rates are often less than 1 gal/min.) Note the power is the actual amount of power being supplied to the element by the controller, not the element's maximum rated power.

You can see that high element power requires excessively high flow rates to keep from overheating the wort on the way thru the RIMS tube. Here's a cropped version of the chart that just covers reasonable recirc rates for home systems:

If your grain bed limits your flow rate to less than what you would like to see, then you can split the flow out of the RIMS so that some bypasses the grain bed (returned under a false bottom, or outside a malt pipe.)
______________________________________________________________

I also did a chart showing how much power is required to support different temp ramp rates as a function of liquid volume in the system. Liquid volume is all the liquid heated in the system = strike water volume + grain volume (0.08 gal/lb * grain weight in lb) + any water in an external heat exchanger loop. This chart assumes that you lose 10% of your input power to the environment.

If you study the charts above, you will find that the usable power for a RIMS is going to limit what kind of ramp rates you can get when stepping.

Brew on