There are 3 parts to consider:
1. Heating the HLT
So you want to heat 500L of water. If you use a 6000W heating element and water has a heat capacity of 4200 Joule per degree per liter, heating by 1 degree will take:
500 * 4200 / 6000 = 350 seconds.
So 6000W will be slow. 15000W will still be a bit slow, but probably workable if you can start with hot water and don’t have to heat up from 20C. 2x 10kW could perhaps be a good option.
2. Moving heat from HLT to mash tun
You want to move heat from the HLT to the mash tun, by recirculating through the coil. The amount of heat that you move depends on 3 things:
- HERMS coil efficiency
- Flow rate
- Temperature difference
You buy a big pump so you can pump 30 liters per minute (8 gallons).
Let’s assume your HLT is 10 degrees higher than your mash tun and that the exit temperature of the HERMS coil is 5 degrees higher than the mash (50% efficiency due to the high flow).
The amount of heat that you are moving is: 30 * 5 * 4200 / 60 = 10500W.
As you see, as long as you keep the temperature difference high, you are moving a lot of heat.
3. Having an equally distributed mash temperature
If you have 500L in your mash tun and every minute you pump 30L, it will take about 15 minutes before the wort you pump in at the top reaches the bottom.
The temperature at the top will be closer to the HERMS coil exit temperature and the temperature at the bottom will be lower because of heat losses in the mash tun.
You’ll want to maximize flow. With more flow, you can work with a lower temperature difference between HLT and mash tun to get the same amount of heat transferred. For this you need a strong pump, a grain bed that wort can pass through easily and not much friction in your hoses and coil.
You only need to ensure that your heat exchange keeps up with your HLT element
Your heat exchange from HLT to MT needs to be enough to move away the heat the element produces. There is not much reason to be able to move more, you’ll just cool down your HLT.
There is no point in getting a very long coil, because after a certain length that extra meter of coil will reduce flow for only a marginally higher output temperature and a net reduction in heat exchange. That’s why our high flow coil has 3 tubes in parallel of only 5m each. This reduces the velocity in each tube (compared to a single 15m tube) and results in less friction and thus more flow. The total volume inside the coil is the same.
Keep your HLT small to prevent overshoot and keep the system responsive
You’ll want to be able to work with a high temperature difference between HLT and MT. But the energy you put into the HLT will eventually be divided over HLT and MT. So if your MT is only 1 degree too low, you don’t want to heat the HLT to 10 degrees above the mash setpoint, because you will overshoot. But what you can do, is make sure the volume in your HLT is a lot less than in your MT. If the HLT is only 50L and the MT is 500L, you CAN heat the HLT 10 degrees too high to bump up the MT by 1 degree. If you the HLT is much smaller than the MT, the system will act more like a RIMS, but without direct heat.
Please also note that your HLT is heating both the HLT and MT when you are recirculating. If you would have a HLT as big as your MT, you are now heating 1000L and will take twice as long.
Our software can be configured to manage the HLT setpoint for you, based on the MT temperature. The PID driving the HLT setpoint will be set to a gain of MT volume / HLT volume. That way, you’ll have no overshoot.
If you want to have a big HLT for heating your sparge and cleaning water, mount the HERMS coil very low. You can fill only a small part of it to have a fast responding HLT while managing mash temp and later fill it up more. You can also fill it slowly at a rate that your heating element can keep up with while you slowly sparge.
I hope that gives some insight.
Some more thoughts:
- 33 gallons per minute sounds like too much. I don’t think you’ll get that kind of flow through your mash without problems. But from the calculations, you can see that it is not needed to move the heat. Your HLT element will be the bottle neck. I only had 30 liters per minute (8 gallons) in my calculation and was moving enough heat (given that the HLT temp is high enough).
- 6 kW is not enough.
An alternative approach for step mashing is that you stop the pump during a mash step, heat up the HLT and then make a quick step when you start pumping again.
Please also note that the volume in the coil is only 1.5L. With 30L per minute, the wort will spend about 3 seconds in the coil. You won’t be overheating the wort in the coil. The temperate at the top of the mash tun will be a problem before the temp in the coil is an issue. Some stirring mechanism in the mash tun and isolating the mash tun will help with temperature difference inside the mash tun.