The heart of any HERMS is the heat exchanger.
Many people put a coil of copper in the HLT and use a PID controller to control the recirulation pump to maintain the set temperature in the mash tun. I don’t like this idea as my HLT holds, typically, 60-70 litres of sparge liquor.
This volume of water cannot be controlled quickly enough to react to any temperature change required. This is why I designed and built my own heat exchanger that holds just under 4 litres. This can be heated with a single 2.4kw element from 20°C to boiling in a matter of a few minutes.
My first heat exchanger was made from a 30cm length of 100mm copper pipe. I sealed one end with a disc of copper sheet (glued on with JB-Weld) then drilled this and fitted a 2.4kw kettle element. I then made a coil of 8mm copper pipe and stuck it in the other end of the pipe and filled the 100mm pipe with water.
This heat exchanger worked fine in the summer but weather cooled down I found it didn’t really have the flow to keep the mash tun at the right temperature. I finally replaced it with a 150mm diameter copper pipe and a coil made from 10mm copper (previously a Brupaks immersion chiller). This enables me to recirculate at a higher flow rate.
The heat exchager is what the wort is pumped through in order to maintain the mash temperature or to raise it for step mashes.
The power to the kettle element in the heat exchanger is controlled by a PID controller and a solid state relay.
The PID controller is a great piece of kit. It uses a mathematical algorithm to compare the desired set temperature to a measured temperature taken from the outlet of the exchanger. The bottom line is that I set 66°C and the kettle element comes on until the measured temperature matches it.
It’s not quite that simple as I also have to set the differential between the temperature measured and the temperature in the mash tun but you get the idea!