The HLT or Hot Liquor Tun, as many call it, is where water is heated for the mash. In a H.E.R.M.S. the HLT also allows boosting the temperature of the mash without the need to infuse more hot water and thinning out the mash or the chance of scorching the mash by using direct heat. Since you are already going to need hot water to dough in the grain, the HLT can be used further by having a coil that the mash water can be circulated through, to not only maintain even mash temps, but allow fairly quick temperature boosts and the ability for mash out. Here is how it works:

A coil of 1/2 inch soft copper tubing is wrapped around some sort of a mandrill to make a coil that will stay under the water level in the HLT. I used an oxygen bottle for my cutting torches for the mandrill, wrapping the tubing tightly around it. At first I used about 15 feet of copper tubing to make the coil or heat exchanger. Since then, I have found that by using a longer coil of 30 feet makes the temperature boosts go even faster. After making the coil, elbows were put on each end of the coil and rigid copper pipe which go to the top of the 15 1/2 gallon converted keg and attach to a manifold going to the Mash Tun. Pictures of this manifold are shown on the Mash Tun page.

The pictures above show the 60 RPM motor that drives a fan blade used to stir the water in the HLT. By doing this a slight turbulence is created, causing the heated water in the tank to maintain a consistent temperature and a more efficient exchange of heat to the mash liquor. Also you can see the float gauge mounted on the  HLT which is used to accurately monitor how much hot liquor is being added for dough in and sparge water being pumped to the Mash Tun. The lower 2 pictures show the top of the HLT and a 1/2" soft copper line coming from the frame which is used to fill the HLT using a valve on the filter manifold.

In order for the PID to monitor the temperature of the Mash Liquor exiting the heat exchanger a RTD temperature probe is mounted in the copper manifold at the top of the HLT as shown in the picture below. The PID uses this temperature reference to fire the burner under the HLT to maintain what ever set temperature is programmed for that particular step.