Heat Exchanger

In brewing all grain, precisely controlling the mash temperature is paramount to making a great beer. This is where the Heat Exchanger comes in. The Heat Exchanger is the heart of the whole system. It is the device that controls the temperature of the recirculating wort and keeps it at the proper temperature. The Heat Exchanger vessel contains water that is heated by 2 standard water heater elements. A copper coil submerged in the water carries recirculating wort that is heated by the water. The amount of heat imparted to the water by the heating elements is controlled by the temperature of the wort that is exiting the coil as measured by a thermocouple. The location of the thermocouple is critical to accurate temperature control. Putting the thermocouple at the coil exit will yield the tightest temperature control. As the thermocouple is located farther and farther away from the coil exit, there is an increasing delay in the time that it takes for the thermocouple to "see" the temperature of the wort coming out of the heat exchanger. This delay causes temperature swings in the mash. The largest temperature swing that can occur is when the thermocouple is located right at the coil inlet. See the HERMS Design Specifics for more details. Lastly, the Heat Exchanger has an agitation device built into the lid to provide turbulence to the water and maximize heat transfer.

I built my Heat Exchanger from a 5 quart paint can. It was not easy to find, but the industrious and resourceful homebrewer could find a suitable container. This made the mass of heating water fairly small (about 1 gallon) so it would be quick to heat. I used two heating elements wired in parallel: a standard 1500 Watt/120 VAC element, and a 2000 Watt/220 VAC element. Both are run at 120 VAC which yields 2000 Watts at a total current draw of 16-17 Amps (cutting the voltage by 1/2 on the 2000 watt element reduces the power to 1/4, or 500 Watts). Both elements were installed from the bottom using 1" conduit rings (sprayed with polyurethane to minimize rusting) and the gaskets that came with the heating elements.

Next I insulated the can with pink fiberglass insulation. I wrapped the insulation tightly around the circumference of the can to a thickness of about 2 inches. I then secured this insulation with duct tape. To water proof the insulation, I "painted" the can and the lid with polyurethane foam. I had access to some two-part polyurethane foam, but you could use epoxy or other water resistant coating. Whatever insulation you use, water proofing is essential.

Agitation of heating water is important. It substantially improves the heat transfer and helps temperature distribution in the Heat Exchanger. To agitate the water, I obtained a small 120 V electric motor out of a bathroom vent fan and installed a shaft extension on the motor with a small paddle on the end (actually, a plastic wing nut). The motor is fastened to the lid of the can and plugged into a switched outlet on the Mash Tun stand. This is very quiet and does a great job of agitating the water.

For the coil. I used 3/8" copper tubing coiled so that it just fit into the ID of the can. I was able to get 29 feet of tubing into the confines of the can. To get the maximum length of coil, you want to make the OD of the coil as big as possible. To do this, make sure that the leg that comes off the bottom of the coil extends up the inside the ID of the coil. This means that you should start with a straight piece of tubing about 18 inches long, then start wrapping around the outside of the straight piece. The photo explains it better.

The coil ends are plumbed so that the straight vertical piece is the outlet and the curved end is the inlet. This is because the thermocouple has to be inserted into the coil, so a 4 or 5 inch straight section is needed, hence the straight section is the outlet. I installed a 3/8" compression fitting with 1/4" male NPT threads onto the end of the coil. Then I installed a 1/4" female tee onto the fitting with one end pointing up, the other end pointing horizontal. In the vertical leg of the tee, I installed a 1/8" compression fitting with a plastic ferrule. This is what the thermocouple fits into. The thermocouple is 1/8" diameter. It slides into the fitting, then the compression nut is tightened just enough to form a seal. This is not permanent since the thermocouple is removed after each brew session, so the nut on the fitting is not tightened very tight at all, just enough to form a seal. Do not use a metal ferrule on this. If you can't find the right fitting, I've seen folks use a rubber stopper with a small hole drilled through it. The thermocouple is inserted into the stopper, then the stopper is inserted into the end of the coil.

The final step was to make sure the Heat Exchanger could stand upright. For this, it would have been possible to hang it from something, but I chose to install legs on the bottom and have it free standing. The length of the legs was adjusted to clear the hardware installed on the bottom and to match the connection point on the Mash Tun.

Putting a slit in the lid of the Heat Exchanger made room for the inlet and outlet tubes plus the agitation motor shaft. The completed Heat Exchanger is now free standing with two tubes protruding out of the lid, one is the inlet, the other is the outlet. The Control Box reads the temperature from the thermocouple and turns the heaters in the Heat Exchanger on and off so that the exiting wort is at the desired temperature.