Heat Exchanger Redesign

>> Sunday, March 10, 2013

Finished HEX
I recently decided to build an upgraded version of my heat exchanger. The old one is based on a Rubbermaid cooler and has worked well but I know at some point the plastic is going to crack. I also thought I could have done a better job enclosing the element connections. Another issue is the half coupling used to mount the element; it has a tendency to pool water around the base of the element resulting in some rusting. This time I’ll be using a stainless nut that should allow the water to drain away from the element base. More on mounting the elements later.

My redesign is inspired by "The Electric Brewery" brew system. Kal's system is extremely well thought out and I’ve borrowed some of his ideas in the past for my own setup. Kal uses a HERMS coil in his HLT and I believe this design works well, but I really like the size of my HEX and the flexibility that comes from using a standalone HEX (e.g. the ability to change mash temps independently of the HLT temps). I figured I could take Kal’s design and implement it on a smaller scale and retain the flexibility and functionality of my design.

I started looking into vessels that I could modify to suit my needs. I wanted stainless steel so that it was easy to clean and would last a long time. I ended up settling on a 2.25 gallon Bain Marie pot that I picked up at my local Asian market. It seems to be well-made and at $10.99 it was pretty cheap. The material is thin but these things are made to be repeatedly submerged in boiling or near boiling water, so it should hold up to mashing temperatures.

I also bought some used Greenlee radio chassis punches off eBay. These punches were used to create the holes for mounting the HERMS coil (13/16”) and heating element (1 ¼”). The 13/16” Greenlee punch creates perfect sized holes for mounting ½” fittings which is what I’m using. The punches are really nice because they give you a perfectly round burr-free hole. They can be a little pricey if you buy them brand new, but if you’re patient you can usually find used ones on eBay for half the price of new.

All fittings on the HEX are weldless and are Blichmann-style as described on Kal’s site. If you’re considering doing something like this I highly recommend checking out Kal’s site first because he as some great information.

Drilling the Bain Marie Pot
Marking the pot for drilling
My design called for mounting the HEX input/output through the wall of the pot so I needed to drill a couple holes. Going through the side of the pot will allow the coil to drain better after each use. Compared to my original design, it will also allow for better clearance when attaching hoses or moving the temp probe from the startup position to the recirculation position.

I marked the pot 1.25 inches on vertical center from the top and bottom of pot. I started out with smaller bits then worked up to larger bits until I was able to use the 13/16” punch. If you haven’t drilled stainless steel before, it’s worth mentioning that it work hardens; the hotter it gets, the harder it gets. When drilling it’s important to go slow, use a fair amount of pressure, and use lubricant to keep it from heating up too much. 4-in-1 oil works well for this as does vegetable oil and I’ve heard of people also using motor oil.

Like my original build, the heating element will be mounted through the bottom of the pot. The bottom was drilled out like the side and the 1 ¼” punch was used to create the hole for the water heater element.

Mounting the Heating Element
Sink drain wrench
Due to space limitations, I mounted the heating element through the bottom of the pot rather than the side like in Kal’s HLT. Except for that, the mounting design is basically the same as Kal’s. Kal has great step by step directions on his site, so I won’t repeat them here. The key components are a 1” stainless nut, o-ring, washer, stainless double gang blank cover, double gang box, and an all-weather cover.

One thing I had to contend with was due to the shape of the pot there wasn't enough room to tighten the element nut with a regular wrench. This isn't an issue when you mount the element through the side of a pot. I ended up buying a sink drain wrench and modding it into a custom crowfoot-style wrench. I used my grinder to remove some material from the jaws to get it to fit the nut. I also trimmed off the excess where needed to get it to fit inside the pot. Th image to the left is what I ended up with and I must say that it gripped the nut very well and allowed me to really tighten it down.

Forming and Mounting the HERMS Coil
I decided to use copper tubing again for my HERMS coil. I was considering using stainless tubing but it is notoriously difficult to bend by hand without kinking; the tighter the radius the more difficult it is. Considering the relatively small diameter of my pot, even the copper was somewhat difficult to bend without kinking. I used 10 feet of ½” I.D. copper tubing and formed it in three stages. First was shaping it into a rough cylindrical-shaped coil. Next I wrapped the tubing around a mini crockpot to refine the shape and tighten it up a bit. Lastly I used a roll of duct tape to form an even tighter coil. There was some slight deformation of the tubing, but not too much and no kinks.

Ports for input/output
I used 90 degree elbows on the ends of the coiled tubing to transition to rigid copper pipe. Stainless compression fittings attach to the rigid pipe, which connect to stainless nipples to form the bulkhead. This is attached to the pot Kal/Blichmann style which forms a very solid and water tight connection. It’s also worth mentioning, I had to use an assortment of 45 degree fittings to clear the heating element at the bottom of the coil.

Miscellaneous Fittings and Final Assembly
Test fitting the coil
Pot filled with water and testing for leaks
The fittings on this project are by far the most expensive part. Stainless isn't cheap, but I think it’s worth it in this case. All the threaded fittings where wrapped in Teflon tape then everything was assembled and leak tested. I still need to order a 1/4" stainless fitting for the temp probe, but it's basically ready to use. Next week will be the inaugural brew using the new heat exchanger.