It's been awhile since I posted any updates on the evolution of my brewing stand. I recently made a major conversion to a HERMS. If you're not familiar with HERMS, it stands for Heat Exchange Recirculation Mash System. Basically what it means is that there is a heat exchanger in the HLT which wort from the mash tun is pumped through during recirc to raise the mash temperature. The biggest reason I went with HERMS is that I was paranoid about using direct heat to raise mash temps because I scorched one batch really bad and completely ruined it. Since then, I've been so cautious about direct heat that temp steps take forever. Infusion isn't a practical option because the mash tun isn't big enough to add enough water to significantly raise the temperature of the mash, unless I'm brewing a particularly small beer (which I almost never do).
So, here's how I went about converting my rig. I made a coil from 30 feet of 5/8 inch flexible copper tubing. I wrapped the tubing around a corny keg to get the proper diameter. I then soldered 90 degree bends on the ends of the copper coil with short sections of rigid copper tubing that would connect to the fittings in the side of the HLT.
To drill the holes in the HLT I used a step bit or "Unibit." The secret to drilling through stainless steel is to go slow and keep it cool. If you go too fast and let the steel get hot it will harden and make it nearly impossible to drill through. I actually did the drilling over my deep sink and ran cold water over the bit as I was drilling the hole.
For bulkhead fittings through the HLT wall I used the weldless fittings that are commonly sold at homebrew shops and online. Since space inside the HLT to make connections was limited, I turned the fittings around from the typical installation so that the stainless steel coupling is on the outside of the tank and the threaded nipple is on the inside. 
Connecting the coil to the fittings was just a matter of using 1/2" female iron pipe to 5/8" compression fittings. On the outside of the tank I used a 1/2 inch 90 degree street elbow with a hose barb to direct the flow upwards at the outlet of the heat exchanger and just a hose barb on the bottom fitting for the inlet.
I've brewed in the converted rig once since conversion. It was a simple mash schedule - saccharification rest at 150 and mash out at 170. To raise the temp for mash out, I heated the HLT to about 180 and started recirculating. The temp in the HLT quickly dropped to around 160 and I had to stop recirculating to raise the HLT back up to around 180. I think there was temperature stratification in the HLT and although the thermometer was still reading around 180, the temp was stratified and the average temperature was lower. When I started the recirc, I also stirred the water in the HLT to improve heat transfer, which gave me a more accurate temp reading. Next time I'll time things better and ensure the HLT is actually at 180 before I start recircing, and I'll also keep a flame on it when I recirc to help keep the temperature up.
I'm already considering some modifications to my newly modified rig. The biggest being a bypass in the recirc line that would allow me to recirc w/out running the wort through the heat exchanger in the HLT. When I was trying to raise the temp to mashout on my last brew I stopped the recirc for a while to let the HLT heat faster. If I had a bypass I could have kept it recirculating while the HLT heated.
Finally, I wanted to mention another minor mod. I don't think I pointed out that I removed the hard-piped connections to the brewing vessels. I did this before I even brewed on this rig the first time. I simply cut out short sections of copper pipe below the valves on each of the vessels and replaced it with flexible tubing (high temp thermo-plastic tubing). Along with the HERMS conversion, I also cut out a section of the hard pipe that cross-connects the discharge piping of the two pumps. This section of piping allows the wort pump to pump back into the top of the mash tun as well as kettle. Having all the hard piping connections was putting stress on the pump heads, which are made from plastic, and I was concerned that over time the stress would crack the heads. One added benefit of some of the modifications I made is that the liquor pump now does not lose suction at low flows. For some reason, before these mods, if I throttled down on the discharge of the liquor pump it would end up getting air bound. If I kept the discharge valve wide open it pumped fine. This is backwards from what you would expect if the pump were losing suction due to cavitation, so the behavior was a mystery to me. In any event, after the HERMS conversion I was able to throttle the sparge water flow to a trickle and the pump never lost suction. It doesn't make sense that the HERMS conversion corrected the problem because all it did as far as that pump is concerned is add 30 feet of piping to the system, increasing the head on the pump. The same effect could have been accomplished by throttling a valve on the discharge, which is exactly the situation where the pump would lose suction. I think that putting soft piping in the cross-connect pipe is what fixed the problem. My theory is that the stress on the pump head was warping it slightly and that air was getting sucked in past the O-ring seal on the back of the head. Or maybe through a threaded fitting on the suction side of the pump (which didn't leak with static head on it, so I don't believe that was the problem). In any event, my theory may be way off base, but it's nice to be able to throttle the sparge water flow. Now I can match flows in and out of the mash tun rather than having to cycle the sparge water flow full on and off manually.
