Easy Aeration
TroubleShooting
Jesse Bavaro - Rocky Hill, Connecticut asks,
In the December 2013 issue you described a low-cost method for wort aeration consisting of a barbed tee set up in-line on the way to the fermenter. I recently tried this method and ended up with a wort geyser! I thought that it may have happened because I had created too much back pressure by leaving the standoff at the end of the racking cane. I experimented again later without the standoff, using only water and had the same issue. I am pumping wort into my heat exchanger with 1⁄2-inch tubing and it exits into 3⁄8-inch tubing, through a 3⁄8-inch in-line thermometer, then to the 3⁄8-inch tee with the open end facing up, to the 3⁄8-inch racking cane. What am I doing wrong?
The low-cost method I described uses a tee where the wort flows horizontally into the center branch of the tee and down from the bottom leg. As liquid flows through the tee, air is sucked into the liquid flow from the branch that is exposed to the air. I described the use of a cotton wad filter and explained how microbiologists have used these simple yet effective filters for the past 150 years in microbiology labs across the globe. I like the history of science and suggest reading up on some of the laboratory methods used by Louis Pasteur as well as the design and explanation of Venturi nozzles.
When I was a graduate student in Dr. Michael Lewis’ brewing lab in the early ‘90s at University of California-Davis we used this type of device to aerate wort that was gravity flowing from a 5-gallon (19-L) hop back through a wort cooler and into the glass carboys used for fermentation. No wort geysers that I can recall ever set loose from the aeration tee because the fill tube extending into the carboy was a few inches shorter than the carboy and flow was not restricted. The main differences in your method are your pump and reduction in line size from the heat exchanger and into the tee. The pressure drop downstream of the tee is greater than pressure developed by your pump, so the excess pressure and accompanying liquid flow is pushing up and out of the tee.
There are many ways to “scrub” pressure from pumps. Control valves, orifice plates and choker lines are examples of things that can be added in line before the tee to reduce pressure. A simple needle valve is probably the easiest solution since it is adjustable and you can see how closing the valve down prevents the wort geyser. Orifice plates and choker lines need to be sized to work properly and require a pretty solid understanding of fluid dynamics to get right. Or you can use trial and error to size.
In commercial operations, pump speed is often varied with the use of a variable frequency drive (VFD). Over the last 20 years the price of VFDs has dropped low enough that many process systems use them to modulate motor speeds for a wide range of controls applications. Modulating pump speed is a very convenient way to control discharge pressure. You can purchase single phase AC VFDs for small motors for about $100. This is really pretty inexpensive considering the control provided. With a VFD-controlled pump you can vary flow rate and use a single pump for a wide range of applications. Mash-in, sparging, whirlpooling, and wort cooling usually require different flow rates, and a pump sized for the worst case duty can easily be dialed back with a VFD. I realize this information is not directly related to your aeration question, but the topic of pump pressure control relates to other homebrewing applications.
