Maintaining a Vacuum in a Still
Updated: Mar 29, 2022
Outer space is a vacuum, though not a perfect one. Even in interstellar space, there are still at least a few hydrogen atoms in every cubic meter. If a human being stepped (or floated) into outer space without a suit, that person wouldn't die instantly, it would take anywhere from 15 seconds to perhaps as many as two minutes for death to occur. Almost immediately, the warmth of the human body coupled with an instantaneous drop to almost zero pressure would cause any fluids in the body to boil and vaporize, and any dissolved gases or oxygen would expand out rapidly as they try to escape the body and spread out into the void of space. Once the warmth of the body is transferred away, the body would simply be frozen to a temp of roughly -455F. Vacuum is a harsh and unforgiving condition. And yet, those very features that make it impossible for life make it an amazingly useful state to distill within if controlled on Earth.
In a previous article, I discussed the feedstock considerations for vacuum distillations. This one is more technical regarding the maintenance of a vacuum condition in a still. Of course, you can spend a boatload of money on an engineered machine where this has already been done for you, but I'm more of a "bushcraft" engineer and it's always empowering to know why things are done a certain way.
Vacuum leaks are your enemy!
Vacuum distilling equipment MUST be tight. Small leaks are not necessarily a big issue or concern; we're not trying to create a perfect vacuum here. Larger leaks, however, will make obtaining and maintaining vacuum next to impossible below a certain point. Furthermore, large leaks in the wrong places will just end up siphoning off your product into the atmosphere, or worse, into your vacuum pump!!
On the other hand, your equipment must be serviceable, so a balance must be struck. There are various types of clamps, flanges, etc. that you can use of various sturdiness, tightness and expense. High vacuum clamps and fittings, Sanitary clamps and fittings, bolt flanges, etc. I'll just discuss the first two. Bolt flanges only come into play if we're talking larger diameter equipment as these are going to be too expensive for smaller equipment.
High-vacuum clamps are ideal, and these are what are used on high-vacuum pumps typically. These consist of a metal ring with an o-ring around it that gets sandwiched between two fittings and creates a secure vacuum seal capable of ultra-high vacuum applications. However, the fittings and clamps are far more expensive and less readily available than standard brewery/distillery tri-clamps.
Sanitary tri-clamp fittings and clamps are more than adequate for the vacuum levels required to distill spirits. These consist of tubing with grooved flanges on the ends that will seat a gasket. Once placed together, you clamp around the gasket with the hinged clamp and tighten the nut, providing equal pressure around the two flanges pushing together against the gasket. It's important to use these with an ethanol-resistant gasket material that is soft enough to provide some flex. I've used PTFE gaskets before as well (fluorinated gasket materials provide the best chemical resistance), and these can work but they are rigid and rather expensive. Viton can be used as well, which is softer but also more expensive. Do not use Buna-N or EPDM as these materials are not very compatible with ethanol long term. I use platinum-cured silicone gaskets on my equipment. They have just the right amount of flex, are vacuum tight, and I can easily take sections apart for cleaning or modification and reuse them. Also, consider getting them with an outer edge flange as this makes them easier to connect larger pieces together as they have some additional contact with the tubing flange and are less likely to fall off when connecting two pieces (a very frustrating thing that happens frequently).
Sight glasses are another notorious potential leak point, but we need them to see what the hell is going on inside. There are several options here as well. Fused glass, sanitary clamp connect, screw on sight ports, inline sight glasses. Here are some considerations here...
Fused glass is ideal, but super expensive! I use one of these on the reboiler section of my production still, because there's zero chance of leaking where the glass is connected as it's literally fused to the stainless steel plate. So all you have to do here is connect it with a gasket and clamp just like you would any sanitary fitting. As stated, they're very expensive, and the sight glass tends to be smaller than you want. I got two of these used on eBay for a good price, and that's why I use them here. I would use them everywhere but that's not practical.
Sanitary clamp connect sight glasses are great as well. These are more of a cheaper version of fused glass and easier to service version of the screw on type, and equally in between on price. The thing is, once you screw a sight glass on and get it vacuum tight, you will not want to go through that process again. You don't ever want to touch it again. So these are great for setting up once and you can then remove them without going through that again.
The screw on type are usually what is on sight glass tees that you can buy relatively inexpensively. The sight glass literally is a metal ring with a sight glass that sits between the ring and the tee section with a teflon gasket on either side. My stripping plates have one of these on each to see if they're operating properly, flooded, etc. They're very hard to seal for vacuum though because of the Teflon and there isn't much tightening one can do lest you break the glass. In order to make them work better, apply high vacuum grease from Dow onto each side of the gaskets before tightening. Woila! Perfect seal.
Inline sight glasses are good as well, to a certain size. Above probably 6" diameter or so, these become a bit precarious and rather expensive. However they're great for areas you need to monitor levels (collection lines, etc.) and see the product as they have 360 degree light and visual access. Basically a glass pipe between two stainless steel tri-clamp flanges with four long bolts to hold it together and gaskets on the glass ends to seal the glass. Use vacuum grease on these as well and be careful not to overtighten the bolts, or you will break the glass. If the glass is broken anywhere, throw it away and get a new one, it's useless.
Some combination of these different equipment considerations should be taken into account along with serviceability and your overall budget. Really high end stuff (like the fused sight glasses I found) can sometimes be found on Amazon, eBay or at equipment auctions. I'm always on the lookout for stuff I might not need now, but might need later. Equipment like this doesn't show up frequently because it's not often used or is custom to the application. If you purchase your equipment from a manufacturer, custom or otherwise, pay attention to some of these technical considerations on where connections are made, how they're made, etc. Why did they use an inline sight glass here? What kind of gasket is this? If I need to replace this, can I get something readily available online on short notice that can accomplish the same thing?
Thanks for reading! In the next article I'll delve into the complications of vacuum distilling and my experiences with the equipment to work around them. Cheers!