Understanding Vacuum

Understanding Vacuum

Understanding Vacuum (noun): Space in which there is no matter, or in which the pressure is so low that any particles in the space do not affect any processes being carried on there. It is a condition well below normal atmospheric pressure and is measured in units of pressure.

In this post we’ll discuss the curative industry’s interaction with vacuum vs other industries, and some ways vacuum can be used. First, we will talk about the vacuum systems themselves. The differences between laboratory vacuum systems, and HVAC vacuum systems, are huge. The thing to remember is that achieving 29 inHg of vacuum (~ 20,000 microns) is not difficult; going past 29.9 inHg (~150 micron) is the hard part.

There is either an analytical or utilitarian design of the hardware and systems. For example- take an EasyVac, Bullet, Robinair, Harbor Freight, or comparable vacuum pump. These are commonly used in the HVAC industry, and are rated on pumping down an empty chamber. They may get to 50 microns in a day or so based on volume of the chamber. They are simple rotary vane pumps and don’t have splash back protection, or even a proper design.

An analytical pump, like an Edwards, Axiden etc. can establish 50 microns in seconds or minutes, instead of days. They also can be tested with a capped gauge to nearly 1 micron, after an hour or less of running.

Remember vacuum speed (CFM) is a useful tool; however, past a certain vacuum depth there is no perceivable CFM. When vacuum becomes very deep, the flow rate drops significantly, and CFM is no longer an issue. Ultimately, it’s the vacuum pump’s horsepower and real-life achievable vacuum depth that matters. A high-horsepower pump that can aggressively remove discharged vapors while they are present, is an important factor in efficient distillations and other vacuum-assisted work. When fractioning, having ample horsepower means the vacuum can remove pressures faster than they are be made available from the solution being distilled. This is a critical effect necessary with fractioning. More power also partially compensates for working with vacuum at higher altitudes.

So, in fact – if you wanted to repeatedly and accurately pump down a system loaded with $2000+ worth of volatile-releasing product, you would want to aim for the best vacuum pump for the money. In our experience, we find rotary vane pumps have the best CFM evacuation curve and depth ratings for the cost. Understanding vacuum comes in many forms.

Types of Vacuum Pumps

Scroll Pumps:

Scroll pumps have a corkscrew rotation, and the pump faces sit on cams that also spin as the rotary action spins. The idea is vacuum can be reached by the molecules being directed along the rotating surfaces and creating a void. An issue with this style is scroll pumps have low CFM ratings relative to their size. Additionally, they do not reach their ultimate depth nearly as fast as other style pumps.

Scrolls are typically used in high temperature gas discharge environments, like in semiconductor distillations. They are rebuilt almost every few uses. We have always felt that this pump is ideal for applications where oil-based pumps cannot be used. For these reasons, we don’t like using them. When visiting local vacuum shops, scroll pumps are seen in the dead piles more often than any other pump.

Rotary Screw Pumps:

These are like industrial superchargers but for vacuum conditions. They are used to pull enormous amounts of CFM, and generally act as a roughing pump for higher depth vacuum applications. Imagine an experiment inside of a vacuum chamber, where you are testing a rocket motor under space conditions. You need to remove vapors faster than the rocket creates them. Rotary screw pumps are great for this.

These pumps are also used in vacuum condenser systems, kiln operations, semiconductor production, and other industrial applications. Rotary screw pumps are normally very large and require several computerized systems to operate a massive package of valves during roughing and depth phases. These can be used with turbomolecular pumps, but the turbo pump would only be brought online after the rotary screw and/or secondary backing pumps reached the required vacuum level. We do not suggest this style pump for obvious reasons.

Rotary Vane Pumps:

Rotary vane pumps are unique in how simple yet powerful they are. By using a vane, or a set of pores at the end of a vane, the pump draws displacement and pulls significant CFM. Be aware that running high amounts of CFM on a rotary vane pump will damage it. In high CFM applications, an initial scroll or screw-type pump is used. In the case where a sole rotary vane is in operation, the pump can accommodate nearly 25% of its CFM repeatedly, without damage. For example, an Edwards E2M28 that runs 24 CFM, can pull about 4 or 5 CFM without damaging it.

So, displacement and horsepower to achieve CFM and good ultimate depth is very easily handled by rotary vane pumps. Unlike many other types of pumps that require complex fore traps, cryotraps, etc. – a well maintained rotary vane can withstand great amounts abuse; making them very affordable and usable without much worry. Even with a damaged vane on the stator, they can still reach 100 microns. Remanufacturers will claim 10 microns at the pump inlet is considered “freshly rebuilt”, but these pumps usually operate around 1-2 microns with clean oil.

The second most important feature of these pumps is when they heat up, the tip of the vane will capture molecules within oil vapor, similar to an oil diffusion pump. This makes its operation similar, but not as efficient or effective as a true diffusion pump.

In many cases, rotary vane pumps are used in banks or on their own with no support hardware to remove gases while processing under vacuum. They are also great pumps when using inert gas, as the gas helps dry out the pump oil. Many other pump styles cannot keep up with off gassing during distillation, but a rotary vane easily can. Molecular pumps can definitely keep up as well, but we’ll discuss those further down below.

Diaphragm and Dry Piston Pumps:

These pumps use an oil-less piston or a diaphragm to make displacement- thus creating vacuum. The vacuum depth is subject to the materials used in the pump, and unfortunately these cannot get that deep. With improper use they also can get loaded with particulates and condensate. This gums up surfaces and eventually can generate heat due to friction from the contaminant(s). Generally, this can be remedied by flowing an appropriate solvent vapor through the pump. Diaphragm pumps especially are very safe pumps, but if operated improperly aren’t as reliable as say, rotary vane pumps.
They are safe to use in many casuistic environments but don’t flow nearly as many CFM as rotary vane. They are suitable for pulling the solvent/terpene fraction, simple degassing and inert gas feeds, but can’t keep up to the full requirements of fractional distillation.
As a side note- if you’re seeking absolute precise control over vacuum down to ~100 microns, with consistent repeatable results, Summit Research sells an amazing product called the VARIO Controller. Pictured above on the left, the controller pairs to high-end Vacuubrand pumps. It can not only regulate vacuum to very precise setpoints, it also has a super easy to read display and solid controls layout.

Turbomolecular Pumps:

First let’s talk about what is needed to turn one of these on.

Fore trap->vapor trap ->cold trap->cryogenic condenser->roughing pump(s)->turbomolecular pump

These pumps are quite expensive to own. A true setup can cost $10k for a refurbished turbo assembly, $1-3k for a controller, and around $10k for the manifolds, sensors, and valving systems required for operation.

Using a bent blade, a turbomolecular pump knocks molecules outside of the system being evacuated. This is similar to an automotive turbocharger- creating pressure by spinning blades to compress air. In our case of operating within vacuum, it is not grabbing “air” that is realized; but grabbing the vapor molecules themselves. The pump spins around 100,000 RPM on magnetic bearings while generating vacuum; so fast that it keeps external atmosphere out by the action of the blades.

There are several ways to run a roughing pump on a turbomolecular, but the general idea is to bring vacuum down to roughing levels, then further down again with an oil diffusion pump. Only after that, can a turbomolecular pump can be brought online. The pump-down times needed to even begin to engage a true turbomolecular vacuum pump can be minutes, hours, days, weeks, etc. depending upon the size of the chamber being evacuated.

These pumps are used semi-frequently in other industries, and the main problem is that without the initial fore line and multiple condensate traps, the turbo molecular pump will burn up or fail prematurely. Contamination can be in the form of terpenes, hydrocarbons, and even noble gases. I have seen numerous pump cores destroyed by eager people who didn’t have a proper grasp of the technology. They often would initiate their turbo pump, while still inadvertently removing butane or terpenes. The flow of so many molecules causes friction. The pump then gets so hot, that it is liable to throw turbine blades. It is an insane sight to see, though I have only seen the aftermath. The energy released when a blade is thrown, approaches that of a bullet leaving a pistol. Thankfully, for 9 out of 10 failures, the turbine assembly will seize and turn into a glowing piece of metal without releasing the blades. Fires are often seen at this point.

Turbomolecular pumps operate at a “near Earth outer space” level vacuum, which is around 10^-3 microns (0.001 microns). Deep interstellar space is around 10^-14 microns (0.00000000000001 microns). The thing to remember is we aren’t looking for space depth. We just want stay close to 5 microns, or in that range. These pumps are impressive in that they can reach roughly 10x the depth of a diffusion pump, but are very cost and safety prohibitive.

Oil Diffusion Pumps:

Using a roughing pump like an Axiden or mid-series Edwards, the vacuum line is first evacuated. Under maximum depth, there is an additional vacuum accumulator at the top of the roughing pump inlet. This accumulator, an oil diffusion pump, has a heater that reaches very high temperatures. It uses a special vacuum oil, and the vapor from the heated oil is directed into a thin cone that sprays downward into the path of incoming molecules. This hot cone of oil is traveling past the speed of sound, bonds with molecules, and drags them out of the system. With the assistance of a roughing pump, a diffusion pump can very quickly pull an incredible level of vacuum.

At our repair shop we saw a smaller Edwards-style 2 stage oil vane pump. It was being used as a roughing pump, residing under a 20-liter oil diffusion pump. What this means, is that a standard 1-5 micron pump is used to reach roughing levels. Then once engaged, an oil diffusion pump will reach 0.01 microns and deeper.

These pumps come with additional dangers, more so with certain configurations. Fire is by far the largest threat. A fire can occur due to vacuum leaks, issues with the pump, heater, etc. Oxygen in the atmosphere can actually ignite some diffusion pump oils, so a strong understanding of this technology is necessary before diving in. Heat is a concern too, as some of these pumps use 1,000+ watts just to power the oil heater.

With the appropriate steps taken, oil diffusion pumps work very well for distillation purposes. They are recommended on our top-tier build outs and for experienced users. The oil is incredibly expensive, with performance oils costing over $3500 per liter. Though there are a lot of upfront costs and concerns to be aware of with this technology; proper care, usage, and maintenance will keep these pumps running for decades.

Cryogenic or Lyophilizing Vacuum Pumps:

The concept is as the roughing pumps establish a depth needed to operate, a cryogenic environment is created in a carbon bed or even on rods. The environment is so cold that molecules bunch up closer and closer, basically increasing vacuum depth by compressing the molecules with ultra-low temperatures. This effect is observed on a smaller scale when using a cold trap during normal vacuum-assisted work.

With lyophilizing pumps, often the roughing pump must be isolated from the cryogenic section, as during operation the cryogenic section heats up and discharges condensate. After the discharge, the pump is rough pumped again, the cryogenic section re-initiates, and the condensing action of the molecules begins again.

These pumps can be used easily to crystalize an extract using low temperatures and vacuum. This occurs due to removal of oils and ultra-high boiling point molecules that normally would become destroyed at higher temperatures. For instance, lyophilizing T H C acetate can be accomplished by taking the compound and using a cryogenic pump, and time, to selectively remove oils and other compounds that can now slowly vaporize at the low vacuum depths.

These pumps are generally used more for R&D, exotic formulations, etc. and are not suitable for use in distillation.

Conclusion

In conclusion, we feel rotary vane is the best vacuum pump technology when it comes to fractional distillation of curatives. With simple oil changes, the pumps last a long time. Most are made to run for 12-18 months on the analytical side, nonstop, with nothing but regular oil changes. Also, they can be boosted exponentially with the addition of an oil diffusion pump.

When working with solvents at vacuum levels above 1,000 mbar, diaphragm pumps are generally the best solution. For harsh or reactive chemicals, a chemical-rated diaphragm pump is required. Understanding Vacuum and controlling it:

If you want us to elaborate more on any of these items, or need help understanding vacuum at a “deeper level”, send us an email at info@summit-research.tech — JBV