The Quest for High Vacuum in a Maple Tubing System (Part 1)

The variety of vacuum pumps on today’s market is extensive. Although vacuum has become a mainstay in maple production, our utilization of vacuum pumps and equipment is small compared to their use in the broader industrialized world. Maple production is just on the tip of the iceberg when it comes to vacuum utilization. For this reason there is a lot of misunderstanding about the laws of physics (Quantium Mechanics) that govern the science of vacuum. Wikipedia defines the word vacuum as “void of matte.” The English word vacuum stems from the Latin vacuus which means “vacant.” The study of vacuum goes back to the Greek Age and the time of Aristotle. Several basic scientific principles apply when it comes to vacuum. Due to pressure exerted by the earth’s atmosphere (15 lbs per square inch) you can only achieve a maximum vacuum level of 29.92 inches of mercury (Hg). You actually can only achieve a vacuum level equal to the barometric pressure on any given day at any given location. Barometric pressure changes with the elevation above sea level and with the prevailing weather pattern. Another principle is how we measure vacuum. The level of vacuum is a negative measure (because you are creating a negative pressure inside of a vessel) and is read in inches of mercury (Hg). The rate of air being removed from a vessel by a vacuum pump is measured in cubic feet per minute (CFM) on an English measurement scale.

Even though it has become the Holy Grail in the maple industry, the term “High Vacuum” is largely misunderstood. High Vacuum or perfect vacuum exists only at 29.92 inches Hg. This is the highest level of vacuum achievable in our atmosphere and occurs only when every molecule of matter is removed from a vessel. This is extremely hard to achieve because once all of the air is removed there are still other gases that qualify as matter and are very difficult to remove. In fact the closest thing to a perfect vacuum only exists in outer space and we are not producing syrup on the moon.

Wikipedia states:

There are three levels of vacuum achievable with modern vacuum pumps. Low Vacuum (vacuum cleaners), Medium Vacuum (achieved with a single pump) and High Vacuum (achieved with multi-staged pumps and measured with an ion- gauge).

As you can see the vacuum we use falls in a range of somewhere between Low and Medium. And thankfully, the average maple producer does not live in the scientific world of vacuum, nor does he need to. The reality is that we are not dealing with a closed vessel but rather miles of tubing where the introduction of air occurs at every tap, fitting, and squirrel chew. The range that most maple producers should be comfortable with is around 20 to 27 inches of vacuum depending on their system and the pump they are using.

This is where the discussion and the debate begin. As I have stated in an earlier post, the producer must consider the entire system before he decides on the type and size of vacuum pump to use. Even though we are increasing the volume of sap being produced by increasing the level vacuum closer to 29.92 inches of Hg, we need to be more concerned about the ability of the whole system to remove air from the system efficiently. Rather than concentrating on achieving the maximum level of vacuum, we should be paying closer attention to the system’s ability to overcome leakage and everyday wear and tear.

There is a wide variety of vacuum pumps that can be used to apply vacuum to a maple tubing system. In fact, with the use of 3/16″ tubing, you may not even need a vacuum pump to achieve your vacuum goal. Most of the pumps used in the maple industry are adapted from some other type of use. The first pumps came from the dairy industry and were originally used to milk cows. These were rotary vane pumps that were designed to produce around 16 inches of vacuum. The vacuum was produced as the air trapped between the vanes held in an offset rotor was expelled to the outside via the exhaust. As vacuum level increases, heat builds up, and as a result, the system needs some kind of lubrication to absorb the heat. The pump is lubricated with oil that was contained in an oil reservoir. Once you went above 16 inches vacuum, the strain on the pump produced more heat that it was designed for. For that reason, oil coolers and oil-reclaimers were used to make pumps more efficient. Bearings need to be lubricated with a precise amount of oil to maintain function. When running above 20 inches Hg, if any of the above are neglected, you are headed for a Chernobyl-type melt down. There are commercial rotary vane pumps (running a flood vacuum) on the market that are capable of achieving up to 27″ of vacuum. One of the most popular pumps being used is the liquid ring pump. The liquid ring pump uses an impeller running in a ring of liquid producing close to 29 inches of vacuum. As the air is drawn in, the air becomes trapped in a compression chamber that is formed between the impeller veins and the liquid. The air is expelled to the outside as the liquid (oil or water) is recycled. These pumps achieve as close to 29 inches of vacuum as any pump on the market. The downside of this type of pump is that a water source is needed and that source needs to be kept above freezing.

One of the most recent pumps to come on the maple scene is the rotary claw pump. The rotary claw will produce 27 inches of vacuum, just under the level of a liquid ring pump. Rotary claw pumps are designed for continuous duty and require minimal in-season maintenance. The claw runs at a very close tolerance to the chamber and traps air in-between the claws and the chamber expelling it to the outside. A small amount of oil is used for lubrication. The downside is that these pumps are very expensive. They are designed to be run year round. Long layover periods may allow the pump to develop a rust layer inside to the pump resulting in excessive air. Because they run at a very close tolerance this may lead to early breakdowns. If you buy a rotary claw you need to fog the pump with anti-oxidation oil in the off season to prevent premature wear.

The last pump is the new-era rotary vane pumps that are designed to run continuously and to produce a vacuum of 29 inches. These appear to be highly efficient pumps. These pumps are similar in design to the older rotary vane pumps but have very close tolerances. They lubricate with oil.

So let’s rate the pumps on their ability to produce high vacuum from top to bottom. At the top is the liquid ring and the new-era rotary vane with the edge going to the liquid ring – especially one of the two-stage models on the market. These pumps consistently reach 27 to 29 inches of vacuum. Not far behind is the rotary claw which will produce 27 to 28 inches of vacuum. Next is the improved rotary vane with a flood system at 27 inches. At the bottom is Bessy’s favorite – the old style rotary vain used in milking systems. She liked it because it produced no more than 16 inches of vacuum. Any more and Bessy would send it across the room with one swift kick. No matter what you use, you will get more sap from your trees. Collecting maple sap with a vacuum system not only saves time and labor, but the vacuum will increase your sap yield somewhere between 50% and 150%. In the next post, I will cover things you need to consider before you hook your pump into the system.

Author: Les Ober, Geauga County OSU Extension

One thought on “The Quest for High Vacuum in a Maple Tubing System (Part 1)

  1. I had zero clue that there were so many different types of vacuum pump out there. I always thought most of them worked about the same, and was completely oblivious to the fact that the interior of them looked so different. I’m looking to get a vacuum pump for a project that I’ve got coming up, but now I know I probably need to do some more research. Thanks for sharing!

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