The Quest for High Vacuum Part 2

When you bring up the subject of vacuum, one of producers’ first questions are “What size vacuum pump will I need to run my system?” They might add “Is the old rotary vane pump my granddad left in the barn good enough?” The question I ask them in return is “What vacuum level do you want to run today and into the future?”

As I stated in Part I, there are two ways to measure vacuum pump performance, inches of mercury (Hg) and CFMs (cubic feet per minute). Inches of Mercury (Hg) measure the negative pressure produced when air leaves the line. For example if 50% of the air is removed, then the inches of mercury should be 15 given the maximum pressure possible is between 29 and 30. At 25″ Hg, approximately 85% of the air has been removed from the lines. CFM measures the amount of air being evacuated from the lines measured in cubic feet per minute. Pump ratings for CFM are assigned based on their ability to remove air, and this is largely determined by the size of the pump.

Two other factors come into play when comparing vacuum pumps. One is the horsepower rating. As the air is removed from an enclosed area, the molecules of air in that air become very sparse. The pump has to work harder as the air becomes thinner. The pump also has to overcome the force of the negative pressure inside that area. This requires more horsepower. A larger CFM rating does this faster but requires more horsepower. The other factor is pump speed. If you turn a pump faster, you will move more air and will increase the capacity. However, over speeding a pump can cause excessive wear on the pump.

So to answer the second question first, Granddad’s pump is not designed to produce anything over 15 inches of vacuum and that is not high vacuum by today’s standards. Most of the liquid ring, flood vacuum rotary claw and new-era rotary vane pumps are designed to run at vacuum levels up to 29 inches. Remember all of the pump ratings and their ability to increase vacuum level are done at the factory removing air from a sealed vessel. Based on this information, a performance curve can be developed. What makes this whole process more confusing is that many maple equipment companies are now listing there pump sizes by motor horsepower instead of by CFM capacity. As pointed out earlier, motor horsepower is only one factor determining pump capacity. When questioned about CFM, one dealer told me his pump will develop 11 CFM at 29 inches of vacuum. This has to be a specification taken off of a performance curve taken at the factory. The more important question is how likely is that pump to ever reach 29″ of vacuum in a working maple system? The fairest comparison should be made when the CFM is measured on a pump being run at 15 inches of vacuum. Otherwise, without having the performance curve data in hand for every pump you are considering, how can a customer make a fair comparison – quite simply, he or she can not.

So are the dealers wrong when they tell you that your pump will produce 29 inches vacuum? The answer to that question is both yes and no. As stated, most pumps are capable and have been tested to deliver 29 inches of vacuum. This is clearly shown on the pump performance curves. However, because the performance curves are standardized to barometric pressure at sea level, an adjustment for elevation above sea level needs to be made. For every 1000 feet of elevation you lose 1 inch of vacuum, this means the highest vacuum level achievable at an altitude of 2000 feet is around 28 inches depending on the barometric reading on any given day.

So what is going on when a producer tells you that his pump gauge mounted somewhere near the inlet of his pump is reading 28 inch of vacuum but out in the woods it is 18 inches or less? Is he wrong? Is the gauge broken? The answer to this question is no. Because line diameter restricts flow (termed line loss), a vacuum pump has the ability remove all of the air from the system within a short distance of the pump inlet. This phenomenon occurs because the pump can pull air out faster than the line can deliver it, thus creating a small area of high vacuum close to the pump, and the gauge at the pump measures only the vacuum in that area. But further out in the woods, the same is not necessarily – and in fact is unlikely to be – true. This is graphically displayed in the line loss charts used in the Cornell New York State Tubing and Vacuum System Notebook. A 60 CFM pump set at 15 Hg hooked to a 3“ line can maintain over 40 CFM out to 5000 feet. That same pump hooked to ¾” line is incapable of delivering 15 inches of vacuum at 2500 feet. This information is covered in a previous post titled “How Can I Get More Vacuum Where I Need It?”  Bottom line is that if the line diameter is too small, the pump’s capacity to remove air will be compromised, and the only vacuum reading that counts is the reading that is taken out in the woods at the last tap.

How do you determine the CFM capacity of the pump that will best fit in your operation? The New York State Tubing and Vacuum System Notebook (NSTVN) written at Cornell University by State Maple Specialist Steve Childs states that to go from 15 inches to 18 inches of vacuum you need to increase the CFM capacity of your system by 50%. Let’s start with the number of taps you have on the system. Let’s say you have 3000 taps. You know that for every 100 taps you need 1 CFM to keep up with the air and gases coming into the system primarily from the trees. This means that it would take at least a 30 CFM pump to remove the air that is coming into the system from the outside. The vacuum level under these conditions would be somewhere around 12 Hg. The NSVTN states that for every 1″ of vacuum you will lose 10% of the capacity of the pump. In order to increase that vacuum level to 18 inches or beyond, you would need to increase the pump size by at least 50%. That would now mean that you need a 45 CFM pump. This is only 18 inches of vacuum and you want to produce a high vacuum rate of at least 25 inches to achieve near optimal sap production. To get to 25 inches of vacuum, you still need to add another 7 inches of vacuum. Starting with a 45 CFM pump running at 18 inches of vacuum, using the 10% loss for every 1 Hg gain, you would end up with only 13.5 CFM (4.5 X 7 = 31.5 – 45 = 13.5 CFM). If upgraded to a 75 CFM pump, you would still only achieve 22.5 CFM (7.5 X 7 = 52.5 – 75 = 22.5) which still falls short of your goal. Not until you install a 100 CFM pump (which translates to 30 CFM; 10 X 7 = 70 – 100= 30) are you able to run your 3000 tap sugarbush at 25 inches of vacuum.

Now let’s look at the yield side, this time based on research done at University of VT’s Proctor Research Center. Proctor researchers set out to calculate yield up to 25 inches of vacuum. The study shows that sap yield doubles when vacuum is taken from 0 to 15 inches. From 0 to 15 inches, there was a 8 gal per tap increase, from 15 to 20 inches a 3 gal increase, and from 20 to 25 inches a 2.5 gallon increase. At 25 inches of vacuum a producer can cumulatively add 14 gallons of sap per tap. And at 20 inches vacuum, you have still added 11 gallons of sap. So what would happen if you settled for working at a lower vacuum level? If you backed down to 22 inches of vacuum, a 45 CFM pump would deliver 27 CFM – just short of the amount needed. Going up to a 60 CFM pump would deliver 36 CFM, adequate to run the woods with some reserve. You would raise your production by 12 gallons per tap per season. That is over 85% of your original goal of 14 gallons per tap.

You have now made all of the calculations and are beginning to understand the logic and principles behind setting up a vacuum tubing system. The one thing we did not mention was the importance of reserve vacuum. You also need to factor in the vacuum that is needed to run a manual releaser (at least 5 CFM) and any other features such as lifts or vacuum piston pumps. All of these chew up CFM. You do not want to be maxed out on CFM capacity when Mr. Bushy Tail shows up. Factor in another 3 – 5 CFMs in reserve vacuum and hope he does not bring his relatives. Your system needs capacity to recover from leaks and other unforeseen problems and it needs to do so as quickly as possible.

In my small world of maple production I am not comfortable with anything under 35 CFM. Here’s why! Our home woods only have 400 taps and the requirement to run those taps is only 4 CFM, but I have maxed out a 35 CFM pump. Here is how we did it. First, we have long mainlines because the woods is spread out. Second, most lines drain to a low point that is totally inaccessible to sap pickup so we use a lift to bring the sap forward to the releaser. Third, we then move the sap from the releaser tank to road via vacuum-operated piston pump. No one in their right mind would have put tubing in these woods, but we did and it works! We maintain 25 inches at the releaser, 22 inches of vacuum at the lift and 18 to 20 inches at the end of the mainlines. I will replace that pump with a bigger one someday, but in the meantime we are constantly looking for new and innovative ways to conserve vacuum and utilize what we have in the best way possible. Just like everyone else, we are spending countless hours looking for what Mr. Bushy Tail and his friends have done to our tubing. I cannot over emphasize the importance maintaining your entire system. There is simply no substitute.

Footnote: Many producers are successfully running their vacuum systems over 25″ Hg. They are successful because their system is properly designed and maintained.

Author: Les Ober, Geauga County OSU Extension

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