Ohio State University Extension
When you bring up the subject of vacuum, one of the first questions producers ask is what size vacuum pump will I need to run my system? They also ask if the old rotary vane pump their grandfather abandon in the barn is good enough? The question I ask them is; what vacuum level do you want to run at today and into the future. As I stated in Part I there are two way to measure vacuum pump performance, inches of mercury and CFM. Inches of Mercury 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. At 25 inches of mercury approximately 85% of the air has been removes from the lines. CFM on the other hand measures the amount of air being evacuated from the lines measure in cubic feet per minute. Pumps are rated (CFM) on their ability to remove air and this largely determines 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 Hp. The other factor is pump speed. If you turn a pump faster your will move more air and will increase the capacity. However, over speeding a pump can cause excessive wear on the pump. To answer the second question first, Grandpas pump is not designed to produce anything over 15 inches of vacuum and that is not high vacuum. Most of the liquid ring, flood vacuum rotary claw and new age 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. From this information a performance curve is developed. What makes this whole process more confusing is that many maple equipment companies are now listing there pump sizes by motor horsepower instead by CFM capacity. As pointed out motor Hp 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 question I have how likely is that pump will ever reach 29 in of vacuum in a maple system? The fairest comparison should be made when the CFM is measured on a pump being run at 15 inches of vacuum. Otherwise unless you have performance curve data in hand for every pump you are considering how you can make a fair comparison. Will the pump be able to deliver 29 inches in the woods at a higher elevation, hooked to thousands of feet of line that is anything but leak free?
Are the dealers wrong when they tell you that your pump will produce 29 inches vacuum? The answer that question is 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 curve. However because the performance curve is read at the 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 nears the inlet of you pump is reading 28 inch of vacuum but out in the woods it is 18 in. or less? Is that wrong? The answer to the second question is no. Because of line diameter is restricting the flow (Line Loss), the 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. The gauge at the pump measures only the vacuum in that area. 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 inch line is incapable of delivering 15 in. of vacuum at 25 feet. This is covered in a previous post; April 24, 2013 How Can I Get More Vacuum Where I Need it? The pump is only one part of the total system. If the line diameter is too small it will reduce the capacity of the pump to remove air. The reality is that 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 NY State Tubing Vacuum Notebook (NSTVN) written at Cornell University by State Maple Specialist Steve Childs states that to go from 15 inches to 18 inches of vacuum of vacuum you need to increase the CFM capacity of your system by 50%. You 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 a 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 in of vacuum you will lose 10% of the capacity of the pump. In order to increase that vacuum level to 18 in or beyond you would need to increase the pump size by at least 50 %. That would mean that you would 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. This is based on research done at UVM Proctor Research Center you need to have at least 25 in of vacuum in the lines to get near optimum sap production. However to get to 25 in vacuum you would need to add 7 more inches of vacuum. Starting with a 45 CFM pump running at 18 in of vacuum, using the 10% loss for every 1hg gain you would end up with only 13.5 CFM (4.5 X 7 = 31.5 – 45 = 13.5 CFM). If you go to 75 CFM pump it translates 22.5 CFM (7.5 X 7 = 52.5 – 75 = 22.5), which falls 7.5 CFM short. A 100 CFM pump translates to 30 CFM (10 X 7 = 70 – 100= 30), enough to run the 3000 tap woods at 25 in of vacuum. Again all pumps are not created equal motor size and pump revolution speed come into play. These are just guidelines, some systems are easier to operate than other and it depends on well you manage your system for leaks.
Now let’s look at the yield side, again based on research done at UVM Proctor Research Center. In their study determining the yield up to 25 in of vacuum was their goal. The study shows that sap yield doubles when vacuum is taken from 0 to 15 in. From 0 to 15 in. there was a 8 gal per tap increase, from 15 to 20 in. there was a 3 gal increase and from 20 to 25 in. a 2.5 gallon increase. At 25 in. vacuum you have added 14 gallons of sap per tap. However, at 20 in of vacuum you have added 11 gallons of sap.. So what would happen if you settled for working at a lower vacuum level? If you backed down to 22inches 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 80% 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 principal 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 anything else like lifts and vacuum piston pumps. All of these eat up CFM. You do not want to be maxed out on CFM capacity when Mr. Bushy Tail shows up. Factor in another 3 – 5CFM in reserve vacuum and hope he does not bring his relatives. Your system need capacity to recover from leaks and other unforeseen problems and it need to do it as quick as possible. In my small world of maple production I am not comfortable with anything under 35CFM. Here’s why! Our home woods only have 400 taps, 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. Secondly most the lines drain to a low point that is totally inaccessible to sap pickup. We use a lift to bring the sap forward to the releaser. 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 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 innovative ways to conserver 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 system. The most important time you will invest in you maple syrup operation will be the time you spend in the woods managing your tubing system.
Footnote: Many producers are successfully rnning their vacuum systems over 25hg. They are successful because their system is properly designed and maintained.