This is an update for an article I wrote on the Ohio Maple Blog in 2013. It was entitled “Is It the pump or the mainline size that is effecting the performance of you tubing system”. A lot of knowledge has been gained since that original article. In fact, a whole new type of gravity tubing system, 3/16, has been introduced and overwhelmingly accepted by sugarmakers.
When we talk about tubing systems we have two roads to travel. One is a gravity system and the other is a vacuum system. A conventional 5/16 gravity system is not much different from running sap into a bucket. It does save labor but the yield is much the same. When we add vacuum to a tubing system we increase the sap yield 5 to 7% for every inch of vacuum we generate in our system. For example, if we produce 15 inches of vacuum in a line we should be able to double our sap yield.
The definition of vacuum is the absence of air. The level of vacuum that is achievable is determined by the barometric pressure for any given day. This means that our vacuum level can never exceed the barometric pressure in the location of our sugar bush. There are factors that have a direct effect on Barometric pressure. One is altitude. As the altitude increases the barometric pressure decreases. At sea level, 0 altitude, the average barometric press can be 29inches and at 2000 feet the average barometric pressure is approximately 28 inches. In addition, barometric pressure changes under different environmental conditions. It can change multiple times during the course of a day. This is more important when we are boiling syrup because it changes the boiling point of water. If we are running a vacuum pump under a low barometer at an altitude of 2000 feet we might struggle to maintain 27to 28 inches of vacuum on a very tight, well maintained tubing system. This statement also emphasizes the importance of managing leaks in a vacuum tubing system. Every leak adds additional air to the system making it harder for the vacuum pump to achieve high vacuum. The amount of air moved out of a system is measured in Cubic Feet per Minute CFM. It is important to be able to differentiate between Inches of Vacuum and CFM. To successfully raise your vacuum level, you have to be able to remove the air from your tubing system. Once the air is removed, your vacuum level will increase unless you are letting air in through leaks.
Now let’s look at what happens inside a maple tubing line. A conventional vacuum pump is designed to move air not liquid. This means that a vacuum pump is pulling air out of the system while the trees and the leaks are adding air into the system. A properly sized vacuum pump with a proper CFM rating will be capable of removing air faster that it is introduced. The only thing that will slow that process is line size. If your line diameter is to small, the air movement will be restricted requiring more time for the pump to clear the air from the lines. This is commonly referred to as Line Loss. The smaller the line the higher the line loss and the longer it will take to re-establish your peak vacuum level. That is why tubing design and pump size are so important in a conventional vacuum system. It is also very important to note, in a vacuum system, liquid does not need to be present to create a higher vacuum. The movement of sap is secondary. As the vacuum level builds it creates a siphon that pulls the sap along with the air. In fact, when we look at the space inside a cross section of tubing we need to maintain a ratio of 60 % air and 40% liquid. If the liquid level increases or is uneven (wavy) the air movement is restricted and the inches of vacuum drop. The pump will then have to work harder to keep up and maintain peak high vacuum.
Let’s look at other alternatives to move sap in a tubing system. One of the more popular alternatives to conventional vacuum is the diaphragm pump. Let’s look at what happens with a diaphragm pump. Diaphragm pumps are water pumps that unlike vacuum pumps, are designed to move liquid. They move water not air and their capability of creating CFM is minimal at best. Manufactures tell us that these pumps are capable of creating 20 plus inches of vacuum. How do you create a vacuum with these pumps when their ability to move CFM air is limited? In the sugar bush our lines are sloped toward our tank this allows sap to flow toward the pump. Once the pump picks up the sap on the intake side it accelerates the flow in the line. The pump simultaneously pushes the sap under pressure through the outlet. Because the pump is pulling hard on the sap, pushing it through the outlet, it creates a solid column of sap. As this column of sap moves down the line the air and the liquid combine. This creates a negative pressure on the backside of the column. This negative pressure can be measures with a vacuum gauge. This continues until the sap flow slows down. As the sap flow slows the vacuum level begins to drop. Once the flow is terminated the pump can no longer push sap through the outlet the negative pressure will ultimately disappear. If you run the pump without liquid, you risk damaging the pump. The big thing to remember is that a $200.00 diaphragm pump will not remove air from the system by itself. It has to move liquid to create a negative pressure on the backside of a column of sap. I know the above statements will create controversy from those that are using diaphragm pumps successfully. There are ways to tweak a system to create increased vacuum during low flows but the ultimate end is reduced or no vacuum. The other thing to keep in mind, if you want to be successful with a diaphragm pump, keep your tubing system free of leaks. Leaks will result in poor pump performance. Also protect you pump from freezing and ice in the lines. Ice can damage diaphragms. Diaphragm pumps are a good choice in small operations where an increased level of vacuum during a good run is better than no vacuum. They were never intended to a replace a conventional vacuum system and they never will.
The second part of this article will address the use of diaphragm pumps in a system that can preform as well as a conventional vacuum system under the right conditions. This hybrid system is a different animal, because it is used with 3/16 gravity tubing.