Vacuum: an Explanation

The concept of vacuum is the exact opposite of what most people think of when they see a pump and some lines. Most people think of air being pushed though a line, similar to a compressor with an air line. Air can be compressed to an infinite level as long as what is holding that air does not explode. With vacuum it is the exact opposite. Vacuum is pressure based on the force that the earth’s atmosphere exerts on all of us. This amounts to about 15 pounds per square inch of surface or 29 inches of mercury. This pressure is also referred to as barometric pressure. As the atmosphere fluctuates, we might know by watching the weather that barometric pressure goes up and down with changes in atmospheric air movement. If you remove air from a container, you will produce a vacuum inside that container. That lack of air creates a negative pressure that is measured in inches of mercury (element abbreviation Hg) and will never exceed the outside barometric pressure.  As molecules of air are moving toward the pump and that air is ejected at a volume over a period of time, in this case Cubic Feet of air per Minute (CFM), the capacity of the pump will determine how fast this will happens.

What we are trying to create inside our sap lines is the absence of air or a perfect vacuum. Most producers grasp these basic concepts they also realize that there is no way to maintain a perfect vacuum inside their sap lines. Damage from wildlife and aging equipment introduces air into the system. Even the tree allows air to be introduced. For this reason we always allow for 1 CFM of air movement for every 100 taps. The problem with most systems is that we are getting way more air into the system than we want. This puts a greater burden on the pump to remove the air. The speed at which this is accomplished is largely determined not only by pump capacity, but how the tubing system is constructed. Line length and diameter in relation to the pump and the amount of liquid in the lines has as much to do with it as pump size. Couple this with the fact that most producers are attempting to run at high vacuum (as close to the daily barometric pressure as possible). The problem with this is that it is counterproductive to pump efficiency. To go from 12 to 15” (Hg) vacuum requires 20% more system capacity, 12 to 18” requires 50% more system capacity and from 12 to 20” requires 80% more capacity. Placing your woods on vacuum can yield more sap per tap, up to a 50% increase; however, this greatly increases the demand on your pump and everything behind the pump – from the shed to the tree – has to be in optimum condition. You can see in one short paragraph there is more to running a vacuum system than simply hooking a line to a vacuum pump.

The simplest way to design a vacuum system is to start with a tubing system and then install a pump that will effectively handle the tubing system. First, you need to determine how many taps will be on each mainline. You need to know the slope of those mainlines. Sap flowing in a relatively flat woods will move more slowly than sap moving down a mountain side. Each line has a volume capacity for the liquid it is conducting. For example, a 1 inch line on gravity will conduct 50 gallons per hour on a 2% slope and 75 gallons per hour on 6% slope. A good rule of thumb is that you want no more than 40% of the space inside the tubing holding liquid. The rest is needed to move air. The vacuum line is dual purpose, but its main function is air movement which facilitates the actual vacuum effect. If the sap level rises to the point that it blocks that air movement then the vacuum level quickly drops off. This along with excessive leakage are the main reasons for vacuum level drop from the pump out into the woods. In other words, using too small a diameter line will result in lines running full of liquid and dropping your optimal vacuum levels. One of the best ways to overcome this problem is to use dual-line conductors, using the top line for air movement and the bottom line for liquid. The use of this type of system is vital in flat woods with very little slope. Getting your lines sized correctly is the first step in creating an efficient vacuum system. In the next post we will discuss the importance of vacuum line sizing and distribution.

Author: Les Ober, Geauga County OSU Extension

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