Vacuum
Depending on whether the tubing system will use natural or artificial vacuum, lateral lines will vary in the number of taps they carry. While producers are using larger mainlines, they are also reducing the number of taps in lateral lines. Natural vacuum systems require more taps per lateral line to produce sufficient head pressure. It is important to remember that the number of taps per lateral line in a natural vacuum system depends on the slope percent.The slope of the lateral line must be a minimum of three to five percent. A practical maximum number of taps per lateral line is a natural vacuum system, based on the slope percent, ranges between 20-30 taps. Natural vacuum will only work in a tight, leak-free system. Sagging lateral lines restrict the flow of sap. The benefits of increased sap flow in natural vacuum systems are realized only during good sap flow periods.
The merits of vacuum pumping are several fold and should be considered over natural vacuum if at all possible. An artificial vacuum system requires a vacuum pump with a vacuum controller and a vacuum release unit. The vacuum pump is operated at all times when the air temperature is above freezing and there is potential for sap flow. Lateral lines in a artificial vacuum may have shallower slopes. Artificial vacuum is not a remedy for poorly designed or improperly installed tubing systems. A leak-free system substantially increase the volume of sap collected. The benefits of increased sap flow in artificial vacuum systems are found throughout the season, but are best realized during poor sap flow days.
Artificial vacuum systems do not pull or suck maple sap from the taphole; The process is more complex. Sap flow is generally initiated as temperatures rise above freezing. along with an associated increase in sap pressure within the tree. When tree pressure is greater than atmospheric or positive pressure, sap flows freely from the taphole. Conditions often exist during the production season when internal tree pressure is slightly below or equal to atmospheric pressure. This results in low volume or "weeping flows". Vacuum pumping, in effect, lowers the atmospheric pressure in the tubing system. This creates a pressure differential or positive pressure in the tree, which encourages sap to flow more freely. Research has shown more sap can be collected with an artificial vacuum with no significant reduction of sap sugar concentration.
Though the benefits of artificial vacuum are realized in a well designed and maintained tubing system, sometimes the distance from the sugarbush to the location of the vacuum pump is great. In this case, substantial vacuum is lost to friction. Vacuum-transfer systems or booster tanks can be used to reduce the loss of vacuum and increase the volume of sap collected by moving the source of the vacuum closer to the trees. In a vacuum-transfer system, a second tank is placed in the sugarbush. Mainlines from the second tank serve different parts of the sugarbush. The transfer tank is connected to the vacuum pump by two lines. One line carries that sap, while the other evacuates gases, therefore creating a vacuum. The distance between the vacuum-transfer tank and the vacuum pump determines the effectiveness of the system. Distances less than 600 feel may only show slight advantages. Where trees are 1200 feet or more from the vacuum pump, the transfer system is more effective.
