When you install a vacuum tubing system, you need to understand a few basic principles that determine how air moves through a mainline. First you must consider line loss. Line loss is caused by the friction of air moving through the line. A general rule of thumb is that the narrower the diameter of the mainline, the slower the air removal at long distances. A smaller line will restrict the pump’s ability to quickly remove vacuum and recover from leakage more than a larger line. If you have a 1000 foot, 1 inch mainline attached to a 60 CFM (cubic feet per minute) pump, you will lose 50% of your CFMs within the first 200 feet. If you attach the same pump to a 2” line you will retain 50% of your CFM out to 1500 feet. Simply stated, air moves easier through a larger diameter line.
You also need to consider how a vacuum pump works. The measure of vacuum pumps efficiency is not actually how many inches of mercury (Hg) it can obtain but how many cubic feet of air it can remove from the line in one minute. Remember even the smallest vacuum pump can remove the air from an air tight system and obtain high vacuum if you give it enough time to work. But one also needs to remember there is no such thing as an air tight maple tubing system, so ability to recover from leakage quickly is critical. Bigger pumps can remove air from the line faster but only if that air can move down the mainline quickly – which gets us back to mainline size. If the diameter of the mainline is too small, the air flow will be restricted by line loss.
So using an identical 1000 ft. mainline, how many taps can we run? Let’s consider a 1 inch line, 1000 feet long hooked to a 60 CFM pump. The 1 inch line will only allow 8 cubic feet of air to move through the line in one minute’s time at 1000 feet. If you follow the rule of 1 CFM for every 100 taps that would mean that you could not exceed 800 taps on that line, even though you have a vacuum pump capable of running 6,000 taps. The only way to solve this problem is to go to a larger diameter line. If you move up to a 1 1/4 line of the same length hooked to the same pump you would have 12 CFM available at 1000 feet into the woods. You could theoretically run up to 1200 taps on this line. The major problem here is that many producers feel that they can solve their vacuum problems by buying a bigger vacuum pump. The truth is that at 1000 feet with a 1 inch line, hooked to a much smaller 15 CFM pump, your system is still capable of transferring 7 CFM. If you replace the smaller pump with a much larger pump (60 CFM) it will only be able to transfer 8 CFM. A larger pump under a fixed scenario will not necessarily transfer more CFMs.
Yet another factor to consider is that most modern vacuum pumps are capable of maintaining a high level of Hg or inches of vacuum, but once a leak develops the vacuum level declines. It is now up to the vacuum pump to overcome that leak by removing the incoming air faster than it is entering the system. The pump must be able to do the job quickly to maintain an optimum level of performance (high vacuum). As demonstrated in the above example, a big pump can only be as efficient as the line capacity behind it.
Up to this point, we have only considered air flow through an empty line with no sap in it. What happens when we add sap? The optimum goal is to maintain 60% air and 40% liquid inside the vacuum line. What happens during peak flow when the ratio is often reversed? Under low flow conditions, there is very little liquid inside you mainline and air can move freely. Under peak flow conditions, sap builds up and air blockage often occurs. This blockage could be in the form of waves or even worse slugs of sap that seal off a portion of the line. This is a real problem especially on slopes of 2 % or less. The solution to this problem, especially on flat ground or where large volumes of sap are entering the primary mainline from secondary main lines is a dual-line conductor or Wet-Dry line. The bottom line conducts the sap and the top line removes the air from the system. The bottom line is sized based on its liquid capacity and the top line is sized based on air flow and CFM capacity. When figure the CFM capacity for a Wet-Dry system only consider the capacity of top line. The advantage of a Wet-Dry system is that you should never have liquid in your top line, that means that it will always transfer air at full capacity and the bottom line will have greater capacity to transfer sap.
To get a more in-depth description of how to install a vacuum system, including line loss charts for both single and wet-dry mainline, consult the New York State Maple Tubing and Vacuum System Notebook from Cornell University.