The goal of the previous article (Part I), this article, and the next is for you to realize that there are many factors that go into installing and running a maple vacuum tubing system. All the factors are interrelated and each one needs to be careful considered on the part of the operator. The below information is contained in the Cornell New York State Tubing and Vacuum System Notebook (NSTVN) written by Cornell University’s Maple Specialist Steve Childs. Much of the information is these three posts is a synthesis of past content with some more recent best practice guidance.
Part I introduced basic concepts of vacuum in a tubing system, some different variants within vacuum systems, and the different factors (most well within the control of the producer!) that influence vacuum levels throughout a system. Part II will walk you through how to calculate vacuum levels within your system and how to ensure your production needs are met by your system’s capacity.
It is not uncommon during a peak or flood run for your vacuum to drop. If you maintain your lines and are running a tight, leak free system what is the possible explanation for this sudden drop in vacuum? One possible reason is CFM Allocation (air flow measured in Cubic Feet per Minute). In the most basic systems, all vacuum lines are properly and equally sized with the same number of taps per line and all running to a single collection point. The CFM requirements to maintain optimum vacuum will be equally distributed across the whole system. For example, if you have 4 lines of equal diameter connected to a 60 CFM vacuum pump each line would receive 25% of the vacuum CFM (15 CFM). According to theory that would be enough vacuum to run 1500 taps on each line. To use another example, if you are using a 20 CFM pump on a system with 4 equally sized lines and each line serviced 200 taps each for a total of 800, then you would be allocating slightly less than 5 CFM to each line – still more than enough to run each line. However, Total CFM utilization is not always dictated by the number of taps in the woods. One must account for the CFMs utilized by other components of the system, such as if you run a mechanical releaser and other add-on features like lifts or reverse-slope releasers. This reduces the number of available CFMs to accommodate tree loss and leak loss.
Caption: Vacuum Pump with Vacuum Gauge
Now let’s add some complexity to our scenario. Let’s say you expand your 800 tap operation by adding 600 taps to the backside of one of your 200 tap lines. What happens to your 20 available CFMs if you remove a 1” line and replace it with a 1 ¼” line to service the line that now has 800 taps. Now you have 3, 1” lines and the new 1 ¼” line servicing 1400 total taps. Now you must calculate your line allocation to determine proper CFM distribution.
The first step is to calculate the cross-sectional area of each pipe which is easily accomplished with basic geometry’s “area of a circle” equation.
|Cross-sectional Area of a Pipe|
|1 ¼”||1.23 in2|
|1 ½”||1.77 in2|
Second, you need to determine the percentage of your total vacuum going to each line. As a reminder, our example has 4 mainlines: 3, 1” lines and a single 1 ¼” line. Here is a simple way to determine vacuum distribution.
The cumulative cross-sectional area of our 3, 1” lines = 0.78 + 0.78 + 0.78 = 2.34 square inches. And for the single 1 ¼” line, 1.23 square inches. The grand total sums to 3.57 in2.
Now divide the cross-sectional area of each line by the total to see what proportion or percentage of vacuum is being applied to each line. You will find that each 1” line is pulling 22% of your overall CFMs which leaves 34% of the vacuum for the 1 ¼” line. By CFMs (remember you started with 20 CFMs), each 1” mainline is pulling a maximum of 4.4 CFM and the single larger line is hovering just under 7.
You can quickly see that you are sending way too many CFMs to each of the 1” lines and not enough to maintain good vacuum on the 1 ¼” line. A quick solution would be to combine the 3, 1” lines into a 1 ¼” manifold with the existing 1 ¼” line going directly into the releaser. That would result in the releaser with just two lines coming out each equally sized at 1 ¼”. This solution would re-allocate 50% of the CFMs to each line solving the problem of line allocation.
It is important to remember, you need to account for leaks that will introduce more air into lines. You might be able to maintain peak vacuum on most average days, but will your system keep up with sap flow when the big run hits and you need to move as much air as fast as possible to maintain vacuum levels. If you have your lines sized properly, you now need to take the next step to determine what size pump you should purchase.
Stay tuned for Part III (What Pump to Purchase?) on Thursday and be sure to leave questions or comments!