Why should dry lines be larger than wet linnes?

[Brian Ryther]

The way I understand it, there is 4x the amount of gass being released from the tree as there is sap but gasses move 100x faster than liquids in our vacuum systems. So why do we oversize our dry lines?

[Thompson's Tree Farm]

Good question Brian, I have asked it of many experts and can't seem to get a very definitive answer.

[Daren]

Not sure if this is hogwash, but I have heard and it seems to make at least a little sense is that the oversized vac line creates an almost "reserve" vacume copacity to absorbe vairations is pressures within the system resulting in a more consistent delivery of vacuum pressure. Or it may be akin to standing on your left foot while facing north on the last day in February on leap year.

[Brian Ryther]

A related question. Does the speed at which air wants to contract under vacuum increase as deeper levels of vacuum are applied? If so then despite the loss of efficency a vacuum pump experences as deeper levels of vacuum are acheived, are we removing air faster?

[Thompson's Tree Farm]

Steve Childs addresses this in some of his lectures. As there are less air molecules in the system, less are removed each minute so in effect it is less efficient as the vacuum becomes deeper. I don't explain this very well, sorry

[802maple]

Mostly Brian it comes down to restriction loss and creating the same amount of vacuum at the back of the system as at the pump. If you can create the same at both ends then you don't need a bigger line. For example a 3000 foot piece of inch line will lose in the vicinity of 3 inches of vacuum at the end just dead headed. 1 and a quarter will lose less in the area of 1 inch of mercury and 1 and half will not loose any, I believe with out going to my chart those are the right figures. That is the answer in a nutshell.

[Brian Ryther]

Jerry. I think you are getting closer to the truth. In the NYS maple tube guide by Steve Childs all of his charts are based on cfm at the end of open ended pipe. We do not have open ended pipe. We have dead headed pipe with lots of micro leaks. The further away from the pump the fewer leaks, the closer to the pump the greater amount of air to be removed. line sizing should consider these factors, and what I want to know and cant find is how different vac levels effect the removial of un wanted air. Like Doug said the deeper the vac levels the further apart the air molicules are. Does the distance between the air molicules influence the speed at which they travel towards PSIG 0. If so then vac levels at which a system is operated at will factor into the size of dry line needed to remove bad air. All of the charts in the NYS maple tube guide are all at 15". Are they relevent to a system at 25"?

[Beweller]

Maple addict: Consider your vacuum pump to be a positive displacement pump with a displacement of V and a clearance volume of C. As the piston retreats from, tdc, the air in the clearance volume expands to a pressure (isothermal assumed) of p = pb*C/(V+C) (pb, barometric pressure). (This is the lowest pressure the pump is capable of producing.) At bdc, the inlet valve opens and the pressure rises to po (po, pressure at the pump inlet). The amount of air taken into the cylinder can be calculated as A = V*(1 +C -pb*C/V*po). This is the amount of air discharged per stroke--measured at the pump inlet pressure. The amount of air pumped falls as the pressure falls, slowly at first, then more rapidly as the pressure nears the ultimate pressure.

If you wish the volume of air at barometric pressure, multiply the above A by po/pb.

Example: V = 1, C = 0.1, po = 10 (approx 20 inches Hg vacuum), pb = 30 A = (1 + 0.1 - 30*0.1/1*10) = 0.8. Measured at pb, this is 0.8*10/30 = 0.267. The capacity measured at barometric pressure falls very rapidly with falling inlet pressure (higher vacuum).

[maple flats]

I'm new to wet/dry, only having used it 2 years, but I believe it is all related to distance. Any given diameter of main line has a theoretical max length. To even approach that length you need your best vacuum delivered to the manifold where that main to the wet dry. If you are running fairly short mains it may not be an issue. In my case I have 1.25 wet and 1.5" dry conductors. So far I have just 4 mains this year, but it will grow. 1 main ties directly into the releaser and is just 600'. The others go into the wet dry, 1 (1000' 3/4" (Leader told me 1000' of 3/4" is the max for good vacuum transfer) increased to 1"the last 200' at about 300' from the releaser. I have another main of 850' of 3/4 connected at about 500' (I'll call it C) to the wet/dry and my last is 100' of 3/4" going to 250' of 1" and tied to the wet/dry at 1400' from the releaser. As I finish this woods there will be 1 more line directly to the releaser, maybe 2 and about 5 or 6 more spaced at 100' intervals starting about 100' past C. For me to get full vacuum to each of these the larger is very much needed. If you have a much shorter system, maybe you might possibly get by with less, but can you afford to experiment. Each " increase in vacuum adds 5-7% more sap according to Dr. Tim, if you lose 3" on your last main you lose over 15% and maybe almost 21.5%. This loss will be for the life of the system. If you are going to the trouble of adding vacuum why give up that much sap?

[802maple]

Quote from Maple flats (I'm new to wet/dry, only having used it 2 years, but I believe it is all related to distance. Any given diameter of main line has a theoretical max length. To even approach that length you need your best vacuum delivered to the manifold where that main to the wet dry. If you are running fairly short mains it may not be an issue.)


Bingo, we have a winner. I had a 4000 ft length of 3/4 inch mainline when I got into this vacuum gig. I could open that wide open at the end and the pump wouldn't show any drop at all, because resistance it still was running 20 inches of mercury. That is why you need to run the larger drylines if you have long distances.