Table of Contents
Advertisement
The conductance speed of 1/4" OD refrigeration tubing is too small to be useful as a
connecting line in a system; so small we will not even discuss it. Never use it if you can avoid
it as it is too costly in excessive pump-down time. Review of Vacuum for Service Engineers,
Saunders and Williams Copyright 1959, Revised 1988, p. 60.
Mathematically it is important to understand that increasing the diameter of a hose does not have a
linear effect but rather that to a ratio of its diameter to the third or fourth power. By diameter, we're
talking ID not OD.
Comparing 1/4" ID hose to TruBlu 3/4" ID hose shows a Conductance Speed increase of approximately
96 times faster! (.25
In addition to higher conductance, we need to consider the type of flow. In vacuum there are three types of
flow to consider: viscous, laminar, and molecular. Viscous flow means we have a steady stream of molecules
flowing through the pipe. As we get to laminar and molecular flow things really slow down, so staying in the
viscous flow region is critical for quick evacuation times (and hence, our focus!).
Viscous flow ends where the microns are approximately 100/d" or 100/internal diameter of the tubing in
inches. When comparing viscous flow in various diameters, again you can see that 3/4" tubing has a huge
advantage. Viscous flow stops in 1/4" tubing at 400 microns (100/.25" = 400 Microns) where 3/4" viscous flow
continues to 133 microns (100/.75" = 133 microns). This is important because with a larger diameter we get
a 3X deeper vacuum in the viscous flow region (400/133= 3.00). A deeper vacuum means faster degassing
and much better dehydration. Dehydration is critical more than ever for systems contaminated with POE oils
since moisture breaks-down the oil forming sludge and acids.
So how much faster is a 5 foot, 3/4" ID hose than 1/4" ID at 500 microns? Approximately 80 times faster.
The conductance for air at 68°F is expressed as 520 x d
length is expressed in feet of hose.
Where 500 microns
(520 x .25"
x .500)/5' = .2 CFM
4
(520 x .75"
x .500)/5' = 16 CFM
4
16 CFM/.2 CFM
What does all this mean?
Short fat hoses trump long
skinny ones, every time!
4
= .00331) vs. (.75
= .31972) or (.31972/.00331 = 96.5)
4
4
= .5 mm Hg abs
= 80 CFM faster at 500 microns of Hg
x P/L where P is the pressure in mm Hg Absolute and
4
Advertisement
Table of Contents
