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Oh Vacuum, Where Art Thou?

Since refrigerant charging is typically much faster than evacuation, we often find excess charging capacity sitting right behind a bottleneck at the evacuation stage.  As a result, we frequently hear our customers ask what they can do to reduce their evacuation time.

There are several methods, each with its own advantages, drawbacks and strings attached.  The best answer must be tailored to your specific application.  The hard truth is however, that there is no magic evacuation machine (yet?).  It comes down to basic physical principles.  But fret not, there are things you can do!

One of the first things that many people turn to is a larger pump size.  Yes, this can decrease your evacuation time, but there is a major caveat here:  the maximum pumping speed (usually measured in CFM) is only relevant when there is still enough air in the system to fill that pipeline.  Imagine you’ve got a circuit with an internal volume of 5 cubic feet.  You currently use a 5cfm pump.  Why does it take significantly longer than 1 minute to fully evacuate the system?  Because life isn’t fair!  Furthermore, once you get down to lower micron levels, the remaining air molecules become further and further apart which means that the low pressure generated by the pump is less and less effective in “grabbing” them and pulling them out of the system. (More on this further down.) At this level, the size of your pump could be 100cfm or 5cfm, and your pumping speed will be almost identical. (this is, of course, comparing similarly constructed pumps.  Two-stage pumps perform better at lower micron levels than single-stage pumps).  A larger pump may evacuate faster, but only if you’ve got an internal volume that is quite a bit larger than the cfm speed of the pump so that it operates longer at full capacity.

We mentioned the pump “grabbing” molecules a few lines back – that’s not quite accurate, especially at lower micron levels.  This is because gas behaves differently from liquid in a straw and, at a certain point, the pump does little more than provide an open exit door, and we hope that these molecules find their own way out after bouncing around enough times.  One of the best things you can do then, is to provide as free and unobstructed of a path to a very large opening as you can.  Every obstruction, foot of hose, or tiny orifice adds up to more time!

Before anything else, we recommend increasing your evacuation hose diameter and reducing restrictions in the evacuation line.  The jump from 1/4″ ID hose to 3/8” is quite significant, nearly double by some estimates.  Reducing your hose lengths will also reduce friction – another cause for slow evac times.  Similar improvements can be made if you remove Schrader cores, or using quick connect Hansen fittings or CoreMax fittings which are designed to improve flow.  Reducing elbows and using lines with smooth internal surfaces will improve the chances that those molecules won’t get hung up on snags and start bouncing the wrong direction and against the flow of traffic towards your pump.  While we’re looking at your hoses and fittings, a gentle reminder here to try evacuating from both high and lose sides – this reduces the need to pull vacuum through those tiny cap tubes or expansion valves.

One of the more effective and lowest cost ways to reduce your evacuation time is to really take a hard look at your vacuum requirements.  I know, this isn’t a hardware change, but it really matters!  If your current requirements call for vacuum down below 100 microns for instance, each additional micron can add a LOT of time.  So, get with your product engineers and really zero in on what your specific product evacuation requirements are.  Sometimes you’ll hear “I don’t know, we’ve just always done it that way”, and there may be some room for dramatic improvement in your evacuation times simply by tailoring your requirements.  If you’re at a loss, Airserco can help recommend some baselines as a starting point.

While the next option does not reduce the evacuation time overall, it does help with throughput.  Try evacuating more units at once, so that the charging station has a steady supply of evacuated (or rough evacuated) units to keep it busy.  Some highly efficient operations will have their units being evacuated on a roller or carousel as they move from one area of the plant to another, so that when they reach the final evacuation and charging station, they can be processed in a minute or less.  We offer both pre-vacuum (a.k.a. “rough vacuum”) stations as well as final evacuation stations which can perform rise/decay tests and record results, and even go straight to the charging phase in the case of our iRockall series, and these can even incorporate trace gas leak detection to combine several steps into one.

There are other ways to improve your process times, such as nitrogen sweeps or even adding heat.  While these can provide some benefits, they are difficult to implement on a large enough scale or with sufficient automation to make the process efficient.  Typically, the simpler solutions mentioned above have the highest overall ratio of bang for buck.

If you need help finding the right solution to cure what ails your production line, send us a note and let us know what your situation is, and we’ll see what we can come up with!

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AHR Expo 2019

On January 14-16, Airserco will be attending the 2019 AHR Expo in Orlando with our partners, H2O Weld! Please stop to say “hi” and see this amazing water-based brazing system in action for yourself!