Priming the Pump

Yesterday, I had the opportunity to talk to a customer who was looking to prime a pump to remove a liquid out of an 8 foot deep tank.  He was pulling in the liquid through 1 1/2″ pipe and wanted to prime the pump in 5 seconds.  This was an interesting application for me, because it had some tangible numbers for the design.  In most applications with the E-Vac Vacuum Generators, a customer needs to lift something heavy and they need the job done fast. Those terms are very subjective, and we have to try and prognosticate what vacuum generator will work for the customer. Yesterday was different, the customer had some really defined limits, and I knew what I needed to do in order to satisfy the application.  The engineer in me was very happy with the situation.

Let’s start with the parameters.  He was looking to lift water 8 feet vertically.  8 feet of water corresponds to 7.06 inches of mercury.  The porous E-Vac can generate vacuum up to 21 inches of mercury and the non-porous E-vacs can generate 27 inches of mercury, so all of the E-Vac models can easily draw the liquid up the 8 feet of piping.  This is where most vacuum generator applications stop, but not this one.  If time is not a concern the 800001 will use the least amount of air (1.5 SCFM @ 80 PSIG of inlet) and get the job done, but how long will it take?

This is where you need to use the evacuation charts, from EXAIR.com. Below are two charts from the “Specs” tab for Inline E-Vacs.

E-Vac porous evacuation time

Non-porous Evacuation time

The 1 1/2″ Pipe that is 8 ft. long has volume of around .1 cubic foot.  We need to generate at least a 7.06 inches of mercury, so we will look at the 9 inches of mercury column. It will take the 810002 17.85 seconds to evacuate 1 cubic foot of pipe.  The 800001 will take 14.40 seconds to evacuate the same volume, and  it will use less air.  We will want to use the porous vacuum generator, because we don’t need a very high vacuum to get the job done. If the 800001 can evacuate 1 cubic foot in 14.40 seconds it should be able to evacuate .1 cubic foot in 1.44 seconds which is easily fast enough for the customer.  The math also told us the customer could use up to 27.7 feet of hose to lift up that 8 feet, if he needed to take a non linear path.  If we know that the customer needed to move the fluid more that 27.7 feet, we could move up to the next vacuum generator to get the job done faster, but it was not necessary in this application.

Dave Woerner
Application Engineer
Davewoerner@EXAIR.com
@EXAIR_DW

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