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Universal pump laws

Tattered Old Graywolf

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Let’s discuss universal pump laws!

There is a lot of discussion about which recovery pump is best suited for our application and my first thought is to ask which one of our applications?? Different processes and budgets have different needs.

While our industry got started using pumps designed for refrigeration recovery and from those humble beginnings different pumps evolved, in our legitimacy we now have available to us pumps developed by the petrochemical industry specifically for transfer and compressing the liquid hydrocarbons like propane and butane that we use in our processes.

Amidst the new plethora of options, we are also faced with exorbitant claims, reminiscent of the “This is primo shit man” claims prior to legalization and lab analysis, soooo let’s review the universal laws governing pumps, and discuss design features.

The pumps typically used in our industry are displacement pumps, which expel a given volume with each stroke of the piston.

The amount of actual gas within that volume depends on its density and thus the volumetric efficiency of the system, as well as the pressure on the pump intake.

Volumetric efficiency relates to the systems resistance to the flow of gas on the intake side and is influenced by intake pressure, line and port sizes, as well as valve size and timing and pump speed. That is different for each pump.

While the internals of the pump head may not be easily examined, one measure that is typically visible is the discharge port and line size. As a rule of thumb, for a given cfm there is less resistance to flow in a big line than a small one, so look at both line and port sizes. Some smaller ports are bushed up to accommodate larger line sizes, but the small port itself remains as a restriction.

Vapor density is a variable that affects not only the volume pumped, but the horsepower required to do so.

If we look at a pumps free air cfm rating, the intake to the pump is at atmospheric pressure, or about 14.7 psi absolute and there is no resistance to flow on the discharge side beyond line friction and static pressure from bends and elbows in the internal piping.

Under those circumstances, each stroke of the piston pushes its displacement volume from the cylinder out through the discharge port, so ignoring volumetric efficiency, if the piston diameter and stroke of the pump was 1 cubic inches and it was running at 1725 RPM, then total cfm would be 1 X 1725, or 1725 cubic inches per minute discharged. 1725 cubic inches is a cubic foot so output would be 1 cfm.

Now consider what happens when the pump intake is below atmospheric pressure. 14.7 psi is about 29.92” HG/760K microns and gas laws tell us that density and pressure are directly proportional, so if the atmospheric pressure on the intake is half the 14.7 psi or say 14.96” Hg/380K microns, then the density of the vapors in the cylinder will be half, so each stroke of the cylinder will discharge half the vapor.

Conversely, if the intake is under positive pressure, the opposite is true. Each stroke of the piston will deliver a proportionately higher volume.

When the vapor being pumped is from a boiling pool, when the liquid turns into vapor, it absorbs the latent heat of vaporization, which drops the temperature of the boiling pool and slows boiling to a crawl as it approaches its boiling point under vacuum.

The way to offset that and maintain recovery speed, is to replace the btu’s lost to vaporization so as to maintain atmospheric to positive pressure on the pump’s intake.

That brings us to what happens on the discharge side of the pump. The gas discharge will be at a higher temperature than the intake because of heat of compression and the gases physical contact with the hot pump. Unless we remove that heat, the tank pressure on the discharge side of the pump will rise and though the pump with an unfettered intake will still put out one cylinder volume per stroke, the back pressure on the discharge side will require more horse power to do so.

Once the horsepower required exceeds the amp rating of the motor, the motor will overheat and trip the overload. Increasing horsepower will solve the back pressure issue, but will not add to the pumps output, only an increase in volumetric efficiency or rpm will do that.

Some two cylinder compressors can be run either single stage or double stage to address the back pressure issue with existing horsepower. In single stage both cylinders discharge directly to the exhaust and in double stage, one cylinder discharges to the intake of the second cylinder, which discharges to the directly to the exhaust, so that the pump puts out about half the volume, but at twice the pressure using the same horsepower.

 

2broke2smoke

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Great information GW, but how do you decide where to apply this knowledge in relation to 2 different processes i.e. say reclaiming ethanol from a rotovap and then short path distillation? :D

2b2s
 

RosterTheCog

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Will the pump still have correct pressure in high altitude,
ie: Denver?
 

Tattered Old Graywolf

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Great information GW, but how do you decide where to apply this knowledge in relation to 2 different processes i.e. say reclaiming ethanol from a rotovap and then short path distillation? :D

2b2s
Although those are different process than LPG reclaim, the same principles apply more so reclaiming ethanol than short path because of the volumes of vapor handled. In both those cases you also need a chemical duty pump and with the short path you need to be able to pump under 100, preferably under 10 microns for best results at minimum temperatures.

What the laws tell you is that if the inlet is under pressure the pump puts out proportionally more output, and if it is below atmospheric pressure it puts out proportionately less, regardless of whether it is a piston, diaphragm, or dry scroll pump.

Diaphragm and dry scroll pumps are better designed to operate in a solvent vapor atmosphere without bypassing the piston rings and contaminating the crank case. They are also less apt to add contaminants that would hinder reclaiming the constituents of their exhaust.

We did Kugelrohr short path under 10 microns using a Welch Duoseal 1402 piston pump with a cold trap, but that is old technology. An Edwards nXDS6iC dry scroll is chemical duty and will pull to 15 microns, and a Welch diaphragm chemical duty pump will typically pull to 75K microns.

The dry scroll would work on either ethanol short path or rotovap, but the diaphragm pump would be limited to the rotovap.
 

2broke2smoke

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So if I am understanding you correctly, by using the Edwards nXDS6iC I would not have to change the crankcase oil after every run like in the welch 1400 and it can be used for both processes? If so that would be a huge improvement:D

2b2s
 

Tattered Old Graywolf

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So if I am understanding you correctly, by using the Edwards nXDS6iC I would not have to change the crankcase oil after every run like in the welch 1400 and it can be used for both processes? If so that would be a huge improvement:D

2b2s
Yup. The 1400 is venerable and gets the job done, but good point about frequent oil changes, even with a cold trap and there have been some innovations since its creation.

A big difference in price too, but aside from performance, the Edward is a thang of beauty and low maintenance by comparison, not to mention how amazed your family and friends will be, once they cast their eyes upon it. They offer a smaller one as well, but looks like it only pulls to around 250 microns, which is not low enough for good short path separation.
 

Tattered Old Graywolf

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Edwards is damn proud of that pump. Thats a lot of oil changes lol. Looking at the big picture it only makes sense to upgrade even though the difference in price is so large:D

2b2s
They are proud of it for sure. Too proud for me to own one for my personal joy and amazement, but a good choice for commercial installations.
 

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