Acolyte in training
- Sep 18, 2020
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- Rip City
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.