Calculating Force required for a simple air piston pump 1

[IdeadGuy]

Network engineer, so not a real Engineer or a Student Engineer; is it allowed to ask a Mech. Eng. question/s?

If so, I'm looking to build a simple air piston pump w/low CFM (5.5), delivering air to ~9' below water surface.
Q1- Since 9' is ~1/3 Bar, does that mean I need to provide PSI at 0.3 Bar, or 1.3Bar at cylinder outlet?

Aside Q1, Q2 is figuring out Force (N) required on piston.

5cmf = 8640 ci
Cylinder 8"ID x 12"L = 603ci
8640 / 603 = ~14 stroke/min
60/14= 4.28 sec/full stroke
2.14 spf = 0.142 mps piston speed
Cylinder exit orifice 0.75 in

There seem to be a multitude of different Air Cylinder Force equations, however they all seem to be for closed cylinders.
One online calculator yielded a Force of 4K N....

I can mix bore size, length etc, and mix and match motors and gearing if I have to. Maybe smaller PVC prototypes.
But I am just shy of understanding the physics/fluid dynamics and I think I've got 90% of what I need to get a properly newbie engineered solution*. For various values of Engineered...

Would anyone be willing to help a guy out?


Thanks!

 

[rb1957]

maybe you need a shark ?

 

[pierreick]

Hi,
For aeration, a blower roots type or other will do the job.
Other systems in the link attached.

https://www.alibaba.com/premium/aerator_fish_pond.html?src=sem_ggl&field=UG&from=sem_ggl&cmpgn=20789349274&adgrp=162646800944&fditm=&tgt=kwd-302885078153&locintrst=&locphyscl=9197688&mtchtyp=p&ntwrk=g&device=c&dvcmdl=&creative=681592639643&plcmnt=&plcmntcat=&aceid=&position=&gad_source=1&gclid=EAIaIQobChMIs4Org-7-igMV_80WBR30aAfIEAAYAiAAEgIgQfD_BwE

Note: Here in Thailand shrimps' ponds are equipped with surface aerators
Pierre

 

[LittleInch]

BOAFP
Based On A False Premise??
May be less reliable than a WAG?

Back Of A Fag (cigarette) Packet.

Simple calculation.

 

[LittleInch]

"Note that at the end of your stroke the air pressure will still be 1.3 bar a, so on the inlet stroke the first 23% of the stroke will not pull in any air so account for that when you do the air flow calculation."

That part is not correct. It is only the "unswept volume" between the piston and the outlet check valve that will have to expand 23%. This volume can be (and should be) designed to be close to zero.

If you are not an engineer or avid tinkerer you should not try to design one yourself. There are many cheap inflators on the market, both hand and battery powered. Your question, though, is still relevant to selecting a pump that will work. If this is for breathing air, you need to pay attention to air contamination from lubricants or wear particles, particularly if there is a motor.

Ah. Sorry. Yes it's 23% of the volume left at the end of the stroke, which we don't know.. The pint being that there is a certain volume which might be quite small so maybe 5% or less before you start to draw air into the system.

 

[Snickster]

Snickster, thanks very much!
I'll admit that'll take a few to figure out.
But net-net I need 196 ft/lbs to compress, and power and stroke time are dependant upon available power and motor size, gearing?
Unless I misread what Force on Piston is 196 @ 9' is referring to.

196 pounds (not ft/lbs) is the force equivalent to balance the pressure in the pond at 9 feet depth. Additionally once the air in the piston is compressed to equal the pressure in the pond, the air as it is forced out of the piston at the point where it reaches 0.3 bar will experience additional back pressure due to the flow from the piston to the pond which will add a little to the 196 pound force on the piston. Note that this is the maximum force which starts off at approximately zero for before the compression begins.

Waross pointed out some issues in designing the drive train between the motor and the pump. I am not a compressor designer and drive train expert I just understand thermodynamics of compression cycles.

It is possible to estimate the required motor size (HP) for a given motor speed using ideal thermodynamic equations for compression of a gas and assume an efficiency of compression. Then you would need to add power required by the drive train.

 

[waross]

196 pounds (not ft/lbs) is the force equivalent (On an 8" Dia piston) to balance the pressure in the pond at 9 feet depth.

Use a much smaller piston.

 

[IdeadGuy]

Yes, a smaller piston is going to have to be the way to go.

4" is pretty common and cheaply available, so I'll start there.
Then I expect heat to become more of an unavoidable problem as speed ramps up to get near desired cfm.
And an internal oiler/sprayer.

Design is the art of turning constraints into opportunities still seems to be true.

I appreciated everyone's help.

I won't keep surfacing this thread unless anyone wants to see how its goes/fails spectacularly.

 

[waross]

And an internal oiler/sprayer.

Will you be using edible oil in deference to the fishes?
Consider a diaphragm pump, either a flat diaphragm such as an air brake chamber or a rolling diaphragm such as is used on air suspension units on some large trucks and trailers.

 

[Lacquit]

Oh, this discussion helped me

 

[Whavillat]

If you know the pressure and the area of the piston, you can use the simple formula F = P × A, where P is the pressure and A is the cross-sectional area of the piston.