Air pump for achieving higher pressure ?

[europus2002]

Hi all,
A simple question..... If I have a 100Psi air pump (Pump # 1) & i enclose that pump in a sealed enclosure held at 100Psi. What pressure should i expect out of the enclosed pump if i am measuring the output pressure at atmospheric pressure level? Will it be simply 100Psi or 100+100= 200Psi or will it be 100x100 = 10000Psi ? I am an electronic guy thinking of above scenario... Thanks in advance

 

[zdas04]

Your "pump" (let's call it a compressor so I don't get confused further) is doing 7.8 compression ratios now assuming sea level operations.

If you raised the suction pressure to 100 psig (call it 114.7 psia), then the same 7.8 ratios would give you 880 psig discharge. In the real world, you'd have many auxilliary considerations (e.g., first stage clearance, vessel and piping pressure rating, etc.), but that is the magnitude of the change.

David

 

[hydtools]

And large rise in temperature.

Ted

 

[zdas04]

Temperature rise is a function of compression ratios, starting temp, and ratio of specific heats. 7.8 compression ratios (with the same starting temp and the same gas) should be the same temp rise in either case.

David

 

[europus2002]

Thanks David & Ted for throwing some light on this situation. So it will do what i want it to do but with lotsa changes , caution and consideration like you guys suggested . I agree it may not even be possible . But thanks for solving the puzzle. Btw , David, you mentioned about "first stage clearance" . Can you please tell me what that is ? I understand the vessel and piping pressure etc...

 

[zdas04]

I'm making the huge leap of faith and assuming that the compressor is a recip. For separable recips (i.e., the compressor and driver are in separate casings) that are air cooled the maximum compression ratios tends to be around 4.5. If you add jacket water cooling and strengthen the connecting rods, some machines can do over 12 ratios. In my statement I was assuming a separable, air-cooled, recip so to do 7.8 ratios you need two stages. The first stage takes the suction pressure to some intermediate pressure and, then the gas is sent through an intercooler, then it is compressed the rest of the way in the second stage.

Compressor clearance is the unswept volume in the cylinder. The larger this number is, the fewer compression ratios the cylinder can do. On a multi-stage compressor you can balance the capacity of the stages by adding or subtracting clearance. [I just wrote a detailed example and realized that compressor guys would get it but no one else would and deleted it.] The arithmetic is CR=(V1/V2)^k so increasing clearance will decrease the compression ratios (i.e., if V1 was 6 in^3 and V2 was 2 in^3, then if I increase clearance 10% of V1 then the ratio goes from 3.0^k to 2.5^k or 4.66 compression ratios to 3.7 compression ratios for air).

Now if your compressor is a flooded screw then none of this matters. Flooded screws are positive displacement machines like a recip, but they tend to have pretty low maximum suction pressure and maximum discharge pressure (e.g., a Gardner Denver SSY air compressor has a max suction of 30 psig and a max discharge of 250 psig and you couldn't safely give it 100 psig suction).

David

 

[rconner]

I'm not sure exactly where you are headed with this inquiry, but in any case may need to be very careful with high air gas pressures. If you did a search under "air multiplier", I think you might conceivably find specialized pumps that may be sort of capable of doing what you are talking about safely and off the shelf.

 

[europus2002]

Devid... You are right .The compressor is a recip & not rotary screw. Ok , i got it... so it all sounds like a challenge and i can definitly sense the difficulty in using some commercially available compressors to modify them to give out 3000Psi plus.... Also you explained clearence well & I understood it . Anyway , any other suggestions for getting 500CC cylinderical volume to 3200Psi within 15 minutes ?

 

[BigInch]

Sink it to 1400 feet underwater.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[hydtools]

Connect the 500cc to a nitrogen tank. Fill the volume from the N2 tank.

Ted

 

[MikeHalloran]

Beware of running oil-lubricated recips on high pressure air.

What with the oxygen content, the oil vapor and the heat of compression, sometimes they think they're Diesels.

They're not built to contain Diesel explosions.





Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

Mike,
What kind of oil have you seen that autoignites below 350F? Are you lubricating with Ether?

I've never seen a recip that did not have a high temp kill and they've all been set at 350F or less. The heat of compression calculation is T(out)=T(in)(CR)^((k-1)/k). So high temp is only a problem with a lot of compression ratios, not with any particular discharge pressure.

David

 

[MikeHalloran]

Point taken.

... but we're talking about a homebrew rig here; no kill switch guaranteed to be present.



Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

Good point. Roll your own may skip some normal safety precautions.

David

 

[europus2002]

Ok ... I love this forum .... People are so supportive and open to discussion...and almost all of you are highly experienced . So i have taken all points . But i still don;t understand "hydtools" concept of using N2. Even though it gives us simplicity engineering wise , what about the cost ? Anyone aware how much is the cost of N2 ?

 

[MikeHalloran]

The deposit on the bottle will probably far exceed the sale price of the N2. Call your local gas supplier.





Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

A nitrogen bottle is the simplest way to go but if you need to pressurize to 3200 psi a 3000 psi bottle will not get you there. the bottle pressure drops rapidly as it is used so you still need a pressure booster like these

http://www.haskel-usa.com/air_pressure.htm

If you have a pressure booster and can use air then you may as well start with a 100 psi compressor and boost that.

 

[europus2002]

Thanks compositepro.... I called Haskel 3 days back to inquire about the pricing . $2900 starting price. . I saw some used ones on ebay and the ones that looked like they would work without a hassle were priced upwards of $600.... So to avoid all these issues, i thought of buying a used stewart warner military compressor . Whole unit costs $ 1050 as compared to the brand new ones not below $ 1800 .

 

[Compositepro]

Are you sure that you can't use liquid to pressurize the chamber? That would be a lot cheaper and, perhaps,safer.

 

[europus2002]

Are you pointing towards pressure washers by any chance ? I saw them all over the place but i was not able to formulate how i would use it... Any suggestions welcome... That sounds interesting though .