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tripplec
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Hydraulic air pump

Sat Mar 16, 2019 1:54 am

Someone tell me why it wouldn't work to use a big hydraulic cylinder and pump to pump air into a scuba tank? First concern is the oil. However, undamaged cylinders dont leak oil past the orings, and it would run much cooler than an oil lubed piston compressor. So what miniscule amounts of fumes or hydrocarbons should be easily captured by the carbon filder. Oil seepage should be even further reduced by having the same psi of air on the fill side as oil pushing on the pump side. Gotta be a reason people aren't doing it other than it sounds as scary as a lp steel tank with 1/2" threads.

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tripplec
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Re: Hydraulic air pump

Sat Mar 16, 2019 3:34 am

The other thought I've had is mounting two cylinders ram to ram using hydraulic in the primary to push air with the secondary which is lubed with a little silicone grese. But im not sure how well the orings will seal with a full tank of air on the outlet side of the secondary cylinder and ambient psi on the intake. Preasure is preasure I know. I keep coming back to the idea of hydraulics because cylinders are cheaper to buy and cheaper to rebuild than a three stage compressor.

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tbone1004
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Re: Hydraulic air pump

Sat Mar 16, 2019 8:03 am

a lot of people have used them to make gas boosters. No reason it couldn't/wouldn't work it would just take an astronomically long time.

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tripplec
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Re: Hydraulic air pump

Sat Mar 16, 2019 10:52 am

I was wondering about the gas bosters. As far as time, I have to wonder how long it would take a 4"x24" slow moving piston to fill a tank vrs the tiny third stage piston in a compressor at 1800rpm.

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antique diver
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Re: Hydraulic air pump

Sat Mar 16, 2019 12:38 pm

I understand your reasoning, but it seems to me that the heat produced by compressing air at such high ratios pretty much eliminates the practicality of performing that in a single stage of compression. High pressure compressors compress the air in stages, which keeps the compression ratios much lower in each stage.

Consider a 3 stage compressor for example of keeping compression ratios managable. A normal range for compression to 3000 psi varies some among given compressors, but let's say the atmospheric air is compressed to 80 psig in the first stage. Then the higher temperature of that air is moderated by passing through cooling coils on the way to the second stage. The second stage may compress that somewhat cooler 80 psig air to say 600 psig before going through cooling coils and expanding for additional cooling as it enters the moisture separator/condensor. The third stage then takes the cooled air to 3000 psi, which can create head and valve temperatures well in excess of 300 F before going through the after cooling coils and expansion in the final separator. An simplified calculation of the compression ratios (using 15 psia for atmospheric pressure) would give estimates of compression ratios of 6.33, 6.47, and 4.9 in the successive stages. Much less heat of compression is produced overall.

The compression ratio of the air being compressed to 3000 psig in a single cylinder by taking air in at atmospheric pressure is about 200:1,
( a simplification without figuring all other minor factors that might be involved). The heat produced would likely be enough to ignite any lubricants and orings at least, introducing hydrocarbons into the expelled air. I don't know how to calculate the potential temperature in the proper manner, but consider that a Diesel engine generates enough heat of compression (Adiabatic Heating) to ignite diesel fuel vapor at compression ratios from about 15:1 to 20:1.

I hope all our engineer friends will please forgive my simplified and possibly inaccurate drug-store physics calculations. In fact I would love to be enlightened on the temperatures potentially created at 200:1. I don't want to try to wade through those formulas myself!
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tripplec
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Re: Hydraulic air pump

Sat Mar 16, 2019 2:39 pm

I thays one of the things I wondered about antique diver. I wish there was a way of removing the cylinde wall and piston heat from a conventional compressor to see what the actual air compression heat is. I was hoping it would be about the same as the heat you get from filling an empty tank off a storage bank over a period of 30min. I also have wondered about setting up hydraulic cylinders in a three stage setup going to a smaller diameter each time.

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tripplec
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Re: Hydraulic air pump

Sat Mar 16, 2019 2:43 pm

On the dieseling thought, you can run an engine on diesel down to about 10:1. Not verry well. Thats around 160psi compression I think. I use kerosene to store my gas engines instead of our crappy government mandated gasoline we have here in Oregon.

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tbone1004
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Re: Hydraulic air pump

Sun Mar 17, 2019 9:22 am

the compression ratio is something that can be managed with speed though. there is a LOT of thermal mass in those pistons, so while a 1500+rpm compressor generates a lot of heat, quite a bit of that is normal friction due to metal moving past metal, compounded by adiabatic heat from compression itself. At the painfully slow speeds that the pistons run, there may well be enough thermal mass to keep it cool.
Giving the pistons a 120psi shot of air from a compressor with some filtration on it would help the pistons return back, but also lower the ratio to less than 30:1. Still well above the recommended 6-7:1 ratio used in most air compressor designs, but it essentially mimics the first stage of a normal compressor.
A lot of heat still being generated, but there are ways to keep those pistons cool and if you aren't hunting for speed out of it, which you certainly won't be considering the use of hydraulic pistons, you should be OK with air.

On the cfm calculations, pretty easy to do given the volume of the cylinder and approximate speed of the piston. Gettin the volume of that piston chamber may be a bit hard since it's not a normally quoted specification, but you could just fill one with water when you get it to validate.

Of note, this is the same principal as a Haskel gas booster, though that does require a minimum inlet pressure to cycle the piston back

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tripplec
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Re: Hydraulic air pump

Sun Mar 17, 2019 11:44 am

Maybe I can get creative and fill scuba tanks while splitting fire wood.

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antique diver
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Re: Hydraulic air pump

Sun Mar 17, 2019 12:54 pm

tripplec wrote:
Sun Mar 17, 2019 11:44 am
Maybe I can get creative and fill scuba tanks while splitting fire wood.

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Now that would be an interesting and useful project!
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SurfLung
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Re: Hydraulic air pump

Mon Mar 18, 2019 10:53 am

I wonder what the compression ratio is on those duplex cylinder airgun compressors we see on Ebay?
SurfLung
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