Gas Compressor Snorkle questions

[cool_hardware52]

OK, I'll set this up. So there is a 3/4 fitting at inlet of compressor, will it work if step that fitting up and add the vacuum guage there?

Yes, a "tee" at the head will provide an easy means to have a test port for the gauge.

I have a 18 cfm pump that has about 1 1/4" diameter hole in the first stage casting. From the factory it had a smallish filter connected to the compressor via a ~5" long piece of 1 1/4 " pipe.

I added a ~20 ft long remote using 2" id tubing with about 3 large radius 90's total.

The 2" dia tube is more than 2.5 times the cross sectional area of the inlet to the compressor.

This totally failed. I was pulling about 6" of vacuum at the compressor inlet.

I had to replace the 2" with 3" to get the vacuum back down to near zero.

3" dia is almost 6 times the cross section of the 1 1/4" dia inlet.

As the remote intake gets longer it needs to be huge to keep the pressure drop near zero.

Short answer, *Go Big* to start with, much easier, and cheaper than doing it twice.

Tobin

 

[WVMike]

Yes, a "tee" at the head will provide an easy means to have a test port for the gauge. <snip> Short answer, *Go Big* to start with, much easier, and cheaper than doing it twice.

Tobin

Thanks again, will heed your advice and let you know how the testing goes.

I filled a set of double AL80s last night with this setup and the fill seemed hot to me. They were at 1000 psi at start and I filled to 3000,( took about 20 minutes) the tank measured over 110 degrees F at end of fill. Maybe a dumb question, but would hot gas in the compressor from a vacuum cause the fill to be hotter?

I am waiting on a CO monitor, that is in the mail, and I will check this tank before use.

 

[cool_hardware52]

Thanks again, will heed your advice and let you know how the testing goes.

I filled a set of double AL80s last night with this setup and the fill seemed hot to me. They were at 1000 psi at start and I filled to 3000,( took about 20 minutes) the tank measured over 110 degrees F at end of fill. Maybe a dumb question, but would hot gas in the compressor from a vacuum cause the fill to be hotter?

I am waiting on a CO monitor, that is in the mail, and I will check this tank before use.

Yes. 3000 psi = 207 bar so pumping from 1 ata to 207 ata is a 207:1 compression ratio

OTOH if you start at say 1/2 ata (due to restriction in the intake) the compression ratio will double! This is an extreme example, but the idea is the same.

From another forum post by "Swampdiver"

Tout = R^0.286*Tin (R=press ratio and T = temperature Both absolute).
The R is the output pressure divided inlet pressure absolute.
Where for a 3-stage compressor you use the cube root of the overall R. Square root for a 2-stage and the 4th root for a four stage - and so on.

Assume inlet air is 27C and outlet pressure is 4000 psig, and ambient is 14.7 psia. You get an overall R = (4000+14.7)/14.7 = 273.11
For 3-stage use R^0.3333 = 6.486. Inlet temp is 273 + 27 = 300. The outlet temp = 6.486^0.286 x 300 = 512C abs. Which is 239C.

Now assume the suction pressure is quite high (small duct / blocked inlet filter) and is half ambient say 7 psia. (and as you said this vacuum also sucks in additional oil from the sump)

The pressure ratio becomes (4000 + 7)/7 = 574.5. or 8.3 per stage. Tout = 8.3^0.286 x 300 = 549.5C abs. Which is 276C. Therefore 37C hotter.

That is assuming full inter-cooling between stages. If not, you add the extra temp to ambient and use that as the input temp. This of course is the instantaneous temperature of adiabatic compression. The air soon cools down in contact with the piston and cylinder walls - as long as they are kept cool that is. If the outlet valves leak - hot air flows back into the cylinder - which becomes the starting temp next time - which leaves that much hotter - to leak back again


The real nonsense can be seen in the recreational dive industry with small compressor rooms, inadequate compressor cooling, long narrow remote intakes, continued use of mineral oils, etc. The end result is an overheated compressor resulting in dieseling of the compressor oil to produce carbon monoxide and early moisture saturation of the desiccant, hopcalite, and AC beds.


Tobin

 

[rcontrera]

Mike

On our 5 foot standard snorkel we use 1 1/4" hose with the filter at the end. When I have done boat installations and long snorkels, I have used 1 1/2 to 2". And this is on the little guys (4.2 CFM)