? about air flow restiction, and piping size

I am a service tech at a scuba shop, currently we have three compressors two 10.6cfm @ 4700psi and one 13.4cfm @ 4700psi all three are manifolded together into 3/16th I.D. stainless tubing with a run length of approx 100 foot. I know that we have major volume loss due to restriction, nut I need numbers to show the owner.

Dear Randy,

FORMULA PIPE SIZING WATER & AIR/GAS :

Q = 3.142/4 X A2 X V X 3600/1000000

Q = M3/HR A = MM V = VELOCITY M/S 250 = 62500/ 300 = 90000

V = WATER 1-10 KG/CM2 = 1.5 – 3 M/S

AIR 1-2 KG/CM2 = 8-15 M/S

AIR 200-300 KG/CM2 = 5-7 M/S (Q = A2 X V X 0.0028)

4700 psi = 324.14 Kg/cm2

You can take the same velocity 5-7 m/s to be on the safe side.

Calculation:

CFM – M3/H X 1.6990

1M3/H = 0.588 CFM

30 cfm x 1.6990 = 51.0 M3/hr

35 cfm x 1.6990 = 59.5 M3/hr

Therefore: (Q = A2 X V X 0.0028)

2.5"/ 65 mm pipe = 4225 x 5 x 0.0028 = 59.0 m3/hr x 0.588 = 34.8 Cfm

4225 x 6 x 0.0028 = 71.0 m3/hr x 0.588 = 41.7 Cfm

4225 x 7 x 0.0028 = 82.8 m3/hr x 0.588 = 48.7 Cfm

Your ideal Pipe Size to pass 35 cfm for this Compressor works out to be a

2.5"/65 mm pipe with Velocity 5 m/s.

For 2 Compressors:

30 cfm x 1.6990 = 51.0 M3/hr x 2 = 102 M3/hr

35 cfm x 1.6990 = 59.5 M3/hr x 2 = 119 M3/hr

3"/80 mm pipe = 6400 x 5 x 0.0028 = 89.6 m3/hr x 0.588 = 52.7 Cfm

6400 x 6 x 0.0028 = 107.5 m3/hr x 0.588 = 63.2 Cfm

6400 x 6.75 x 0.0028 = 121 m3/hr x 0.588 = 71.1 Cfm

6400 x 7 x 0.0028 = 125.4 m3/hr x 0.588 = 73.8 Cfm

Your ideal Pipe Size to pass 70 cfm for this Compressor works out to be a

3"/80 mm pipe with Velocity 6.75 m/s.

Rgds,

Duja

PS

For 100 Rft to be in the safe side I'd tap out from the Compressors with a 2.5"/65mm line each with a Disk Check Valve into a 4"/100 mm line then run 100 ft and at the end due to restriction fit a reducer 100mmx80mm to further minimize any pressure drop problem.

Thank you very much.

I guess that the 3/16 I.D. that they have now is even more lacking than I thought. I was just guessing but I was thinking that 1/2 to 3/4 I.D. (inch) would still be way small.

Thanks

Randy

Please confirm if we are considering 35 CFM at standard conditions (1 atm at 20C or 14.7 psia at 68F) or 35 cfm at 4700 psia.

Randy, are you refering to 35 SCFM or ACFM?

If ACFM, then yes a 2" or 2.5" pipe is needed.

If SCFM, then 3/8" or 1/2" is adequate.

Yes you are right. I had a feeling he was mistaking between cfm compressed air and cfm free air. I believe his 35 cfm is free air, henced compressed will occupy much much less space at 4700 psia and then a 1/2" would be ideal.

Basic geometry math will show you that doubling the ID of any pipe will multiply it's cross sectional area by four, basically increase it's volume carry by 4x

Your 3/16" tube has lower parasitic drag than pipe and will flow more volume than normal rough wall pipe but it is still too Small for your desired volume......you pay a time penalty in fill time and an economic penalty of longer motor run electrical costs.

In your scuba tanks that may not mean much...ie: small tank volume....short fill time.

Larger tube to handle that 4700 PSI pressure can run into money very quickly.

Suggest the compressors, if possible, be moved much closer to use point...re wiring is cheaper than high pressure tubing for 4700 PSI...this will reduce the parasitic drag factor of excess length.

Or an accumulator at fill point will give some reservoir surge to speed up the lower fill pressure in the bottom half of the fill pressure accomplishment.

MR. GUY

2nd attempt to comment.

Good suggestions,Mr. Guy. Find out the cost of larger pressure tubing (1/2"), the cost of installing an accumulator near the tank fill point (my guess is the accumulator should be 2 or 3X the size of an average scuba tank), the cost of moving the compressors nearing the fill point, and the cost moving the fill point to the compressors.One of these alternatives will be economically preferable.

Questions. Does the compressed air need to be cooled before filling the scuba tank? Do you have air monitoring on the compressed air line or fill point for CO, CO2 or hydrocarbons (oil mist, HCs, other nasties)? I have designed breathing air systems for paint booth breathing systems, and I remember OSHA having requirements for this.

Also make sure the compressor air inlet is at a clean point. I have heard horror stories about air intakes picking up diesel fumes and poisoning the poor sap at the end of the line.

Yep my mistake it is "free air".

Right now we have three compressors running, all air is clean filtered to grade "E" specs (O2 compatible), the "accumulator" (we call banks), consist of four banks:

bank 1 ; ten 345 cubic foot cylinders @4500 psi

Bank 2; ten 345 cubic foot cylinders @ 4500 psi

bank 3; three 345 cubic foot cylinders @ 4500 psi

bank 4; six 345 cubic foot cylinders @ 4500 psi

banks are at the fill stations, bank 1, 2 & 3 at the air station, and bank 4 at the enriched air station

can't move the compressors closer not enough room at the fill stations, (one 7 whip fill station and a 4 whip enriched air (Nitrox) fill station), also noise from the compressors is an issue.

We try to keep the air as cool as possible by using the bank system, however on a busy weekend we will fill as many as 600 -700 scuba cylinders (most are aluminum 80 Cubic foot cylinders @ 3000 psi)

So on a busy weekend you pump around 48,000 to 56,000 cubic feet of air.

With 35 CFM compressors, they need to run flat out at 24 to 27 hours to keep up.

Definitely a new compressor and larger pipe to get rid of the pressure drop.

yep

another compressor is on the way 25cfm to go with the 35cfm total that we have now.

All that said, what size pipe am I going to need to handle all the air with minimal restriction?

last weekend was fairly slow, so I just ran 2 of the compressors 1 (10.6cfm) and 1 (13.4cfm) the small one ran 26 hours and the larger one for 20 (that is Saturday and Sunday together)

http://www.swagelok.com/downloads/webcatalogs/EN/MS-01-107.PDF

This pdf has data on pressure ratings for various tubing at various wall thickness.

I don't have a chart handy, but 1/2 or 5/8 is a LARGE improvement over the current 3/16 tubing.

Yep. I agree. A larger diameter is not detrimental and minimizes pressure drop with reduced in velocity

If you really mean that your air pressure is 4700 psi I am flabbergasted.

Could this be 4700 p/ft² = 32.64 psi ? Let's pretend.

I converted a formula for compressed air flow used by multiple manufactures of pneumatic valves, normally presented in Cv to use a known diameter.

Qscfm= (D2/.03443) x √[(P1 - P2) x (P2 + 14.7)]

To solve for the internal diameter of the pipe allowing 5% pressure drop (Dp)

Re-arrange the formula: D² = .03443 x Q / √[(P1-P2) x (P2+14.7)]

D² = .03443 x 35 / √(2 x 45.34) = .1265 .............√.1265 = .3557 Inside Dia

With an added flow making the total 70 scfm

D² = .03443 x 70 / √(2 x 45.34) = .25309 ...............√.25309 = .503 Inside Dia

This is based on a short distance. Use larger pipe, especially for long runs. Double the diameter is a good idea. From a table in Norgren Helpful Engineering Information the "maximum" recommended flow (based on 10% pressure drop in 100 feet, straight an no fittings) This is referenced to pipe size not the ID of that pipe.

At 40 psig, 34 scfm = 3/4 shed 40

At 40 psig, 62 scfm = 1" 135 scfm = 1 1/4"

At 100 psig 54 scfm = 1/2"

At 100 psig 80 scfm = 3/4"

Your options would be to get rid of restrictions, use larger pipe (cheap in the long run) and increase the pressure.

Sorry! I just read the question off the list. Please disregard my previous reply and for gosh sakes take a hard look at the force produced at 4700 psi which increases as the diameter gets larger. Very special pipe or tubing is required.

Which is why I posted the Swagelok link for pressure ratings of various tubing. Be very careful selecting and purchasing tubing and fittings.

Given ren's response to my query on air quality for breathing air, I suspect he understands what he needs to install and just wanted help with the sizing. If he wasn't sure, the catalog gives working pressure ratings for tubing of various materials and wall thicknesses.

Thank you very much for the swaglock link. Currently On the 3/16 stainless we use now it has a work rating of 10,000psi and we are using "ALock" fittings (same as the swaglock) but I did not have a tube chart. Still want to know so that I do it right the this time, on what Diameter.

And we try to always err on the side of caution when dealing with high pressure air (and I did mean 4700psi or psig if you want). And what is really scary that a full (3000psi), aluminum 80 cubic foot scuba cylinder in a catostrophic failure will release 1.5 million foot pounds of energy. (and people carry them in ALL ways)