Minimum useful air tank volume

Submitted: Friday, Jul 04, 2003 at 04:43
ThreadID: 5805 Views:5885 Replies:6 FollowUps:10
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If going the airtank + compressor route, what would be the minimum volume of tank that would be required and the pressure needed to inflate 4 tyres from say 15 to 35 psi. I have just been given two tanks, one 300 mm dia x 250 mm tall (~ 14 llitres), the other 300 mm dia x 150 mm tall(~ 7 litres), and the plaque on both says "W.P.680 kPa (working pressure?? 680 kPa = 100 psi) and another pressure T.P. 1030 kPa (??? pressure 1030 kPa = 150psi). Both the tanks have blowoff valves attatched, is this the "TP" value? Would either of these be suitable for my intended purpose? Any guidance appreciated.
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Reply By: Member - Wherethehellawi - Friday, Jul 04, 2003 at 07:48

Friday, Jul 04, 2003 at 07:48
TP is test the pressure the cylinder is tested to. Note it is NOT the working pressure.

How big a tank you'll need is dependant on how big your tyres are.....are they 10" or are they dumpster truck tyres? Not being sarcastic but emphasizing that tyre size is relative to how much air is required.
If your using a airconditioning compressor as the air compressor then cylinder size is not important. Dont believe its important with the mini compressors either as its only time (minutes/hours) that is required to pump tyres to 35 psi.
No doubt there'll be more usefull replies to your question as time goes by.Richard
AnswerID: 24168

Reply By: Tony - Friday, Jul 04, 2003 at 07:59

Friday, Jul 04, 2003 at 07:59
If it helps, I made up a tank to use with a Blue Tounge compressor, 300mm dia x 450mm long to run my diff locks, it took about 5 min to fill, good for seating tyres back onto rims but once it lost that inital burst was back to pumping at 1 psi.

The compressor had a 120 psi cut out, I never bothered to work out the volume cause I didn't know how. The wall thickness was 6mm with welded caps on each end.
AnswerID: 24171

Reply By: wazza - Friday, Jul 04, 2003 at 08:50

Friday, Jul 04, 2003 at 08:50
Gary, I have a truck air brake tank off a small truck (mabye Mitso or Isuzu). It would be around 300mm diam x 350mm long (19 litres ??). It is run off the arb and uses the cutout to limit pressure to 100 psi. It is enough to pump one tyre from about 18psi to 30psi, as I tested a couple of days ago. Takes about 2 minutes to fill to 100psi.

Good for reseating tyres, or it will operate my diff locks for ages should the compressor crap out (if I remembered to leave the tank at full pressure at the start of the day).

Don't forget a shut off valve between the compressor and the tank as it will leak the pressure back through the piston on the compressor over a few hours.

I reckon you would need a mega tank to do all four tyres.

AnswerID: 24172

Follow Up By: Member - DOZER- Friday, Jul 04, 2003 at 09:22

Friday, Jul 04, 2003 at 09:22
Im only nit picking here, but make that a check valve instead of a shutoff, then the compresser will be isolated and the possibility of forgetting it is shut and stuffing the pump wont occur.
Have this setup on my cruiser with air cond pump as compresser, and it holds pressure for days/weeks.
Pumps 35's up in 30 seconds aswell:-)
Andrewwheredayathinkwer mike?
FollowupID: 16265

Follow Up By: wazza - Friday, Jul 04, 2003 at 09:38

Friday, Jul 04, 2003 at 09:38
A check valve would be good to stop the 20 odd litres from flowing back through the compressor. Doesn't matter if I forget the stop valve is shut, as the ARB is designed to run it's own header tank to 100psi with a cutout stopping the compressor when it reaches this pressure. This is part of their design to have a header tank of air to drive the diff locks.

FollowupID: 16266

Follow Up By: Member - DOZER- Friday, Jul 04, 2003 at 10:03

Friday, Jul 04, 2003 at 10:03
Sorry Wazza
so with this shutoff valve, you have the option to pump up the extra vessel or only pump up the ARB to run lockers etc :-)
Sounds good
I use a fire extinguisher as the vessel, Dry powder ones are rated at 100psi, t/p 150 aswell,and the alloy top makes for easy tapping of threads.
Andrewwheredayathinkwer mike?
FollowupID: 16267

Reply By: Janset - Friday, Jul 04, 2003 at 19:12

Friday, Jul 04, 2003 at 19:12
Hi there.

I have had 2 troopies over the past 15 years and both had blue Tongue compressors before I fitted an A/C type compressor, to both.

My observations are basically the same as already stated but I now state appears to have been over looked.

(1) 12 Volt compressor: With an air cylinder, as stated you only really get benefit with the first tyre but when the pressure in that cylinder drops to the existing tyre pressure, you now have to fill up TWO vessels (the cylinder and the tyre) to the desired pressure, as the total volume has increased. In effect the cylinder is now working to a degree against you.

(2) A/C compressor....Only way to go :-) A cylinder is really a necessity rather than an option.

Before anyone jumps down my throat let me explain why.

The A/C compressor does not in most cases have an oil sump and thus it has to be lubricated by pouring oil (about 1/3 pint) directly into the compressor via the plug on the body.

As the compressor works it pumps the oil out quite readily either as straight oil or as oil vapour. Either-way this is not a desirable event for the tubes.

Now if you have a storage cylinder, a truck brake size is good, fitted preferably some distance from the compressor and you can draw the air from the top of the cylinder, then there is a very good chance the the oil vapor has by now liquefied and/or any oil travelling down the air line fall to the bottom of the tank reducing the oil discharge considerably.

In my setup, I have between the compressor and the truck air cyl a small intermediary cylinder/manifold made up of a 6 inch length of rectangular thick walled tubing. This manifold has an oil drain tap, and the blow-off valve set to 120 PSI.

This system works so well that the air coming out of the brake cylinder is oil free to the extent that when I blow air onto my fingers and rub them together I can not feel any oil, there is a small amount of water vapour, but I can live with that.

I hope this helps in you decision.

AnswerID: 24222

Follow Up By: GaryInOz (Vic) - Friday, Jul 04, 2003 at 19:19

Friday, Jul 04, 2003 at 19:19
Thanks Janset, what size is the cylinder that you have attatched to your A/C compressor?
FollowupID: 16296

Follow Up By: Janset - Friday, Jul 04, 2003 at 19:36

Friday, Jul 04, 2003 at 19:36
Hi Gary.

I think it is about a 1 & 1/2 cubic foot. It measures about 18 inches long and about 10 inches in diameter. It is mounted and fits very nicely between the gear box and the chassis rail and protected by the belly bash plate. I can still access the filler plug without drama.

FollowupID: 16300

Follow Up By: Member- Rox - Friday, Jul 04, 2003 at 22:42

Friday, Jul 04, 2003 at 22:42
Janset,Do you also have an air con as well/ 1 for aircon/ 1 for air pressure?
If so how did you mount themAround Oz 06/2004
FollowupID: 16312

Follow Up By: Janset - Saturday, Jul 05, 2003 at 11:59

Saturday, Jul 05, 2003 at 11:59
Hi Rox.

Yes I do. From memory I wrote up an article in this forum or Overlander 4WD Technical forum on the installation about 8-10 months ago.

In a nut shell, the windscreen washer bottle is removed and a Marlows all purpose rectangular bottle is secured to the other side beside the battery.

The A/C compressor is then bolted to the inner mudguard in the space now vacated. I faces backwards. Another pulley is machined and tack welded to the standard Air conditioner and a belt is then fitted between the 2 pulleys.

I can not leave the belt on all the time as it is rather noisy and as the motor rolls the belt loosens slightly before re-tensioning it's self again as the motor settles.

It is a slight inconvenience of having to fit and remove the belt each time, but the big plus is that I can fit the belt, blow all 4 tyres from 15psi to 35 psi, remove the belt and stow the hose, while the others with a 12 volt comp. are still on their 2nd tyre, plus I have untold air for what ever the need.

If you want to drop me and email, I can send you a couple of photos showing you the set up.

But here I have to say in all honesty. It was a MONGREL of a job setting it up as there are so many angles to consider in the alignment, but you only have to do it once, and if you are patient, it's well worth it.


FollowupID: 16332

Follow Up By: Member- Rox - Monday, Jul 07, 2003 at 20:59

Monday, Jul 07, 2003 at 20:59
My adress is
Love to see the photos as traveling next year and will be changing tyre pressure daily/weekly.
Thanks in advance RoxAround Oz 06/2004
FollowupID: 16422

Follow Up By: Janset - Monday, Jul 07, 2003 at 22:34

Monday, Jul 07, 2003 at 22:34
Hi Rox.

In the mail. I wish you luck with it.

FollowupID: 16425

Reply By: Member - Mal B - Friday, Jul 04, 2003 at 19:44

Friday, Jul 04, 2003 at 19:44
how did you mount the air conditioner compressor to the land cruiser safe driveing mal
AnswerID: 24225

Follow Up By: Janset - Saturday, Jul 05, 2003 at 12:00

Saturday, Jul 05, 2003 at 12:00
Hi Mal.
See above post.

FollowupID: 16333

Reply By: dave slater - Thursday, Jul 24, 2003 at 15:15

Thursday, Jul 24, 2003 at 15:15
Hi GaryInOz,
Lots of great comments and ideas, but I was surprized nobody pointed out the mysterious. It is a fact about air pumps and receiving tanks that if the connection between pump and tank is on the opposite end of the tank from the outlet connection, then somehow the pump capacity is increased. I believe that this applys only when the outlet is being used and the pump is pumping. It does make sense to keep the inlet and outlet on opposite sides of the tank. Perhaps an industrial air compressor guru can explain this.
Another disturbing fact is that a tremendous amout of energy is stored in a compressed air tank. If the tank is pressed up to full value then the sun heats it you are really asking for vehicle/personel injury if you dont have a safety relief valve installed where it cant be shut off.
As metioned before quite a few necessary additions are required to your air system to make it right and make it last. Safety relief valve, pump inlet filter, pump oiler, pressure gage (or schrader fitting to measure pressure with tire gage), pump outlet water/oil filters, tank drains (auto or manual), check valve between pump and tank are really necessary.
Years ago I installed a small tank with copper lines in my car trunk to press up my air shocks. Ignorance is bliss. After seeing industrial application copper tubing fail so easily under vibration conditions I believe that copper tubing has absolutly no business being in a 4x4 in Australia. Stainless tubing or steel pipe is the way to go for all connections.
Some comments on your original question.
Taking a tire from 0 to 15 psig requires adding 1 extra volume of air into the tire.
I just measured my car tire a 4highx6widex60middlecircumferance inches or about 2040in^3 or 33 liters. So it would take 33 liters just to get it to 0 psig. Add another 33 liters takes it to 15 and another 33 liters for 30 psig. Another 10 takes it to 35 psig for a total requirement of about 76 liters. (I think Len Beadle said it took about 450 strokes on his manual air pump to press up one tire.)
You have 21 liters to work with in tanks that were once tested and proved to hold 150 psig (TP) but built to be used at 100 psig (WP) More than likely the existing blowdown (I hope they are really relief valves not water blow down valves) valves are set to 125 psig. Thus it should be possible to safely press those tanks up to 100 psig.
0 = 21 liters
10 = 28
10.7 = 30
15 = 42
30 = 84
35 = 98
60 = 168
90 = 84 + 168 = 252
100 = 280 liters of air
120 = would be 336 liters but dont take the tanks that high
At first look the 280/76 would yield 3.6 tires but that would entail draining the tanks to 0 psig but you have to stop at 35 (acutally higher so air will flow from tank to tire). So we only have 280-98 = 182 liters available as the tank pressure drops from 100 to 35 psig. At 76 liters per tire you would get just 2 tires done and be leaving 98 + 30 = 128 liters in the tank at a pressure of 45.7 psig. The second tire would have been slow to fill.
What happens when the tank pressure drops to the pump motor cut in pressure (assume 90 psig)? If there is no check valve between tank and pump the pump piston will have 90 psig on it and the motor will have to be able to overcome that 90 psig to stroke the piston. If it is an electric motor it will likely stall and burn up since it was probably designed to start with 0 psig on the piston. An electric motor does not build up much torque until it is running full speed, thus the need for a check valve to unload the piston via piston leakby
To summarize; your tanks will do a couple tires, but do be careful and be sure you get working relief valves on each tank, and since you will have to run the pump while filling tires you should have the inlet and outlet on opposite sides of the tanks. I am not certain if putting the tanks in series would also add to the effective pump flow rate, but I bet it does.
later, slater
AnswerID: 25792

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