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post #1 of (permalink) Old 04-02-2002, 12:25 PM
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Onboard air - 2.5 or 1.25 gallon tank???????

Hello again all,
I am putting a York compressor in my 2001 jeep and wanted to start a thread on tank size. Everyone wants to focus on the pluming and the compressor but I have yet to see a discussion on the size of tanks used.

I see most installed kits have the 2.5 gallon. Why is this? Just cause? or is there a underlying reason other than more is better? I figure that air tools need 4-5 CFM to work correctly and from what I have read the York produces this without a problem. Seems like air stored in the tank would not make a huge difference if you are pushing a high CFM from the compressor. I would think the tank would even out the flow of air output to tools because there would not be the patter of air each type the cylinder cycles. I also have a 2.5 gallon tank that does not have a compressor on it and it only increases the PSI in my tires about 2-3 PSI before it it empty.

Overall I was thinking of going with a 1.25 gallon tank that is smaller so I won't have to deal with mounting space, weight (Not much I know), cost, and the 2.5 gallons look like when mounted it is about 1" below the frame rail when installed on the dirver's side (A air tank would not be good as a rock slider).

Looking for thoughts and mostly input from someone who runs tools with a York and a smaller tank than 2.5 gallon.

Thanks everyone!!!
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post #2 of (permalink) Old 04-02-2002, 01:48 PM
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

the reson most folks use a 2.5 gallon tank is because it fits nicely under the hood between the hood supports. I have found that you need at least 2.5 gallons when trying to reseat a tire too... you cant ever have too much air...
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post #3 of (permalink) Old 04-02-2002, 01:51 PM
 
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

Go with whatever fits better, but you would be happier with the 2.5 gallon. The size of the tank equals how long you'll be able to run tools before low pressure kicks the compressor back on. Volume is not the same as pressure, the York needs to fill a volume to create the pressure. You woudn't be happy with it at all without any tank. Think about the difference between the 25 gallon home units on wheels and the 80 gallon uprights. The compressor fills a larger volume, and thus has to cycle less while you work. You get higher useable pressure for a longer time. If you plan on using an impact wrench, for example, the larger tank will allow you more full-pressure "hits" than the smaller one before you have to wait for the compressor to build pressure to a useable level so you can work again. Even filling tires you'll have to wait less with a 2.5 gallon tank vs. the 1.25 gallon, but you still will have to wait for the compressor to fill the tank. The tank isn't just a volume in the system, it's the storage vessel for the pressure. Your system (basic) should be compressor > check valve > pressure switch > tank, so you have a storage tank of compressed air. Your compressor will fill the volume, hit the high limit of the pressure switch and shut off. When the pressure in the tank reaches the low limit of the switch, the compressor will turn back on, and fill to the high limit. Looking at this, you can see that the larger tank will allow more time between compressor runs.

I know this is long but hopefully it gives you the info that you're looking for.

Ken
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post #4 of (permalink) Old 04-02-2002, 02:43 PM
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

Do I have the deal for you! Just about 24 hours ago I posted a formula and a spreadsheet for calculating how big a tank you need to air up your tires. It's titled 'OBA formula and spreadsheet' or something like that.

You'll be amazed at how much air it takes to pump up a big tire. 20 gallons at 150 PSI for 4 tires is a ballpark figure. Whether you start with a 1.25 or 2.5 gallon tank won't make much difference. You're going to be waiting for the compressor anyway.

By the way I just tested my program with a 7.5 gallon carry tank. The test came out very close to the calculation, but I had to be conservative with the tire dimensions I put in. After all, the sidewall and tread don't hold any air. I am modifying the spreadsheet to "Diameter less tread" and "Width less sidewalls."
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post #5 of (permalink) Old 04-02-2002, 03:03 PM
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

I do agree that the air in the tank helps but it will be gone in just a few seconds then the York would kick on anyway. I want to know if the tools that state they need 4CFM to run (Impact wrench, air hammer) can run straight from the air coming from compressor, if the tank is empty anyway isnt all the air coming straight from the compressor anyway? The York puts out 4+ CFM so shouldn't this do the trick? If so I could use an air hammer continuously as long as the output from the compressor was at least 4CFM? Yes or no?

Thanks.
post #6 of (permalink) Old 04-02-2002, 03:15 PM
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

Maybe yes, probably no. Compressors are usually rated in SCFM - Standard Cubic Feet per Minute. That is a cubic foot of air at standard atmospheric pressure and temperature. It really defines how much air goes into the compressor.

Air tools are usually rated at "4 CFM at 90 PSI". If the compressor sucks in 4 cubic feet of air and "compresses" it to 90 PSI, it will only be 2/3 of a cubic foot after being compressed.
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post #7 of (permalink) Old 04-02-2002, 06:57 PM
 
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

The York may be able to put out 4+ cfm, but at what pressure? Like Jim said, tools are rated for "4cfm at 90psi". Running the tool right off the compressor might seem like a good idea because hey, this one needs 4cfm and this one puts out 4cfm. The 4cfm output is at a low pressure, that's why you need a tank where the pressure is allowed to build to a useable psi. So the answer is no, you wouldn't be able to run an air hammer continuously just off the compressor. Well, maybe it would run but it wouldn't have enough force to do any work. Once you have your York installed, try it. In order to do work you need a tank.

Ken
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post #8 of (permalink) Old 04-02-2002, 11:43 PM
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Re: Onboard air - 2.5 or 1.25 gallon tank???????

Thanks for all you input everyone. [img]images/icons/smile.gif[/img] [img]images/icons/smile.gif[/img]
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