Air Compressor flow rates question

Submitted: Tuesday, Jan 01, 2019 at 14:46
ThreadID: 137629 Views:6051 Replies:8 FollowUps:2
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Serious question.

For tire inflation, Whats the maximum air flow rate that a tire valve will handle without removing the valve core?

Just seems to me that if the valve will only handle X litres/minute then having a compressor that will produce significantly more than X litres/minute is probably overkill.

Or are we getting to the stage where we'll soon be removing the valve core to increase inflation rates just like we do with the rapid tire deflators.
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Reply By: Bob Y. - Qld - Tuesday, Jan 01, 2019 at 16:45

Tuesday, Jan 01, 2019 at 16:45
Not a pyshics scholar by any means, but I’d suggest it would be a relationship between supply pressure & FAD, free air delivery. No good having 120psi going out of your compressor, if you haven’t got the volume to back it up.

No doubt it all depends where you’re pumping up these tyres? If in the Simpson, putting another 10-20psi into them probably calls for a bit of smoko, but if you’re topping off 70 x 11R22.5 truck tyres on a triple fuel tanker, you’d like the flow to be almost instantaneous!


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Follow Up By: Allan B (Sunshine Coast) - Tuesday, Jan 01, 2019 at 17:45

Tuesday, Jan 01, 2019 at 17:45
Bob's right. The flow rate of air from the compressor into the tyre is a function of the differential pressure between the compressor output and the pressure within the tyre. This flow is further affected by the restriction of the tyre valve and other restrictions in the flow path. The valve does not limit the air flow to a particular value but it does reduce the flow much as a resistance in an electrical circuit.

As Bob says, you may have a compressor capable of attaining a high pressure but not capable of delivering a decent volume. By the same token, it is futile if a compressor is capable of high flow volume but not being able to maintain sufficient pressure to deliver air into the tyre.

The published pressure and flow figures from the compressor manufacturers/retailers can give false impressions of the capabilities of their products. They usually quote 'maximum output pressure' and 'free-air delivery rate' neither of which provide adequate information of their compressor's practical performance in inflating a tyre at the operating pressure of typically 30 to 50 psi. The real test is how long the compressor will take to inflate a tyre up to operating pressure. Some reviews have tested using that as the comparative criteria, which gives a realistic assessment of the performance.

I do often wonder just what gauges are used in these compressor tests. Pressure is pretty easy to measure but air flow does require somewhat more complex instruments to achieve a reasonably accurate determination. I suspect that they may simply compute the theoretical air flow by multiplying the piston displacement by the shaft revolutions, or even worse just pulling a suitable figure out of the air. (Tell 'em what they want to hear!)

But to address your actual question..... removing the valve core would always improve the air flow just as it does in deflation, but probably without as much benefit. In both deflation and inflation the flow rate will progressively decrease as the process proceeds. However, with a decent compressor, the differential pressure can be maintained at a higher value than when deflating so the flow is also better maintained and it may not be worth the trouble of valve core removal when inflating. Better to purchase a compressor which is capable in the first place.

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Reply By: RMD - Tuesday, Jan 01, 2019 at 17:06

Tuesday, Jan 01, 2019 at 17:06
All the air entering is passing through the two little side holes of the flats and the spindle clearance of the valve core. Having plenty of pressure increases that flow rate. However, the flow rate varies, similar to voltage/ampere flows in a battery. If flat the air flow rate is higher because the differential pressure is greater.
Many cheaply compressors can/may develop high pressure but with a small piston they can’t deliver the speed of flow, so tyre inflation is slow. A better compressor doesn’t make as much pressure perhaps but delivers more flow rate at a suitable pressure.

If concerned, just plumb your compressor onto you tyre deflator and with the valve removed the input flow will be the max of whatever is supplying it. Plenty quick.
I am certainly not going to be removing the vave to inflate. I don’t own a B double. How many times do you see a driver doing all the tyres of a B double from low to fully inflated?
AnswerID: 622947

Reply By: Hoyks - Tuesday, Jan 01, 2019 at 17:19

Tuesday, Jan 01, 2019 at 17:19
There is flow rate and there is pressure.
A lot of the compressors will quote a high cubic foot/minute (cfm) flow rate, but it is at very low pressures. Start upping the pressure and the cfm will drop markedly.

You also need a pressure gradient to fill the tyre quickly.

So using some ball park figures and an online calculator...
28 degrees
120psi supply
14psi in the tyre
and a flow rating of the valve Cv of .12 (it must be right, I got it off a forum)

a schrader valve will give you a flow of 9.3 CFM at 14psi, of course the flow will drop off as the inside pressure gets closer to the supply pressure.

Thats also the main reason for pulling the valve core. The 40 PSI comes screaming out of the tyre, but as the pressure drops to 30 and then 20 the pressure differential is flattening out, so that's the bit that takes the longest.

A restriction in the line is a good idea though, stops you rapidly over pressurising and blowing up the wheel assembly.
AnswerID: 622948

Reply By: Batt's - Tuesday, Jan 01, 2019 at 17:59

Tuesday, Jan 01, 2019 at 17:59
You will most likely find the average 12v compressor be it 72lpm or 160lpm would not supply enough air to be restricted by the valve so you won't have a problem there. An endless air model might if you have the engine idle turned up but I haven't used one before so not sure.

Depending on how important the compressor is to you as in reliability and how often your likely to use it the size of your tyres you can buy a good quality one that will last for 10yrs plus or one that's good for a yr or two. Just remember they do draw a bit of power some more than others so idle up if your able turn off spotties. Some of the cheaper ones have very low cut off temps so avoid under bonnet installation unless you pop the bonnet when using as the excessive heat under their can shorten it's life as well.

AnswerID: 622949

Reply By: smwhiskey - Tuesday, Jan 01, 2019 at 18:54

Tuesday, Jan 01, 2019 at 18:54
Thanks everybody.

Currently not looking to buy one as I'm more than satisfied with the ARB one that I've had for the past 7 years.

Just trying to work out the rational of having air flow volumes as a selling point if they they are variable during use and/or exceed the limitations of the environment that they operate in.

Been a slow day recovering from the excesses of the night before

AnswerID: 622950

Follow Up By: mountainman - Tuesday, Jan 01, 2019 at 19:09

Tuesday, Jan 01, 2019 at 19:09
Its about time.
Trying to inflate 4 tyres from 10 to 16 psi up to road pressure
Takes time
Arb twin comp rules
The old style arb comp was slow as

But the boss compressor had the flow rates for many years
Untill arb did the new one and then doubled them up as a twin pack
Uses some decent battery amps but i have never ever melted a fuse holder compared to the old style ?
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Reply By: HKB Electronics - Tuesday, Jan 01, 2019 at 20:05

Tuesday, Jan 01, 2019 at 20:05
Depends on the valve too, I have too different type of internal TPMS sensors, one type fills considerable slower than the other type as the hole on the base of the valve stem is smaller. A compressor with grunt is handy if your airing up 6 tires, using an air tool duster etc or try to seat a tire.

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Reply By: Member - Barnray (NSW) - Tuesday, Jan 01, 2019 at 21:16

Tuesday, Jan 01, 2019 at 21:16
What's the point of loosing sleep over it.R

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Reply By: Alloy c/t - Wednesday, Jan 09, 2019 at 11:07

Wednesday, Jan 09, 2019 at 11:07
Volume vs Pressure ....just watch a tire fitter , no valve in-place when seating the bead , maximum fast volume required , then insert valve to hold and regulate pressure ...
AnswerID: 623081

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