Calculating run time

[Thrashsmith]

Sorry if this is a painfully obvious question but is there an easy way to work out how long I can run an air tool before my compressor kicks back in? I'm new to compressors and have so far only run a little die grinder off mine and I've been surprised by how hungry it is and how soon my compressor needs to kick back in. I want to get a small blast cabinet but don't want to go for a size where the compressor has to kick back in every 2 seconds and do my head in. Cheers for any help.

 

[addjunkie]

Mathematically possible, but you wont like the answer. As you will need a huge tank, a huge compressor and a small blast cabinet.

Blasting is very air hungry. If compressor noise is an issue, you might want to consider a quiet type compressor ( lots of money) or fitting your compressor in a sound proofed enclosure. Screw and vane compressors are a different noise to piston.

 

[hotponyshoes]

Have you got a regulator (s) attached to your compressor?
If so, then for the grinder you can turn it down until you notice the grinder dropping off. This will save you a fair bit of air over running tools directly from the compressor.

As said, blast cabinets use masses of air. Some compressors have a switch so they run all the time for jobs like this and never turn off.

If you want to work out how long the air will last you need to know:
Rating of the tool in working psi and cfm.
The capacity of the compressor tank.
The cfm of the compressor head.
The psi that the compressor kicks in / kicks out at.

If you want to make the maths a bit simpler then adjust your compressor so it comes back on at the maximum working psi of the air tool.

 

[rtbcomp]

It's not the size of the cabinet that uses the air, it's the size of the nozzle on the gun.

I used to operate a blast room with a blasting pot using various nozzles, the largest, 4mm IIRC, needed 2 3hp compressors running all the time. 4mm is a small nozzle in terms of industrial equipment.

 

[optima21]

even the smallest bench top blast cabinet with a 3hp compressor will run continiously. the from the compressor isn't an issue for me though, as the wet/dry vacuum cleaner used to dust extraction is louder.

you'll probably find that in reality the consumption of the cabinet is more that that quoted in the specification. also you use less air if you turn the working pressure down, 70psi makes a big saving in air consumption compared to 90psi.

the nozzle size needs to be taken into account as it dictates the air requirement of the blaster, this gives you an idea of this (14 cfm free air delivery is a standard 3hp compressor)

http://www.blastitclean.co.uk/page6a.html

 

[RonA]

Sorry if this is a painfully obvious question but is there an easy way to work out how long I can run an air tool before my compressor kicks back in? I'm new to compressors and have so far only run a little die grinder off mine and I've been surprised by how hungry it is and how soon my compressor needs to kick back in. I want to get a small blast cabinet but don't want to go for a size where the compressor has to kick back in every 2 seconds and do my head in. Cheers for any help.

The formula you need is P1 x V1/T1 = P2 x V2/T2
P= Press(absolute i.e + atmospheric 14.7 lb/sq in) V=Volume (cu ft) T=Temperature for your purposes you can ignore the temp element, so we are left with P1 x V1 = P2 x V2.
Using the formula work out how much air is in your tank at full pressure & then again at the point the the pressure switchbtrips in at. Subtract the two to get how much air you have to supplement your compressors output.
Then using your tools air consumption subtract your compressors output, this is the amount of air you need to draw from the receiver/min. Using the air available between full pressure & point at which pressure switch trips in divide by the amount of air being drawn by the receiver to feed the tool, this gives the time before press switch kicks in.
RonA

 

[Thrashsmith]

Thanks folks. Sounds like I'll just have to lump the noise & have it running all the time. Like I say I'm new to compressors so wasn't sure if having it running all the time was a no-no or not. I've been pausing with the air grinder when the compressor kicks back in so that the noise stops quicker, but also because one day when I didn't stop I noticed a bit of a smell & wondered if I was overworking the compressor motor or something.

 

[brightspark]

you haven't said the size of your compressor or what type .in a nutshell blasting with a dirct drive comp will kill it in no time. a 14cfm belt drive comp will run a 3mm jet but will run continuous to keep up any sort of decent pressure . any variation in pressure when in use will upset the sand to air ratio and will be ineficcient. using 2 comps together at 14cfm will give a good result

 

[Thrashsmith]

you haven't said the size of your compressor or what type

It's a Fiac Workhorse, 200L, 3ph, belt driven, 21 CFM, 16.5 FAD, 150 max psi, kicks back in at 120 psi. No adjustment on the tank, just a regulator on the wall.

 

[brightspark]

It's a Fiac Workhorse, 200L, 3ph, belt driven, 21 CFM, 16.5 FAD, 150 max psi, kicks back in at 120 psi. No adjustment on the tank, just a regulator on the wall.

it should work quite well using a 3mm jet for blasting or any other airtool at that its preferable when blasting keep the pressure up to 90psi unless your doing delicate parts and aren't in a rush . outgoing air hose causes problems if the bore is 2 small and restricts the flow of air to your tools or cabinet

 

[addjunkie]

Something like this might help

https://www.acscustom.com/uk/products/hearing-protection/other/total-blocks

 

[rcx132]

Calculating run time is easy but only if you know the true output and input of your tools. Small compressors wildly exaggerate their output and many tools understate their input.