Compressor pressure, increase or not?

[julianthegypsy]

I spent the day sorting out the new compressor, abac 200 litre 5.5hp. got a new 300 psi glycerine filled gauge and new fittings for the outlet, fitted a plug (it had been hard wired and plumbed into pipework running around the workshop before it came to me). Anyway, deep breath, hit the go button and off it went, quietly and fairly quickly up to about 9 bar and cut out. Opened the tap, let some air go and it cut back in. Hurrah!
My question is, it's rated at 11 bar. Is there any point turning up the pressure? It's easy enough to do, but as long as it keeps up with the plasma table, which it should do easily, will it be easier on it having the max pressure a bit lower than it could be, or not?

 

[brightspark]

leave as is as long as you have enough pressure from it. 9 bar is well enough run mine at about 130 psi and cut back in at about 100 psi

 

[Edward Teach]

I have my 3hp, 14 cfm, 150 ltr compressor set at 100 psi, the same as Brightspark.
I do not need any higher pressure or capacity.

 

[mtt.tr]

More pressure uses more energy, ideally you'll set it to the lowest that you can for your requirements

 

[julianthegypsy]

Perfect. Thanks for the replies. The plasma runs at 80 psi, and with 200 litres in the tank and 18cfm delivery I can't see it needing more either but I wasn't sure whether it should be turned up or not and it's now abundantly clear.

 

[Ross365]

For what it's worth, I'm not too sure about the accuracy of some of the pressure gauges that come in from the far east. I've just had to replace the main gauge on my compressor which is only about 18 months old. Stashed away somewhere, I have a gauge made by Budenberg which I used to keep traceably calibrated when I was still in business, and I'm going dig it out and check the three modern (cheap) gauges on my compressor and gas line. I've just bought a needle-scaler which came with dire warnings not to exceed the rated working pressure of 6bar.

 

[mtt.tr]

For what it's worth, I'm not too sure about the accuracy of some of the pressure gauges that come in from the far east. I've just had to replace the main gauge on my compressor which is only about 18 months old. Stashed away somewhere, I have a gauge made by Budenberg which I used to keep traceably calibrated when I was still in business, and I'm going dig it out and check the three modern (cheap) gauges on my compressor and gas line. I've just bought a needle-scaler which came with dire warnings not to exceed the rated working pressure of 6bar.

Surprisingly accurate, I had a cheap dirty one that I'd happily loose 5 times, when compared to the transducer on the machine near enough matched.

Close enough for to be valid atleast

 

[not done it yet]

A piston compressor is as close as one can get, for this application, to a positive displacement device.

As mtt.tr, the cost of squeezing the same volume to a smaller volume costs money and the smaller the final volume, the more costly (and slightly less efficient, as well) it will be.

The volume of the receiver is irrelevant if the pump is unable to deliver more than is being taken out (apart from the buffer volume between usage pressure and the pump cut-in pressure). If the compressor motor is not continuously rated (it should be, but…), there may be a possible further issue lurking, if the pump were not to cut out within a reasonable timescale.

With a plasma cutter (and most other air tools) the design air pressure is at the machine (tool), not necessarily at the receiver.

Common sense (sadly lacking, these days) is needed when sizing compressors. Compressed air is a costly commodity and more so, if the machinery is not sized adequately.

That new gauge may well be reasonably calibrated at 300psi, but may be progressively less-so at lower pressures. Be aware also, if raising the cut-out pressure, that the cut-in pressure may be changed at the same time.

 

[carbon]

I would check the motor plate for the current rating and check what current the motor is drawing at full load when close to the shut off point. If the two are close then leave the pressure set as it is

 

[mtt.tr]

A piston compressor is as close as one can get, for this application, to a positive displacement device.

As mtt.tr, the cost of squeezing the same volume to a smaller volume costs money and the smaller the final volume, the more costly (and slightly less efficient, as well) it will be.

The volume of the receiver is irrelevant if the pump is unable to deliver more than is being taken out (apart from the buffer volume between usage pressure and the pump cut-in pressure). If the compressor motor is not continuously rated (it should be, but…), there may be a possible further issue lurking, if the pump were not to cut out within a reasonable timescale.

With a plasma cutter (and most other air tools) the design air pressure is at the machine (tool), not necessarily at the receiver.

Common sense (sadly lacking, these days) is needed when sizing compressors. Compressed air is a costly commodity and more so, if the machinery is not sized adequately.

That new gauge may well be reasonably calibrated at 300psi, but may be progressively less-so at lower pressures. Be aware also, if raising the cut-out pressure, that the cut-in pressure may be changed at the same time.

The last firm would audit and adjust max pressures and differential the saved energy was unbelievable. And the smart controller's would do similar after learning patterns was really impressive.

 

[julianthegypsy]

I've shelled out £35 for a glycerine filled gauge from our local specialist. Taking no chances. It cuts out at 9 bar and easily keeps up with the plasma. Happy days!

 

[hotponyshoes]

It's been a long time since I did any air compression maths (I used to be into scuba diving) but if I remember correctly compressing air too pressure was not linear.
Ie. Going from 10bar to 20bar takes more than twice the energy.

Even if you get free electric then running at a higher pressure than you need means extra wear & tear on the compressor, possibly more moisture in the air lines, and, as its less efficient, it will actually take longer to reach 20bar than get to 10bar from nothing twice.

Obviously most of use just get the biggest compressor we can and leave it as it came from the factory but if you were only running one tool regularly then I expect some useful savings (money and equipment life) could be made by having the compressor set at the bare minimum you require.
If you could be bothered to do the maths or trial & error it until you got it running right!

 

[julianthegypsy]

It's been a long time since I did any air compression maths (I used to be into scuba diving) but if I remember correctly compressing air too pressure was not linear.
Ie. Going from 10bar to 20bar takes more than twice the energy.

Even if you get free electric then running at a higher pressure than you need means extra wear & tear on the compressor, possibly more moisture in the air lines, and, as its less efficient, it will actually take longer to reach 20bar than get to 10bar from nothing twice.

Obviously most of use just get the biggest compressor we can and leave it as it came from the factory but if you were only running one tool regularly then I expect some useful savings (money and equipment life) could be made by having the compressor set at the bare minimum you require.
If you could be bothered to do the maths or trial & error it until you got it running right!

I haven't cut anything big enough yet to see if it will outrun the plasma table, but I have no doubt it will. It cut in just as the torch was returning to zero after cutting this. The air runs on for a minute after the torch stops cutting and a minute after that it cut back out again. That's about 25cm tall for reference.

 

[Morrisman]

It's been a long time since I did any air compression maths (I used to be into scuba diving) but if I remember correctly compressing air too pressure was not linear.
Ie. Going from 10bar to 20bar takes more than twice the energy.

Even if you get free electric then running at a higher pressure than you need means extra wear & tear on the compressor, possibly more moisture in the air lines, and, as its less efficient, it will actually take longer to reach 20bar than get to 10bar from nothing twice.

Obviously most of use just get the biggest compressor we can and leave it as it came from the factory but if you were only running one tool regularly then I expect some useful savings (money and equipment life) could be made by having the compressor set at the bare minimum you require.
If you could be bothered to do the maths or trial & error it until you got it running right!

When I worked offshore my job was operating and maintaining huge compressors, 1800cfm @ 2000psi. We used 5 stage compressors, first stage was a 24" diameter double acting piston. All it did was increase the air pressure to 1.8bar from atmospheric. Second stage took it to 7 bar, followed by 32, 75 then 140. 5th stage piston was just 3" diameter.

We wanted to see if they could be used to refill the SCBA bottles for our fire fighting gear, but the Rarus 827 synthetic oil we used in the compressors was carcinogenic, so the safety man, he say no.

 

[not done it yet]

Going from 10bar to 20bar takes more than twice the energy.

Of course it will require more than twice the energy, to compress to twice the pressure. Higher compression produces more heat (think diesel engine as a good analogy), which means higher temperatures reached which, in turn, means more heat has to be dissipated to the surroundings.

 

[knighty]

contrary to every other post here... I always turn compressors up to the max (or close) - I run at 250psi at work, could go a bit higher but not all the compressors can - tbh some not rated for 150 but I turn them up anyway (tanks are, just not the compressors)

I want as much air available as possible when I need it, and want constant output pressure (after end regulators etc. where they're used)


to be fair.... I don't have anything right now that really uses a constant air supply

 

[brightspark]

contrary to every other post here... I always turn compressors up to the max (or close) - I run at 250psi at work, could go a bit higher but not all the compressors can - tbh some not rated for 150 but I turn them up anyway (tanks are, just not the compressors)

I want as much air available as possible when I need it, and want constant output pressure (after end regulators etc. where they're used)


to be fair.... I don't have anything right now that really uses a constant air supply

if u want more air just buy a bigger compressor. no point in killing a smaller one running it at a higher pressure than it is designed for and cooking the motor as well

 

[knighty]

I've got a 30cfm hydrovane, a 14cmf hydrovane and a 30cfm piston compressor, hooked up to a 1500litre tank (pluss I guess a 250litre on the piston pump)

you only need to blow one tyre up to 100+psi to regret not running at higher pressure

 

[8ob]

I've got a 30cfm hydrovane, a 14cmf hydrovane and a 30cfm piston compressor, hooked up to a 1500litre tank (pluss I guess a 250litre on the piston pump)

you only need to blow one tyre up to 100+psi to regret not running at higher pressure

Thats painful, especially when the differential is set low.

Bob

 

[knighty]

I can remember blowing tyres up when I still lived at home with a little hobby direct drive compressor thing, the type with wheels on one end buyt it's easier to pick it up with one hand

had to blow the tyres up to 90... compressor cut off at 90

you had to blow them up as far as you could then dump air out of the inflator till the compressor kicked in, stick the inflator back on the tyre and keep the trigger pulled 'inflating' until the compressor kicked off again.... if the tyre was really low you sometimes had to do it twice :-(