Author Topic: Calculating heat dissipation requirements for high pressure compressor... ?  (Read 5295 times)

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Offline max_torque

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I'd also add that typical pneumatic actuator systems run at about 10 bar.  Very few systems run any higher than this becase the efficiency tumbles, safety is difficult to maintain and you can get plenty of power / force from "just" 10 bar.  A robot using even 10 bar sounds reasonably dangerous to me (and you'll find that valves and other components are expensive and difficult to source)

it seems to me that this is the classic case of "i've made up my mind i'm doing it like this, now help me to do it exactly like this" when it's frankly immediately obvious that you REALLY don't want to do it like that at all..........

BTW, you will also need to check what machinery and HS&E directives cover high pressure systems, even in domestic settings  (like for example those SCUBA tanks, which despite being "recreational" in useage are still explicitly covered by various safety directive due to the very real risks they pose)
 

Offline pipe2nullTopic starter

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An alternative is to do destructive pressure tests with water, which can be done with an off the shelf pressure washer.
er, nobody tests high pressure air systems to failure with high pressure air!!

If you need to carry out burst/failure tests, do it hydraulically like everyone else.  Costs are small, safety is maximised.   In the UK, i would be in breach of a large number of laws if i were destructive testing high pressure air systems in any domestic environment.

This makes a helluva lot of sense, and is the direction I will go for destructive tests.

it seems to me that this is the classic case of "i've made up my mind i'm doing it like this, now help me to do it exactly like this" when it's frankly immediately obvious that you REALLY don't want to do it like that at all..........
Yes and no... You are partially correct, and completely correct on the first "I've made up my mind I'm doing it---" part, but I am pretty flexible on the "how to do it the best way with minimal resources, while maintaining safety requirements" side of things.  I've been running down the HPA path because that's the only idea I came up with for destructive testing and until now I was unaware of the alternative.

I hate being wrong, but thank you for correcting my ignorance.  ;)
 

Offline skylar

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For 3k to 120PSI reduction there are dedicated HPA products aimed at paintball/airgun market. I have one, works ok for low volume use. Also paintball gun regs will drop 4k to 800psi and you can go from there as an intermediate stage. Welding regs take 2-3kpsi to 15psi or so, so not good for your needs.

Keep in mind that if you use the HPA for breathing at all, you really should not connect it to a LP system to prevent contamination. Also keep track of what lubricants you use, particularly in the HPA side to avoid an O2 ignition as I mentioned in my first reply. Sure you can get away without these considerations, until you don't, rapidly.


PV = NRT is correct

Not so much, actually. It is sort of true in high school science classes, but it leaves out a lot of thermodynamics that happens in real world scenarios. In particular it doesn't apply to air compressors as it tells you how any two variables are related if the other variable is held constant. So you can do P vs V at constant T, or P vs T at constant V, for example. If you compress air then P, V and T are all changing at the same time and a different formula applies ( \$Pv^\gamma=\textrm{constant}\$ is the basic one).

This is it! This equation and compressors are not a static condition is why my thermal numbers are so much higher than others using just PV=nRT. You have to consider the change in condition, not the final state. PS: The 840W required energy is the compressor input and not the air heat output, therefore you need to cool the heat from compressor inefficiency (35% minimum I'd guess; 15% from motor, 20% from seal friction/valves) and the air compression heat.

er, nobody tests high pressure air systems to failure with high pressure air!!

If you need to carry out burst/failure tests, do it hydraulically like everyone else.  Costs are small, safety is maximised.   In the UK, i would be in breach of a large number of laws if i were destructive testing high pressure air systems in any domestic environment.

If you want an air gun, fair enough, just use the normal small capacity cylinders designed explcitly for this. Your local dive shop or air products place will fill that cylinder for you  very cheaply!

Seconded, also hydraulic failure is less likely to send the broken bits to the extremes of your safety cage making post-failure analysis much more useful. Pneumatic failures often leave little intact to access, particularly in brittle plastics.

An interesting side note, often you will see HP hydraulic systems (oilfield) tested with HP N2 (in R&D, not in production/service) so as to ensure that an air tight seal will surely contain an oil/water fluid (due to the relative molecular/atomic sizes in play, high vacuum systems are often tested with helium). 15kpsi of N2 is fun stuff and these days I hear (I changed industries 5 years back) they're up to 30kpsi in the offshore safety equipment, no thank you.
 
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Offline tautech

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For 3k to 120PSI reduction there are dedicated HPA products aimed at paintball/airgun market. I have one, works ok for low volume use. Also paintball gun regs will drop 4k to 800psi and you can go from there as an intermediate stage. Welding regs take 2-3kpsi to 15psi or so, so not good for your needs.
Actually oxy regs will allow for 150 PSI IIRC (not got one in front of me) but their flow rates will be only suitable for dusting without another storage cylinder to buffer the flow rate losses and certainly not enough to run other than small air tools.
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Offline skylar

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Actually oxy regs will allow for 150 PSI IIRC (not got one in front of me) but their flow rates will be only suitable for dusting without another storage cylinder to buffer the flow rate losses and certainly not enough to run other than small air tools.

Fair, I was thinking about your cheap MIG/MAG/TIG/etc. regs that only have flow adjustments, no PSI adjust.
 

Online coppercone2

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flow adjustment is preferred for those welders I thought. I mean two regulators, one to regulate pressure, and one to regulate flow is better (I think, and you might need a big expansion tank thats run at a intermediate regulated pressure to benefit, and the large size of the cylinder might make it irrelevant all together), but I learned somewhere that overall flow control would beat a pressure control regulator for gas shielding. I do not think its the cheaper solution unless you get a dual regulator system,, and I believe that would only be useful if you are welding REALLY long beads, like robot welders.

For whatever reason, they are more expensive anyway
https://www.harrisproductsgroup.com/en/Expert-Advice/tech-tips/controlling-gas-flow.aspx

Not to mention when you bend the hoses and stuff, you change resistance of the gas path, and the important parameter is mass transfer (gas transfer), so a flow regulator if it works will give the same amount of gas output even if you bend the hose around. If a slight obstruction occurs, you still get the same amount of shielding gas if you have fixed flow, be that sputter on the gas nozzle or bent hose.
« Last Edit: May 26, 2020, 10:30:35 pm by coppercone2 »
 

Offline skylar

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flow adjustment is preferred for those welders I thought.
For whatever reason, they are more expensive anyway
https://www.harrisproductsgroup.com/en/Expert-Advice/tech-tips/controlling-gas-flow.aspx

Flow is the important metric as it will be a roughly ambient jet at the welding nozzle. I misspoke about the flow adjust, indeed on the cheap models you do adjust the pressure across a fixed orifice so as to change the flow; really thought they were the other way abouts. That aside, the use of the orifice is gonna really limit your delivery in any typical 120PSI use case. Without a buffer tank as big as any standard LPA compressor you're not gonna find a 3kpsi reg that can handle the flow rates you would want on shop tools unless you are only using incredibly short bursts. I've heard of guys dunking the regs in a warm water bath, but that's not practical.

More to the point of the main topic, I would also like to mention that SCUBA fellers usually have their compressors inside something hefty. Seen lots of old intermodal boxes converted into a tank room, with the tanks themselves also in individual heavy wall pipe with water or at least misters on for heat and explosion dampening. I bring this up to try once again to convince yall to keep the HPA outside and away from yourself where it belongs, all this multistage pressure buses and tanks is just gonna bite ya.
 


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