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Calculating heat dissipation requirements for high pressure compressor... ?
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IanB:

--- Quote from: pipe2null on May 18, 2020, 02:08:07 am ---Right now, I'm stuck at trying to figure out how much heat must be dissipated by the compressor to pressurize the air, temporarily ignoring heat generated by the compressor itself.  These types of compressors are water cooled and you normally use a couple large trashcans of cold water to dump heat to, or something similar.
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Just saw this thread, though I notice it has been going for a few days.

This is basically a mechanical engineering question. When you compress air it gets hot, really hot. Have you ever come across those fire sticks where you put some tinder in the end of a small cylinder and then press a plunger down sharply with your hand? The air gets hot enough to light the tinder and start a fire. And the pressure you can generate with your hand is nowhere near 3000 psi. So when you run an air compressor to get your air up to 3000 psi the air is going to get hotter than you can possibly imagine, which is why massive cooling is required to stop things seizing up, melting or catching fire. What's more, all that heat that has to be removed with cooling is power you have to feed into the motor. So you take electricity out of the wall, convert most of it to heat, and then throw away the heat in a cooling system.

To find out just how much power is needed, and therefore how much cooling is required, you can almost certainly find mechanical engineering toolbox sites on the web. Just look up compressor power calculations. Put in the standard cubic feet per minute of air to be compressed and the desired pressure, and the toolbox will tell you the power requirement and the temperature reached. The compression has to be done in stages, since you can't go much above 10:1 in a single stage without some intermediate cooling.

In summary, think big, think heavy, think expensive, think plumbing...
IanB:

--- Quote from: coppercone2 on May 18, 2020, 09:47:52 pm ---PV = NRT is correct
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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).
pipe2null:
@tautech:
Sorry, it was not intended as a lecture...  I get extra wordy when attempting to be clear, especially when people are reasonably concerned about safe use of HP.

@IanB: Thanks!
The claimed "specs" for one of the cheapo chinese brand HPA compressors I was looking at:
Model : YH-QB01
Power Rating : 1.8KW
Inflating Speed : 2800R/Min
Noise : MAX 85DB
Air Flow Rate : 40-50L/MIN; 1.5-1.8CFM
Working Pressure : 100-300BAR; 1500-4500PSI
Cooling System : Water Cooling
Compression Stage : Two Stage
Lubrication Mode : Splash Type
Material of Cover : Cast Aluminum
Shout down : Manual stop/automatic stop(for optional)
Pressure indicate : Gauge
Filtration : Water/oil separator
Lubricating oil : ISO VG46 or AW 46
Air hose connection : 8mm quick connect fitting

Using the calculator: https://www.engineeringtoolbox.com/horsepower-compressed-air-d_1363.html
With the claimed "spec" numbers of 2 stages at 1.8 cfm to compress from ambient to 3000psi yields: "Power Required (Theoretical): 0.927 HP, +10-20% for friction"
So 0.927HP x 1.2 => appx 830W

I'm not sure if that number seems off or a little low, but that's what I got.  If it is in the right ball park, it does not seem too extreme to use a closed loop water cooler with an outdoor radiator instead of the trashcans of cold water normally used (non-industrial use).

Most product listings do not include the full spec, and this is the first time I've seen an actual number for the noise level.  I was expecting something in the upper 70'sdB, but 85dB is quite a bit worse than I thought...  So, first order of business: safety valves/devices/etc.  Second order of business: find appropriately sized radiator and figure out overall cooling system.  Third order of business:  Minimize operational noise level by any reasonable means...

Damn...  I can get away with making a lot of noise once or twice a week, but not everyday without getting lynched by the neighbors.  Looks like I'll be getting both the HP and LP compressor after all, maybe sell the LP if/when I get HP noise level sufficiently reduced.  Damn, I was really trying to avoid that.  On the bright side, I can use the LP for preliminary tests of the HP assembled parts, at least make sure everything is connected tightly with no leaks prior to taking the HP compressor out of its crate.
NiHaoMike:

--- Quote from: pipe2null on May 21, 2020, 03:37:41 am ---Plain old PVC can safely contain a substantial amount of pressure, with >1000psi rupture point.
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Main problem with PVC is that the burst pressure can decrease dramatically due to stuff like UV aging, and fail very dramatically if it does. Hence why PVC is not recommended for compressed air.

--- Quote ---I have an outdoor steel enclosure with steel grating over vents that is suitable for failure testing parts with very thin plastic walls, but even that basic testing will require a little more pressure than a typical consumer air compressor can produce.  Doing failure testing of parts with thicker walls will require a trip to an outdoor gun range or similar location for obvious safety reasons.  There is no practical way to test the reliability and safety of a design without destructive testing in a safe and controlled manner, like placing the DUT in a vented steel enclosure and then over-pressurizing the thing until it fails.  I'm sure I'll come up with more uses for HP as time goes on.
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Have you considered repurposing a discarded (but still working) HVAC compressor? One designed for R410a should be able to go up to 600PSI or so if the inlet is pressurized to 120PSI using standard shop air in order to keep the compression ratio reasonable. You'll have to change the oil to some sort of mineral oil since modern HVAC oils turn acidic if exposed to moisture.

An alternative is to do destructive pressure tests with water, which can be done with an off the shelf pressure washer.
max_torque:
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!

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