Calculating heat dissipation requirements for high pressure compressor...  ? - Page 1
Forum section: electronics / projects

author: pipe2null
author_rank: Regular Contributor
timestamp: May 18, 2020, 02:08:07 am
content: Yea, this is not exactly an electronics question, but thermodynamics is part of electronics, right?I'm trying to figure out the actual cost and complexity of setting up a high pressure air compressor (3000-4500psi), small high pressure holding tank, plus a couple pressure regulators downstream to step down to normal shop air 40-90psi. I have occasional use for high ~3000psi pressure, but not often enough to merit the cost of a high pressure system unless I'm also getting shop air out of it too. Yes, I am fully aware you can rent high pressure tanks, like $15/day from a local scuba shop. The other immediate advantage is the equivalent storage capacity of a small 3000psi tank compared to a normal 120psi tank: 1 gallon at 3k psi is roughly equivalent to a 25 gallon 120psi tank, if I did my math correctly...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.For simplicity, let's assume the high pressure holding tank is a 100 cubic foot scuba tank being pressurized to 3000psi. I'm pretty sure scuba tanks are rated by volume of ambient 1 atm air compressed into the cylinder, so 100 cu.ft of 1 atm air compressed to 3000psi. I vaguely recall from high school chemistry class the whole PV=nRT thing, so with zero heat transfer, the compressed air would be 204xAmbient degrees (Ambient temperature x 3000psi/14.696psi), so the heat energy that the needs to be removed from the system is equivalent to the temperature delta for the given volume of air... I think... But that's where I'm getting stuck, and I'm not even sure PV=nRT is the right equation to use for this calculation. Any pointers on the subject would be much appreciated.Ultimately I'm trying to evaluate how feasible closed loop water cooling might be, and if a PC watercooling radiator would be sufficient or if I would need a much, much beefier radiator. For my purposes, it is acceptable to shut off the compressor once or twice during a fill so the water cooling can catch up, but not every other second.Thoughts?

author: skylar
author_rank: Contributor
timestamp: May 18, 2020, 09:24:28 pm
content: A junkyard car radiator with 12V electric fan attached is probably cheaper, easier, and more effective to dump that sort of heat load than a CPU cooler ever will be. Also, most of your assumptions are on adiabatic compression which is valid for piston compressors, though a water jacket will change that slightly due to the Q!=0 of the walls.As for your temp calc, you ignored the volume change and your number is way off. My math shows your final volume to be 2.27% of initial and a final temperature of 1994 degF which is reasonable for adiabatic cycles. This illustrates why many 3kpsi systems are triple piston and run at low RPM, you don't want to hit the ignition temp of your lubricants (O2 compressors and tanks/valves even use a non-hydrocarbon grease due to the lower flash point of high O2 environment).I would suggest that for your high pressure needs you get a small electric 3kpsi system for <$500 on the bay and use a separate compressor for shop air. You will see much better service life for both systems and you won't have to worry about how to get 3000 psi air down to 120psi without freezing your regulators, etc.

author: coppercone2
author_rank: Super Contributor
timestamp: May 18, 2020, 09:47:52 pm
content: PV = NRT is correctDelta P is 3000V = Volume of tank, 18 LitersN = Number of moles as corresponding to amount of air you compress, value taken at 77 cubic feet of airR = Constant, 8.314 J/molT = Solve forConversions:77 cubic feet, 2180.4 liters18 liters to 0.018 cubic meters3000 PSI to 2.0684e+7 pascalSTP Mole of air = 22.4 L2180.4/22.4, 97.33 Moles of airFormula = PV=NRT2.0684*10^7*0.018=97.33*8.314*t372312=809.2*tt=460.09 KT final = 186.948 CThats what I got. It assumes a perfect system.I filled tanks to 4000+ PSI. What you do is fill it slow and it comes off kinda hot, you can add extra when it cools down so long you respect pressure rating if you fill too fast. Fill it in a blast proof chamber.1.225 grams per liter, * 2180 = 2670 grams of air. Rough because its a compressible gas.If you are interested in heat, then calculate how much energy it takes to raise 77 cubic feet of air by 160 C, that is how much will be removed from system when it cools down.You use a regulator to slowly fill it.

author: Picuino
author_rank: Super Contributor
timestamp: May 18, 2020, 09:50:07 pm
content: The maths are very simple.If you use a 1kW motor for the compressor, the power to be dissipated is 1kW.The non dissipating power will serve to heat the air inside the tank, which should be as little as possible.

author: wizard69
author_rank: Super Contributor
timestamp: May 19, 2020, 02:18:32 am
content: I can't answer your heat questions but ca offer a bit of practical advice. If the opportunity is there to turn up the air pressure somebody will do it when they shouldn't!!!Thus blowing out your 80 PSI air lines and damaging anything else pressure sensitive. Working in the automation field I've seen this happen multiple times, sometimes a qualified engineer on the knob. It isn't just air lines either but that is getting off track.Instead keep the high pressure systems separated from your low pressure systems. If you need more storage for low pressure air put a tank on the roof of your building,,

author: pipe2null
author_rank: Regular Contributor
timestamp: May 19, 2020, 11:22:06 pm
content: Thanks for the feedback!@Picuino and coppercone2: Thanks for the assists. I swear I used to know how to do this stuff. I'm not too old for this damnit, just a bit hazy on details occasionally. Hah@wizard69: Very good advice. I'm the only person who will have access, so I'm the only idiot who can potentially cause problems. Hehe. I joke, but I take the safety factor seriously.Unfortunately, I rent and do not have a garage, so outdoor or roof mounting is not an option.@skylar: I've been checking out Yong Hengs on ebay and amazon. They seem to have consistent "good unit for the cost" kind of reviews provided you can interpret bad Engrish. I'm specifically looking at the 2 stage with auto shutoff/preset. Seems like a feature that reaaallllly should probably be standard. Thanks for pointing out the valve freezing from decompression, I totally forgot that part. I like the car radiator idea, now to figure out how I might mount that in a window without looking like the beverly hillbillies.?: Any idea a decent company and/or search terms for pressure regulators capable of 4500psi down to 40-90, or at least down to the 200psi range? I've been digging through amazon and there are tons of things either at HPA pressure range for paintball and air rifles, or for lower conventional pressure ranges, but not really in-between at all, for understandable reasons. The closest I've found is made for nitrogen tanks and not really an in-line pressure regulator.I've looked at using separate air compressors for low and high pressures, and cost-wise it would probably break even or possibly be a little cheaper to keep them separate, depending on cost of the HPA to low pressure regulator (and keeping the regulator from freezing). The main reason I'm seriously looking at a single HPA compressor with step down to LP is the total space required. A pony tank of HPA is on par with a very large LP tank you normally would only see outdoors or installed in a garage. I rent a 2 bedroom apartment with the second bedroom a dedicated home lab and workroom. It is not large, and space is a premium even with the ridiculous floor to ceiling workbench/shelving unit I built. Right now I'm trying to figure out where I'm going to move my 3D printer to so I can finally setup a down cam/microscope to work with surface mount components and whatnot. A compact HPA setup would take a similar volume of space as a computer in a tower case and have similar air capacity as a large garage-installed LP system. This is also why I'm attempting a closed-loop water cooling system since using buckets of water indoors is a PIA and not something I want around my scope, power supplies, new bench DMM, computers, etc.Safety-wise, I'm intending on everything being rated for 4500psi but only using 3200-3500psi of the capacity to provide a decent safety margin. Obviously, initial pressurization and testing will be an outdoor task. I'm debating whether or not adding a big red emergency shutoff is prudent or if it's overkill, especially if used in conjunction with a normally open solenoid to slowly dump pressure in the event of shutoff (or power outage or compressor tripping circuit breaker, unfortunately). Putting in a new electric circuit is not an option, but there is already an existing unused 20A 240V circuit to the room for an unused electric baseboard heater, and I can add an outlet for that no problem. I was extremely disappointed to discover it's wired with 2 conductor plus ground, so I can't do a split phase, so only way to make it 120V is with an expensive transformer. If you know of a cheap option for 240V->120V with decent amps I would be very grateful.I might have no choice but to go with 2 separate compressors, but I need to figure out the HPA-only option before making a decision.

author: coppercone2
author_rank: Super Contributor
timestamp: May 20, 2020, 12:12:31 am
content: keep in mind the compressor is like, 2 big ass refrigerators next to each other and runs at 110dB

author: pipe2null
author_rank: Regular Contributor
timestamp: May 20, 2020, 12:44:54 am
content: The 4500psi compressors I'm looking at are around 350mm x 175mm x 360mm. They are loud, but somewhat comparable to conventional compressors, at least that's what the owners have said in reviews. It will certainly be louder than California Air Tools ~60dB compressors, but much quieter than a heavy metal concert.

author: coppercone2
author_rank: Super Contributor
timestamp: May 20, 2020, 12:54:16 am
content: will that air be considered clean and dry? I am thinking about it differently because I thought about it before to make plasma cutter air bottles, but it needs to be very pure to be better

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 20, 2020, 01:34:14 am
content: What about a booster to further pressurize 120PSI air?http://www.brianb.org/images/Scuba/Homemade/Gas%20Booster/gas_booster.htmQuote from: pipe2null on May 19, 2020, 11:22:06 pmDepending on your definition of "cheap", a Prius inverter available for about $90 can be easily modified into a general purpose power stage with a buck/boost converter and 2 3 phase inverter/rectifiers, good for at least 20kW continuous. But if you just want a dedicated 120V 20A circuit, just move one of the hots to the neutral bar. If it's going to need more than 20A at 120V, it would really be worth considering redesigning for 240V. (I once had to rewire an old test rack that had a 120V, 50A feed. Most of the equipment would also run on 240V, so most of the work was already done. I then added a 15A breaker to the PDU for the few devices that were 120V only. End result was that it would run on 240V 30A which was a lot more common in that place.)There is, however, value to using an inverter for compressors and pumps that have to deal with a wide range of pressures. The load on the motor is more or less proportional to the pressure, so at half the maximum pressure, only half of the available power can be used. With an inverter, the motor can be run at a higher speed to deliver more flow, making the most of the power available. (Ryobi recently started making pressure washers with that technology, the main benefit being that they could design it for a higher pressure without the flow at lower pressure settings becoming uselessly low.)

author: jmelson
author_rank: Super Contributor
timestamp: May 20, 2020, 01:49:47 am
content: This is just insane! The amount of energy required to compress air to 3000+ PSI is huge. The only reason people do this is to carry large amounts of air in small packages, ie. SCUBA tanks and Scott air packs. Compressing air to3000 PSI and then regulating down to 80 PSI is an enormous waste of energy. I'm not an expert in thermodynamics, but it seems like you compress 200:1 and then expand 32 X to get back to 80 PSI Gauge (95 absolute). So, you recover32 / 200 = 16% of the energy you spent compressing it. This is skipping over the adiabatic effects, which are going to be huge, so it actually is much worse than these numbers.Also, regulators can stick, leak or get debris holding the orifice open. This will cause an explosion in your low pressure system!Jon

author: jmelson
author_rank: Super Contributor
timestamp: May 20, 2020, 01:53:57 am
content: Quote from: pipe2null on May 19, 2020, 11:22:06 pmThis one is easy. Go to a scrap yard and ask for "step down transformers". These are used on all sorts of industrial equipment, and usually have internal straps or connections for combinations of 480:240 and or 240:120 conversion.They are usually scrapped for the copper, so they will sell them for about the copper value, by the pound.Jon

author: pipe2null
author_rank: Regular Contributor
timestamp: May 20, 2020, 07:05:25 am
content: @jmelson:If the incredibly electrically inefficient "Use HPA for everything" approach works out, it will save a significant amount of space in my cramped apartment. It's trading one efficiency for another. But I fully agree with you, the idea is at least a bit nutty. To the best of my knowledge, no one has ever accused me of being 100% sane...But jokes aside, I take the safety factor seriously.Quote from: jmelson on May 20, 2020, 01:49:47 amExcellent feedback, thank you! Would this be the correct/safe way to do it (all pipe/hose/fittings prior to the LP hose connection point all rated for max pressure):HP shutoff valve->HP to LP step down pressure regulator to 120-200ish psi-> LP over pressure relief valve -> HP-rated shutoff -> LP pressure regulator for LP adjustment (40psi vs 90psi tools) -> connection point for LP shop hose.I'm not sure if I'll need a small LP holding tank or not. The only reason for needing one that I can think of is to help keep the HP/LP step down from freezing by reducing short term spikes in LP air flow demand. If I end up needing one, I can make it from schedule 40 or 80 PVC pipe, with dimensions I can stash in the corner behind the closet door, literally.Thanks for the junk yard tip, I didn't know they had those types of things. The transformers I looked at were bran new on digikey, and very much not at all cheap to get normal household current capacity.@NiHaoMike:That booster is pretty cool, a bit more space than I have though. And I am very very curious about the Prius inverter, maybe not as a 120V solution, but as a multi channel multi phase AC power supply for my lab. Could it take un/rectified 240VAC direct? How bad is the THD? Where can I get tech and/or hacking info? Hehe... I think I need to attend a TEA meeting. But seriously, more info please!My issue with electrical power in my apartment is that decades ago they wired half the lights and outlets in the unit to a single 15A 120V circuit. Nearly all the other circuits in the breaker box are dedicated to specific things, like dishwasher, kitchen outlets, water heater, etc. My second bedroom/lab/workshop is not exactly wired to be a lab/workshop. Even if it can handle my addition of an air compressor, I keep adding equipment and it won't be long before I'm tripping the breaker on a regular basis. The property owner of my rental unit doesn't care about the improvements I make as long as they are up to building code and legally speaking do not require a building inspector visit. So, I cannot modify the breaker box, but i can put an outlet on the existing unused 20A 240V circuit in my lab. Good tip on the "flip the 110V->220V" switch on power supplies... That didn't even occur to me.What I consider a "cheap" solution for 120V: The price difference between the 120V and 220V HPA compressors I've been looking at is around $150 for the same thing at different voltages, the 220V costs more. So, a solution that let's me (optionally) stick with 120V equipment purchases that are easier to resell later is worth at least $150ish.@coppertone2:I would not use it to fill a scuba tank I intended to use for breathing without a minimum of 2 stages of filtering the output. I am not a health nut, I just don't like the taste of oil. Heh. Supposedly the Yong Heng brand is the better of the cheapo HPA compressors. Back in the day I got my tanks filled at a scuba shop that used cheap chinese "brand" compressors to save a few thousand dollars, I think it was Yong Heng or Tuxing, but I don't remember exactly. Anyway, they used 3 or 4 filtering stages between the compressor and the holding tank and their air was always good. So, the cheap compressor by itself probably not clean enough for your purposes without a few add-ons. Did you happen to come across any in-line pressure regulators that can drop 4500psi to 200ish psi when you were looking into HPA before?

author: coppercone2
author_rank: Super Contributor
timestamp: May 20, 2020, 02:05:00 pm
content: there are tons of regulators that go from high pressure to medium pressure, you might be used to seeing O2 and acetylene regs, but they easily have 0-300-500 PSI stuff for nitrogen and other gasses used in medium quantities, commonly used for chemical analysis systemsmy hydrogen tank output regulator was up to 300PSI iirc (way too high). Like it would be used for supplying forming gas to a huge kiln or something I think.you might also want to consider a restrictor to increase safety, that is just a resistive dropper. (choke the flow by orifice restriction, its a precisely drilled hole) Beware of things like the supply pressure effect when dealing with high pressure and regulators (research this). It's almost like inductive kick if you think about it when dealing with certain regulators.Keep in mind compressed gasses can be way more dangerous then hydraulics because they are compressible even if they run at the same pressures. The entire system pneumatics should be armored too, don't model after a SCBA pack because its designed for humans to do really crazy stuff in (its like at the limits of design of utility for firefighters dealing with insane situations).You need alot of steel plate IMO

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2020, 02:35:15 pm
content: Quote from: pipe2null on May 18, 2020, 02:08:07 amDo tell what for ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 20, 2020, 07:02:44 pm
content: Quote from: pipe2null on May 20, 2020, 07:05:25 amhttps://openinverter.org/forum/viewforum.php?f=14You can just connect two phases on one of the inverters to 240V, but unless you're planning to use it to backfeed power (as I will for my solar inverter project, carefully controlled by a FPGA and sensors) it's a waste of inverter phases. Just add a bridge rectifier in order to use the buck/boost converter as a PFC stage.QuoteCheck if the stove circuit has a neutral, which they often do. In that case, make some sort of adapter with circuit breakers for overload protection. Then run some good quality 12AWG extension cord(s) to your lab. You can either have a pass through to allow the stove to still be used (it will only pull near its rated power if all burners in use, which you can avoid doing) or buy an induction cooker.

author: max_torque
author_rank: Super Contributor
timestamp: May 20, 2020, 10:08:09 pm
content: It seems crazy expensive and crazy dangerous to have 4000 psi in your house, in an untested / certified system, in a domestic setting.What do you need the HPA for? and how much do you actually need? You can simply get an account with one of the usual HP gas providers and buy a HP cylinder when you need it, or, get your local scuba shop to fill small tanks. Given you can buy a fully certified 10l scuba tank for a couple of hundred $ and a typical fill is 5$ why would you mess around doing it yourself?I'd also say that "i'll test it outside" is in no way a suitable level of safety for testing a prototpe HP system operating with a compressabel fluid (Cylinder cert is done with un-compressabe fluid) You'll also have to check your local laws that set the requirement for HP cylinder visual and hydraulic testing (in the UK, it's visual every year, and Hydraulic every 5 years for steel tanks)

author: max_torque
author_rank: Super Contributor
timestamp: May 20, 2020, 10:11:39 pm
content: Oh and trying to use a HP compressor as a LP compressor is unlikely to work, simply because the HP compressor will have a tiny flow capacity. If you only use small amounts of LP air, then you can of course run that off a small HP cylinder though a suitable adjustable regulator. Even the large, powerful HP compressors used by commercial businesses dealing with compressed gases tend to be used to continuously trickle fill a large "bank" of HP storage cylinders, and those cylinders provide the higher flow rate to quickly fill the end-users cylinders.

author: pipe2null
author_rank: Regular Contributor
timestamp: May 21, 2020, 03:37:41 am
content: Quote from: tautech on May 20, 2020, 02:35:15 pmI was wondering when someone was going to ask that one. Heh.First of all, I take safety very serious, and I hope I've made that point clear to everyone who has posted here. Improperly set up HPA is extremely dangerous and can result in explosions, various types of missiles, and high velocity shrapnel. So, yes, I take related safety seriously. But like any motor vehicle, HPA is safe when properly and responsibly used. A crazy driver can get on the highway and cause massive property damage, injury, and death, similar to improperly set up HPA. The big difference is the danger involved with a crazy driver is very obvious, but the dangers involved with HPA are NOT obvious and require not only good sense, but also experience in identifying the danger points. When it comes to the safety of others, I am good at exercising good sense, but I lack sufficient experience working with HPA to design a guaranteed-safe system. Some required safety bits and pieces are obvious, others are not.Some posters on this thread have shared some of their valuable experience, and I greatly appreciate that (and so do my neighbors). Please be assured I am applying that good advice to make sure all required safety mechanisms are in place and tested as much as feasibly possibly long before even plugging in a HP compressor.To answer the actual question: Well, I have several uses for high pressure, and that is assuming I do not get back into SCUBA any time soon. I still have a closet full of underwater gear that I should probably either use or get rid of... Spring cleaning for another day. But I digress. I've been trying to get my nephews interested in building a robot with me, since they speak some new fangled language my older ears do not understand, figured robotics might be a good rosetta stone so to speak. I'm not going to give them any HP components, but I have some ideas I want to try out adding pneumatics to a mobile platform. I also have a few friends that are into guns and go to an outdoor range pretty often. I'm not really into the whole firearms thing, but picking up a high powered air rifle to "me too" without compromising on the "fire"arm part would be good I'm thinking. The main reason for high pressure, and my immediate need for air compressors in general is adding pneumatic part development capabilities to my lab. I'm currently working on several projects for 3DP pneumatic parts and I need several different pneumatic power rails (aka low pressure manifold with several low pressure regulators putting out different pressures). Plastic has been used to convey pressurized material for a long time. Plain old PVC can safely contain a substantial amount of pressure, with >1000psi rupture point. The single biggest problem (due to safety) with 3DP pneumatic parts is the unpredictability of the rupture point, and where in your design the rupture will occur. The second biggest problem with 3DP for pneumatics is very poor dimensional accuracy and repeatability, but much of this can be overcome using o rings, normal metal springs, and other non-printed bits and pieces. It is not a small challenge to come up withpractical3DP pneumatic designs. But being a responsible designer requires significant testing, including destructive testing. 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.To put my HPA "system" in perspective, it's basically a small 350mm x 175mm x 360mm HP compressor, a pony sized HP tank, plus all the necessary bits and pieces for safety. Compressor and tank will be located directly next to each other, no long haul HP piping. No matter what, I will still need an adjustable regulator for HP to LP so destructive tests can start at very low pressure and brought up slowly until DUT rupture. A pony tank should have enough capacity for a single destructive test, but if also using "system" for LP, will probably use a normal sized scuba tank, but in no scenario will I have larger than a 120 cuft scuba tank (like I used to own).As far as trying to use an HPA system also for LP: The thought is simply that once I have asmallHPA system designed and SAFELY built, not much else is needed to also use it for LP. Since my home is space constrained, minimizing square footage by using a single HP compressor with HP tank of conservative size instead of both an HP and separate LP compressor with comparatively very large LP tank would be a very good thing, although not a 100% requirement (more like 95% required).It is extremely inefficient to use HP for LP, but no reason you can't do it. The tricky bit here is not how to use HPA for everything, but how to design and construct the HPA with all safety concerns addressed, with or without the LP step down. I owned and used HP scuba tanks before, even had them turned and certified for nitrox, but setting up that same tank for continuous use and connection plus the compressor (for attended use only), has significantly different safety needs that I am not sufficiently familiar with yet.

author: tautech
author_rank: Super Contributor
timestamp: May 21, 2020, 06:05:15 am
content: Thanks for the lecture but you're a braver man than I.Played with HP hydraulics, diesel injectors and air compressors most of my life and even built a couple but 3k PSI air I won't go there....shit it rhymes !

author: IanB
author_rank: Super Contributor
timestamp: May 21, 2020, 06:35:11 am
content: Quote from: pipe2null on May 18, 2020, 02:08:07 amJust 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...

author: IanB
author_rank: Super Contributor
timestamp: May 21, 2020, 06:43:53 am
content: Quote from: coppercone2 on May 18, 2020, 09:47:52 pmNot 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).

author: pipe2null
author_rank: Regular Contributor
timestamp: May 21, 2020, 09:50:42 am
content: @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-QB01Power Rating : 1.8KWInflating Speed : 2800R/MinNoise : MAX 85DBAir Flow Rate : 40-50L/MIN; 1.5-1.8CFMWorking Pressure : 100-300BAR; 1500-4500PSICooling System : Water CoolingCompression Stage : Two StageLubrication Mode : Splash TypeMaterial of Cover : Cast AluminumShout down : Manual stop/automatic stop(for optional)Pressure indicate : GaugeFiltration : Water/oil separatorLubricating oil : ISO VG46 or AW 46Air hose connection : 8mm quick connect fittingUsing the calculator:https://www.engineeringtoolbox.com/horsepower-compressed-air-d_1363.htmlWith 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 830WI'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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 21, 2020, 01:36:26 pm
content: Quote from: pipe2null on May 21, 2020, 03:37:41 amMain 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.QuoteHave 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.

author: max_torque
author_rank: Super Contributor
timestamp: May 21, 2020, 05:17:22 pm
content: 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!

author: max_torque
author_rank: Super Contributor
timestamp: May 21, 2020, 05:22:44 pm
content: 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)

author: pipe2null
author_rank: Regular Contributor
timestamp: May 21, 2020, 08:50:05 pm
content: Quote from: NiHaoMike on May 21, 2020, 01:36:26 pmQuote from: max_torque on May 21, 2020, 05:17:22 pmThis makes a helluva lot of sense, and is the direction I will go for destructive tests.Quote from: max_torque on May 21, 2020, 05:22:44 pmYes 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.

author: skylar
author_rank: Contributor
timestamp: May 26, 2020, 05:05:04 pm
content: 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.Quote from: IanB on May 21, 2020, 06:43:53 amThis 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 thechangein 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)andthe air compression heat.Quote from: max_torque on May 21, 2020, 05:17:22 pmSeconded, 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.

author: tautech
author_rank: Super Contributor
timestamp: May 26, 2020, 08:06:46 pm
content: Quote from: skylar on May 26, 2020, 05:05:04 pmActually 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.

author: skylar
author_rank: Contributor
timestamp: May 26, 2020, 09:30:39 pm
content: Quote from: tautech on May 26, 2020, 08:06:46 pmFair, I was thinking about your cheap MIG/MAG/TIG/etc. regs that only have flow adjustments, no PSI adjust.

author: coppercone2
author_rank: Super Contributor
timestamp: May 26, 2020, 10:19:44 pm
content: 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 anywayhttps://www.harrisproductsgroup.com/en/Expert-Advice/tech-tips/controlling-gas-flow.aspxNot 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.

author: skylar
author_rank: Contributor
timestamp: May 26, 2020, 11:41:00 pm
content: Quote from: coppercone2 on May 26, 2020, 10:19:44 pmFlow 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.