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  #31 (permalink)  
Old 09-01-2008, 07:13 AM
F&J F&J is offline
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There is one basic statement lacking in this discussion. My comparison of the welding gas bottles needs explanation.


The storage of very high welding gas bottles is all about "duty cycle"; meaning we store those ultra high pressues so we don't need to swap bottles every time we weld. Same is true with the "higher than needed" pressures put out by two stage compressors...it keeps the compressor in it's duty cycle, and the compressor stays "off" for longer periods.


That's the best I can do
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Old 09-01-2008, 07:23 AM
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OldRed

Geesh, don't go and get your hackles up over a discussion!
I posted no bs, all hard facts.


You ask what turbines and rotaries have to do with the single vs 2 stage discussion? Just more facts on compressors and what is better. did it relate to the original discussion -NO, but either does your support of a 2 stage air compressor when the OP asked "Respectable paint on a 120v compressor".

You toss out 30 years of experience but do not substantiate it..

I toss out my 30 years of experience and give a small background with some brief info on OTHER types of compressors I have worked on and you call bs and say it is gibberish.
That is just a half baked way of calling me a liar.
Your true colors have shown Red, and they aren't to pretty. If someone does not agree with you - you resort to name calling , a very mature way to participate in a debate-NOT.

To all the other users here, I apologize for jacking this thread. I only wished to answer the question the OP asked about single/2stage.
I hope that the info presented by all parties helps others learn a bit about air compressors.


btw, the answer to the OP's original question was given many posts back.
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Old 09-01-2008, 08:00 AM
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Quote:
Originally Posted by F&J
One very imortant, yet short paragraph, was extremely informative to the basic home hobbiest, in my opinion:



In his other post, he explained that the 2 stage units produce much higher PSIs even though that max pressure is not needed to run tools. Put the two statements together, and you bascially can compare the concept of "storing" these higher than needed pressures to the "compact" storage of high pressure gasses that we use; oxy, acey, co/argon.

Like it or not, the home user nearly always runs a single stage for a variety of reasons, and they will get the job done.

Yes, there was a lot of info posted on high end, large manufacturing plant type compressors that did not apply to the home user. However, I just feel that the entire post should not have been condemned for that reason.


Like it or not? No one said anything about not using a single stage and in fact due to the lower cost quite often they are the most logical choice when air demands are not that high.

The problem here is in saying that buying a two stage compressor amounts only to the same as buying extra tank storage when in fact that has little to do with the main purpose for the two stage pump, how would that explain the two stage compressors that have a cut-out pressure only slightly higher than the single stage? The main purpose for the two stage pump is to maximize the use of the available power through a more efficient system of compression. By the logic that was presented two comparable compressors, one single stage and one two stage, would start out empty and pump at the same pressure and volume until reaching the cut-off pressure of the single stage at which point the two stage would continue to compress air until it reached a much higher pressure just so that it could store more air. It simply does not work that way, the motor to pump drive ratio on a single stage compressor is arrived at by determining the most efficient cut off pressure and then selecting the proper ratio that would have the motor nearing it's maximum torque at that pressure. Just as with the one speed transmission comparison a compromise must be made as to the most efficient volume that can be produced at low pressure while at the same time not overloading the motor at the desired cut out pressure. With the more efficient two stage design the available power can be utilized to provide a higher volume at the lower pressures while at the same time allowing the motor to drive the pump to a higher pressure before nearing it's maximum torque, thus providing both higher volume and pressure. Some compressors, my Quincy is an example as is some Eatons, choose on some models to develop even more volume at the lower pressures thus providing a higher CFM in that range and a faster recharge time but at the expense of less maximum pressure and storage. Telling someone who may need a lot of air that the only difference between the two types is just in the storage capacity is just plain bad advice because there is a heck of a lot more involved than just storage.

I guess I did get a bit ticked off but trying to prove his point by portraying himself as an authority by using a lot of engineering drivel that explained little in the way of what we were discussing and then posting pictures of mega-dollar industrial units lifted from compressor web sites as an example of his expertise did not sit well with me. Heck I could sit and rattle off that garbage all day too as could several others here who are probably better versed on the subject than either of us and the pics I have are of units that I had installed and/or built not just something lifted from the net. But why do that? Is anyone here really interested in looking at a screw compressor mounted on a mining shovel or dragline? How about trying to get the most CFM out of a hydraulic driven compressor on a mine mechanics service truck? No one is interested in that and I am sure if I had of posted those pics and started rattling off specs on this and that then someone would have called me on it too, and rightfully so. Heck I have been called down several times for getting a little (well more than a little) carried away with talking about welding experiences.
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Old 09-01-2008, 08:15 AM
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Quote:
Originally Posted by Old Fool

did it relate to the original discussion -NO, but either does your support of a 2 stage air compressor when the OP asked "Respectable paint on a 120v compressor".

The question WAS directly asked about the differences between the two types of compressor.



What I called BS was not your info, and I said the engineering data was correct just that it did not make the point, it was the attempt to make your point by impression that I took exception too.
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Old 09-01-2008, 10:38 AM
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You wouldn't think this would be such a confusing subject.

Willys36 has a good explanation of 1 stage vs 2 stage on this thread:

Compressor - Single vs. Dual Stage

However, I'm not sure it's accurate, because he says the following:

For a given horse power rating a single stage compressor should put out incrementally more air @ 90psig than a two stage since the former uses all the cylinders for the rated volume and the latter only used the first stage cylinder for that and the second cylinder and related horsepower to increase pressure at the expense of less low-pressure volume.

That 18.1cfm IR unit is on it's way to me from Northern Tools and I would love to resolve the debate about it once an for all. Is there any way to measure flow with tools I have lying around in the garage?
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Old 09-01-2008, 01:52 PM
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Quote:
Originally Posted by wyomingclimber
For a given horse power rating a single stage compressor should put out incrementally more air @ 90psig than a two stage since the former uses all the cylinders for the rated volume and the latter only used the first stage cylinder for that and the second cylinder and related horsepower to increase pressure at the expense of less low-pressure volume.

In most cases (not sure if that is what he means here or not) when someone tries to compare a two stage vs a single stage they do so with the mistaken assumption that given the same HP both the two stage and single stage would have the same displacement and RPM. This is almost never the case so it becomes next to impossible to compare them in this manner, besides it truly is a case of comparing "Apples and Oranges" because of the major differences in design. Because the two stage is a more efficient design the same amount of power available can be used to drive a larger displacement pump or maybe run a similar displacement pump at a higher RPM, either would accomplish the goal of increasing air flow but it pretty much negates a direct comparison of the two stage vs single stage from a displacement standpoint.


I have said more than once that I am skeptical of that IR unit's CFM claim however I also said that I have never checked one and this assumption is just based on my past experiences. Even if I am right about that thing , and that is IF, that in no way means that outfit would be low in CFM. It most likely would rate right up there with best single stage units in that power range, indeed it should be one of the best! That is a very robust pump and certainly should be capable of producing that much air flow with the right motor but I personally just don't think they can do it with only 5 HP. Regardless that thing should put out a LOT of air and there is no way one of the 6 or 7 "peak" HP outfits that are so common would come even close to doing what it will do.
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Old 09-01-2008, 04:35 PM
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Quote:
Originally Posted by wyomingclimber

Is there any way to measure flow with tools I have lying around in the garage?
Yes there is. This simple test will tell the truth:
article written by Richard J. Kinch

Evaluating True Horsepower and CFM Ratings of Air Compressors

Don't Believe the "Peak Power" Advertising!


Air compressors for the home or small shop have been advertised and sold with ridiculously inflated horsepower ratings. This was especially true before the year 2004, when the US government said it would enforce more honest ratings (see the end of this essay), and it seems to be returning to the marketplace after a year or two of spotty compliance. The specs and stickers on the unit likely do not tell the truth, and add confusion instead of critical information to buying decisions. But you do not need a testing laboratory to calculate true horsepower or CFM delivered. I will explain below how to estimate these ratings from pressure readings and elapsed time measurements.

The way to measure true power is to measure the time it takes to pump the reservoir tank of known volume from a known starting pressure to a known ending pressure. Then you can figure the true CFM from the difference in starting and final pressures, times the volume of the tank, divided by the time it took to pump up. You can also time the pump-up cycle from the cut-in to the cut-out pressure, since that's how one usually runs a compressor. These true performance measurements are impossible to fake.

Example


--------------------------------------------------------------------------------
My Chinese-import compressor bears a big yellow sticker claiming the unit delivers 6.5 HP, and 10 CFM at 90 psi. Let's see what it really delivers.
My compressor says it has a 25 gallon tank, and I confirmed that with some rough measurements and volume calculations. If I trigger a refill cycle by bleeding out air slowly with the relief valve, I observe on the tank gauge (not the downstream gauge) that the compressor "cuts in" at 85 psig and "cuts out" again at 102 psig, a difference of 17 psi. It cranks for 35 seconds to build up that pressure.

Divide the tank volume in gallons by 7.48 (1 cu-ft = 7.48 gallons) to get the tank volume in cubic feet. Thus the tank volume is 25 gallons / 7.48 gal/cu-ft = 3.34 cubic feet.

In units of atmospheres of pressure, since 1 atm = 14.7 psi, the 17 psi of pressure added during the cycle is 17/14.7 = 1.16 atm of pressure during the cycle.

When a compressor pumps one "CFM" (cubic foot per minute), that means the intake port inhaled one cubic foot of "free air" (air at atmospheric pressure). (Note: A CFM does not mean in any sense the compressed volume.) So the unit really measures the mass of air flowing per minute, not volume. Some people labor under a stubborn misundertanding that these units refer to the flow of compressed volume (as opposed to free air volume), but this is flatly wrong.

Thus in one cycle, the rate at which air is being pumped into my tank, is the pressure rise times the volume of the tank, or 3.34 cubic feet * 1.16 atm = 3.87 cubic feet per 35 seconds. To proportion the 35 seconds up to minutes, to get the pumped volume per minute, multiply by 60/35, or 3.6 * 60/35 = 6.6 CFM (at 85 psig).

The error range in our estimate is perhaps about 30 percent (the true value might perhaps be as much as 8 CFM or as little as 5 CFM). Certainly this is not performing at 6.5 HP like the advertising sticker says, or the 10 CFM on the data plate. I was hoping for better, especially since it is wired for 240 VAC.


--------------------------------------------------------------------------------

Now you know why the data plates on the electric motors have blank boxes for horespower ratings. A true power rating from the motor manufacturer would expose the lie of the advertised compressor power.

Tip: Any motorized device that takes power from a 120 VAC outlet, surely delivers less than about 2 HP, and likely far less. Why? Standard AC cords are limited to 15 amps of current, or about 1800 watts. At 746 watts/horsepower, and considering efficiency losses, 2 HP is all you can get, and even then the starting currents might be tripping circuit breakers.

Tip: CFM ratings are meaningless without an associated delivery pressure. I have a 600 CFM compressor in my garage that uses only 1/3 HP! (It's a fan delivering 0.1 psi.)

Rules of thumb:

A good compressor, per true HP, will deliver about 4 true CFM at 100 psig.
The tank should be sized to be at least 1 gallon of volume per CFM of the compressor.
Uncooled compressed air is hot, as much as 250 to 350 deg F!
Many tools require more CFM at 90 psi than what is physically possible to get from the power available through a 120 VAC outlet. If you don't observe this physical reality, then either your tool won't run right, or you won't be able to run it at a decent duty cycle.
Beware also, that the CFM figure given as the required air power on many tools (e.g., air chisels/hammers, sandblasters) is for an absurdly low duty cycle. You just can't run these constantly on anything but a monster compressor, but the manufacturer still wants you to believe you can, so you will buy the tool.
Assumptions: We have assumed a single-stage compressor, which is to say, just about anything small or portable; two- and three-stage compressors are somewhat more efficient, and will yield better results, but only become economical in larger sizes. Our proportioning calculations are based on the ideal gas law PV = nRT with isothermal compression (pressure and volume of the compressed air are changed, but the temperature is not, which is the case for cooled compressed air). This method does not account for ambient humidity condensing in the tank, for different ambient pressures, or for heating/cooling of the air; these are relatively minor but not necessarily insignificant factors.

Reference: Machinery's Handbook (26th edition, see http://www.industrialpress.com/mh.htm) has an excellent section on analyzing compressed-air systems, including formulas and tables on the horsepower required to compress air, and losses in pipes and hoses. Marks' Standard Handbook for Mechanical Engineers describes the thermodynamics of expansion and compression of air in the section on "General Principles of Thermodynamics", subsection "Special Changes of State for Ideal Gases", item 5 "Polytropic"; and practical compressor technology in the "Pumps and Compressors" chapter.

Other Terms: An "SCFM" (standard cubic foot per minute) is a CFM produced with input air at 68 deg F, 36 percent RH, and 14.7 psia pressure (the mere letters "SCFM" refer to no official standard, and while various temperature and RH values are in use, these are the most commonly accepted values). "Displacment CFM" is the rate of volume displaced by a reciprocating piston compressor, which is compared to the delivered CFM to evaluate volumetric efficiency. "Peak horsepower" typically means the electrical power drawn by the motor at the instant of starting; this figure is a meaningless specification because it says next to nothing about the sustainable horsepower delivered by the system. "Peak horsepower" most definitely does not mean anything like "what you get if you run this unit full throttle", "what the motor can deliver for short periods of time", or "what the motor can do if heavily loaded". Also, rated CFM at "90 psi" can really mean the inflated value measured from the CFM input during a pump-up from 0 to 90 psi. Such trickery is what you get in the absence of well-defined engineering testing standards and methods, which is to say, "consumer" mentality. This applies to larger systems like the 5 HP 80-gallon units common in auto repair shops, just as well as the homeowner models.

Wet Air: As much as you might want to believe that your carefully-chosen compressor is providing a useful supply of compressed air power, the fact is that without an expensive drying unit, your compressed air is of very limited application. This is because the "air" being sucked into your compressor does not consist of just compressible gases like nitrogen and oxygen, but lots of water vapor. In all but the very driest climates, the compression from atmospheric pressure (14.7 psia at sea level) to the total tank pressure (typically, 100 psig = 115 psia, or so) compresses the atmospheric water vapor to the condensation point inside the compressed air tank and delivery system. This is why you have liquid water collecting in the tank, which must be periodically drained. But the water does not stop at the tank. While you can strip any liquid water out of the air line with an inexpensive coalescing filter, the water vapor remains everywhere in the flow of compressed air at close to the condensation point (100 percent "relative" humidity), and will condense into liquid with the slightest cooling downline from the initial heated temperature. This is why you can have liquid water sputtering from your air lines into your tools, tires, paint spray, etc., even after a so-called filter/dryer; hot, steamy air fills the air line even after the filter, and if allowed time to cool, the humidity condenses into liquid water. While this water-contaminated air spoils the lubrication in your air-powered tools, robs power, throws your tires off balance, etc., it positively ruins the operation of tasks like painting and sandblasting that cannot contaminate the work with specks of liquid water or water vapor. You do not just have compressed air, you literally have compressed steam as well. (Tell your friends your shop is steam-powered!)

Dry Air: The only solution to wet air is a refrigerated air dryer. This is a unit that costs more than many small compressors. It is essentially a small air conditioner that chills the air stream running through a coiled tube, thereby condensing almost all the water vapor to liquid, which liquid is then separated and drained out of the unit. Such a unit is all that is needed for most dry air applications, yielding a relative humidity of about 10 percent in the output air, literally drier than a desert. For demanding applications requiring even less humidity, dessicant filters with a complex system to regenerate themselves with heat are used, typically after a refrigerated dryer, yielding a truly near-zero humidity. The grim thermodynamic reality is that there is no easy or cheap way on a small scale to purify air from contaminating water vapor. Water is a problematic contaminant in compressed air, and wet air is greatly inferior to dried air. Think of it as another thing the Sears salesman didn't tell you about the compressor's performance, namely, that without further expensive treatment, it produces water-contaminated output.

Caveats: Making estimates with the pump-up timing method requires trustworthy measurements. Pressure gauges are often way off calibration. Confirm the specified tank volume by measuring the geometry instead of just accepting the specified value (watch out for imported units that have had design changes without updated documentation). Measure the elapsed time carefully over several cycles. Measuring delivered air power also requires that you consider the resistance and losses involved in the regulator(s) and hose(s) between the tool and the compressor; these can rob significant amounts of power.

The Manufacturers Repent (or Did They?): In early 2004, consumers and the government, organized under a class-action lawsuit, attempted to force several major manufacturers of air compressors to stop advertising inflated values for compressor horsepower. The lawsuit alleged that "the companies knowingly labeled, promoted and sold consumer air compressors with electric motors as having higher horsepower motors than they actually contained." The settlement requires manufacturers to measure horsepower based on the continuous power output of the electric motor shaft, or continuous power input to the compressor shaft. Advertising based on "peak power", "max developed power", "max kinetic power", or "breakdown torque", is no longer to be used. Manufacturers agreeing to this settlement include Campbell Hausfeld, DeVilbiss, Ingersoll-Rand Co., and Coleman Powermate, Inc. While the usual boilerplate in the court settlement absolves them of any illegal actions, these firms implicitly admit that their behavior was deceptive and uneconomic. See http://www.aircompressorsettlement.com/ [broken link now, I guess they gave up on this].

In the years since this settlement, however, one sees just as much advertising and labeling of inflated compressor power as ever. The awards to consumers from the class-action lawsuit consisted of nothing more than discounts for more mislabeled equipment from the deceptive advertisers!

Let us be generous and think of the whole affair this way: perhaps none of those manufacturers wanted to be inflating horsepower ratings, but once specifications started being inflated (however it started), they all had to do so as a matter of marketing self-defense. It took a consumer lawsuit to get them to all agree to return to the most elementary rules of honesty and fairness. The horsepower output of a machine is as certain and standardized as the weight of a bag of apples at the produce stand. Honest weights and measures are as important to prosperity of the air compressor business as any other.

Links: See Kevin Brady's essay The Numbers Game: A Primer on Electric Motor Horsepower Ratings, [5-page, 229 KB PDF file] which performs a similar critical analysis on electric motors in general.


Have a comment or question about my air compressor page?
Need to gloat (or moan) about your unit's performance?
Email me at:
kinch@truetex.com
Richard J. Kinch
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Old 09-01-2008, 05:17 PM
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I'm gonna do it and post back.

I'm a little concerned about his pressure number being wrong, though. Wouldn't his example yield flow numbers for 93.5psi (i.e. the average of the cut in and cut out psi?)

Also, he wrote 3.6 * 60/35 when he seems to have meant 3.87 * 60/35.

Oh, and to answer the question that just popped into your mind: Yes, I drive my wife nuts
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Old 09-01-2008, 05:32 PM
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I did an unscientific CFM test on a 1922 IR 4 cyl gas powered 2 cyl "jack hammer-trailer type unit" (pre-internet days)


I think I found a sandblast catalog with nozzle sizes and a chart to show what cfm was needed to maintain any certain pressures. It gave various pressures and all the different sizes of nozzles. I ended up with a guesstimate of 38 cfm @ the 80 lbs that I was blasting heavier sheetmetal with.

Not accurate, I am sure, but there were no specs on this old unit.
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Old 09-01-2008, 05:54 PM
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Quote:
Originally Posted by wyomingclimber
I'm gonna do it and post back.

I'm a little concerned about his pressure number being wrong, though. Wouldn't his example yield flow numbers for 93.5psi (i.e. the average of the cut in and cut out psi?)

Also, he wrote 3.6 * 60/35 when he seems to have meant 3.87 * 60/35.

Oh, and to answer the question that just popped into your mind: Yes, I drive my wife nuts
Yes I agree he has a typo of 3.6 instead of a rounded 3.8. Yes I would assume you could also say his yield was at a meana average pressure of 93.5psi.

I really doubt that there is a +/-30% error factor. - maybe, + no.
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Old 09-01-2008, 07:37 PM
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Ok:

- Yes, you can paint a car with a small 110v compressor, I did a few cars with my dadís old 1.5hp/10gallon craftsman compressor that he and my godfather have been painting cars with since the late 50ís or so.
- What you want is a combination of compressor and gun that can make it from one end to the other of whatever youíre painting without having to stop for the compressor to catch up. You donít want to ever paint into a dry edge so the faster/less waiting you do the better. The reason that HVLP is not good for this is that it basically uses a very large volume of air to move the paint, where a traditional gun uses pressure, and within reasonable limits, itís easier to build high pressure at low flow numbers, then any kind of pressure at high volumes/flow numbers. Any decent old school conventional gun will do it fine. OTOH, if you were painting something small like a single panel, then something like an HVLP detail gun will work great.
- 10scfm @ 90psi is 10scfm @ 90 psi, it doesnít matter if itís coming from a single or 2 stage compressor. If thatís what both will deliver then thatís what youíll get out of them. At that point the only real practical difference most of us will see is that the 2 stage will typically fill the tank to 175 psi vs something closer to 100psi for the single stage, and pressure ratio roughly equals density ratio, so your 60 gallon tank filled with 175psi is holding 75% more air molicules at that higher pressure. If you use the air from both regulated down to something below the pressure stored in the tank, youíll be able to do 75% more work before that tank needs to be recharged. Doesnít make any difference once you get to the point that the compressor is running constantly to keep up it doesnít make any difference if you have a single stage or dual stage if they both move the same amount of air at the same pressure.

That being said, there _might_ be an efficiency advantage with the 2 stage compressor. When youíre talking about compressor efficiency you can talk about volumetric efficiency (how much air it moves in one pumping stroke or over a set period of time running at a specific rpm) and there is adiabatic efficiency (how much extra heat is added to the air that is pumped). Volumetric efficiency is pretty much a curve of sorts (at lower pressures it tends to look like a flat line), as you turn the compressor faster within itís limits the more air you move. Adiabatic efficiency looks like an island graph. There is a point where it will be turning and moving air with the best efficiency, adding the least amount of heat for the amount of air itís moving. Faster or slower and it is less efficient.

Since you loose whatever percent efficiency at every compressor step (the efficiency numbers multiply, so if you have 85% efficiency at each stage the total will be 72% efficiency), itís fairly easy to end up with a 2 stage setup that is less efficient, but OTOH, if you can run both stages in their optimum efficiency island then you will likely have a total efficiency better than a single stage running at the edge of itís usable range.
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Old 09-05-2008, 03:14 PM
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Well, I got my IR SS5L5 up and running. Here's the skinny:

Time to fill a 60 gallon tank to 90psi: 2:43
CFM @90 using Kinch's calculation: 16.3
Range based on Kinch's stated margin of error: 11.4-21.2cfm@90

Running a 9/64 nozzle in my blaster with no media in it and taping the valve open, the tank pressure stabilizes at 83psi with the compressor running constant.

FYI, the 14.2cfm@90psi rated Harbor Freight unit I returned took about 5:30 to fill a 53 gallon tank.

I'm almost looking forward blasting the rest of my truck
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Old 09-06-2008, 06:00 PM
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Quote:
Originally Posted by wyomingclimber
Well, I got my IR SS5L5 up and running. Here's the skinny:

Time to fill a 60 gallon tank to 90psi: 2:43
CFM @90 using Kinch's calculation: 16.3
Range based on Kinch's stated margin of error: 11.4-21.2cfm@90

Running a 9/64 nozzle in my blaster with no media in it and taping the valve open, the tank pressure stabilizes at 83psi with the compressor running constant.
Here is a nice flow chart, 80 psi with an 1/8 orfice is around 23scfm.


The Ingersoll Rand rating of 18 @ 90 is a fair assessment.
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Old 06-10-2010, 02:01 AM
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The only solution to wet air is a refrigerated air dryer. This is a unit that costs more than many small compressors. It is essentially a small air conditioner that chills the air stream running through a coiled tube, thereby condensing almost all the water vapor to liquid, which liquid is then separated and drained out of the unit. Such a unit is all that is needed for most dry air applications, yielding a relative humidity of about 10 percent in the output air, literally drier than a desert. For demanding applications requiring even less humidity, dessicant filters with a complex system to regenerate themselves with heat are used, typically after a refrigerated dryer, yielding a truly near-zero humidity. The grim thermodynamic reality is that there is no easy or cheap way on a small scale to purify air from contaminating water vapor. Water is a problematic contaminant in compressed air, and wet air is greatly inferior to dried air. Think of it as another thing the Sears salesman didn't tell you about the compressor's performance, namely, that without further expensive treatment, it produces water-contaminated output.
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