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Old 02-16-2006, 10:02 PM
cab cab is offline
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Compressor - Single vs. Dual Stage

All,

Ok, I know, I know - another stupid compressor question, but my question is a stage vs. cfm vs. $$$ question. I am looking at a three different compressors. Here are the specs (note: prices include shipping/tax):

Lowes Kobalt 2-stage, 80 gallon, 5 horse (22 amps)
15.2cfm @ 175, 17.1cfm @ 90, 100% duty, 3450rpm - $963

Ingersoll-Rand Single Stage, 60 gallon, 5 horse (23 amp)
15.5 cfm@135, 18.1 cfm @90, 100% duty - $866

Ingersoll-Rand 2-stage, 60 gallon, 5HP Type-30
14.3cfm @ 175, 100% duty - $1245

As you can see the SINGLE stage IR unit actually produces as much (if not more) actual CFM than the to TWO stage units. The Lowes unit seems like a great deal, but I notice the compressor RPM is 3450. A friend of mine has the 7.5HP IR Type 30 unit and it runs that unit at a paltry 1500RPM - it is relatively quiet.
In any case, what is the REAL BENEFIT TO ONE OF THE TWO-STAGE UNITS as opposed to the IR single stage given that CFM is actually as good or better on the single stage?
I THINK the IR 2-stage type 30 might be quieter than the Kobalt IF it runs at 1500 rpm like its 7.5HP big brother Type-30 (anyone have that and can confirm). If the 2 -stage Kobalt is running at 3450 rpm I find it hard to believe it will be any quieter than the IR single stage (assuming noise is directly tied to RPM).

Anyone?

Thanks,

Chris

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Old 02-17-2006, 09:08 AM
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In normal compressor fashion, there is no consistency to the marketing to try to confuse the consumer! The cfm's listed for the three compressors are not all the same. Lowes is 15.4 cfm @ 175 psi, IR SS is 15.5 @ 135 psi and 18.1 @ 90 psi, and the 2 stage IR is 14.3 @ 175 psi. If you truly want to compare apples to apples you need to find out how much cfm @ 90 psi for both 2 stage compressors. I think that you will find that either of the 2 stage compressors will actually have higher cfm ratings @ 90 psi than the single stage. Slower speed means quieter, less moisture, and potentially, more longevity for the unit. I would have a better comfort level with the 2 stage IR than the others, even though the info says that the Kobalt has a higher cfm at 175 psi, but that's just my opinion. Do a search, there are other brands in those price ranges, and a lot of info on this board concerning them. Goiod luck.
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Old 02-17-2006, 09:45 AM
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OK need a little more info here. That single stage IR at 18.1 CFM@ 90 PSI just don't sound right, and I would be highly suspicious of that rating. I have seen what I believe to be that unit at both Sears and Northern tool and it is simply a common pump found on a lot of lower end compressor models driven at a slightly higher RPM by a 5 HP motor but I find that CFM rating VERY hard to believe! Also that 3450 on the Kobalt is the motor RPM not the pump RPM which will be substantially less. What are the model numbers of the units you are looking at? A single stage pump is simply not in the same class with a two stage unit which is far more efficient than a single stage and thus will produce more air while running cooler than a single stage running on the same power. I also agree with Toy about the two stage IR, this is a very popular unit and for a good reason, but beware of that single stage IR since something about those numbers just plain stinks!
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Old 02-17-2006, 04:40 PM
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All,

The Kobalt unit isn't on the Lowes website, and I can't find consistent psi numbers anywhere (on this site or others):

Here is the IR Single Stage (note: this is 11.5HP peak, and 5hp "true" -not at all the typical cheapie 5 HP single stage you see at Sears, etc.)
IR Single Stage 5 hp

Here is the IR Two-Stage type 30:
IR 2 stage 5 HP

From what I have read, the IR single stage is legitimately producing some good PSI numbers, but it will be louder, run at a higher rpm, and potentially die sooner. Keep in mind, this will be in a home shop - not a business.

Thanks for pointing out the discrepancy on motor rpm vs. pump rpm - really wish I could get apples to apples on these things. Interestingly, my friend's Type 30 7.5HP unit has the exact SAME CFM rating at 175 psi, 90 psi, and 40 psi according to the label on the side of the tank (i.e. 24CFM).

Thanks,

Chris
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Old 02-17-2006, 05:23 PM
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one thing i like on my 2-stage compressor, if you have a line regulator running at 100 psi for you air tools, the compressor will kick on around 145 psi. You never get a lack of air to your tools waiting for compressor to kick on. Most single stage kick on around 90 psi. I have a campbell hosfeld 2 stage, don't recall exact specs..it was around $1400 from TSC, about 9-10 years ago. never been a problem. it get used everyday.
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Old 02-17-2006, 06:09 PM
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That 5 HP single stage IR is the one I have seen before and I am highly skeptical of those numbers. I will not accuse them of out and out lying let's just say I think they are being very reckless with the truth. A 3.5 HP compressor (commonly sold with a 6-7.5 HP "peak" motor ) will produce about 10-12 CFM at best and I don't think another 1.5 HP will produce 50%-60% more air! Those numbers are good for a two stage and are simply out of the ballpark for a single stage. Compressor companies are well known for stretching the numbers to the limit and I would think if someone were to press them on this one that those CFM numbers would turn out to be "tank assisted" and thus pure non-sense! I have always had a lot of respect for IR but lately I have seen some real non-sense from them such as ridiculous HP and "tank assisted" CFM ratings, mostly from TSC and I have to wonder if it is IR or the retailer doing this. I have dealt with air systems for over 30 years as part of my welding and mine machinery repair business(retired now ) and I have never seen a single stage(non-industrial)compressor get anywhere near that kind of performance and I would truly like to check one with a flow meter and make bets as to how close it would come to 18 CFM@90 PSI
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Old 02-17-2006, 07:01 PM
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All,

Well I just got back from Northern Tool and they did not have the IR two-stage unit in stock, but did have a different IR two-stage (about $50 less - the TS4L5 series) which showed 15.6 cfm @ 175, 90, and 40 - just like my friend's Type 30 - same CFM regardless of psi. They did have the single stage unit and it is definitely advertising those same #'s. Amazingly the pump speed on the two stage was a low 1350 rpm....but the pump speed on the single stage was an even lower 950rpm!

I am attaching a Excel spreadsheet (exported to a tab delimited text file) so you guys can see my dillema of choices (and the difficulty in comparing). The Kobalt and IR single stage both look compelling at the $800-$1000 price point.

Thanks for all the input so far - excelllent forum.

Chris
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File Type: txt Air Compressor Comparison Chart.txt (1.3 KB, 977 views)
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Old 02-17-2006, 09:13 PM
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Cab, you are finding why it is so confusing when trying to make sense of the ratings numbers. Unfortunately there are many ways to arrive at these numbers and the manufacturers play on this to mislead the uninformed into thinking he is getting more than he really is. I notice you mention Eaton, have you contacted them? These guys shoot straight and you can count on them to tell it like it is their ratings are more close to fact than most and make a good point to compare from. One of those single stage outfits you are looking at may look like a better deal now when comparing numbers but believe me when you are standing there holding that DA sander or other air hungry tool waiting for the darn thing to catch up it may not seem so good after all. It is simple laws of physics, a compressor stores energy from the motor as compressed air in the tank and the more efficiently it does so the more cubic feet of air per horsepower and a two stage is simply a more efficient design. Remember it is the actual air flow from the pump that will run your tools and not some overly optimistic numbers painted on the tank.
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Old 02-17-2006, 10:47 PM
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Compressor ratings are confusing and I think that is what some manufactures want. The less you know, and the prettier the paint job, the easier it is to sell you a piece of junk for too much $$. Ones to avoid right off the bat are compressors that only tell you the tank volume and horsepower. Tank volume has NOTHING to do with the output of the machine and a single HP rating is usually peak startup HP, not running HP which is a lot less. That kind of advertising is borderline criminal IMHO. Also avoid the 3500rpm or any direct drive units. Junk for serious use. I have a small oil-less direct drive 1 1/2 hp unit that is easy to carry around that I use for my pneumatic framing hammer. Right machine for the right use but would be a disaster on high demand continuous use tools.

Single stage, belt driven units are fine for hobby use. I have had a 5HP/10cfm @ 90psig single stage unit for over 15 years and it is still going strong on regular all-day Sunday use. 'Single stage' does not mean an inferior compressor compared to a 'two stage' unit. The difference is the pressure that you can get out of them. Remember back to your high school science class. When you compress a gas, the temperature rises. In air compression, 100psig is about all you want to do in a single stage because the temperature of the compressed air is at a point that lube oil, aluminum, and gasket materials start to break down. 90psig is fine for most shop uses. If you want higher pressure, you need to cool the air coming out of the first stage cylinder and then run it to the second stage for compression to ~150psig. Again, if you want higher pressure than 150psig, you need to cool that air and run it to a third stage and so on. In industrial compressors, it is not uncommon to have 4, 5, 6 or more stages of compression for really high pressure uses. All need inter-coolers between stages to keep the air temp from melting things to the ground. The first stage cylinder is the largest and determines the cfm rating of the compressor. Subsequent stages are smaller since as the pressure goes up, the volume of the gas is smaller. Multi cylinder single stage compressors will have cylinders all the same size and the piping will be in parallel. Two stage compressors will have a large primary cylinder, a noticeably smaller second stage cylinder and a tell-tale finned cooling tube connecting the two in series. 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.

For a quality machine, always look for number stages, not number of cylinders, horse-power, volume @ 90psig, and peak pressure rating. Tank rating is nice to know and the larger the better since a big tank stores more air and reduces the start/stop cycling of a given compressor. If they don't tell you those things, they have something to hide, period. Belt driver and oiled crankcase are features to get for long, trouble free life.
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Old 02-18-2006, 12:31 AM
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CUDOS Willys36 An excellent rundown on the compressor scene.
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Old 02-18-2006, 09:19 AM
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As I said before it is simple laws of physics, on any given amount of power you can pump high volume at low pressure or low volume at high pressure but not high volume at high pressure. If you were to set up a single stage pump to produce a very high volume at low pressure then as the tank pressure rises the demand on the motor would be exceeded due to high pressure and high volume. Conversely if you were to set up this same pump/motor to produce a high tank pressure then low pressure volume would drop to unacceptable levels. A compromise must be made to allow the pump to produce a usable volume at low pressure but this volume can not be so high as to limit it's max pressure to below a desirable level. A multi-stage pump gets around this by using both a high volume stage and a high pressure stage thus allowing a higher volume at lower tank pressures while still maintaining a decent volume at high pressure. It is a common mis-belief that the only advantage of a two stage is higher cut-off pressure but this is not true the main advantage of multi stage pumps is higher volume due to better efficiency and indeed it is not at all uncommon to find GOOD two stage outfits with a factory set cut-off of 135 PSI. Since most tools are designed to operate on about 90 PSI (we would NEVER run them any higher now would we ) the higher, normally 175 PSI, is used for volume because the multi-stage pumps will operate efficiently at these pressures while a single stage can not. Think of a single stage as a car with one gear, you will have to set it low enough to move from a stop but this will limit your top speed but the same car with a two speed transmission will be able to move from a stop and still reach a higher speed. There is no way around it comparing a single stage to a two stage is, as the old saying goes, comparing apples and oranges.

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Old 02-18-2006, 10:05 AM
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Guys - Thanks for all the great feedback. At this point I am leaning VERY heavily towards the Lowes Kobalt 2 stage unit based on the following:

1. It appears to have adequate CFM @ 90 psi - 17.1.
2. The specs appear to reflect a legitimate 5HP (see tag pic).
3. It is a 2-stage unit and should run cooler, last longer, probably be (relatively) quiet
4. It has an excellent CFM per dollar ratio (under $1K at my door)
5. I am doing this as a hobby. Like many home users I expect this thing to sit idle for weeks on end, and as such I think is a good compromise between the $1300/$1500 compressors, and the $400 "cheapies" - when did $400 become cheap?

The 80 gallon tank is nice, but didn't really factor into my decision that much - I needed a vertical unit for space saving, but 60 or 80 was fine.

The pic isn't on the Lowes website, but I found these two pics on the web. Note the pic of the tag showing the "real" specs - seem fairly respectable.

Any strong disagreement with this choice (keep in mind item 5)?

Thanks!

Chris
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Old 02-18-2006, 01:42 PM
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looks like a super choice for your needs. Should last a lifetime w/ little or no maintenance.
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Old 02-18-2006, 02:24 PM
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Cab, looks like you are making a well thought out decision based on the right criteria. All too often when someone shops for a new compressor they will run straight to the biggest TANK in the store and then base their choice on price, tank size and (quite often) phony HP numbers.
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Old 02-18-2006, 04:40 PM
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Thanks Guys - Just got back from the store. I am now the owner of that Kobalt 2-stage compressor - it should be delivered tomorrow. I'll report back over time with regard to my satisfaction or dissatisfaction of this unit. Time to buy my neighbor (an electrician) a beer or two to install that 230 volt outlet in my garage...

THANKS!

Chris
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