Air Compressor Min. Requirements

[Kevin Whiteley]

I want to purchase an air compressor (my first) for various uses. I need one that is suitable for automotive use as well as household use. Essentially, I want to have an impact wrench, nibbler, grinder, finish nailer, framing nailer, and most important sand/media blaster.

What would be the minimum requirements needed - h.p., psi, and scfm? Although I would like to get the largest available, it still needs to be easily portable. Thanks.

[Joe L.]

Re: Air Compressor Min. Requirements

Kevin-----

If you want the compressor to be portable, you're limited to about 5 hp. That's also the approximate limit for compressors operating on 110V house current. Any bigger and you have to go to 220V.

Most of the compressors on the market in the 5 hp range are going to be competitive in terms of max psi and scfm. A 5 hp compressor will operate just about all of the tools you described. However, it may be marginal for the nailer and the blast cabinet. But, like I say, if you want it to be portable, you're pretty much stuck at the 5 hp limit.

My choice is a Campbell-Hausfeld (or similar) unit with belt drive and an all cast iron compressor. An all cast iron compressor is the most rugged and durable. Virtually all of these units are single stage and limited to about 125-135 psi. Get the largest tank you can consistent with your portability requirements. 30 gallons is about as big as these get for portable compressors.

A unit which has come on the market in the last few years that interests me and I've been giving some thought to acquiring is the new Craftsman Professional-Series unit with 2 stage compressor. It's one of the few 2 stage portables on the market for home use (there are "big buck" 2 stage portables designed for industrial use out there if you're willing to cough up about $1,000+). The downside to the Craftsman unit is that it uses a "direct drive", "oil-less" compressor. These are what I call "toy compressors" since that's what the actual compressor looks like. On the plus side, the unit produces 175 psi, which is the range you can usually only achieve with a 2 stage design.

[John M.]

Re: Air Compressor Min. Requirements

Kevin,
I have had several compressors over the years, and have found that a 2 stage 5HP compressor is about as small as you want to go to be able to be used for all you stated. That being said, you will find that not all 5 HP compresors are not created equal! I have seen many compressors advertized as 5 or even 6 HP comperssors that still have the option of 110 as well as 220 VAC! This is total advertizing hype since it is not possible to have a true 5 HP compressor that runs on 110! The key to selecting the proper compressor is the max operating pressure and the SCFM. Now a lot of compressors will have a pretty high CFM rating, but when you look closely you will see that the rating is at 40 PSI. This is the problem with a single stage compressor, it's CFM drops dramatically as the pressure goes up. All of the uses which you need air for will need way more pressure tha the rated 40 PSI, and the compressor will not keep up.
With a true 5 HP 2 stage compressor you should be able to find a compressor that operates at 150 PSI max and makes about 15-17 SCFM at at least 90 PSI. This will meet most of your needs, but as Joe pointed out, it will be a little short for a blast cabinet and maybe constant use of a grinder.
The real problem here is that you will be tied to 220 V no matter what, so being portable will be of limited use. I finally put in a large 80 gallon stationary compressor a several years ago and keep a little cheap 1 HP portable for those infrequent times that I need to take it somewhere. The next compressor I buy will be a 7.5 HP 2 stage since it will put out on the high side of 24 SCFM at 100 PSI, but I will need to wear out my other compressor first! Buy smart, and research your choices well, because there is a lot of B.S. to filter through when buying a compressor.

Regards, John McGraw

[Ed Jennings]

Re: Air Compressor Min. Requirements

John gives some real good advice. Be aware that some of the tools you want to run, such as grinders, sandblasters, etc. use a lot of air. One of the little 110V compressors will probably leave you waiting frequently for the compressor to catch up.
There are some "Industrial" style compressors available at places like Lowes and Home Depot that are much better suited to this type work. They are in the 5hp and up range with 60 or 80 gal verticle tanks and run on 220V. Needless to say, they aren't portable. I think these are priced in the $5-700 range. I believe Sam's Club also carries a similar unit. I wouldn't put one of these in in my shop, but for at home use one would probably last well enough to bring a few bucks at the Estate Sale.

[Mike M.]

Re: Air Compressor Min. Requirements

My Emglo(johnstown, pa) 7 hp , 2 stage 80 gal tank has been here on the farm bead/sand blasting near a doz vets as well as sandblasting our 6000 sq. ft. brick farm house sine about 1980. Frequent oil changes are all i've done for it.If it ever goes teats up, no question i'll get another Emglo. mike

[Chuck S.]

Re: Air Compressor Min. Requirements

Kevin, you are specifying an extreme range of purposes for this single compressor that is likely to yield something that is unsatisfactory for most of the missions. It's like the military services trying to specify a joint strike fighter that can also transport an Abrams tank and a company of infantry.

A good quality bead blast cabinet will consume about 13 cfm at about 100 psi. Less than that, and your bead blasting is going to be slow going. As Joe said above, 13 cfm is going require 5HP, or probably more. My opinion is that it will take about 6.5HP with two stages to provide additional capacity at higher pressure. If your compressor is running continuously when you are operating your bead blast cabinet, your compressor is too small for the task. It should shut off after a few minutes running time and be able to rest before starting up again.

At the 5-6.5HP level, you will be making a major investment, say $700-$1200, in your compressor. The additional requirement of portability will, as Joe said, force you into direct drive, nonlubricated Craftsman (or similar) compressors. I have the same disdain for these "toy compressors" as Joe. If you run one of these continuously, the plastic piston rings and valves will overheat, leading to shortened service life (been there; done that). The best compressor for shop service is belt-driven, has lubricated internals with steel piston rings and valves, and has a cast iron pump housing or cast iron cylinder sleeves, and the biggest reservoir you can buy (probably 80 gallons). It will probably also require a 220V outlet be added to your garage.

I believe nail guns require a small volume of high pressure air for each shot. After the nailer is discharged, the compressor recovers the lost volume to a very small reservoir before the next shot. I have seen small compressors for this service that are hand-carried to the job, and cost a fraction of a good shop compressor. If your future construction projects are to be further than 50 hose feet from your closest air connection, I believe I would buy another purpose-specific compressor for the nailers.

[Kevin Whiteley]

Re: Air Compressor Min. Requirements

Thanks for the great info! After Joe's response, I looked in my Craftsman catalog (they sell Craftsman, Ingersol, DeWalt, Campbell, & Porter Cable) and found one that looked OK. It is a 6hp, 33 gallon, 150 max psi, which develops 8.6 scfm @ 40psi and 6.4 @ 90 psi. From the other responses, it sounds as if grinders and blasters tax the compressor more than other tools (I'm sure there are others).

I probably won't do much grinding as I don't yet know how to weld (I'm going to learn), so any grinding will be minimal. I think my sand/media/soda blasting limit will be parts that fit into a table top blast cabinet. I really don't see myself do a house (wow!) or something as large as a auto body. At most will be small sections.

Anyone think the above mentioned unit is acceptable for my use. And I really appreciate the advice.

P.S. How do you make the smiley face with the wide toothy grin?

[Craig S.]

Re: Air Compressor Min. Requirements

Kevin - lots of good advice here already. I have gone through several compressors over the years, starting with the Sears portable, currently have professional series Qunicy pressure lube units 10HP and building a 20HP unit (just me, I am crazy...Ebay used stuff could never justify the new). One of the heaviest users of air are the bead blast cabinets and Chicago Pneumatic air grinders/sanders...can take up to 25cfm at 100+ PSI (I keep my bead pressure down though). While the typical guns on the units such as Skat Blast from TIP work reasonably well, I migrated to the production guns for faster cleaning, and they really take the air. For most of your uses, as you noted, a smaller portable unit will do well, but if you think at all you may do much body work, you will want the 2 stage 5HP-7.5HP range (real HP..not this compressor rated stuff)...Craig

[Doug Flaten]

Re: Air Compressor H.P. Ratings

I have wondered at what point Sears and others have been allowed to make these high horsepower claims on 110 volt power. The shop vacs and compressors seem to be way out of line. At 110 volts and a 20 amp breaker, the max load would be 2200 watts or 2.95 hp. Many of your household circuits use 15 amp breakers. A 5 hp unit would have to pull 34 amps on a 110 volt circuit. I have wondered if their rating system means that it could supply a peak of 5 or 6 hp of equivalent air delivery when the compressor is running and a fully pressurized tank in reserve power is openned up to meet a short term air delivery requirement. That would be the only way that I would think they could make these claims and sleep at night. My 3 hp compressor that I bought from Sears 20 years ago requires a 220 volt circuit which makes sense for continuous duty, true 3 hp load. So as John says, take those ratings with a big 'ol grain of salt. You may want to look at the amperage rating of the motor to see if there is any way that it could even pull a 3 hp load.

[Mike Baker]

Re: Air Compressor Min. Requirements

Kevin,

I have the Sear's compressor your talking about.

In my experiance, it cannot supply enough air for my table top blast cabinet to be efficient. It's effective for a couple minutes and then you have to stop and wait for the compressor to catch up. It is very slow going and frustrating. I've given up using it for blasting and pay someone else to blast parts for me.

I will finish my current project (excluding bead blasting) with this compressor but will either buy a new compressor or scrap and replace the mechanicals on this one before before starting on another project.

Before someone cautions me on modifying an air compressor, I would have the regulator caliberated to not exceed the Max PSI of the current setup. Space constraints prevent me from stepping up to an 60 or 80 gal tank so my options are limited.

Good luck.

[Richard P.]

Re: Air Compressor Min. Requirements

Kevin,
I agree with many of the previous posters regarding the range of activities you hope to use the compressor for. You definitely need to be looking at a 240V, 5HP, two stage compressor at least. I don't know of any portables this large. An 80 gallon tank would give you reserve for some of your more demanding applications in a stationary unit. Some insight into the very high horsepower ratings for small compressors is that the rating is based on the breakdown torque amperage of the motor, not the running HP of the motor. That is the point when the motor cannot turn due to an overloaded condition. The direct drive compressors concern me for a couple of reasons. They turn at much higher RPM's (1725 vs 7-800) and one end of the crankshaft is supported by the electric motor bearing.
Just my $.02
Rick

[Doug Flaten]

Re: Air Compressor Min. Requirements

With that rating system, I don't know how they can sleep at night. I suspected the hp rating had something to with current draw at failure or overload since shop vacs have no reserve tanks and are over rated on hp too. What if they used the same logic in rating the compressor tank. A maximum operating pressure would essentially equate to a burst pressure. It may get to that pressure, but it may only do it once.

[Joe L.]

Re: Air Compressor H.P. Ratings

Doug-----

Yes, the horsepower ratings on most of these compressors is bogus. There are 2 things that tipped me off to this years ago. First, the scfm and psi ratings for, say, a "3-1/2 hp" compressor are usually seen as being just fractions lower than a "5 hp". Second, the motors on these compressors, if they have data plates/decals will have the word "special" (or some other such vague term) in the horsepower block. In any event, bogus rating or not, the "5 hp" compressors have a marginally higher capacity than lower hp, so I think that, on that basis, they're preferable if one is going to get a 110V compressor.

I've noticed lately that these compressors have started to "de-emphasize" horsepower in their advertising claims. While the "horsepower" used to be prominently displayed on the units, many no longer have it shown, at all. Instead, they now emphasize psi or something else.

I've had a "3-1/2" hp" Campbell-Hausfeld with cast iron, dual piston compressor, belt drive, and 20 gallon tank for many years. It's served me very well. I don't need or use it for high scfm applications, so that's part of the reason that it works well for me. However, I did research it very well before I bought it. After that, I became a big fan of cast iron, belt drive compressors. If Campbell Hausfeld (or other manufacturer) comes out with a similar unit in 2 stage design, I'll likely upgrade at that time. I don't really need it, but I've always wanted a 2 stage.

[Kevin Whiteley]

Thanks all, I'm going to do more homework *NM*

[Craig S.]

Re: Air Compressor Min. Requirements

Kevin - wide toothy grin smiley face is colon capital D ....Craig