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Kinda OT: Air compressor H.P. ratings


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  #1  
Old 08-20-2008, 09:44:35 PM
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Default Kinda OT: Air compressor H.P. ratings

Hello!

I have a question about compressor ratings - When a compressor is labelled as a "5HP", does the 5HP refer to the electric motor that drives the compressor, or is it a rating applied to the compressor head? If the HP rating does refer to the compressor head, what exactly is the meaning of the rating?

I ask because I have an Ingersoll Rand T-30 compressor (got it for free from a junk removal job), 2 stage with an 80 gal vertical tank, and the motor is 1 1/2HP, which seems underpowered if other compressors are "5HP", assuming that the rating refers to the motor. It just doesn't seem to make sense to me to make a compressor with an 80 gal tank and an underpowered motor. I'd like to set up the compressor to do some sandblasting, but most sand blasters say you need a compressor with a min of 5HP. Based on that guideline, it doesn't seem that my compressor would fit the bill, or am I not understanding how compressor ratings work.

If anyone can shed some light on what compressor HP ratings actually mean, I'd appreciate it.

Thanks...

Chris
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Old 08-20-2008, 10:09:21 PM
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Default Re: Kinda OT: Air compressor H.P. ratings

From my view of attempting to get set up for sand blasting equipment (which hasnt been done yet) I find that the rated H.P. is at the motor and if it's (i think) 2 and a half horse or bigger it's automatically a 220 unit. The bigger the better i've heard from sand blasting guys. That way while your emtying the tank of air which you would be doing by blasting or using a continueus use air tool, the 5 horse 3 to 5 horse can keep up with the lost air and recharge the tank at the same time of using the air tool or blaster WITHOUT the motor burning up and with out the need of stopping to let the tank build preassure back up. You would have to do that on a 110 unit which could be 2 H.P. or less. Most of the time it's 1-1/2 H.P. for a 110 unit which is OK for a tank with little to no use but it's possible to burn it up blasting with a 110 unit simply becouse the 110 motor is to whimpy. It cant turn the compressor fast enough to keep up with lost preassure and if you dont stop to let the preassure build back up then it gets real hot and can and will burn out the motor. 110 units are ok for air wrenches but thats about it. You need the 220 unit "big boys" to do the industrial type work some of us engine guys tend to do. They can turn that pump fast enough and even though they'll get warm, you can work them without worry. A simpler way of saying it of which i should have said in the first place, the 110 compressors are for the compact on the go people or businuess with air guns or for the dainty light duty hobbyist, but the 220 units are for the (get serious) work. Thats the kind of work i do, myself and i need a 220 unit. Trouble is for some people like me that are not wired for 220 so sometimes you might find someone like me venturing into 110 country and just getting by with what we got, taking care of what ever it is the best we can and knowing when to "lay off" the peice of machinery when it starts to get over worked, and let it cool so the process at hand can resume.
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Old 08-20-2008, 10:14:26 PM
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Default Re: Kinda OT: Air compressor H.P. ratings

Sky - My motor IS a 220V motor, which makes me think that the rating is NOT referring to the motor itself.

Chris
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Old 08-20-2008, 10:17:54 PM
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Default Re: Kinda OT: Air compressor H.P. ratings

Chris,
It depends on how fast you are turning the compressor. The rating is for the motor. A 1 1/2 hp motor turns compressor head at a fairly slow speed. A 3 hp motor can turn it faster. A 5 hp faster still. The pressure lubricated compressors have a minimum speed they have to be run in order to provide adequate lube. Speed is normally varied by pulley diameter.
The faster it turns a greater volume of air is produced requiring more hp.
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Old 08-20-2008, 10:25:44 PM
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Default Re: Kinda OT: Air compressor H.P. ratings

By factory the Hp is the motor. But what i have said just comes from my very breif experience which might as well be no experience on trying to get set up for a sand blaster. That was my findings however. Hay, you ask, i told. lol
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Old 08-20-2008, 11:47:04 PM
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Default Re: Kinda OT: Air compressor H.P. ratings

Chris,

Horsepower ratings generally refer to the motor. If the tag is on the motor and it says 1 or 2 or 5 HP, that's what the motor is capable of producing. If there's a decal or tag on the tank that quotes a horsepower, that merely means that the unit was originally equipped with that size motor.

Compressors are rated by output, in cubic feet per minute (CFM). Outputs are further quantified at various pressures - at zero pressure, at 40psi, at 120psi, or whatever. Dealers selling new compressor pumps may specify the size of motor needed to deliver rated output. This is purely to be helpful to the customer.

It's quite likely that your compressor had the motor replaced at some point in time, with a much smaller motor than was originally provided. I'm no expert on the type 30 compressors, but the smallest two stage units I have seen were built with either 3 or 5 HP motors. The 5HP units had a bigger motor pulley and the compressor ran at a higher speed (and made lots of noise). The next size up was built with a 5 or 7.5 hp motor. I used to know the specific model numbers of these pumps, but age has taken its toll on me and that data is gone from the memory bank. In any event, that model number can be found on the pump's tag and can be used to identify just what you have.

Type 30 compressors were made in single and two stage configurations. Both of these are good, reliable machines but the single stage versions are best suited to lower pressure applications and intermittent use. Two stage pumps are easy to identify. The two pistons/cylinders are very different in size. The big cylinder draws air from the atmosphere, compresses it, and then feeds it to the second (smaller) cylinder, which compresses it again and then discharges the high pressure air to the tank. If both cylinders are the same size, and both discharge into a common manifold that in turn supplies the tank, you have a single stage pump.

If your compressor has been retrofitted with a grossly undersized motor, one or both of two things will be true. The compressor will run very slowly, and take forever to make any pressure, and/or, the motor will stall out at a low pressure and will stop running. Obviously, neither of these are good things.

110 vs. 220 volts isn't an issue specific to compressors. Bigger motors make more power by consuming more electrical current. They consume more current in part because their impedance is lower than small motors. Electrical supply wire has a certain resistance per foot, and at high currents the resistance of the wire becomes significant when compared to the impedance of the motor. This means that too much electrical power gets wasted simply supplying the motor. The consequence of this is low voltage to the motor and loss of power, overheating, etc. It's a good idea to run motors of 1HP or more on 220 volts (assuming they are capable of doing so), especially if the load and/or duty cycle are great and/or the supply line is long. (I've skipped a lot here but I hope you get the idea.)

If you plan on doing any serious sandblasting, a good 5HP two stage compressor is, in my opinion, an entry-level must have. This is a compressor that will deliver 17 cfm at operating pressure (125 psi or so). The CFM rating is the important number, because many compressor makers play fast and loose with horsepower ratings. You can't fool with actual output, though.

This isn't to say that a smaller compressor can't handle intermittent sandblasting. I have a single stage Champion three cylinder compressor that I am powering with a 2HP motor instead of the correct 3HP (because that's what I had handy when I was putting it together). It's mounted on a 30 gallon tank (small) and shuts off at 105 psi. I got a good deal on a used small pressure blaster and bought the smallest nozzle I could find for it. This outfit does a great job on small parts. Yes, I have to stop blasting often to allow the compressor to recover. But for small stuff that's not an issue and is a whole lot less bother than firing up my big Chicago Pneumatic 250CFM compressor's thirsty six cylinder engine and dragging out my bigger blaster.

One final observation, I have found that the higher the pressure you blast at, the quicker and better the job gets done. This is good advice for castings and heavy stuff. But if you're going to be blasting tinwork, you'd better drop back to 80psi or so. Any more than that and you run the risk of warping the metal.

Oops, one more thing! If you plan on blasting a lot on hot, humid days, you'll likely have big problems with water in your air and will have to separate it out. I've heard all sorts of ideas on what might or might not work for air separation. A common theme, though, is that all of these methods are complicated and fussy. I have neither time nor patience, so I haven't tried any of 'em. I rely on the method taught me by an old time contractor that had vast experience blasting and repainting his equipment periodically. Simply install the biggest inline water separator/filter that you can find, and leave the drain plug OPEN. This works wonders for my big outfit where I have plenty of air to waste, but may pose a problem on a smaller system. I've not had any water problems with my small blaster, but then I have only used it a few times, and never on a hot, humid day.

Happy blasting,

Steve
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Old 08-21-2008, 12:12:17 AM
Ed Radtke Ed Radtke is offline
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Default Re: Kinda OT: Air compressor H.P. ratings

We have a t30 at work and if memory serves me,its a 15hp. Its a constant run pump with an unloader so you could use a 30hp gas engine.
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Old 08-21-2008, 08:46:09 AM
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Default Re: Kinda OT: Air compressor H.P. ratings

http://www.old-engine.com/abrasive.shtml

And here's a really good thread with horsepower discussion of air compressors:
http://www.smokstak.com/forum/showthread.php?t=4653
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Old 08-21-2008, 01:53:07 PM
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Default Re: Kinda OT: Air compressor H.P. ratings

I have an American made, Quincy air compressor, its 5 h.p. The compressor head is 4 cylinder, so it runs slower to make more air, which gives it a longer run life. It builds to 175 psi, and when I sand blast, she goes a long time before the compressor even cuts in. I would say that your air compressor has the wrong engine, because I have never heard of an 80 gallon tank, with such a small motor. A small motor will have to work to hard once the air pressure is getting higher in the tank.
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