Air compressor trivia

[indyducati]

Hi folks,

I've always wondered how a "best-quality" air compressor compared to my 3hp, 20gal. Craftsman.

The plant I work in is shut-down for inventory. During this time, many large repair jobs can be done without interupting production. The big air compressors are having maintenence done. The crew I work with are rebuilding the gearbox of a BIG stamping press. The company rented a really nice Quincy compressor to provide air to our job. 15hp, 120gal., and the biggest piston compressor I've ever seen on a tank that size. It's about 4' tall, 2' wide. intercooler, aftercooler, unloader, pressurised oiling, spin-on filter, auto drain ect... Loaded.

Anyway, I was using a 1/4" die-grinder, the tank had 125psi and the supply-line had 90psi. I could only run the grinder for 3.5 minutes before the compressor would cycle. It only took 33 seconds for it to recharge.
I kinda thought a tank that big might last longer. My little Craftsman runs nearly the entire time I'm using a grinder.

Steve in Indy

[cntryboy1289]

I would think the tank would last longer than that as well, but like you said, it only took 33 seconds to refill the tank. That would be fantastic in my line of work. I only have a 6 1/2 hp, 80 gallon tank and use it to blast with in a blasting cabinet. Once the tank pressure kicks in the compressor, it runs full time until I quit. If I am running air tools, it takes about 3 minutes for it to shut off.

[Last Old Dog]

I was initially puzzled with the cycle time of the compressor. Intuitively, one might think that big compressor should be unloaded for longer periods. An Ingersoll-Rand 15hp 2 stage is rated at 50 cfm at 175 psi with a 100% duty cycle. However, a good die grinder under load can consume 20 cfm whilst having an advertised �Average Consumption’ as 4 cfm. So, if the pressure switch had been turned down to 125 on your rental unit for longer life (not to mention a smaller motor pulley to slow the compressor a bit), and, you were �working it', then perhaps the observed cycle time is not unreasonable.

[Your Old Dog]

"However, a good die grinder under load can consume 20 cfm whilst having an advertised �Average Consumption’ as 4 cfm.

I thought "average consumption" meant if you turned the power adjustment on the tool to the midway point. That would mean it works similar to an electrical current. The more the load, the more the current draw.

My 4 1/2 hp 20 gallon cycles quite abit when running the small angle cutoff grinder. I have limit switch set at 125 lbs. I was envious of an 80 gallon unit but maybe not after reading this thread. It don't look like much to gain. Maybe Forrest or someone else can jump in here.

I also have to wonder if these air tools are rated as optimisticly as electrical motors? When they say 20cfm do they mean that or is it really more like 40cfm? Anybody ever check the garden variety of air tools?

[This message has been edited by Your Old Dog (edited 07-09-2005).]

[indyducati]

Hi folks,

Just to add a little more information. I was using a 15 year old Ingersoll grinder. I'm pretty sure it was their top of the line industrial unit. I didn't buy it, I don't know the cmf it uses, but as these grinders go it's pretty powerful. On Quincy's web site, it looks like this was a QR compressor.
I didn't want to jump through their hoops to get the specs.

After this brief, unscientific experiment, I've changed my mind on a new compressor. I still want one, I just don't think I'd spring for the biggest tank. I'd put that money toward a better compressor, and a more powerful, lower-speed motor. That was the nicest thing about that big compressor, it was quiet. The fan we were using was almost as loud.

Steve in Indy

[Last Old Dog]

<font face="Verdana, Arial" size="2">Originally posted by Your Old Dog:
"However, a good die grinder under load can consume 20 cfm whilst having an advertised �Average Consumption’ as 4 cfm.

I thought "average consumption" meant if you turned the power adjustment on the tool to the midway point. That would mean it works similar to an electrical current. The more the load, the more the current draw.

My 4 1/2 hp 20 gallon cycles quite abit when running the small angle cutoff grinder. I have limit switch set at 125 lbs. I was envious of an 80 gallon unit but maybe not after reading this thread. It don't look like much to gain. Maybe Forrest or someone else can jump in here.

I also have to wonder if these air tools are rated as optimisticly as electrical motors? When they say 20cfm do they mean that or is it really more like 40cfm? Anybody ever check the garden variety of air tools?

[This message has been edited by Your Old Dog (edited 07-09-2005).]</font>

YOD - Good eye sir. I certainly did not make my point well, thanx for the heads up. My understanding is as follows: The �Average Consumption’ rating is derived from an arbitrarily defined operating environment wherein the tool is cycled on and off during a task rather than running balls out continuously at max load. Therefore, during the off time, low load time, E.G. repositioning, checking work, changing tooling or abrasives, etc, whilst the clock on the wall continues, there are periods in which the consumption may vary from full load to periods of �off time’ and everything in between. Hope I did better this time.

Further, if the task calls for sustained heavy loads, the compressor must be able to keep up on its own.

An unloader on a big compressor is far easier on the motor not having frequent starting loads.

I remember years ago, Sears taking some heat for their �Peak HP’ ratings. For years I have suspected this was a millisecond duration flash reading just as the motor was about to melt into the �debris container’.

[wierdscience]

If it is really 15hp,then it should deliever 25-30 cfm of free air @120 psi.On that small tank it should recover in less than a minute,I'm thinking more like 30 seconds.

I think something is wrong with that pump.

[lenord]

Got this link from the PM site yesterday. http://www.eatoncompressor.com/page/page/504413.htm
Sounds like they make decent compressors.
What do y'all think ? I don't have one, don't see why I need one really.

FWIW
Lenord

[cntryboy1289]

I have been on the phone recently with several dealers talking about buying a new compressor. I have a so-called 6.5 hp 2 stage compressor now and it runs full time when I am blasting with it. From what I can gather and after a trip to Lowe's yesterday, there are a lot of compressors sold under their peak hp rating. I looked at one at Lowe's yesterday that had 7hp single stage listed in large print. After actually looking further, there was a placard on it that said the actual hp rating was 4 hp. This unit only put out 11-12 cfm at 125psi. My compressor that I have evidently is a 5hp motor and not the 6.5 as listed on it. Mine puts out supposedly [email protected] and [email protected] The blasting cabinet I have is suppose to require [email protected] If I was getting the cfm as what it is rated for, the compressor should be filling the tank as well as running the blasting cabinet. Instead, the compressor runs full time until I stop blasting. The folks that I have been talking to say I need a minimum of 7hp to blast and have the compressor shut off. Most of them want to sell me a rotary screw compressor and I think I am leaning toward that if I can find a decent size unit to run on single phase power. They want close to $2200 for the unit I have been looking at. So do any of you have a recommendation for me?

[wierdscience]

Well Sullair makes a single phase tank mounted screw and they do blow a hurricane,but last price I heard was $4800

[cntryboy1289]

Yep, I have been running into that lately. I forget the folks that sent the catalog the other day, but they do have a 7.5hp screw compressor and a 80 gallon tank for just under 3K with all of the accessories like the auto tank drain and the cooler to reduce the temp. Looks like either way I go I am going to drop 3K to change out to a larger compressor, but when you gotta, you gotta.

[Your Old Dog]

LOD, us dog's gotta watch out for each other