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12-24-2001, 12:32 PM #1Admin5 Guest
Excellent info sent in by Forrest (Admin5)
Air compressors for newbies -
Picking a compressor means treading a minefield of fraudulent claims. No matter what it says on the side of the tank, ALL consumer grade compressors are deceptively or fraudulently rated. I'm not suggesting they won't compress air or give good service. I'm saying you have to divide the available specs by a large BS factor get a compressor capable of fulfilling your requirements.
Ignorance will not only kill you but lead to you to squander money.
If you don't understand the basic physics of compressed air you're at the mercy of people who baffle you with an impressive technical vocabulary but who haven't a clue on how to spec out an air system. Be stubborn and skeptical. Compliant customers feed the fraud frenzy. Enough rant.
Hereís an introduction to home shop air compressors
A consumer grade air compressor is actually a unitized system consisting of a motor, a pump, tank, a pressure relief valve (sometimes called a pop-off valve) and a pressure switch. Often thereís a pressure regulator, an unloader, and some ancillary gadgets like a tank drain and a tank stop valve.
There are two basic compressor layouts, horizontal tank or vertical tank with the pump and motor mounted on a bracket welded on top of the tank. If you expect to move the compressor frequently, get the horizontal arrangement because of its low center of gravity. The vertical arrangement uses half the floor space.Theyíre intended to be moved in and left in place because they are so top heavy.
Compressors divide neatly into oilless and belt driven.
The oilless compressor pumps are directly coupled to the motor. Typically they are noisy, not particularly efficient, low first cost units designed for the occasional user where high duty cycle and longevity isnít a major consideration. As the name ďoilllessĒ implies, there is no lubrication required. While they are simple and reliable, they are not intended for daily or commercial duty although a good many serve that exact purpose. The incoming air passes through a rudimentary filter but their crank assemblies and the bottoms of the pistons and cylinders are exposed to ambient air and whatever dust it carries. If the dust is abrasive or contains materials promoting deterioration of the pump through corrosion or seal deterioration the pumpís life will be shortened. When an oilless compressor pump dies itís usually cheaper to replace the whole unit than fix it.
Belt driven compressors feature a separate induction motor driving a reciprocating compressor via a V belt reduction. Belt driven compressors are perceived as quieter, more efficient, and more durable than oilless and my experience has borne this perception out.
The belt driven compressor pump is built along the lines of an internal combustion engine where the crankshaft and other parts run in a sealed crankcase and are either splash or pressure lubricated with oil. There is no particular advantage to a pressure lubricated compressor over a splash lubricated compressor provided they are properly designed. Examples of each have given reliable service for generations with little or no maintenance beyond oil replentishment.
The vulnerable part of any compressor pump is the valves. Itís generally a good idea to buy a valve and gasket kit when you buy the compressor. Youíll need them ten years in the future on Christmas Eve when the compressor dies just before you need to apply the final coat of lacquer on the blanket chest intended for your about-to-be-married granddaughter.
If a belt driven compressor dies any part of it including the motor and the pump can be readily replaced with standard items for lower cost than replacing the whole unit.
The pressure switch senses the tank pressure and shuts off the power when it reaches the set-point. The set-point and the differential are usually separately adjustable. The set point (PSI to turn off the compressor) is adjusted to 150 PSI, for example, and the differential is adjusted to turn the compressor on at 20 or 30 lb below the setpoint. Thus it cycles, turning on at 120 PSI and shutting off at 150.
The pressure relief (pop-off) valve is a safety device designed to open when the tank pressure exceeds its safe working pressure, blowing down the pressure to a safe level, then automatically closing. If the pressure switch failed closed, itís conceivable the unit would keep on pumping until the tanks bursts. Thus, the pressure relief valve is a safety device.
Thereís been some horrific accidents attributed to pressure vessel failures. The energy of the pressurized air is something like a weak bomb. Ductile or fatigue failure of the shell may be sudden and the reaction of a large volume of 150 PSI air released in 1/4 second is enough to shoot the entire compressor off like a rocket, smashing anything in its path. Be sure the pressure relief valve on your compressor is exercised once a year and that nothing is allowed to interfere with its proper operation.
The check valve prevents tank pressure from flowing back to the pump. Its function is often combined with the unloading valve.
The unloading valve relieves trapped pump discharge so when the compressor starts it doesnít have to start against tank pressure. When the compressor comes up to speed the unloading valve directs pump pressure to the tank. The PPSSsssst you hear when the compressor shuts off is the unloading valve - well - unloading..
The main function of the air tank is to serve as a reservoir, radiate the heat of compression, and to condense water entrained in the compressed air. The tank is a pressure vessel whose manufacture and testing is controlled by UL procedures similar to steam boilers and compressed gas cylinders. US Dept of Commerce regulations requires a sheet metal label to be permanently welded to the exterior of any air tank sold in the US certifying its service, safe pressure, hydrostatic test pressure, and other data including the alloy and gage of the sheet metal used for the shell and heads.
A common belief is that a large tank (actually, ďreceiverĒ) is advantageous and will somhow compensate for an undersized compressor. Not true: A large air tank gives you nothing more than a few extra seconds of surge capacity for short term, high demand tools like impact wrenches. As soon as the compressor kicks in, it's only the compressor delivery that runs the tool. The size of the tank determines the length of the charge/discharge cycle.
The main enemy of air compressor receivers is water and the rust it causes. Air under pressure accelerates rust in a bare steel tank. Frequent draining of accumulated water is the best protection against rust. While itís not necessary to blow down the tank completely after every use, accumulated water should be drained before and after use. Since the drain is always inconveniently located under the tank, most users pipe the drain line to a conveniently located valve and route the discharge outdoors or preferably down a plumbing vent.
Compressor pumps vibrate and the frequent charge/discharge cycles linked with internal rust pits sometimes cause tanks to fail through pinholing and/or metal fatigue. If the tank starts leaking through pinholes, chances are if you fix it another will be along soon. Pinhole leaks are like cockroaches. If you find one thereís a thousand others, waiting. The interior of the tank will be dotted with almost rusted through places; the one leak your find is only the first. If you see a streak of rust along a line starting from a weld or seam in the tankís construction, you most likely are looking at the beginnings of metal fatigue. This can be a dangerous condition because the final stages of fatigue failure can be very rapid if not explosive.
This is a long way to convey a short message: if the tank leaks, replace it because it aint worth fixing. They arenít that expensive (compared to a new belt driven compressor) and most replacements have a universal frame to mount your pump and motor on and a plethora of welded-in connections.
Induction motors are the most reliable component in an air compressor but they are not bullet proof. Itís important that their fans and air inlets are vacuumed (not blown) free of dust and lint. A few small pancake compressors are driven by a series wound motor. If you find it necessary to replace the brushes, you may find it maddening to get at them. Pay close attention to disassembly order.
Most any small oil-less compressor will serve a nailor, pump up the snow tires, and supply an occasional blast of air while lasting for a good many years. I have a heavy duty 23 CFM compressor I seldom use except for sandblasting. 99% of my compressed air is supplied by a 7 year old 1 HP Costco hot dog compressor.
As soon as you consider sprayguns and rotary air tools like a 4" sander, you instantly leave the 115 volt plug-in-the-wall-outlet compressor bracket.
Cheap import sanders are under-rated for air consumption. Furthermore any rotary air tool is VERY inefficient, even the expensive models used in industry. They typically require 5 HP of compressor power to generate 3/4 HP of air tool power. If an import sander spec says it requires 6 CFM at 90 PSI, count on 9 to 11 CFM of actual air consumption. If a 4" disk sander requires 9 CFM you need an 18 CFM compressor to run it, otherwise, you waste time waiting for the compressor to catch up.
According to traditional wisdom, you have to size a compressor to about double the largest air demand. Restating: to size a compressor, pick your air tool having the largest continuous demand (as opposed to a tool used in bursts) and double it to spec a compressor suited for your shop.
A three HP compressor is about the point where thermo-dynamic efficiency makes a two stage compressor economical. A two stage compressor pumps 20 to 30% more CFM per motor HP thanks to the heat of compression dissipated by the intercooler installed between the low pressure and high pressure cylinders. Add up the power savings over the 15 year working life of a two stage compressor compared to a single stage and youíll find the 20% represents enough to pay for the two stage compressor several times over.
A two cylinder compressor is not necessarily a two stage compressor. The cylinders may be in a V configuration or side by side. In a two stage compressor a larger first stage cylinder takes atmospheric air and compresses it to about 1/3 the delivery pressure. The intermediate pressure air passes through the intercooler (the finned tube behind the pump flywheel) to be cooled by windage and into the second stage where itís compressed to the delivery pressure. The first stage cylinder head will have a separate pressure relief valve. A common alternative design has two low pressure cylinders pumping through an intercooler into a third high pressure cylinder in a ďWĒ configuration. In this design the low pressure cylinders are only slightly larger than the high pressure cylinder.
A two cylinder single stage compressor will have two side-by-side cylinders of equal size and no intercooler. Unscrupulous marketers may sometimes peddle a two cylinder single stage compressor as ďtwo stageĒ so be alert if you find a ďbargainĒ.
A consumer grade compressor run continuously will fail prematurely. A typical spraygun requires 5 to 8 CFM. doubling the largest rating equals 16 CFM. That requires a real 5 HP two stage compressor whose induction motor draws 22 Amps @ 240 Volts. A 5 HP 60 gallon vertical tank compressor occupies only a little more floor space than a 3 gal pancake but, because itís nearly 6 feet high, it won't fit under the workbench.
Here's a list of applications and motor HP and electrical demand in ascending order:
Fill bicycle tires or run a nailor 1/2 to 1 HP (10 Amp @ 120 Volts)
Spray paint 2HP (9 Amp at 240 Volts)
General automotive use where air rachets and impact tools are employed 3 to 5 HP (12 to 22 Amps @ 240 volts
Running a blast cabinet 3 to 7.5 HP depending on nozzle diameter (12 to 33 amps @ 240 Volts)
Home Depot sells a good 5 HP two stage Ingersol Rand home duty compressor with an 60 gallon tank for $899. I regard it as a good buy for the home shop user (No plug intended).
The Sears oil-less two stage compressor is not suitable to power rotary air tools. While it is a true two stage compressor and will deliver 175 PSI, the Sears two stage compressor, if honestly rated, would be about 2 real HP. Once the Sears two stage is drawn down to cycling it won't quite keep up with an import 4" air sander under load (yes, I ran a test).
As a side issue, I use electric sanders and avoid the whole problem of large compressors and rotary air tools with their carried over oil and water sprayed on my projects. The electric 4" sanders have 115 volt 6 Amp motors which draw about 1/7 the juice of a 240 Volt 22 Amp compressor motor.
By the way and for what it's worth, most two stage compressors are set for 175 PSI service - too high for most air tools and shop service. If air is compressed much over the required line pressure, energy is wasted when when tank pressure is reduced to line pressure at the regulator. If you change out the motor pulley for one about 20% larger (calculate the actual diameter using Boyle's Law and common sense) and reset the pressure switch to kick in at 105 PSI and out at 125 PSI, you'll have extra delivery, lower duty cycle, cooler compressor operation, and lower power bills.
Any extra wear caused by higher pump speed is more than offset by the lower interstage and discharge pressures and lower head and reed valve temperatures.
12-05-2002, 01:18 AM #2Aluminum
- Join Date
- Nov 2002
- Brooklyn, NY USA
This is the best most concise tutorial on air compressors I have ever seen. I wish I could have read it 15 years ago, before I learned it the hard way. Especially about the false claims of HP and CFM. I've put together a few compressor outfits from scrounged and surplus components that work much better than a mechanic friends, (I think six or seven at last count, in about eight years), "commercial" compressors.
One other point, another function of the receiver is to smooth out the intermittant
bursts of air from the pump and combine them so the output flow is continuous and smooth.
IMHO the most reliable pumps are oil lubricated, (although I once saw a 200 HP oilless that was used for food production, that looked nice) and the most important things you can do to prolong its life is change the oil regularly, make sure you have a good intake filter (not always possible with the newer pumps without tapped inlets) and have a filter on the pump breather, otherwise with every stroke you are possibly pulling contaminants into the crankcase.
It is best to have the compressor setup away from the work area for this reason (dirt)and also beacause of the noise.
[This message has been edited by yf (edited 12-05-2002).]
01-16-2003, 07:48 PM #3Junior Member
- Join Date
- Jan 2003
This was awsome
Ive been looking for months for a new compressor.....
Anyone ever heard of Eaton Compressors out of Ohio?????
02-06-2003, 04:56 AM #4Plastic
- Join Date
- Feb 2003
After 12 years of seconary operations in production tapping, I can only share this.
1st compressor-30 gal tank 3hp motor. I destroyed (2)in two years both bought a well known store. These machines generally run at 3400rpm
2nd compressor-60 gal upright 5hp twin alum casting.(destroyed)after 2.0yr. service.Purchased from comercial outfit.
3rd compressor-60 gal upright (destroyed)1 year.Bought at Fleet type store.Cast iron pump, valves and virbartion problems.
4th compressor-60 gal horizontal cast iron head 175psi .Motor 5hp 1750rpm. It runs from
6am - 11pm everyday seven days a week and has been running since 1993 bought new.
5th compressor- 60gal upright cast iron pump 175psi bilt in the fifties. Has run in production since 1996 seven days a week 6am to 11pm.
The 4th and 5th compressor have one thing in common.
Both have pumps with twin cylinders.
Both have motors that run at 1750rpms.
Both cast iron pumps that run between 550-650 rpms.
Both have 60 gal tanks which cycle about 1-2 minutes on and three minutes off as an example, at 80 off and 100 on setting.
Finally, they have a name you can trust
CHAMPION. You probably see them in many automotive paint shops and oil change places.
I am not a dealer- just one serious user.
These compressors run all seconary equipment,cnc,and shop air.
By the way, other than replacement oil and filters they have never let me down.
04-15-2003, 05:39 PM #5Plastic
- Join Date
- Apr 2003
I'll bring it back up... Anyone know anything about Eaton Compressors out of Ohio? They sell stuff on eBay and they look nice and reliable, anyone use them? Thanks
11-22-2003, 05:01 PM #6Hot Rolled
- Join Date
- May 2002
- Athena, OR
From my experience, Quincy makes the best compressors, hands down. Reliable as hell (20+ year lifetime) and some are rated for continuous duty cycle.
01-09-2004, 08:26 PM #7dephenicie Guest
Eaton: they have a great reputation here in central Indiana.
Least favorite compressor: 10hp Kellog-American, had reed plate/valve failure, piston went to TDC and depopulated the crankcase, instant junk.
Favorite compressor: Quincy QR-25 with spin-on oil filter, runs on low rpm, cast iron, fully rebuildable
07-30-2004, 05:10 AM #8Member
- Join Date
- Jul 2004
- Boca Raton, Florida, US
I own a sears craftsman 7hp 175psi two-stage 60 gallon, oilless, and a big piece of junk. I just want to help anyone out there not to make the same mistake I did. After I bought it from sears, I thought there was a problem with the compressor or my electrical wiring. I had never thought a big compressor could really be that slow. It runs like a cheap 2hp 4 gallon compressor except it fills a 60 gallon tank.
07-30-2004, 10:30 AM #9Diamond
- Join Date
- Apr 2002
- Syracuse, NY USA
I will pass this explaination onto the next customer who calls me and says " I can buy a real nice 10 hp DeVilbis at Sams for 800 bucks, why are you asking 2000 for your 5 year old 15 hp Ingersol rand? they seem to be blind to the differences until 6 months down the road they can't keep there shop running......
07-30-2004, 12:03 PM #10Diamond
- Join Date
- Jul 2004
- Asheville NC USA
Yep, the faster they run the less efficient they are. Sears is right at the top of the list of outfits that give false motor ratings. As soon as a motor rating has a * beside it and a note stating *peak hp, the rating is useless. In simple terms, peak hp is measured in the couple milliseconds when the motor is overloaded to stall but hasn't stalled. I've got an old single stage quincy that's sized to take 7.5 to 10 hp motors. It came from a state maintenance garage where there was no 3 phase power and has a single phase 1725 motor with 3 legitimate hp. It just chugs along and never quits, and I'll bet it will deliver as much air as two of the 7hp Sears or equivalent fluttermills. It was built in the early 50's and has never been touched except for oil and filter. I retired it to my home garage a couple years ago and bought a new 15hp 2 stage IR. Seems like a good compressor so far, but I doubt it'll last like the quincy.
10-07-2004, 07:44 PM #11Plastic
- Join Date
- Sep 2004
- San Jose, CA
Awesome turotial! This is some good stuff considering I've been looking at compressors. Can anybody comment on compressor brand names? I've been looking at compressors from Harbor Freight http://www.harborfreight.com but don't want to buy something that I can't find parts for in the future. Are there any 'avoid at all costs' brands or 'best bang for the buck' brands?
10-10-2004, 09:50 AM #12Plastic
- Join Date
- Sep 2004
A well timed report. My 35 year old Crapsman
leaks ..somewhere, but $890 bucks wow. At near 84 maybe I dont need one to last over a few yrs.
I printed out your info. thanks H
10-10-2004, 01:17 PM #13Diamond
- Join Date
- Jul 2004
- Asheville NC USA
Hubert, I've got a twin cylinder Sears compressor I bought about 27 years ago, and they're a better compressor than what Sears sells today. If the leak on yours isn't in the tank itself, then there are 4 most likely, and easily repairable, points to check. These critters are bad to blow head gaskets if a head bolt gets the least bit loose, and they tend to self loosen from vibration. Next, the valves and the little screws that retain them will sometimes get loose or break. Also, the discharge check valve, located at the entrance to the tank on mine, is pretty sorrily made and will leak back if the compressor is run hard for a while because the seat material doesn't seem to be able to cope with much heat. It causes the tank pressure to bleed back thru the unloader port on the pressure switch. The last thing to check is the discharge line itself. They tend to work harden and develop longitudinal cracks at the bend. If the discharge check is the culprit, grainger sells one thats much better made, with a teflon seat, for less than 15 bucks. Its not a direct replacement, but is very easily adaptable to use on these compressors and will outlast the original one by many times over.
Mine is a 3hp with a 20 gal tank, and it's been used hard in the past when I did a lot of paint and body work as a hobby. It's run air DA sanders and air files and a DeVilbiss JGA spray gun for hours on end, and has survived well. Even at its advanced age, if it were to develop a leak in the tank, I'd look seriously at replacing the tank before I'd spend the money Sears wants today for one of their over-rated die cast aluminum fluttermills.
10-16-2004, 10:19 PM #14Plastic
- Join Date
- Sep 2004
Believe I "fixed" mine. found a broken poppit valve. all seats look ok (2 cylndr job) used a
.0075 steel shim stock ,made one, seems to be pretty similar stock, time will tell. hobie
07-07-2005, 02:00 PM #15Junior Member
- Join Date
- Jul 2005
- Fresno, CA
In the past 6 years Iíve gone through 3 cheap oil-less compressors, the current one being a ď1.5 HPĒ Hitachi that wonít run if I use an extension chord longer than 25 feet. I bought the first one because I didnít know any better, and the two that have followed simply because they were clearance deals I found right after the previous one had died. Someday, when Iím in a bigger house with a proper shop, Iíll shuck out the money for something my kids can use when I die, and Iíll definitely use this discussion as reference material. Thanks to all.
In college I used to hang out and study at an all-night truck stop, and got to know a lot of long haul guys who made a point of stopping there as they criss-crossed the country. One guy Iíd gotten to know well stopped by once with a load of trailer-sized air compressors used mainly for road construction projects. They were modified Chevy 454 engines, where one bank of pistons ran the motor while the other 4 cylinders delivered some ungodly amount of air to a tank whose walls were so thick it didnít ring when I hit it with my knuckles, but rather sounded like I was rapping on a solid iron billet 8 feet long and 4 in diameter. The output fittings looked to be at least 1Ē ID, though that was a long time ago and memory tends to exaggerate things a bit. I canít for the life of me remember the manufacturer.
Something like this is obviously a bit too massive for home use, but Iíve got an old Wisconsin V4 Iím going to rebuild someday, and such a conversion would be, at the very least, at lot of fun in the conception stage, if not in actual execution.
07-07-2005, 06:20 PM #16
"Something like this is obviously a bit too massive for home use, but Iíve got an old Wisconsin V4 Iím going to rebuild someday, and such a conversion would be, at the very least, at lot of fun in the conception stage, if not in actual execution."
In order to keep in balance, run the engine on two cylinders and the make the compressor from the remaining two.
It would be interesting to design a special camshaft which retained IC operation for those two cylinders, while optimizing AC operation for the remaining cylinders.
08-04-2005, 08:33 PM #17
Schramm built construction site tractors that used that principle and were basicly self propelled air compressors. They were used primarily for towing equipment and trailers around the worksite to where they were needed, then were parked nearby and used as air compressors. They had 6 cylinder engines that made power with 3 and compressed air with the other 3. Perhaps Schramm modified the Chevy engines you saw.
12-19-2005, 12:48 AM #18Hot Rolled
- Join Date
- Dec 2005
- Orlando, FL
While it is true that two-stage pumps are more efficient if the goal is to compress air to pressures above 120 PSI, two-stage pumps actually have lower specific output ITO CFM/HP at lower, though still usable pressures. Furthermore, if you do not need air compressed to 120 PSI, then this extra pressure, and the energy and equipment required to acheive pressure >120 PSI efficiently (a two-stage pump) is all wasted.
Honestly, few users EVER need >120 PSI, let alone the 150-200 PSI that two-stage pumps can deliver. Most air tools are not recommended above 90 PSI as all of the lubricant blows out of the tool causing accelerated wear. Besides this, the tool generally runs too fast at these pressures. And who spray paints above 60-70 psi? I spray paint for a living and I NEVER do. Most users run either a diaphragm regulator or a restrictor to get the pressure down below 90 PSI. So again, why use a two-stage pump?
Two stage pumps got to be the standard in industry for two reasons: more efficient distribution over long distances (high pressure decreases piping losses) and plenty of industrial equipment can actually use the higher pressures. At a hangar where I often paint, the compressor house is located about a eighth mile away from my work station. Delivery is through 6" steel piping to 1-1/2" drop legs. The pipes have to go up and over the top to give clearance to tall birds, about 60 feet high. So those pipes are even longer than that distance. They NEED the 200psi! The pumps are a pair of two-stage 50HP screws. This is typical of an industrial installation.
I have always ordered single-stage industrial air compressors for use in my shop. All the major compressor makers, with the possible exception of I-R, offer single stage versions of their most popular industrial rated pumps. My favorite is the one offered by Champion. A typical 5 HP (REAL 5 HP, not bogus 'peak' HP listed on home center offerings) two-stage industrial pump will deliver about 18 cfm @ 100 PSI. That delivery will fall to something like 15-16 cfm @ 175 PSI, which is quite respectable if you can use 175 psi, which as we have established, you can't. A Champion 5 HP SINGLE STAGE pump will deliver about 25cfm @ 100psi. This is not a trivial increase in output @ 100 psi, the max we need anyway. And it gets better yet at 80 psi, the typical working pressure for rotary air tools. Of course the output falls considerably above 120 psi and these pumps should never be used above that pressure anyway since they tend to run too hot there. We don't need that kind of pressure for any reason I can think of, so why pay for that capability?
12-20-2005, 06:17 PM #19Aluminum
- Join Date
- Jun 2005
- Southern Indiana
We had a 2 cylinder compressor with a 30gal tank. One day I seen my dad silver soldering a hole in the bottom of the tank, I said " I don't believe you should be doing that" " It will be OK" he replied. All went Ok until 2 weeks later it developed another leak which Dad silvered soldered shut again. A week later the tank blew, it took out 6 2x6 skirt boards on the end of his pole building and blew the complete unit over top his wood stove, luckly Dad was un-injured even though he was standing close to it when it went. Last tank he ever tried to repair.
05-29-2006, 12:12 PM #20Stainless
- Join Date
- Oct 2005
- tacoma washington
when i was a kid back in the early 70's i had a job at a filling station back in kansas, i used to try to do my home work later in the evening when business was slow.
problem was the old quincy compressor, hammering like hell right beside me,,,, like a hammer on a peice of steel plate, bang bang bang..
finally one evening i checked its oil, hmmmm, none,,, just silvery powder on the dipstick..
i added 1 qt of this stuff that was a competitor to STP, you know the engine honey stuff.. compressor quieted right down...
next evening repeat the process.. day after day for about 2 weeks,,, then...
the boss asked if i was oiling his compressor, i said yes and got chewed out for it...
he told me that he would never get a new one from corporate if it never blew up.... lol
a couple of months later they came and remodeled the station, took out the old compressor,, only to return with it rebuilt a couple of weeks later..
i am sure it is still there, maybe hammering like before..