Compressors

COMPRESSOR

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Compressors

TABLE OF CONTENTS

1. PREFACE..............................................................................................................................................................4

2. THE BASICS FOR CHOOSING THE RIGHT AIR COMPRESSOR...........................................................4

2.1 AIR COMPRESSOR ARTICLES...........................................................................................................................42.2 AIR COMPRESSOR + AIR TOOLS ADVANTAGES:.............................................................................................42.3 AIR COMPRESSOR USES..................................................................................................................................4

2.3.1 Home Use:..............................................................................................................................................42.3.2 Business Use:..........................................................................................................................................4

2.4 HOW TO PICK THE PERFECT AIR COMPRESSOR..............................................................................................52.5 STYLE..............................................................................................................................................................52.6 PUMP TYPES....................................................................................................................................................5

2.6.1 Invector Pumps.......................................................................................................................................52.6.2 Direct-Drive Pumps................................................................................................................................62.6.3 Belt-Drive Pumps....................................................................................................................................6

2.7 AIR COMPRESSORS: STAGES...........................................................................................................................62.7.1 Two-Stage vs. Single-Stage.....................................................................................................................6

2.8 HORSE POWER.................................................................................................................................................62.9 PSI...................................................................................................................................................................72.10 CFM................................................................................................................................................................7

2.10.1 SCFM, ACFM, ICFM...What Does it Mean?.........................................................................................72.11 HOW DO I DETERMINE MY CFM REQUIREMENTS?.........................................................................................82.12 TANK SIZE.....................................................................................................................................................102.13 PRESSURE POT VERSUS SIPHON GUN.............................................................................................................112.14 NOZZLE SIZE..................................................................................................................................................112.15 PERCENT OF DAILY USE- JOB SIZE - CONTINUOUS USE VERSUS INTERMITTENT...........................................11

3. WHAT TYPE OF COMPRESSOR SHOULD I BUY?...................................................................................12

3.1 ROTARY SCREW, RECIPROCATING, CENTRIFUGAL, DIAPHRAGM AND OIL LESS..........................................123.2 OIL LESS (~$300)..........................................................................................................................................123.3 PISTON/ RECIPROCATING OIL LUBRICATED (CONSUMER UNITS: ~$400 TO $800; COMMERCIAL UNITS: ~$1000 TO $2500 AND UP)........................................................................................................................................133.4 WHAT'S NEW: CONSUMER TWO STAGE UNITS (~$800)..................................................................................133.5 TANK SIZE AND SHAPE..................................................................................................................................133.6 SOME TYPICAL COMPRESSORS:......................................................................................................................133.7 WHAT FEATURES SHOULD I SELECT FOR DURABILITY?................................................................................14

3.7.1 Aluminum versus cast iron cylinders....................................................................................................143.7.2 Duty Cycle-...........................................................................................................................................143.7.3 Cost.......................................................................................................................................................153.7.4 Brand Name-.........................................................................................................................................153.7.5 Warranty...............................................................................................................................................15

3.8 WHERE SHOULD I INSTALL IT?......................................................................................................................153.8.1 Noise.....................................................................................................................................................153.8.2 Heat.......................................................................................................................................................153.8.3 Draining................................................................................................................................................153.8.4 Air quality.............................................................................................................................................153.8.5 Maintenance..........................................................................................................................................163.8.6 Piping and filtering the air...................................................................................................................16

3.9 CONCLUSION:................................................................................................................................................163.10 FEATURES TO CONSIDER WHEN PURCHASING AN AIR COMPRESSOR............................................................16

4. Q&A.....................................................................................................................................................................17

4.1 MY NEW PRODUCTION MACHINE REQUIRES 110 CFM AT 90 PSI. WHAT SIZE COMPRESSOR DO I NEED?. .17

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4.2 HOW DO I CHOOSE BETWEEN A ROTARY SCREW AND A RECIPROCATING UNIT?..........................................174.3 WHAT DOES “TWO STAGE” MEAN?...............................................................................................................174.4 I HAVE WATER COMING OUT OF MY AIR LINES. WHY?................................................................................174.5 HOW MUCH AIR AM I LEAKING?....................................................................................................................174.6 MY OIL SALESMAN SAYS HE HAS AN 8000 HOUR OIL FOR MY ROTARY SCREW AIR COMPRESSOR. CAN I REALLY GET 8000 HOURS OUT OF MY OIL?...............................................................................................................174.7 HOW CAN I REPLACE MY TIRED, WORN OUT PUMP?  THE MOTOR AND TANK ARE STILL IN GOOD SHAPE....18

APPENDIX A - RETENTION PERIODS FOR QUALITY RECORDS........................................................19

APPENDIX B - Authorized Document Holders...................................................................................................19

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1. PREFACEWork is vested in Mark D.

2. The Basics for Choosing the Right Air CompressorThere is a lot of confusion out there about selecting the right size air compressor to run your air tools.

There is a lot of confusion out there about selecting the right size air compressor to run your air tools. Basically, there are several factors to look at when choosing the proper air compressor for your shop or garage.

2.1Air Compressor Articleshttp://www.aircompressorsdirect.com/catalog/default.php

2.2Air Compressor + Air Tools Advantages: Air tools are more powerful than traditional electric tools

Air tools deliver higher torque and higher RPM to help get the job done quickly

Air tools are versatile and easily interchangeable

Air tools are a safe alternative to other sources

2.3Air Compressor Uses

2.3.1 Home Use:

Compressed-air power tools can dramatically decrease the amount of time spent on a project. Many homeowners will use smaller, portable units (either hand-carried or wheeled) that move easily from the garage to the house, yard or roof. Common air tools such as brad nailers, reversible drills, and impact wrenches require about 0–5 CFM (Cubic feet per minute — units in which air flow is measured). It's important to know if the compressor you purchase will drive the air tool you'll need for your project. For example, smaller air nailers run on 2 to 5 CFM at 70 to 90 PSI, while larger shears, sanders and polishers require up to 10 or more CFM and 100 to 120 PSI.

2.3.2 Business Use:

Air compressors used for business/professional applications are often classified “industrial grade.” They're packed with features that let you take on more demanding jobs with more horsepower, more CFM, more PSI and more (longer) run times. Though some industrial-grade compressors are portable, many are stationary units with large-capacity, ASME certified air tanks (up to 120 gallons), greater HP, more CFM air volume and faster recovery time to run a wide variety of air tools for as long as you need them to do the job.

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2.4How to Pick the Perfect Air Compressorhttp://aircompressorsdirect.com/catalog/comparingcompressors.php

Finding a selection of air compressors is easy. Selecting the ideal air compressor is just as easy with a little knowledge.

"Naturally, price is an important factor to consider in purchasing an air compressor," says Jon Hoch, founder of Air Compressors Direct. "But what good is a great deal if your compressor can't do the job?"

He says that savvy shoppers should consider four major attributes before purchasing an air compressor: Style, Pump, Tank Size and Power.

2.5StyleAir compressors come in a variety of styles, ranging from tankless inflators to contractor grade models to heavy-duty industrial compressors. So how do you sort through all the clutter?

According to Hoch, you should be able to sum up your needs with a simple prepositional phrase. "We ask our customers where they want to use their air compressor, and they respond with one of four answers."

         Air Compressors Direct even redesigned its web site to help customers find the appropriate model. Customers can now "Shop by Style," narrowing their search to one of the four recommended uses.

"It almost sounds too simple, but it's extremely helpful in narrowing your search," says Hoch. "If you want an air compressor for 'around the house,' you don't have to sort through a bunch of irrelevant products."

2.6Pump TypesThe pump is another important factor in determining the ideal air compressor. There are three basic types of pumps: InvectorTM, Direct-Drive and Belt-Drive.

2.6.1 Invector Pumps

Invector Technology was introduced by Coleman Powermate in 2002. The lightweight models feature a universal motor with a patent-pending air cooling system, which extends the life of the air compressor.

"The Invector models are ideal for first-time air compressor buyers who aren't certain how often they will be using

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it," says Hoch.

2.6.2 Direct-Drive Pumps Direct-drive models feature standard induction motors. These models will last about 500 hours and take up less space.

"Direct drive models are oil free, so they're perfect for the occasional- to moderate-use owner, who doesn't want to deal with messy oil changes," says Hoch.

2.6.3 Belt-Drive PumpsBelt-drive models are quieter than their oil-free counterparts. They are best suited for do-it-yourselfers and professionals who frequently use their air compressor.

Belt-drive pumps will generally last three times longer than direct drive models, but they require oil and filter changes every 500 hours.

2.7Air Compressors: Stages

2.7.1 Two-Stage vs. Single-Stage

Two-stage air compressors provide a reliable air source in commercial, industrial and automotive applications. Multistage units tend to be more efficient, compressing air to a higher pressure than single-stage compressors. This allows more air to be stored for future use while generating less heat, reducing wear and extending compressor life.

2.8Horse PowerThe first criteria is horse power rating. Most people assume the higher the horsepower rating on the box, the better the air compressor. However, all horse power ratings are not created equal and can easily be misrepresented. What I mean is, you go to your local giant hardware retailer to pick out a new air compressor to run your impact wrench. They have a 5 hp unit that is priced really cheap. Why is that 5 hp industrial unit cost so much more? 5 hp is 5 hp, right? Not necessarily. Let me explain. Look at how much power the hardware store unit draws. It probably needs around 15 amps from a normal 110 volt circuit. At this rating, you are really getting only 2 hp. The 5hp rating on the box is inflated. To really produce true 5 hp you need at least 24 amps from 220 volt circuit to get it. If you are looking for 5 hp electric compressor, buy the industrial unit and stay away from the cheaper unit at your local hardware store. I am not saying that every air compressor at your local hardware is junk. A lot of times one of those units is all you need. Just be aware of the true horsepower rating.

Notice that we haven’t mentioned Horsepower ratings, since one manufacturer's 3 HP compressor may deliver the same CFM as another's 5 or 7 HP. The other thing is that most manufacturers advertise a compressor's PEAK HP rather than the actual RUNNING HP. When a compressor first starts up it draws more electrical current than when it's actually running. Inasmuch as the HP of an electric motor is evaluated by the amperage it draws, the peak HP is always greater than the running HP. So it's a bit pointless to evaluate the compressor by HP. This is especially true because the real job of the compressor is to deliver air at the CFM required by the tool it's running. However many HP a particular machine must muster to do that is really immaterial.

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2.9PSIThe next consideration when purchasing a new air compressor is, "How much PSI will I need?" For the newbies, PSI is short for "pounds per square inch" and this is how most compressors in the US are rated. In Europe, you will see them measured in bars. PSI is all you need to worry about on this side of the Atlantic. Most of the commonly used air tools require about 90 PSI to operate correctly. However, you will still need a compressor with a higher shut-off pressure. Most air compressors that you find at the local hardware giant are "single-stage" and shut off at 125 - 135 PSI. Don't let that fool you. You might think all you need is 90 psi, so that should work just fine. Generally, these light duty compressors shut off at 100 psi and don't forget about pressure loss in the line. The little light duty compressor will barely run an impact wrench. It might be fine for light duty garage use, but if you really intend use your air tool, more is definitely better. Many industrial compressors are "two-stage," which means they build up to shut-off pressure in two stages. The first stage builds to about 90 PSI and the second stage builds to 175 PSI.

2.10 CFM

2.10.1 SCFM, ACFM, ICFM...What Does it Mean?One of the terms that is often confused in compressed air and gas systems is the term CFM or Cubic Feet Per Minute. This term is also expressed as SCFM, ACFM, and ICFM. So what are the differences SCFM vs ACFM?Definition SCFM -- The term SCFM stands for Standard Cubic Feet Per Minute, referenced to a pre-specified pressure, temperature, and relative humidity. In most cases, SCFM is referenced to 14.7 PSIA, 68°F, and 0% relative humidity. By specifying these parameters, the mass flow of compressed air and gas systems is clearly defined.Basically it is how much air is being moved. Air tools require a certain amount of air volume to run on. PSI is just part of equation. Don't be confused by different CFM ratings at different pressures. Every manufacturer is trying to make their product look better by giving higher CFM ratings at different pressures. The only real concern is how much CFM you will get at 90 PSI. Remember 90 PSI is what most air tools require to operate. To find out what your air tool needs to run, just look on the box for the manufacturer’s specs. Generally, air tools require 4 - 6 CFM. A good rule of thumb on air compressors is you should get 3 -4 CFM per real HP at 90 PSI.

Many do-it-yourselfers often rate the power of an air compressor by PSI (pounds per square inch) or HP (horsepower). According to Landsly, these novice shoppers are overlooking the acronym that truly measures an air compressor's strength.

"CFM (cubic feet per minute) is the best gauge in determining power," says Landsly. "You should select an air compressor that exceeds the CFM of your most powerful air tool. This will ensure that you're never under-powered."

Air Compressors Direct created a great guide to help you gauge CFM ratings for common air tools. For example, if you are interested in using a spray gun, you should consider getting an air compressor with a 20-gallon tank (minimum) and 4 CFM @ 90 PSI or greater.

The real issue with regard to a compressor is how much air flow it will deliver at a given pressure.

It's very important that the delivery capacity of the compressor exceed the consumption, because it’s unwise and unduly hard on a compressor to have the motor running continuously in an attempt to "just keep up" with the air demand.

Most compressors are rated in SCFM  (Standard Cubic Feet per Minute), rather than ACFM (Actual Cubic Feet per Minute).  This means that they are rated using certain criteria – (In the United States) Air at 14.696 Pounds per Square Inch (PSIA)/ 60 Degrees Fahrenheit (°F) / 0% Relative Humidity (RH).  If your application is different from this, for example if you live in Denver or Albuquerque at high altitude, you will need to de-rate the rating.  If you'd like to see how altitude effects pumping, Tom uses these charts

Several choices will drive your air needs:

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ACFM Definition -- The term ACFM stands for Actual Cubic Feet Per Minute and is often improperly used by some air compressor vendors. For example, an air compressor vendor may state that a 200 HP air compressor is rated for 1000 ACFM at 100 PSIG. Taking this phrase literally, the natural assumption is that 1000 Actual Cubic Feet Per Minute of compressed air is leaving the discharge of the compressor at 100 PSIG. This is incorrect. The compressor vendor should have stated that this 200 HP air compressor is rated for 1000 Inlet Cubic Feet Per Minute (through the inlet air filter) when operating at a discharge pressure of 100 PSIG. The actual volume flow of compressed air leaving this air compressor is about 125 ACFM, depending on ambient conditions.

If in the above example, the atmospheric air pressure was 14.7 PSIA, the ambient air temperature was 68°F, and there was no water vapor in the ambient air (0% relative humidity), then this air compressor would produce 1000 SCFM when operating at 100 PSIG discharge pressure.

This entire subject can be very confusing and, if you are like most people, this paper will require more than one reading. In fact, we have spent many hours explaining this subject to consulting engineering companies.

At Fluid Energy, we believe that understanding basic fundamentals is important when analyzing and troubleshooting compressed air and gas systems. Please contact your Fluid Energy representative if you would like further definition on this subject.

2.11 How do I determine my CFM Requirements?The chart below includes some common air tools and their average CFM measurements. Reference this chart to determine what continuous CFM rating is required of your air compressor. Air tools are often rated as “Average CFM.” “Average CFM” is typically based on a 25% duty cycle (15 seconds out of a min.). This sometimes creates a problem with tools that are used on a continuous basis (ex. orbital sanders, die grinders, etc). Nailers and impact wrenches typically don't have the same problem since by their nature are used more intermittently. If you plan on using an air tool on a continuous basis, it is a good idea to multiply the “Average CFM” x 4 to get a continuous CFM rating for the tool.

Air Tool Description Average CFM @ 90 PSI

Angle Disc Grinder - 7" 5-8

Brad Nailer 0.3

Chisel/Hammer 3-11

Cut-Off Tool 4-10

Drill, Reversible or Straight-Line 3-6

Dual Sander 11-13

Framing Nailer 2.2

Grease Gun 4

Hydraulic Riveter 4

Impact Wrench - 3/8" 2.5-3.5

Impact Wrench - 1/2" 4-5

Impact Wrench - 1" 10

Mini Die Grinder 4-6

Needle Scaler 8-16

Nibbler 4

Orbital Sander 6-9

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Ratchet - 1/4" 2.5-3.5

Ratchet - 3/8" 4.5-5

Rotational Sander 8-12.5

Shears 8-16

Speed Saw 5

Tool Minimum Tank Size Air Requirement

0GALLONS

0-5CFM @ 90 PSI

2GALLONS

1-4CFM @ 90 PSI

4GALLONS

3-6CFM @ 90 PSI

20GALLONS

4-9CFM @ 90 PSI

30GALLONS

6-11CFM @ 90 PSI

30GALLONS

9-15CFM @ 90 PSI

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"Remember, this is just a guide," says Landsly. "You should find out the exact air requirements for your most powerful tool first, and then pick your compressor," warns Landsly.

Just click on the "Air Tools" tab to find out the CFM ratings for more than a hundred different air tools.

http://aircompressorsdirect.com/catalog/airtoolsa.php

2.12 Tank SizeWhen selecting a tank, allow 2 to 4 gallons per cfm

Air compressor tanks vary widely in size, ranging from 2- to 120-gallons. The longer you need to continuously use an air tool, the bigger the tank you'll need.

"Air compressor tanks are similar to hot water tanks in your home," says Andy Landsly, a homeowner who recently switched to pneumatic tools. "If you want short bursts of hot water, a small tank will suffice. If you're going to tap the tank for extended periods, you'll need a large one."

Short burst tools like impact wrenches, staplers and nailers work well with a small tank. Continuous use tools like grinders, sanders, sprayers and sandblasters will require a large tank.

As a general rule, if you can't decide between two comparable air compressors, go with the larger tank, suggests Landsly.

The final factor to consider is tank size. Tank sizes are generally stated in US gallons. For example, 30 gallon is a common tank size. So the question remains, how large a tank do I need? First of all, don't confuse a large tank with more run time for your air tools. If you use your impact intermittently, a large tank is fine. However if you have need for continuous use, you will need a small tank with big enough pump and motor. If the pump and motor are powerful enough, you shouldn't run out of air. You can save some dollars by purchasing an air compressor with a large tank and smaller motor for intermittent use. If you need to run a 1" impact wrench (about 20 CFM) intermittently, and have a small compressor with a large tank, you might have enough air stored in the tank to do the job. However, if you are constantly running your air tool, you will need to invest in a more powerful air compressor to do the job. You should be able to select the air compressor armed with this information. Basically, you need to decide ultimately what you going to use the unit for and pick the appropriate model.

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2.13 Pressure Pot versus siphon gunPressure pots work efficiently at much lower pressures and flow rates than do siphon guns. So the bottom line there is that a siphon system will require a compressor capable of delivering more air at a higher pressure than a pressure pot (the operating pressure of a siphon gun is generally in the 80-90 PSI range when blasting glass, whereas pressure pots are generally operated at 20-40 PSI for glass).

2.14 Nozzle sizeBasically a larger diameter nozzle consumes more air than does a smaller diameter; AND you have to remember that grit will enlarge the nozzle diameter over time during blasting. So, the CFM requirement increases as a nozzle gets used, and eventually the nozzle must be changed. Remember that when you double the diameter, you quadruple the CFM required!

Some folks use ceramic nozzles; their very light weight makes it easier to do very delicate work.  While ceramic nozzles wear away much faster, their initial cost is less than carbide nozzles.   If you’re using ceramic nozzles on a large job, the wearing of the nozzle is something to contend with in respect to overall uniformity of the blast (due to the rapid eroding of the nozzle).  With carbide nozzles the wear of the nozzle is so minute that it’s not much of a factor.

Most glass blasters start out with a 3/32" nozzle, and generally don't go larger than 1/8". A 3/32" nozzle requires 5 CFM @ 40 psi and the same nozzle requires 9 CFM @ 80 psi. If that same 3/32" nozzle enlarges to 1/8" because of grit wear during blasting, the air requirement of the now 1/8" nozzle is 9 CFM @ 40 psi, and 17 CFM @ 80 psi.

Nozzle size CFM @ 40 psi CFM @ 90 psi Suggested Compressor  1/16 2 4 2 HP  3/32 5 9 3 to 5 HP   1/8 9 17 5 HP

data extracted from Dobbins Etched Glass Techniques and Design (see Resources section)

You'll find additional data in the lower right-hand corner on the second page of Tom's reference.   

2.15 Percent of Daily Use- Job size - continuous use versus intermittentThe amount of time that you spend daily at the blast cabinet or in the blast room does become a factor when you’re choosing a compressor. You can afford to wait a few minutes for a small compressor to build up more air and cool

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down if you’re doing an occasional ornament or sun catcher. If you’re doing larger work where you’re blasting for hours that wait time begins to feel like an eternity and a larger portion of your day is spent waiting. In that case you will want a compressor that keeps up with your work load.

By the time you crank these numbers, you'll probably need a compressor that will produce at least 8 CFM at 90 psi, and want at least a 60 gallon tank.  Folks who blast rocks, who do large pieces, have siphon guns, or who do long production runs will need more. 

 

3. What type of compressor should I buy?

3.1Rotary Screw, Reciprocating, Centrifugal, Diaphragm and Oil less. There are others, but unless you’re to use this compressor in a plant turning out high volume work then we’ll primarily focus on oil lubricated reciprocating.   We'll also mention oil less compressors, even though most people in the Sand carving E-group that have used one won’t vouch for their longevity. (They are also REALLY LOUD.)

Here’s what Ingersoll-Rand has to say about reciprocating air compressors:

3.1.1 Reciprocating

Reciprocating air compressors are positive displacement compressors. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by using a piston within a cylinder as the compressing and displacing element.

The reciprocating air compressor is considered single acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting.

The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve.

Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure.

A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ratio (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other design problems.

Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections. 

3.1.2 Oil less (~$300)  Oil less compressors are designed with a low-friction coating on their moving parts to eliminate the need for oil, are generally less expensive than their oil-lubricated counterparts and don't require you to change the oil on a regular basis.  And, the air they produce is cleaner because there is no residue from the lubricating oil (although you should still use an inline filter).  These compressors are really LOUD (89 decibels and higher), and generally have duty cycles of 50% or less.  A few years ago, 60 gallon oil-less compressors were common; that "class" of higher-end consumer compressor has been replaced by the oil lubricated single stage compressor, generally with an all-aluminum compressor.   

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3.1.3 Piston/ reciprocating oil lubricated (consumer units: ~$400 to $800; commercial units: ~$1000 to $2500 and up) As the name implies, these compressors use oil for lubrication. They're significantly quieter than most oil less compressors and, if taken care of, can last a lifetime or two. You will need to change the oil periodically (once per year is enough for home shops, but follow your manufacturer's directions) and you'll need to install a high quality inline filter because of small amounts of oil that get in the air. Life expectancy generally runs anywhere from 2,000 to 6,000 actual pumping hours between rebuilds.  Both single and two stage units are available; they generally feature all cast iron construction for durability and noise reduction. 

3.2What's new: consumer two stage units (~$800)  The most interesting news in air compressors is the recent availability for consumers of higher pressure, 2-stage machines. In the past, the maximum pressure developed by a compressor was around 125 psi. This was the pressure that the compressor would "cut out." Recently Porter-Cable, Campbell Hausfeld and others have introduced compressors capable of developing 175 psi. What's the big deal, you say? After all, you can't use any kind of glass gun at those pressures! Well, imagine that tank below the compressor. It will hold much more air at a higher pressure. You're essentially squeezing the air in tighter. Effectively, there can be as much as 140 percent more air delivered. According to Porter-Cable, their new 2-stage model CPLKC708V2, which has an 80-gallon tank and a cut-out pressure of 175 psi, has the equivalent air storage of a 194-gallon tank!  The "cut-in" pressure (the pressure level that kicks on the compressor), in compressors currently on the market, is  about 95 psi, while the new 2-stage compressors have a cut-in pressure of about 145 psi, so the compressor keeps a significant reserve for long blasting jobs.  

3.3Tank size and shape Sandcarvers generally use fixed location tanks, and favor larger sizes.  Most of us therefore use vertically oriented tanks, of at least 60 gallons, with 80 preferred.   The full range of available compressors is staggering. There's a "buyer's guide" to compressors (dated 2001) here: American Woodworker compressor guide

Ingersoll Rand also makes fine compressors, and should be investigated as part of any selection process.  For some reason, they're missing from this list.

3.4Some typical compressors:  

Table shows comparison of Porter Cable Oil Lubricated Compressors  

Compressor Porter Cable 7.5 HP Porter Cable 7 HP Porter Cable 7HP Peak Model: CPLMC7580V2C CPLKC7080V2 CPLC7060VPrice $1,499 $789 $396

Maximum Pressure 175 175 135Stages: 2 2 1

Lubrication Oil Oil OilCylinder body: Cast Iron Cast Iron (sleeve) Cast Iron (sleeve)

Crankcase and Head Aluminum Aluminum AluminumVoltage 240 240 240

Magnetic Starter Yes not required not requiredTank 80 Gallon 80 Gallon 60 Gallon

Duty Cycle Continuous Continuous Intermittent, no duty cycle listedCFM 25 SCFM @ 100 psi 16.8 @ 90 psi 9.7 @ 90 psi

 

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No, we're not necessarily pushing Porter Cable; but the range of features across different models in one manufacturer's product line makes for interesting reading. 

3.5What features should I select for durability?  So, why do those "professional grade" compressors cost so much, and why would I want one anyway?   Durability and ease of maintenance distinguish commercial units.   

3.5.1 Aluminum versus cast iron cylinders

Sandblasting per se can be an industrial activity that exceeds the air delivery and robustness of many of the "consumer grade" or "homeowner" compressors sold at the "big box" home improvement stores like Lowe’s or Home Depot. Some of the larger compressors sold in those stores are machines that are deliberately made with aluminum pump cylinders so that the prices can be kept below the more robust machines built with all cast iron pump heads. (Note that cast iron compressors are also being sold at these stores.) Despite the CFM ratings of some larger compressors sold in those stores, many of those machines are definitely NOT made to run at continuous duty as are more industrial cast iron compressors.

The bottom line is that even a large compressor equipped with a 60 gal tank that is rated at maybe 10-12 CFM @ 40 psi, will have to run for longer time periods than the machine is capable of sustaining without overheating. The result would be that the pump cylinders get quite hot, and, because they’re made of aluminum, the heat would cause warping and structural changes in the metal that would eventually lead to early failure. This is in stark contrast to an industrial grade compressor, some of which are rated for continuous duty (meaning they can run all the time and not have heat failure) that are made of cast iron, and will likely cost more and have to be bought at a tool shop that specializes in industrial grade tools.

Porter Cable's hybrid cast iron sleeved, aluminum body compressor is interesting, in that it provides cast iron for durability, and aluminum for heat rejection.  If it proves as durable as most Porter Cable tools, it would be an interesting choice.  

Speaking of compressor design – another thing to consider: whether a twin cylinder compressor is of v-twin design or parallel design. With the v-twin design the cylinders are easier to uniformly cool.

3.5.2 Duty Cycle-

The duty cycle of an air compressor is what percentage of the time it is designed to run before it must rest and cool off. Each manufacturer lists the duty cycle in the specifications of that compressor. If he doesn’t know, have him find out before purchasing the unit.

 By figuring what CFM you’ll be blasting at (nozzle orifice size and  PSI) and the tank size, pressure cut on, and maximum pressure you can figure how often the compressor will be running and resting.  Some compressors are listed as continuous duty. For those you can expect to pay more than for a comparable CFM rating compressor with a duty cycle less than 100%.

Compressors generate a lot of heat in doing their job. The heat is tough on the pump components and they'll wear out quickly if the compressor runs all the time. If you don't want to over work your compressor, and risk burning it out, you MUST get a compressor that is capable of delivering about 50% more air than the tool you’re running will demand at its operating psi. So from the example above, if a person is using a pressure pot with a 3/32" nozzle requiring 5 CFM @ 40 psi to blast glass, then the compressor must deliver about 7-8 CFM @ 40 psi in order to meet the usage criteria of 50% excess air capability. As the nozzle enlarges to 1/8", the same compressor should deliver about 12-13 CFM @ 40 psi.

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3.5.3 Cost

If you don’t want to look back and say “ I wish I had bought a bigger or better compressor” now is the time to really think. If you get that cheaper compressor simply because of the cost factor it’ll be a mistake.  We have all made this same  mistake. Take the time to weigh all the factors of  many compressors from many vendors.  Choose the one that fits you and get it. It’ll be the cheapest in the long run.

Cost is also not just the initial price of the compressor. What’s its track record? Does it have a lot of maintenance? Does it require a special oil and if so is there a reason? Does it have a cheap looking inefficient air filter for the intake? Does it have a Baldor motor or is it something you’ve never heard of made in China? These are considerations.    

3.5.4 Brand Name-

A brand name has a lot to do with parts and service. If you need a belt for an off brand compressor and can’t find one locally will you be able to order it from the manufacturer? Can you get it worked on if need be?  Ingersoll Rand, Porter Cable, Campbell Hausfeld, and Quincy are four brands that have generally been well received.

3.5.5 Warranty

Last, but not least what warranty does the compressor that you chose have? Most compressors run for years with little or no work that needs to be done except for regular oil and filter changes. Occasionally a motor, compressor, or pressure switch will go out. Find out if it’s covered and for how long.

3.6Where should I install it?Just when you thought you had this all figured out, we'll throw in some real food for thought.  Choosing a location for your compressor is at least as important as any other single consideration.

3.6.1 Noise

.  If you have neighbors, or if you share your living quarters with anyone else, you better think this one through.  The 89 decibel staccato clatter from an oil less compressor can be deafening, or at least a major irritation (assuming you're partly deaf already).  This fact alone is enough to consider an oil lubricated unit.

3.6.2 Heat

Compressors make heat.  Putting one in a closet is a sure way to overheat the unit, and damage it.  You will need air circulation

3.6.3 Draining

Every year, folks die from compressor explosions in home shops.  They need to be drained regularly to avoid corrosion.  If it's tough to do, you probably won't do it.  Most of us install pipe and a ball valve, running the drain line to a convenient location, so we can open it at the end of the day.  Do this, it's cheap.

Another alternative is to install an automatic drain valve. At somewhere around $150 before installation they’re somewhat expensive, but for the person that doesn’t remember to drain his tank daily it’s the solution to his problem. Tom uses one and swears by it.

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3.6.4 Air quality

Sand carving generates fine abrasive dust.  If you discharge your exhaust system next to your air compressor, and don't filter it down to 0.3 microns, expect to replace your compressor sooner than you would like.

3.6.5 Maintenance

Oil lubricated compressors require oil changes on at least an annual basis.  You will need access to do this.

3.6.6 Piping and filtering the air

This is a section all by itself.  Tom will maybe tackle this when he gets a "round tuit".  Someone please send Tom a round tuit so he can enlighten us.

3.7Conclusion:A simple fact is that we can’t choose a compressor for you. That’s something that you have to do for yourself. Weigh all the factors and get what you think you need. If you have to save up and wait a while you’ll be better off. It’s a lot better than getting something that just won’t do the job.

Need more resources?  Think about folks in your area who have similar uses for compressors.  Probably an auto paint shop has similar demand for continuous duty, moisture free air.  See what works for them, and what maintenance sources are available in your area. 

3.8Features to consider when purchasing an Air Compressor ASME (American Society of Mechanical Engineers) tank

Belt-driven Air Compressor — Belt-driven compressors generally have a longer life, with less vibration and heat transferring from the motor/engine to the pump.

Cast iron cylinder or cast iron pump sleeves — Cast iron is typically the standard material for cylinders

Fully packaged unit with premounted and prewired magnetic motor start, dual control, belt guard mounted air cooled after cooler and electric condensate drain

Low-oil shutdown prevents downtime and costly repairs resulting from low oil levels

Oil-Free (or Oil-less) compressors are virtually maintenance-free performers for light- to general-duty applications. No need for periodic oil changes.

V-design cylinders — V-design cylinders typically run cooler that in-line cylinders. Heat is a compressors worst enemy.

Vertical vs. horizontal air compressors — Vertical compressors usually take up less floor space than horizontal compressors. Typically, vertical compressors are stationary, whereas smaller horizontal compressors are better for portable applications (such as a truck-mounted compressor).

100% Continuous Speed operation vs. Auto Start/Stop operation — Auto start/stop operation has preset cut in/cut out pressures. The compressor actually stops and turns back on as air is needed. With constant speed operation, the compressor runs 100% of the time and simply adjusts the air output. Dual control compressors offer both auto start/stop and continuos speed operation.

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4. Q&A

4.1My new production machine requires 110 CFM at 90 psi. What size compressor do I need?A generally accepted rule is that you will get about 4 CFM of air per horsepower of electrical power at around 90 to 100 psi discharge pressure. A 30 HP unit should be large enough to supply this machine. Remember to subtract any other requirements which may be on this new system (for example, an air purge desiccant air dryer). Also, be sure to plan for pressure loss through the clean up equipment and system piping.

4.2How do I choose between a rotary screw and a reciprocating unit? If you have a pressure requirement below 200 psi and a flow requirement of 125 SCFM (30 HP) or less, reciprocating units are normally less expensive to buy and are cheaper to maintain than rotary screws. Each supplies about 4 to 4.5 CFM per horsepower. A disadvantage to reciprocating units is the noise they produce. A disadvantage to rotary screw units is the concentrated heat stream they produce. Above 125 SCFM and below 200 psi, reciprocating units begin to get very expensive to buy and maintain and rotary screws are usually the better choice.  If your compressed air requirement is greater than 200 psi, a reciprocating air compressor is about the only choice you'll have.

Get recommendations from someone you trust who has expertise in compressed air systems.

4.3 What does “two stage” mean? Two stage air compressors compress the air in two "pressure steps." The first stage compresses the air to about 30 psi and the second stage compresses to the final discharge pressure (in systems operating from 100 to around 125 psi). The air is normally cooled between the stages which increases the efficiency of the unit.  Many types of reciprocating air compressors are two stage.  Most oil-flooded rotary screw air compressors are single stage. 

4.4I have water coming out of my air lines. Why? The air being compressed contains water vapor. As the air is compressed it heats up and becomes saturated with moisture. As the air cools, moisture condenses in the air system. You can remove the moisture from your compressed air system with an air dryer.  

4.5How much air am I leaking? Good question, tough answer. Give us a call at 800-323-8090 for more information. Ask for the Air System Audit Department.  

4.6My oil salesman says he has an 8000 hour oil for my rotary screw air compressor. Can I really get 8000 hours out of my oil? Oil life in a oil flooded rotary screw air compressor is a function of many things, but particularly affected by the type of environment the unit is in.  You can not correctly assume you will get 8000 hours use from your compressor oil. A better approach to determine oil life is to change the oil based on periodic oil sampling.

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4.7How can I replace my tired, worn out pump?  The motor and tank are still in good shape.The information we need to find a replacement pump is found here.

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APPENDIX A - Retention Periods for Quality Records

RECORD NAMERET.PERIOD LOCATION DESCRIPTION

APPENDIX B - Authorized Document HoldersThe following list represents the job titles, functions or service areas authorized to receive controlled hard copies of the QMS document listed. The service area Document Controller and any requests to be added to the lists should be directed to this individual maintains this list.

TITLE OF COPY HOLDER NUMBER OF CONTROLLED HARD

COPIES ISSUED

COPY NUMBER

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