Choosing an Air Compressor

Air Compressor

[: a device that converts electrical power or gas into kinetic energy by pressurizing and compressing air, which is then released in quick bursts]

Things to consider when selecting an air compressor:

Cubic Feet per Minute

Amount of air required, cubic feet per minute  (CFM) is probably the single biggest consideration when buying a compressor. What CFM will your air compressor need to produce in order to power all of your air tools?compressors.

Cubic Feet per Minutes, Free Air Delivery 

This unit of measurement is the volume of air actually discharged from the air compressor to the output (i.e your air tools). Free air delivery is far more useful as it’s the actual amount of air (CFM displacement) or  ‘power’ available for tools etc

     · Low-oil shut down prevents downtime and costly repairs resulting from l ow 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 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 continuous speed operation.

· Two-stage air compressors provide a reliable air source in commercial, industrial and automotive applications. Multi stage 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.

· Identify the tool requirements. Most air powered tools will have a measuring unit indicating minimum air pressure required to operate. Most common is the SCFM (Standard Cubic Feet per Minute). Ensure your air compressor is capable of providing this value compressors.

· To find the minimum required CFM of the compressor you need multiply your tool’s required SCFM by 1.2

· Identify the power requirements. Air compressors come in either 120/220Volts, 120V compressors are suited to home-improvement and small projects while 220v means it are an industrial compressor. 120v/220v air compressors will need a 15AMP dedicated circuit to power them. This means you cannot use your washing machine and the compressor in the same circuit.

· Figure at least $100-200 for the air delivery system; stationary hard air line (galvanized, black iron piping or copper pipe, don't use PVC). You'll need air fittings, a good quality oil /water filter / pressure regulator, valves for drop lines to bleed off water, flex line from the compressor to the hard stationary air line, etc

Check - horsepower (HP) air delivery rate (CFM) pressure (PSI) power requirements (AMP) – amperage requirements (single or three phase)

Generally speaking, the higher the HP, the greater flow rate you can use on a constant basis (greater number of tools, or tools with higher flow rate requirements). The greater the tank volumes, the longer you can run without having the air pump kick in. In evaluating air compressors, beware of misleading specifications.

For example, flow rates specified in CFH (cubic feet per hour) instead of CFM (cubic feet per minute). This is often used on compressors with very low output to make them sound ‘like they have a larger air volume.  Worse yet are compressors that spec the maximum pressure they can output. While pumping 250 psi is interesting, what use is it if you only need 30 psi? Typically, what this means is that the flow rate is so low that the mfg. doesn't want to publish it, instead using the max pressure because it’s a big number.

Sizing an Air Compressor

Most compressor companies base their CFM output @ 0PSI but then sneak there "peak" ratings in the big bold print on the side of the compressor along with the "peak" HP ratings. Divide the tank volume in gallons by 7.48 (1 cu-ft = 7.48 gallons) to get the tank volume in cubic feet.  Atmospheric pressure (atm); 1 atmosphere (atm) = 14.7 psi, the 17 psi of pressure added during the cycle is 17/14.7 = 1.16 atm of pressure during the cycle.

Tank Size

The tank size, measured in US gallons, tell you how much air is compressed and give you something of an idea of how long you can operate your air tool for at a time. The other factors that affect how long you can run your tool for are the PSI and HP of the engine itself as these factors determine how quickly your compressed air supply is replenished

The compressor says it has a 25 gallon tank, and if I trigger a refill cycle by bleeding out air slowly with the relief valve, I observe on the tank gauge (not the downstream gauge) that the compressor "cuts in" at 85 PSI (G) and "cuts out" again at 102 PSI (G), difference of 17 PSI. It cranks for 35 seconds to build up that pressure.

Thus in one cycle, the rate at which air is being pumped into the tank, is the pressure rise times the volume of the tank, or 3.34 cubic feet * 1.16 atm = 3.87 cubic feet per 35 seconds. To proportion the 35 seconds up to minutes, to get the pumped volume per minute, multiply by 60/35, or 3.87 * 60/35 = 6.6 CFM (at 85 PSI (G)).

Here are some of the key attributes you'll need to pay attention to when choosing an air compressor

Horsepower (HP)

Air compressors, as their name implies, compress air. This takes an engine and engines have horsepower. The horsepower of the engine you're considering has direct impact on all the other aspects of your compressor's performance. Horsepower is often inflated, and you should be paying closer attention to the engine's rated amperage for an idea of how powerful it is. (See also Electrical Formula)

Rule of thumb, for compressors under 10HP figure on getting 2-3 CFM of compressed air flow at 90 PSI for each HP of motor size

Air Pressure (PSI)

PSI stands for pounds for square inch. Of course as you remember from high school physics class PSI is a measurement of pressure. To be exact, how many pounds of pressure are applied per square inch? The PSI rating is one of the crucial ratings in the air compressor to understand as air tools have a minimum amount of PSI required to run. Typically that's 90 PSI, but it depends from tool to tool.

Air Volume (CFM)

CFM stands for Cubic Feet per Minute and it's a unit for measuring the rate of air out of your air compressor. Air tools typically require 4-6 CFM for proper operation.

It is also common to size receivers at 1 gallon for each ACFM (Actual Cubic Feet per Minute), or 4 gallons per compressor hp (horse power)

Note: Standard cubic feet per minute (SCFM) is the volumetric flow rate of a gas corrected to "standardized" conditions of temperature, pressure and relative humidity, thus representing a precise mass flow rate at standard conditions ( 14.73 PSI (A) and a temperature of 68. °F, with a relative humidity of 40% RH)

Air Tools

In addition to pressure, each tool needs a certain amount of air (airflow) to operate. The volume of air needed to operate a tool is based on the size of the tool and the speed you want to use it. Any excess pressure is absorbed by the tool which causes premature wear that shortens tool life. It could also increase the level of noise and vibration exposure to the operator.

· 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 (minimizing hand fatigue from vibration)

The important things to look for is the CFM output at a given PSI rating, to ensure the air tools you plan to use will not exceed this rating in order to operate properly. Generally speaking, the higher the HP, the more flow rate you can use on a constant basis (greater number of tools, or tools with higher flow rate requirements). The greater the tank volumes, the longer you can run without having the air pump kick in.

Tool air requirements SCFM @ 90 PSI

· 6-inch pneumatic palm Sander / Polisher - 15-20.0

· 3- inch  pneumatic palm Sander / Polisher – 4- 6.0

· Impact Wrench - 2.5-10.0

· Mini Die Grinder 4-6.0

· Grease Gun -  4.0

· Touch-up HVLP air spray gun 4 – 6.0

· Air Foamer HP Foam Gun – 5.0

· Tornador Car Cleaning Gun – 5.0

Add the CFM of the tools that will be used at the same time plus 10%, add this number to the total CFM required. The number you come up with will be the highest rating of air compressor you should be looking for. This CFM rating will allow you to run your tools without over-taxing the compressor.

Air Compressor

For most detailing tasks a small air compressor will suit the job. Generally this means a compressor with a capacity of about 5 to 20gallons, to run pneumatic a palm hand sander / polisher, and other air-powered tools. A volume detailing shop with multiple operatives will require an industrial air compressor for heavy duty work can go up to 60 gallons.