I recall many trips to the corner service station as a kid to fill my bicycle tires or to top-up my basketball before a game. These days, compressors are everywhere and in every size, from the behemoth at the local garage that runs all day, to the small emergency units that plug into the cigarette lighter in your car.
There has never been a greater selection of compressors for the woodworker to choose from. All compressors accomplish the same basic operation; they compress free air into a tank to a preset pressure, which is then dispensed to the air driven tools via a regulator and a hose. As you may expect, there are several variations on compressor design, and these will determine which type and style of compressor would suit the work you intend to do with it.
Compressors have evolved to suit the needs of the trades’ people. Some situations, such as a service station or production shop, would need a large capacity of high-pressure air. Other situations require portability for working remotely at a jobsite. Different applications call for different compressor designs.
The tank on a compressor, which hold the compressed air, can be mounted either horizontally or vertically. A horizontally mounted tank has a lower center of gravity, so is preferable if the unit is going to be moved occasionally. A vertical orientation uses less floor space, which can be a great advantage in a smaller shop. It is also easier to drain completely. The names of various styles of portable compressors are based on the storage tank’s shape and orientation. The smallest of these is the pancake compressor (Porter Cable C2002- DeltaPorterCable.com). As you might guess from the name, the tank is a small round flattened disc that has the motor mounted on top. A horizontal (or hotdog) compressor (Porter Cable CPF4515), has a longer cylindrical tank, usually mounted on rubber feet, with the motor and handle on top side.
The twin (or side) stack compressor, (King 8488 – KingCanada.com) has two horizontal tanks beside each other to provide additional storage capacity. An upright compressor (King Canada 8498) is similar to the horizontal version, with the motor located on the top end of the tank. A wheelbarrow style compressor (DeWalt D55570, DeWalt.ca) is usually gas powered, and typically used on construction sites. Industrial compressors (Porter Cable C7550) are the largest and most powerful compressors available, suited for continuous high air consumption applications.
When looking at a portable compressor, keep in mind that gauges facing up are much easier to read than ones that force you to squat down to look at them head on. If your compressor will be moved about in vehicles, choose one that has adequate protection for the gauges and fittings in the event it should be a roll over.
If you want a compressor that will serve your needs for many years to come, there are several important ratings that you must carefully evaluate when choosing the right model for the work you expect to do. In addition to the horsepower values we normally look at when selecting tools, there are additional values for PSI and CFM to consider. PSI refers to the pressure in pounds per square inch to which the air is compressed for the tool to do its work. While most compressors deliver air at anywhere from 0 to 200 PSI, most air tools require 90 PSI. CFM refers to the volume of air the pump can deliver in cubic feet per minute. Because air pressure varies with temperature, humidity and atmospheric pressure, manufacturers report a ‘standardized’ level of air pressure, SCFM, which takes these variables into account. To properly size a compressor to the job at hand, you will first need to make a list of all of the air tools you will be likely to use.
Manufacturers will state how much air (SCFM) that a tool will consume at a certain pressure (PSI) needed for the tool to do its job. For example, a finish nailer may consume 3 SCFM at 90 PSI. When selecting a compressor, first choose the tool that has the highest demand, and to allow you a slight margin for comfort, add 25% to this value. So if the tool with the highest demand is an impact wrench at 6 SCFM, then you should be looking for a compressor that can deliver at least 7.5 SCFM at 90 PSI. If you will be running more than one tool at a time, for example, two roofing nailers, add the requirements of both tools and then add a 10 to 15% margin for safety. Using a compressor with a lower rating means you will need to use the tool intermittently to let the pump recharge the reservoir.
With the need of the various tools established, the next consideration would be tank size. Compressors come in a wide range of sizes, ranging from the 1 to 3 gallon pancake compressors, 4 to 6 gallon twin stack and small portable (horizontal and upright – King model 8498 upright shown left) compressors, 6 to 35 gallon horizontal and upright portables, and larger 40 gallon and over, stationary industrial models.
Having a larger tank gives you the ability to store a greater reserve of air, and will reduce the amount of time your pump must cycle to keep up your working pressure. As the maximum storage pressure increases for a given tank size, the volume of stored air increases as well. A compressor that is set to a maximum pressure of 135 PSI will store more air than a compressor that shuts off at 120 PSI.
The heart of any compressor is the pump that does the actual work of compressing the air. Here you will find the greatest variation in design. Compressor pumps fall into two categories; either oil-less or oil lubricated.
Again, the use of the compressor will determine the type of pump that is best suited to your needs. When weight is not a concern, cast iron is the material of choice for the pump. This hard-wearing material will stand up to constant use and it is very efficient at dissipating the heat generated as the air is compressed in the cylinder. The pump functions much like the engine in your car; the air is pulled into the piston and it is then compressed into the tank for storage. Cast iron pumps come in either single or two-stage systems. With a single-stage system, one cylinder is responsible for the entire compressing operation. A two-stage system will use a larger primary cylinder to compress the air to a certain level, after which it is passed on to a smaller cylinder that finishes compressing it the rest of the way. In operation, two-stage oil bath compressors are the quietest of all the available designs. These pumps run at the lowest RPM and the mass of the cast iron helps reduce noise to a bare minimum.
Cast iron pumps filled with oil are a great solution for a stationary shop compressor, but when portability is required, some compromises have to be made. Both the 20-gallon horizontal and the 20-gallon vertical compressors on wheels can have a full cast iron pump. Although the wheels make them somewhat portable, they are not very light. To reduce the weight of the compressor aluminum is often substituted for the body of the pump. Aluminum is a softer material, which will wear faster than cast iron, so manufacturers will often insert a cast iron sleeve into the cylinder for additional wear resistance. The compressor in the ‘Air filter’ photo has an aluminum pump, cast iron cylinder section and an aluminum head. These run at a slightly higher RPM with the result that these compressors are not as quiet as the ones with the solid cast iron pumps, but they are still pretty tolerable in the enclosed environment of a shop.
If ease of portability and compact size is the key issue when it comes to a compressor, then the small portable pancake or hotdog style units win hands down. Pump construction plays a large part in the small size and light weight of these portable units. Manufacturers have substituted lighter weight plastic and aluminum parts for the iron, and they use Teflon coated piston rings and lubricated bearings to make them oil-less. These compressors typically have smaller pistons that operate at much higher speeds than the oil bath models, making them the noisiest class of compressor. Using one of these will require the use of hearing protection if the compressor is in the immediate area; if possible, place the compressor in another room away from your workspace to reduce the noise.
Unlike purchasing a table saw, which will happily cut wood all day long non-stop, compressors normally come with a duty cycle. This refers to the percentage of time the compressor can run in a given length of time without damaging itself. As mentioned earlier, as the air is compressed it heats up, and this causes the pump, motor and associated piping to heat up. With the motor correctly sized and protected with thermal overload devices, there is no need to worry about damaging the motor. If the motor exceeds its maximum operating temperature it will automatically shut off. The large cast iron compressors are the most robust of the various models and can dissipate the heat built up. As the pumps become lighter in weight and the RPMs increase, the heat will build up faster than it can be dissipated. To avoid damaging the piston rings and bearings the manufacturer will have determined an acceptable duty cycle that will keep the heat below critical levels. A duty cycle of 30% means that in every hour the compressor can run for 18 minutes, a 50% duty cycle would allow 30 minutes of operation per hour. Exceeding the duty cycle will cause excessive wear and premature failure of the pump. The interval of time can vary between manufacturers. Some use an hour while other average the use over 30 minutes. Check to see which time frame the manufacturer specifies in the owners manual.
Moisture is a problem with compressors. As the air is compressed it is warmed and then it is blown into a tank. When this air hits the cold outer walls of the tank, it condenses and forms water droplets, which then collect at the bottom of the tank. With a larger tank there is a greater distance that the air will travel before it hits the cold steel of the tank, and in the process it will cool off, resulting is less condensation in the tank. With a twin tank compressor such as the King 8488, the air is pumped into the bottom tank and removed from the upper tank. This creates additional separation between any pooled water in the bottom of the tank and the air outlet. As a percentage of its volume, a small tank will fill up faster with water than a large one, and the confined space of a small tank will mean that more of this moisture is then picked up in the turbulence and sent on to the tools through the air hose. Moisture in the supply air can cause contamination and corrosion in the tools, resulting in a loss in safety and efficiency. If you are using your compressor to drive a spray gun for finishing, moisture in the line will become atomized with the finish resulting in serious problems when using oil-based finishes. If you routinely use a shop compressor to spray finishes, add a drier to remove water and a filter to separate any oil from the air stream, to prevent any contamination of your coatings.
Once the air has been compressed and stored in the tank, it is ready to make your life easier. The only downside is that you still need to get it to the tools to use it, and this can be a frustrating experience. If you have a stationary compressor, a distribution system using copper pipe and providing outlets at each of your workstations might be your best option, but if you have a portable unit, an air hose of some description is in order. Just as there is a range of compressors to choose from, there are a number of different hoses that you can choose from. Hoses come in ¼”, ⅜” and ½” internal diameters. A hose with a larger internal diameter will pose less of a restriction to the flow of air from the compressor and result in less of a drop in pressure when using longer runs of hoses. Air hoses are typically made with three layers of material, a smooth inner core that is covered by a layer of reinforcing material which is then in turn covered with a protective outer jacket. As with most products, there are several different levels of quality to consider, and buying the least expensive hose will leave you frustrated in no time.
Inexpensive hoses are made of vinyl, and while they may be cheap ($20 for a 50 foot length) they are not very flexible. Using this type of hose in a cool shop will leave you with a stiff tangled mess every time. At the other end of the scale are the professional rubber hoses. These are of a heavy-duty construction designed to withstand job-site abuse and remain flexible in all but the coldest temperature conditions. Bostitch introduced a new hose when they brought their TrimAir compressor to market several years ago and this has quickly become the favourite in our shop. The compressor is made of a translucent yellow material that stays flexible across a wide range of temperatures and allows you to see any contaminants in the line. It has a set of ridges that run the length of the hose and these keep the hose from hanging up on objects and getting tangled as the hose is dragged around the shop, job site or finished interior. The outer jacket of this hose is also non-marking so it won’t leave scuff makes on trim or furniture as it slides by.
Short vinyl hoses are available in a coiled format that has some spring to it. These can be very effective when hung from the ceiling over a workbench. However, when used in place of a standard hose on the ground, the recoil of those hoses can often pull the tool out of your reach whenever you put it down, if you are more than a few feet from the compressor. The hoses are not sold with the male and female disconnect fittings, so they must be purchased separately. There are several different types of fittings; choose one style that is readily available, and use that for every hose and tool so they are all compatible.
The control panel of most compressors comprises: an on/off switch, a tank pressure gauge (showing the pressure of air in the tank), an air pressure gauge (showing the air pressure available to the tool), an air pressure control or regulator knob (by which you regulate the air pressure available to the tool), a safety valve (that enables you to quickly release air from the tank), and one or two air hose fittings (that deliver air, through hoses, to the tools). Most compressors will shut off when the pressure in the tank is between 120 and 125 PSI, and will turn on again when the tank pressure drops below 90 PSI. You will find that most air tools run at around the 90 PSI level. Use the regulator to set the pressure to that specified in the operators manual of the tool you are using. When the proper pressure is unknown, start at a low pressure and increase it until the tool performs properly.
To keep the compressor running in top form you should perform regular maintenance on your compressor. A lot of air moves through this machine so it is a good idea to clean out the air filter regularly to maintain maximum air flow. After every use, crank open the valve at the bottom of the tank and let the water in the tank drain out. Water that is left to accumulate in the tank will cause corrosion on the tank walls, and as the water accumulates it is more likely to get passed on to the tools being used, especially if you have a small tank. If you have an oil filled pump, check the oil level on a regular basis and change the oil based on the schedule supplied in the owner’s manual. Oil that is left in the pump can break down and cause premature failure of the internal seals in the pump.
Entry level portable compressors that are ideal for light trim work, filling tires and some occasional framing can be had for $100 to $200, with smaller portables ranging from $200 to $400. Expect models in the 30 gallon and larger sizes to be in the $500 price range and higher, while stationary models with a 100% duty cycle capable of supplying several tools at once can run to several thousand dollars. A unit between two and five hundred dollars would likely cover the needs of most non-professional woodworkers.
An air compressor can be an extremely useful tool in your shop. Choose one that will best suit the size of your shop and the kind of woodworking that you do.
|Average CFM Requirements at 90 PSI|
|Impact Wrench – 3/8″||2.5-3.5|
|Ratchet – 1/4″||2.5-3.5|
|Ratchet – 3/8″||4-5|
|Mini Die Grinder||4-6|
|Random Orbital Sander||16|