Air Compressor Purchasing Guide

Trends

Your specific application and the horsepower required to achieve it effectively will largely determine the air compressor you purchase. But there are a number of trends that may also factor into your purchase decision. The incorporation of one or more of the following has the potential to significantly increase the productivity and cost-effectiveness of your process.

Which of the following could apply to you?

Variable speed drive
This development uses variable speed technology and a special drive to control the speed of the unit, saving substantial energy – up to 35% less with some models – when compared to a fixed speed air compressor. With this vast potential for savings, some dealers cite variable speed drives as “superior to all other control technologies” on the market today.

In short, a variable speed drive automatically pairs its output to the user’s demand for air. The continuous, real-time nature of this development is what provides its peerless energy efficiency, reducing demands for power and fuel while also cutting emissions. Further cost and energy savings are found in its ability to provide unlimited motor starts, progressing from zero to full load without spikes in electric current or generating surplus heat.

Tier 4 engines
The Tier 4 category is known for its adherence to EPA guidelines regarding emissions control. But Tier 4 engines – often manufactured by leading brands like Caterpillar and John Deere – also provide fuel efficiencies that can save you on the total cost of ownership for a diesel-based air compressor.

Many of the top air compressors that incorporate Tier 4 engines are easily capable of 300 to 425 cfm at 100 to 200 psig. They feature advanced electronic control systems that include easy-to-read gauges and an LCD display, allowing the user to quickly adjust specific factors to achieve the optimum combination of performance and cost-effectiveness.

Though not standard to all, some models further extend their value through design enhancements. These can include a frame engineered for full fluid containment as well as easy access service doors that simplify service and maintenance for the fuel tank, engine, air filters, and other vital compartments.

Greater capacity, lower power requirements
The goal of almost every technology is to produce more with less. Air compressor manufacturers have pursued this goal by delivering ever higher levels of output on less fuel. One way they do it is through lower operating temperatures. By redesigning their compressors with lower internal temperatures, new models provide noticeable improvements in volumetric and electrical efficiencies.

Another aspect is seen in a trend we covered above: variable speed drive. Consider for example a rotary screw air compressor. Often integrated into woodworking and spray painting operations, auto body service and repair shops, and even larger industrial operations, they are often not a continuous use machine that has to operate at full capacity. In these situations, a variable speed drive is able to compensate for shifting power demands and monetize them – saving fuel, not to mention unnecessary wear and tear on machine parts. As mentioned above, this technology can reduce energy costs by more than a third, with some variable speed compressors even capable of operating on 40% less energy than their standard counterparts.

Oil-free rotary screw compressors
Essentially maintenance-free, oil-free or “oil-less” air compressors are permanently lubricated through a long-lasting treatment of Teflon or a similar synthetic resin. While oil free models aren’t ideal for continuous use applications, they’re great for short duty and home use. They’re also becoming one of the preferred types for spray painting outfits and other operations that require clean work environments. The reason is that oil-lubricated compressors discharge a small amount of oil in the form of mist, a design limitation that makes the mess difficult for some applications.

In addition to maintenance costs that are practically non-existent, oil-free compressors also tend to be less expensive due to a design that requires fewer parts. This minimalist design also makes them lighter, great for those needing portability.
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Types

As mentioned briefly in the introduction, air compressors are divided into three main types:

1. Reciprocating compressors
2. Rotary screw compressors
3. Centrifugal compressors

Within these three types, there are two different varieties. The more common of the two, encompassing two of the three types, operates through the use of positive displacement.

Positive displacement is a mechanical design that generates air pressure through a pump that’s split into two sides, suction and discharge. These two sides or cavities expand and decrease as air is pulled into the suction compartment and released on the discharge side during the compression stroke. Positive displacement is common among reciprocating and rotary screw air compressors because of its suitability to compressing small amounts of air into high pressures, as well as its ability to quickly disperse the heat resulting from compression.

With that in mind, the air compressor you select will be determined by a number of specific factors including power output requirements and portability.

Here’s how the three types compare.

Reciprocating air compressors
Reciprocating compressors are positive displacement models. The same volume of air that enters the compression chamber leaves the cylinder, pressurized to the necessary PSI. Usually falling on the smaller end of the output scale, reciprocating models feature one of two different cylinders:

• Lubricated – An abbreviated term for “oil-lubricated compressors,” lubricated models use oil to maintain the integrity of the cylinders, piston, and piston rings. The upside to this design is that it usually requires little maintenance aside from having to periodically service the filtration system.
• Non-lubricated – Using Teflon piston rings in place of oil, this variety requires no lubrication and is often lighter in weight due to the use of aluminum components in place of cast iron. Similar to the service requirements for lubricated models, the Teflon rings have to be continuously replaced.

Positive displacement aids in the dispersal of heat, often leveraging water-encased cylinders to avoid buildup. Reciprocating compressors also incorporate intercoolers between each stage as well as after-coolers that act as a final filter on heat and moisture before discharging the pressurized air. In terms of capacity, these are the short duty models of the group, capable of a max of 50 hp and 12,000 CFM at 125 psig.

Rotary screw air compressors
The second type of positive displacement compressor, rotary screw air compressors are built with two or more interlocking screws that draw the air into the system. As the outside air is pulled through the system, it’s compressed at increasing degrees and discharged at the desired psi. Similar to the reciprocating variety, rotary screw models are also split into two unique types:

• Flooded – Flooded compressors pair oil with the air being pressurized as it moves through the system and then filters it out before discharge, recycling it back to the sump for continued use. Regular maintenance for this type of compressor includes routine changes for oil, filters, and separator.
• Oil-free – This type is exactly similar to its flooded counterpart with one major exception: oil is replaced with non-contacting carbon ring seals that eliminate the possibility of oil entering the air stream within the compression compartment. The one disadvantage to this type is that it tends to suffer from excessive heat build-up as it does not have the capability of inlet throttling, a feature that often makes the flooded version preferred.

Rotary screw air compressors have a far greater capacity than reciprocating models, offering anywhere from 7.5 to 100 hp with displacements of up to 2,500 CFM at 125 psig.

Centrifugal air compressors
The odd one out, centrifugal compressors are the largest of the three and powered by electric motors or steam turbines to produce outputs up to 500 hp and 15,000 CFM at 125 psig. Used primarily within large industrial manufacturing processes, this massive amount of pressure is generated through staged compressions, often requiring 2 to 5 stages.

Unlike the positive displacement type, centrifugal models contain at least two impeller assemblies that rotate to compress incoming air. This rotation causes velocity, creating energy that’s used to pressurize the air. The upside to this design is that its capacity is highly customizable, easily regulated by adjusting air inlet or outlet as well as the speed of velocity.

Centrifugal compressors are substantially more costly than the other two models. And the upfront cost is only the beginning. They require constant maintenance and often need costly repairs on individual parts resulting from high velocities and continuous use practices.

Other options
While all air compressors are represented by one of the three types mentioned above, they are also available in different designs to suit needs related to portability, size, and weight. Among the most common designs are:

• Portable – Designed for easy transport from one job to the next, portable air compressors are lightweight and often come mounted on carriages containing 2 to 4 wheels. The obvious advantage is the ability to take it anywhere, powering air tools in an endless range of work environments without needing extra long air hoses to stretch from a stationary location or truck mounted compressor. This type usually falls within the reciprocating family and therefore produces less power overall when compared to larger models.
• Stationary – Designed for long-term projects, stationary air compressors provide substantially more CFMs than their portable cousins. They also require special installation considerations that often include extra hosing drilled through walls and mounted above work areas. Higher end rotary screw compressors are commonly among this variety.
• Truck mounted – Truck mounted compressors are similar to stationary models except they can easily be taken from one jobsite to another without disassembly and reassembly. Plus, larger models are capable of powering multiple air tools at once. Somewhat more environmentally-friendly, electric options are common among mounted compressors, enabling you to save on the cost of fuel during continuous use. But, like a stationary model, you’ll have to run hoses from the truck to the work area and account for a loss of air pressure that can range between .7 to 25 CFM depending on the length of the run.
• Towable – Often found on construction jobsites or within forestry applications, towable air compressors are mounted atop trailers featuring heavy duty steel chassis and fully galvanized canopies. Often featuring a simple connection like a lifting eye hitch, they can be hitched onto any large work vehicle and provide outputs from 14 to 327 hp and 50 to 1,200 CFM (at 125 psig).

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Specs

There are a few key areas that can be used to compare both the value and usability of an air compressor. So when comparing different models, here’s what you should be on the lookout for.

Stages
The term “stages” is used to describe the cylinders in an air compressor. You have two options: single stage and multistage.

Single stage compressors are built with only one cylinder that compresses the air, often capping out around 120 psi. This is usually adequate for home use or smaller shops that only power one tool at a time. If you’re looking for portability (and an air compressor that can easily be carted and lifted), single stage is the way to go.

Multistage compressors are designed for multiple pneumatic air tools and those that require more than 120 psi. With more than one cylinder, the air is first compressed in the initial cylinder and then compressed in each successive cylinder to achieve higher levels of power. Due to the additional mechanical processes involved, multistage compressors generate more heat and require a cooling component, often in the form of a radiator.

Power
This may be decided for you if you plan on using the compressor in an enclosed area – which immediately disqualifies gas and diesel-powered compressors due to the exhaust they produce. Output also often determines the power source, with diesel engines capable of greater horsepower than electric models.

In general, you’ll have a choice between these three options:

1. Gas – Often portable, gas-powered compressors are a strong alternative in rural areas and jobsites that aren’t connected to the power grid. Unfortunately, they can weigh in excess of 200 pounds in some cases. You’ll find two different types: those with an electric generator that powers a cylinder and those that power the cylinder directly. The downside to this setup is they tend to generate more noise than other models and also require the storage and transport of additional fuel. But if you need a multi-stage compressor, this will probably be one of your only options.
2. Diesel – Diesel models usually provide the highest levels of output, commonly ranging from 185 to 1,600 CFM – far and above that of gasoline or electric compressors. This is why diesel is often the power source behind truck mounted and towable models, enabling them to provide higher torque and greater capacity for a wide range of air tools connected simultaneously. Similar to their smaller gasoline counterparts, you are required to keep extra fuel on hand. But diesel models are often engineered with smaller tanks that allow them to be mounted efficiently on a variety of work vehicles.
3. Electric – Reportedly one of the most common types of air compressor, as they’re typically lighter and more compact, electric models are environmentally friendly and can be used safely in enclosed spaces. Most are powered simply by plugging them into a standard wall socket, though larger models that produce higher CFMs may require access to higher voltage. It’s worth noting that this type of compressor requires longer air hoses as some experts discourage the use of extension cords with air compressors.

Purchasing tip: Make a list of every air tool that will require power simultaneously and plan ahead. Under-buying will quickly burn out your compressor, regardless of its power source. And over-buying can be just as bad. You’ll be paying unnecessarily for surplus gas, diesel, or electricity.

CFM, PSI, and horsepower
CFM or cubic-foot-per-minute is the measurement of delivery with respect to inlet. In short, it gauges flow, not volume. PSI or pounds-per-square-inch is the pressure that results from one pound-force directed at an area of one square inch. This results in a measurement of resistance to flow.

Mathematics aside, when evaluating air compressors you’ll need to be aware of both the CFM and PSI – but one more than the other. Many air tools operate at 90 psi, with compressors providing anywhere from 75 to 175 psi as a result.

CFM is more important. In fact, some experts cite it as one of the most important purchase considerations when buying an air compressor. If you plan on powering more than one tool or pneumatic device off of a single compressor, you’ll have to add up the CFMs for each to ensure you have enough output to power them all. You’ll also want to consider exceeding the total by anywhere from 25% to 40% to allow for a buffer and prevent overworking the compressor. CFM requirements are usually listed on the packaging and can also be found on the manufacturer’s website.

Finally, horsepower. According to a leading air compressor manufacturer, horsepower is important… but not nearly as important as CFM when comparing different compressors. Larger air compressors often boast higher horsepower and lower CFMs. Unfortunately, these models tend to run hotter and give out sooner than others. So when purchasing a new or used air compressor, pay attention to the CFM rating first and all else second.

As a general reference, here are some popular tools and their average CFM requirements:

[table]Pneumatic Air Tool, CFM @ 90 PSI, Pneumatic Air Tool, CFM @ 90 PSI
Angle Disc Grinder (7"), 6 CFM, Grease/Caulking Gun, 4 CFM
Air Drill (1/2" or 3/8"), 4 CFM, Hydraulic Riveter, 4 CFM
Brad Nailer, .3 CFM, Impact Wrench (1/2" or 3/4"), 2.5 - 7 CFM
Chisel/Air Hammer, 3 - 11 CFM, Nailer, 1 CFM
Drill, 3 - 6 CFM, Ratchet (1/4" or 3/8"), 3 - 6 CFM
Dual Sander, 6 - 11 CFM, Spray Gun, 6 - 18 CFM[/table]Back to top