Compressed air system terms

Before discussing Compressed Air Systems (CAS) requirements these key terms must be understood:

A. Online compressors: all compressors that are available to serve peak load. Online compressors do not include back-up compressors whose only purpose is to be available when a compressor fails. Online compressors are all compressors that are physically connected to compressed air piping excluding back-up compressors.

B. Back-up compressors: are those compressors not used to meet peak compressed air flow loads. Back –up compressors can be physically connected to the compressed air piping system and can be automatically controlled to turn on if one of the other compressors on the system fails. Back-up compressors do not normally operate.

C. Online capacity: total combined capacity in actual cubic feet per minute (acfm) or actual cubic meters per minute (cmm) of compressed air at a given pressure during times of peak compressed air load.

D. Trim compressor: is a compressor that is designated for part-load operation, handling the short term variable trim load of end uses, in addition to the fully loaded base compressor. In general the trim compressor will be controlled by a VSD but it also can be a compressor with good part load efficiency. If the trim compressor does not have good part load efficiency broadly across its operating range, then it will take more compressors to meet the Energy Standards requirements.

E. Base compressor: the opposite of a trim compressor, a base compressor is expected to be mostly loaded. If the compressed air system has only one compressor, the requirements of the Energy Standards require that the single compressor be treated as a trim compressor.

F. Specific power: the ratio of power to compressed air flow rate at a given pressure typically given the units of kW/100 acfm or kW/m3/min. The lower the specific power the more efficient the compressor is at a given compressed air loads.

G. Total effective trim capacity: the combined effective trim capacity of all trim compressors where effective trim capacity for each compressor is the range of capacities in acfm (cmm) which are within 15 percent of the specific power at its most efficient operating point. This is displayed in Figure 10-41.

H. Largest net capacity increment: is the largest increase in capacity when switching between combinations of base compressors that is expected to occur under the compressed air system control scheme. See example 10-54.

I. Primary Storage: are tanks or other devices that store compressed air capacity. Also known as an air receiver, they reduce peak air demand on the compressor system and reduce the rate of pressure change in a system. As primary storage these devices are near the air compressors and are differentiated from remote storage that might be out near an end use device.

Compressed Air Systems

Compressed air systems can be an area for significant energy losses. Some consultants estimate that 20 percent of all power used in industry to generate compressed air is wasted. Most people are unaware that it takes 7 horsepower of electrical energy to produce 1 horsepower of compressed air. Knowing this, operators and plant managers may want to review what tasks could be accomplished more economically by using electricity or steam rather than compressed air. Another misunderstanding is the belief that because compressed air is a necessity in some applications, its availability for other uses is “free.”

Locating Compressed Air System Leaks

An effective compressed air management program starts with the location and repair of leaks. The economic penalty for air leaks can be easily calculated. For example, if electricity is purchased for $0.08/kwh, the full burdened cost of compressed air can reach $3.25/100 cfm/hr. A good analogy to the cost of wasting compressed air can be made by comparing it to hiring several employees and allowing them to stay home sick. Many operators and managers consider leaks as just another cost of doing business. The identification and repair of those leaks, however, could represent important “hard” dollar savings for an organization. In most cases, the largest electric motors are used to drive air compressors and a good leak management program could probably reduce electrical costs to power those motors by 15-20 percent. Another benefit is reduced noise in the manufacturing environment.

A good way to start is to identify the more obvious audible leaks. An audible leak is normally at least 2cfm/hr. Once identified, they should be marked by tags, surveyor’s tape or fluorescent paint. A list should be made identifying each leak, assigning a cost to its existence, and totaling the overall waste cost of all the leaks. The net savings will be the total, less the cost of repairing each leak.

The next step in a comprehensive air leak control program might involve the purchase or lease of more sophisticated leak detection equipment, such as ultrasonic leak detectors. These instruments are capable of locating leaks in noisy production areas as well as in overhead and other limited access areas. The cost of these instruments is most often more than offset by the money they will save by stopping the leaks.

Much of the waste of compressed air, besides leaks, rests with the end user. Compressed air is routinely used for tasks better (and more economically) suited to other methods.

A good example is the use of compressed air to clean up a dusty production area. In one particular organization, an electric leaf blower was purchased for the individual responsible for clean-up to use in of the compressed air. The blower functioned well and the employee was informed after the clean-up was complete that the cost of the compressed air used in the clean-up would purchase the electric blower after only 3 days usage. This made an impression on the employee and focused attention on the true cost of compressed air.

Compressed air management involves leak detection and repair, employee education, and a review and revision of current practices to see if there are other economical alternatives to using compressed air. Here are some suggestions for better control of compressed air systems:

Pressure Drop

A major source of energy waste is a loss of pressure in the compressed air system. If system pressure drops below the minimum operating pressure of tools and equipment, efficiency declines rapidly.

For example, many air tools are designed to operate at an inlet pressure of 90 to 100 PSIG (6 to 7 barg). A ten percent drop in pressure means nearly a 40 percent loss in work output of the tool. Some reasons for pressure drop include:

• Undersized air compressor
• Excess number of tools or pieces of equipment on the system
• Excessive leakage
• Air friction in the piping system
• This last category can be managed knowing a few basic rules: For a given pipe or hose size and length, the pressure loss increases as the volume of air increases.
• Under the same conditions, the pressure loss
• increases with a lower initial pressure and de-
• creases with a higher initial pressure.
• A smooth inner lining of the pipe or hose
• will cause less pressure drop. Conversely, a rough inner lining of the pipe or hose will cause more pressure drop.
• Couplings, fittings and valves increase the pressure drop.

Drying Compressed Air

Moisture can cause problems in any compressed air system. Moisture reduces the efficiency of air-operated equipment and creates excessive maintenance costs and downtime through equipment corrosion and breakdowns. While compression of air reduces its volume, it does not eliminate moisture.

For example, a 25 horsepower compressor delivering 100 CFM at a pressure of 100 PSIG can produce 18 gallons of water per day at fairly standard conditions of 90 degree ambient temperature and 50 percent relative humidity. An after-cooler will remove approximately 66 per-cent of this moisture, still leaving 6.2 gallons of water per day to flow through the system. This will find its way downstream where it can ruin air-operated tools, equipment and instrumentation, foul spray processes and sand blast operations and contaminate food packaging and processing and create air line freeze-ups.

Separators can remove up to 98 percent of free water flowing through the system but do nothing to remove moisture present in saturated air. Complete moisture removal is done by reducing the dewpoint temperature of the compressed air. Dewpoints (temperatures where moisture condenses) are reduced physically by refrigeration or chemical means.

Refrigerated dryers cool the air by mechanical refrigeration to condense water vapor in the air; a moisture separator removes the condensate. The initial cost of these units is relatively low. Ongoing operating and maintenance costs are also low, due to the sealed nature of the unit. These units can operate in an ambient temperature down to 35° Fahrenheit (2°C).

Regenerative or Desiccant compressed air dryers use porous, non-consumable materials (known as desiccants) to absorb water molecules from the air stream onto the surface of the desiccant. Periodically, the desiccant is removed from the air stream to be regenerated for reuse. This process involves removing the entrapped water from the material. Typically, two desiccant towers are employed; one absorbing moisture from the air stream while the other is drying out.

In lubricated compressor installations, oil removal filters are required at the dryer inlet to prevent slugs of water from reaching the dryer and damaging the desiccant. After-filters are recommended at the dryer outlet to keep fine desiccant particles from entering the compressed airstream. These dryers can be used in any application that requires a pressure dewpoint below 35° Fahrenheit (2°C).

Operator’s Checklist for Compressed Air Systems

• Get control of compressed air needs through the generation of usage standard, system controls, operating pressure, increased storage (compressed air is one of the few energy sources that is easily handled and stored), and a program to reduce air-operated systems for clean-up, pumping, etc.
• Another way to reduce compressed air need is through a leakage control program. Also, reexamine clean-up and production start-up schedules to more efficiently distribute the demand.
• Reduce electrical consumption by reducing
• system pressure where possible and disconnecting unneeded equipment.
• Formalize maintenance procedures and monitoring using non-invasive predictive maintenance techniques and automatic controls where economically feasible.
• On larger systems or the existence of multiple compressed air networks, the employment of an Air Optimization Consultant may identify additional ways to cut down compressed air waste.

Diesel Engine Power Cogeneration

Diesel engines are frequent choices for power cogeneration and standby use. They are compact sources of power and can be started and brought into service quickly with a minimum of operator intervention. They are available in a wide variety of sizes from a wide variety of manufacturers.

An energy-saving by-product of diesel engine power cogeneration is additional heat. During operation, the energy input in the fuel is distributed among four endpoints. Some of the energy is converted into mechanical shaft power. The remainder is either absorbed by the water used to cool the cylinders, by the engine lubricating or exits with hot exhaust gases. A small portion is lost to radiation.

The energy and work capability of the exhaust can be recovered to produce steam. In addition, the jacket cooling water can be used to produce hot water or, in some cases, low-temperature steam.

The net thermal efficiency of the diesel engine remains fairly constant down to approximately 50 percent load. After that, it begins to drop rapidly. Also, as loading drops, the majority of additional relative heat production is absorbed in the cooling water. The exhaust heat content, however, as a percent of fuel input remains fairly constant over the load range, within about 5 per-cent. Exhaust temperature remains fairly constant as well, within plus or minus 5 percent.

Using a diesel engine in a cogeneration application is somewhat restricted by the capability to recover heat in a useful form. Process applications are limited to temperature and heat rate characteristics of the jacket cooling water and exhaust gases.

Process heat can be recovered in the form of hot air, hot water or steam. A requirement for hot air is satisfied by capturing it through the jacket water radiator and/or from exhaust gases flowing through a heat exchanger. Where contamination is not a concern, the exhaust can be used directly. A requirement for hot water can be satisfied from these sources as well. Heat ex-changers must be used when capturing heat from the cooling jacket to prevent coolant contamination.

A requirement for steam at a pressure above 205 kPa can be satisfied by an exhaust recovery boiler. Below 205 kPa, the heat energy absorbed by the jacket cooling water can be recovered as an additional steam supply, through the use of a flash boiler or ebullient cooling. With the flash boiler mounted above the engine, jacket water approaches the boiler, the static pressure drops and the water flashes to steam. In an ebullient system, vapor formation is allowed in the engine cooling jacket with natural circulation being used to continually remove the steam bubbles from the cooling surfaces.

A steam separator is required at some point above the engine. Jacket water steam production is limited to approximately 205 kPa because of the high-speed engine jacket operating temperature limit of approximately 90k. However, conventional absorption air-conditioning chillers are designed to use low-pressure steam at approximately 184 kPa and represent an extensive energy-reuse potential.

Air Compressors

Compressor types

Compressed air standards

The two main technical standards are ISO 1217 for displacement compressors and ISO 5389 for turbo compressors.

ISO 1217 : 2009 Annex F - Displacement Compressors - Acceptance Tests

ISO 1217: 2009 / A.1: 2016 - specifies methods for acceptance tests regarding volume rate of flow and power requirements of displacement compressors. It also specifies methods for testing liquid-ring type compressors and the operating and testing conditions which apply when a full performance test is specified.

A.1: 2016 Displacement compressors — Acceptance tests Amendment 1: Calculation of isentropic efficiency and relationship with specific energy

ISO 5389 : 2005 - Turbo compressors - Performance Test Code

ISO 5389:2005 applies to performance tests on turbocompressors of all types. ISO 5389 does not apply to fans and high-vacuum pumps, or to jet-type compressors with moving drive components

Turbocompressors comprise machines in which inlet, compression and discharge are continuous flow processes. The gas is conveyed and compressed in impellers and decelerated with further increase in pressure in fixed vaned or vaneless stators.

ISO 5389 is intended to provide standard provisions for the preparation, procedure, evaluation and assessment of performance tests on compressors as specified above. The acceptance test of the performance is based on this performance test code. Acceptance tests are intended to demonstrate fulfilment of the order conditions and guarantees specified in the contract.

ISO 7183 : 2007- Compressor Air Dryers - Specifications and Testing

ISO 7183:2007 - This standard specifies the performance data that are necessary to state and applicable test methods for different types of compressed air dryers. It is applicable to compressed air dryers working with an effective (gauge) pressure of more than 50 kPa (0.5 bar), but less than or equal to 1,600 kPa (16 bar) and include the following: adsorption dryers, membrane dryers, refrigeration dryers (including drying by cooling) or a combination of these.

A description is given of the principles of operation of the dryers within the scope of ISO 7183:2007.

This standard identifies test methods for measuring dryer parameters that include the following: pressure dew point, flow rate, pressure drop, compressed-air loss, power consumption and noise emission.

This standard also provides partial-load tests for determining the performance of energy saving devices or measures and describes the mounting, operating and loading conditions of dryers for the measurement of noise.

This standard is not applicable to the following types of dryers or drying processes: absorption dryers, drying by over-compression and integral dryers.

ISO 8573 - Compressed Air for general use - Contaminants and Quality Classes

ISO 8573-1:2010 is the primary standard for compressed air quality. It outlines precise classifications for air purity, it specifically outlines specific requirements for compressed air purity, quantifying the acceptable amounts of particles, water, and oil in compressed air.

This detailed classification helps industries determine the level of filtration needed to ensure their air quality meets the necessary criteria for their specific applications.

By categorizing air purity into different classes, ISO 8573-1:2010 allows manufacturers and other users to align their systems with clear, international benchmarks.

Industrial sector uses of compressed air

Typical application ranges

Air Compressor Purchasing Guide

Introduction

Air compressors power everything from large industrial production machinery to individual ratchet and impact wrenches and paint sprayers… This broad application is where you often find a couple more major advantages of an air system: versatility and power. Regardless of the scope or location of your project, an air compressor accommodates the full range of compatible tools – switched as quickly and easily as changing a drill bit. Plus, when compared to standard tooling, air powered models often provide greater torque and higher RPMs.

Three types
Air compressors are available in three basic types:

1. Reciprocating – Positive displacement compressors that provide increased air pressure by limiting the volume of air. Available in single-stage and two-stage, they’re typically capable of outputs ranging from 1 to 15 hp.
2. Rotary screw – Positive displacement compressors that are considered simple to operate and maintain. Favored for their ability to provide continuous duty, their design provides cooling within the compressor’s interior, saving the individual parts from extreme operating temperatures and enabling them to deliver outputs that range from 7.5 to 100 hp and up.
3. Centrifugal – Compressors that do not rely on positive displacement and are most effective when running at their full capacity, making them ideal for operating environments in which demand is continuous and output needs start around 100 hp.

In general, air compressors are highly durable tools. For example, it’s estimated that a rotary compressor will last for 40,000 to 60,000 hours – the equivalent of full-time operation for 10 to 20 years. With regular maintenance, reciprocating compressors have a life expectancy of 10 years. Each of the three types also has a number of advantages and disadvantages we’ll cover in more detail below.

Importance of a professional dealer and independent expert

You’ll quickly discover that air compressor dealers often specialize in a particular brand. The reason is manufacturers typically require exclusivity among their networks. While this entails a little more comparison shopping on your part, it ends up being a plus in the long-run. Specialization enables the dealers to become highly familiar with each of the models they’re selling, the specific applications they’ve been designed for, and the type of service required to keep them up and running.

Good approach is to start with independent compressed air expert, who will assess and audit your compressed air system from all aspects not just new compressor investment and give you directions about purchase.

This expertise allows them to consult on the exact type of machinery best suited to your intended application(s) and also offer value-based service packages at a fixed cost that can help you maintain the investment well into the future. In fact, by some estimates, maintenance and repair can account for as much as 20% of the total investment on a light-duty or industrial air compressor.

Estimated costs
The more power you need, the more you can expect to pay. But given the versatility of an air compressor, a single investment may end up saving you on the purchase of additional tooling and equipment.

Here are some estimated costs on popular specs:

Type,Power, CFM, PSI, Portable / Stationary Mounted, Estimated Cost
Reciprocating, 3 - 5 hp, 10.3 - 15.5 cfm, 135 psi, Portable, “$800 - $1,400”
Reciprocating, 7.5 hp, 24 cfm, 175 psi, Portable, “$1,900 - $2,200”
Rotary Screw, 10 hp, 34 cfm, 175 psi, Portable, “$2,700 - $5,200”
Rotary Screw, 15 hp, 46 cfm, 175 psi, Portable, “$2,900 - $7,500”
Rotary Screw, 80 hp, 185 cfm, 80 - 125 psi, Stationary, “$12,000 - $17,500”
Centrifugal, 20 - 200hp, 14.3 - 998 cfm, 75 - 150 psi, Stationary, “$17,000 - $24,000”
Centrifugal, 250 - 500hp, 1466 - 2444 cfm, 109 - 125 psi, Stationary, “$35,000 - $90,000

Trends

Your specific application and the power 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.

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.

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).

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 power
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:

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

Choosing a quality seller

As mentioned briefly in the introduction, a professional seller is an enormous benefit when it comes to the selection process and post-sale support. Specializing in the specific brand and application you’re interested in, a professional air compressor seller provides additional resources and information that can include the following.

Safety resources and laws
Though compliance isn’t mandatory for air compressor operators, OSHA published a checklist of safety suggestions for air compressors. Largely addressing issues related to service and maintenance, a qualified seller can address each of these concerns and will often build them into a service level agreement that routinely inspects each component for wear.

Warranties and service level agreements
Many quality air compressors are backed by a full warranty. But the specifics of those warranties often vary from company to company. Others offer a selection of warranties that cover popular parts for a set term: 3-year, 5-year, 7-year, and 10-year are common examples. Items covered typically address issues related to:

• Airends
• Variable speed drives
• Drive motors
• Rings
• Gaskets
• Suction/discharge valves
• Air/fluid receivers
• Drive couplers
• Coolers

Service level agreements (also known as “SLAs”) are above and beyond the warranty. They outline a routine service plan designed around preventative maintenance. Offered for an annual fee (or sometimes on a monthly basis), common items included in this type of post-sale support include:

• Oil levels
• Belt tension
• Coolers
• Electrical connections
• Air filters
• Valves
• Traps

Strength of manufacturer
Most air compressors are produced by companies with long-established histories – Ingersoll Rand, Atlas-Copco, Hitachi, Husky, Maxair, Sullair, and Kaeser, ELGI, to name just a few. These brands have extensive dealer networks around the world.

Nevertheless, check into the dealer in your local area. Is it a new business or have they been around for awhile? A seller that’s been offering the same air compressors for 10 or 15 years obviously knows what they’re doing and has an established track record of quality service. This also indicates they will more than likely be around for the foreseeable future to provide the support necessary for your compressor.

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Science of Compressed Air Calculations

Calculating the size of an air receiver.

Calculating rate of change

Calculating flow

Calculating change in pressure

Receiver needed to limit pressure change?