A mechanical device that converts atmospheric air into pressurized air is called an air compressor. Compressed air is denser and has more pressure than regular air. The machines use energy such as chemical or electrical energy for conversion.
This comprehensive post is your ultimate reference for what you need to know before buying an air compressor.
Air Compressor Terminology
Before you can start looking for air compressors to buy, a good understanding of terms is crucial.
Pounds per Square Inch or PSI – It is the unit of pressure of an avoirdupois unit. It is the amount of pressure that is applied by one-pound force on a square inch area. Higher the number, the more air can be compressed inside a tank.
Pounds per Square Inch Gauge or PSIG – It is the gauge pressure and the force that an air compressor delivers relative to atmospheric or ambient pressure at sea level. Normal atmospheric pressure is 14.7 PSI at sea level. Gauges in air compressors are calibrated at zero, so the 14.7 PSI of atmospheric pressure is not taken into account.
Actual Cubic Feet per Minute or ACFM – It is the exact volume of air released by a compressor per minute of operation. The delivery is about its conditions such as pressure, temperature and operating conditions.
Standard Cubic Feet per Minute or SCFM – It is the volume of air delivered at 15.6 degrees Celsius in one minute by a compressor. Air is capable of expanding and contracting. Therefore some of the air compressors come with SCFM rating for accurate result delivery.
Horsepower or HP – It is the unit of measurement of power produced by a machine. The higher the number of horsepower, the more powerful an air compressor will be. More work equals greater ACFM or PSI.
Brake Horsepower – The amount of horsepower needed by the compressor’s shaft to perform the required amount of work. It is also the amount of horsepower conveyed to the output shaft of an engine or a motor.
Indicated Horsepower – This is primarily applicable for displacement type of compressors. It is the horsepower determined from indicator diagrams of the compressor.
Intercooler – The cooling mechanism that cools the air in the compressor between the different stages of compression.
After cooler – It is the heat exchanger that cools the air discharged by a compressor. A moisture separator removes the condensation residue after the after cooler’s function is over.
Pneumatic Tools – Power tools that require air pressure to operate.
Positive displacement compressors – These are usually rotating or reciprocating compressors. Successive volumes of gas are enclosed in a mechanically reduced space. It results in air compression.
Types of Air Compressors
Air compressors are most commonly divided into three categories – Consumer-grade air compressors, Professional-grade air compressors, and Commercial/Industrial-grade air compressors.
Consumer grade air compressors
Power tools that have a PSI of 135 or lower are labeled as consumer grade. They are commonly called single stage models, pancakes or hotdogs.
They are portable but efficient. It can power brad guns and inflators. Consumer grade compressors are ideal for carpenters, hobbyists and other homeowners for executing small tasks.
The air compressors in this category are:
Single-stage air compressors
These are of two types- stationary and portable. The stationary single-stage is a perfect garage accessory. It is most ideal for executing tasks like rotating tires. It is pricier than the movable type but also lasts longer.
Portable air compressors are a carpenter’s friend. It is ideal for nail guns and can be easily carried to work. It is also a great assistant in doing small to medium construction works at home or a boat.
Hot dog air compressors
These compressors have enough power to operate smaller power tools such as staplers and brushes. They have handles for easy transportation and have no oils and belts. They are very easy to maintain and are a favorite among the hobbyists.
Hot dog air compressors can be very loud, in comparison to their size. They are also heavier than pancake compressors.
Pancake air compressors
They have a small tank about one gallon and weighs 16 pounds. It is lightweight and easily portable. It is oil-free and belt free for easy maintenance although the oil-free feature can make it a bit loud.
They are ideal for performing small tasks – filling tires and balls and maybe a raft.
Twin stake air compressors
As the name suggests, it has two air tanks. The twin tanks make the compressor more efficient in its output. It has enough power for running nail guns, brad guns and finish-trim tools.
Professional-grade air compressors
These are air tools made for more demanding applications and longer durations. They are most ideal for auto shops, woodworking shops, and contractors.
Professional grade compressors are designed to withstand rigorous jobs. They are also made portable and more durable than consumer grade compressors.
They can make such activities like painting cars, building decks on a large scale very effortless. It saves time, increases productivity and saves businesses a lot of money.
Commercial or industrial grade air compressors
These are specially made to produce compressed air for long hours at a stretch. They incorporate advanced technology for enhanced productivity, efficiency, and longevity.
Commercial grade compressors are an investment for any business establishment. The right choice of a compressor can easily mean the difference between the company’s success and failure.
Industrial or commercial compressors are of the following types.
Reciprocating piston compressors
It is a compressor with a positive displacement where the crankshaft is driven by the piston to deliver high-pressure gas. The pistons move in a back and forth motion inside the cylinder and create a vacuum. The vacuum retracts as it sucks the air and extends as it pushes air into the tank.
The older versions of these compressors tended to overheat. With the addition of new technology such as a dual piston and oil-less features, it has drastically reduced the noise as well as overheating in the compressors. Reciprocating piston compressors are the most commonly used type among the compressors.
These compressors make use of the rotary type mechanism. They are commonly used where a large amount of pressured gas is required. High-power air tools or industrial tools like impact wrenches and jackhammers make use of this positive displacement mechanism.
When the gas enters the sealed chamber, the two helically opposed screws squeeze it. When this happens, the volume of air is reduced, and compressed air is produced.
The parts of the compressors do not touch and they are also oil-sealed. Therefore, they do not overheat, and it is effortless to maintain.
It is a refrigerant or an oil compressor. In this compressor, two spiral-shaped pieces rotate to compress air inside the compressor.
One scroll is stationary while the other moves around it and together they compress air. Scroll technology is used in a lot of applications. It includes refrigeration, pharmaceuticals, electronics, dental, food industry, and high-efficiency automobiles.
The air produced by scroll compressors is industry-standard and are the cleanest by ISO standards.
These compressors are oil-free and low maintenance. They are also relatively quiet during operation.
What type of air compressor do you need?
When you embark on a journey to buy an air compressor, the sheer number of models and types can overwhelm a first-time buyer. The labels and the specifications on the machines are no help either. However, you need to understand that if you can determine a rough sketch of its intended use, buying the machine itself will not be hard.
The ideal compressor you need to buy will depend on the following factors:
Where are you going to use the compressor – a business establishment or a job that requires constant moving? This is an essential factor to consider because compressors are either stationary or portable.
If perhaps you are a carpenter or a contractor, a lightweight and slim that is easy to transport is ideal. While on this note, it is worth knowing that the weight of a compressor itself does not determine its portability. A compressor with a slim design is more comfortable to carry than those with wider designs.
A lot of folks equate portability with compromised performance. However, modern wheeled designs deliver more CFM, i.e., cubic feet per meter. And this is perhaps the most critical number in a compressor.
Some compressors also come with a hose that can be extended for flexibility. Delivery of compressed air in no way compromises with the performance.
Pressure and Capacity
Regardless of what the salesperson tells you or the labels on the machines say, CFM and PSI are the only two numbers that you should give more attention. For reference, 115 psi / 8 bar pressure, is a small capacity compressor while 115 psi / 7.5 bar pressure, is a high capacity compressor.
Pressure refers to the strength of compressed air during delivery and is referred to as the PSI, pounds per square inch. The amount of pressure required will be
determined by the power tools you use. It should be relatively easy to note this as most power tools are labeled, “The machine operates at 125 psi and so on.”
On the other hand, capacity is how much the compressor can deliver in a minute. It is expressed as CFM or cubic feet per minute. The amount of capacity you need will be primarily determined by the consumption of air by the tools you use. The easiest way to work out the capacity requirement is to check the PSI specifications of all the tools you will potentially use and add it up. In most cases, this resultant number will tell you how much capacity you need in a compressor.
Most air tools can be powered comfortably by 10 CFM at 90 PSI. However, in case you need to operate a sander continuously, a compressor with 16 CFM at 90 PSI at 65% duty cycle is an ideal compressor.
Consider what type of tools you will be using with the air compressor. This will give you a rough estimate of the CFM and PSI that you need.
Two-stage or single-stage
Two-stage compressors inherently have higher PSI than the single-stage models. The reason is that two-stage models have more than one cylinder. The air gets pumped from one cylinder to the next consecutively before it reaches the tank. The air enters the first cylinder at 90 PSI but attains 175 PSI when it reaches the second tank.
Industrial compressors with HP of 5 or more are powerful and efficient because they are made for industrial applications and not necessarily because they are two-stage models.
However, two-stage models are in no way better than one-stage models and vice versa. A single-stage model that is well built can easily outperform a low-quality two-stage compressor.
The rotary screw is the most common screw from many manufacturers. They are different in pressure as well as capacity but rotary screws they are, nonetheless.
Size of the tank
The size of the tank does not affect the performance of the compressor in a significant manner. In a compressor, the tank stores the air; it doesn’t produce it.
That being said, smaller sized tanks are ideal for power tools that are used often like nail guns and staplers. They can attain the required pressure quickly, and they are also portable.
On the other hand, a larger sized tank is ideal for machines that are used occasionally, such as sanders and grinders.
The running time of both the tanks is usually the same. The size of the tanks in the compressor is expressed in US gallons.
Perhaps the only significant advantage a bigger tank has over the smaller sized one is the strain on the motor and condensation. The component that fills up the compressed air in a tank is the motor. In a smaller tank, the motor has to be started up repeatedly which puts an enormous strain on it. This affects the longevity of the compressor. In a larger sized tank, the motor does not undergo such stress, so it lasts longer.
The other is condensation inside the tank. When air is compressed, it is hot and holds moisture in it. In a smaller tank, compressed air is put into use before it cools down. The hot air is not the problem but when hot compressed air is used; the residual air condenses and clogs the airlines. This can significantly reduce the efficiency and performance of the air tools as well as the compressor. In a larger sized tank though, there are components such as intercoolers and after coolers. These effectively cool down the air before they can pass through the airline to be used.
Horsepower is the rating of the engine’s power of the compressor. Unfortunately, marketing strategies employed in the sale of compressors can be devious or ingenious, depending on how you see it. When a salesperson proudly explains that a compressor is 5HP, he is usually talking about the peak performance. So don’t be fooled by this tactic.
A straightforward way to determine the exact horsepower of a compressor is to know how much electrical power it consumes when it is operating. A compressor that has 5HP motor (at ease HP and not peak HP) will consume around 24amps -220-volt circuit. Any compressor that runs on 15amp on a 110-volt circuit has a motor that is around 2HP. When a dealer sells you a 5HP compressor, ask him to plug it in and check the electrical AMP consumption yourself.
There is also a vast difference between how a gas compressor and an air compressor is rated in terms of horsepower. Air compressors are more potent than gas compressors.
This feature is not that of a compressor per se, but it is an essential factor to consider. A sturdy electrical unit with proper plugs must be installed to supply the compressor with electrical needs.
It is not recommended to use a compressor with a generator. The fluctuations in the power supply can cause irreparable damage to the unit. Most manufacturers also nullify the warranty if the compressor was used with a generator.
Extension cords are also not recommended to be used for power supply. It can seriously affect the efficiency of the compressor.
If you must use the compressor further away from the machine, use an air hose that can be extended. It will supply the air without compromising on its efficacy.
Features of Air Compressor.
By this time, the basic idea of purchasing an air compressor should be forming. Let us take a step further and discuss the features of air compressors. This will enable you to make the best decision.
Direct Drive or Belt Drive
The motor of air compressors that use rotary screw is either direct driven or belt driven.
Direct driven air compressors have the crankshaft connected directly to the motor shaft. This is why they run with the same speed as the motor which is typically between 1,725 or 3,450 RPM. These compressors have low RPM and are most ideal for carpenters. These are very lightweight, portable and relatively very quiet. They can be effortlessly used in places that require high energy efficiency but have low-temperature environments. However, these compressors need a lot of maintenance and are pricier.
In a belt driven air compressor, the pulleys connect the air compressor pump and the motor. In this type of compressors, the motor spins faster than the pumps. This allows the compressor to work more efficiently and eliminates the wear and tear of the machine. The airflow and pressure in this type of compressors can be adjusted
as they are more flexible. They are also easy to maintain, and installation is straightforward and much cheaper than direct-driven compressors. However, they cannot be used in harsh weather situation.
Oil lubricated or oil-less
Oil-less compressors are more lightweight, easy to maintain, and more affordable. These compressor designs are straightforward with fewer parts. They also last longer especially if they are built with the right design and have low RPM. These are perfect for non-commercial uses such as small carpentry or household works.
On the flip side, oil-less compressors can be extremely noisy. So, consider your environment before making the purchase. They also tend to get heated after using it for some time.
Oil lubricated compressors are more massive, and not easily portable due to their weight. The oil also needs to be changed more frequently, which means that they requiere more maintenance.
They are more ideal for industrial or commercial use due to the fact that they can run long hours of operation without heating. Oil lubricated compressors are comparatively very quiet as the components are very well lubricated.
If portability is not an essential factor, an ideal air compressor is a belt-driven oil-lubricated compressor. It is the best combination of quality, performance, durability, and price.
Starting up and stop options
These are operational features to use the compressors. They come in a variety of option.
Auto-start and off – An automatic pressure switch turns on the motor as soon as it detects that air is needed. Then it switches off by itself as soon as the air in the tank reaches the maximum limit.
Continuous speed – The air supply runs continuously, and the supply of compressed air gets adjusted according to the demand.
Dual control – The air compressor is built with the option that can be set to an auto start / off option, continuous speed option or dual control option.
Conventional compressor designs primarily made use of iron cylinders. But modern piston style designs incorporate the use of metal in the crankshaft,
crankcase, flywheel or valve seats. The metal components are either made of iron or aluminum.
Cast iron components are more heavy-duty, and significantly denser than other materials. It is substantially stronger, and also more durable than most designs.
The downside of this metal is its susceptibility to rust. This can weaken the component as well as the compressor over time. Its crystalline structure also makes it more brittle than other materials. The heavy weight can also make portability an issue.
Components made of aluminum, on the other hand, are lightweight. The metal has a lower density than a cast iron component. It makes portability very effortless, especially in small working environments. Aluminum does not have a problem with rusting and is resistant to corrosion. This allows the compressor to be used even in harsh conditions. It is also straightforward to recycle, unlike cast iron which requires melting and recasting. This metal also has a high electrical and thermal conductivity which allows the components and the compressor to cool down faster after it is used.
The downside to this metal is the longevity issue. It does not last as long as the components made of cast iron.
Vertical and horizontal tanks
This factor in no way affects the performance of the air compressor. However, if you have an issue with space and storing, consider this factor too.
Compressors with vertical tank need less space for storage. They can also be whelmed or transported from one location to the next very smoothly.
On the other hand, a horizontal tank occupies more space on the floor. But they can also be hidden under cabinets and worktables.
Ball-valve tank drain
Air has a remarkable ability to hold on to water, mostly in the form of water vapor. When air is compressed, it cannot hold the water vapor as much which results in condensation. This leads to a collection of residual water in the tank.
A ball-valve allows the water to be drained efficiently and safely. When the water collects and remains in the tank for long periods, the tank may rupture or develop
leak. A worse case would be the tank forming rust which can ruin your compressor. Ball-valve drain is more efficient and durable than other drain valves.
Low oil protection
A safety device in most compressors shuts down the unit when it detects that the air pressure has fallen below a certain level. This feature prevents downtime and eliminates the need for expensive repairs or worse, replacing the compressor. Two of the most common controls are pressure-sensing electronic control and mechanical differential control.
The electronic control uses a pressure sensor that is mounted on the outlets of the pumps and is connected to it with an electric cable. When the pressure falls below 7 to 9 PSI, the pressure sensor signals the control module. The signal makes sure the control modules open the safety contact points. When it gets tripped, electronic control has to be reset manually.
In a mechanical control module, it is the tube that senses the compressor’s suction pressure as well as the pump’s outlet oil pressure. In this module, the pump’s total pressure is the threshold. This pressure is less than the pressure from the suction. The mechanical module also requires manual resetting when it is tripped.
Some reasons why the control module gets tripped are
Problems with the electrical unit.
Technical problems inside the compressor.
Incorrect setting of the control modules.
Low level of compressor lubricant.
Faulty design and sizing of the refrigerant piping system.
Low refrigerant or winter charge.
The duty cycle in a compressor tells you the duration of the time it can run in 10 minutes. If a compressor shows a 50% duty cycle, it should not be made to run more than 5 minutes. Making the compressor run more than its duty cycle will result in overheating the unit and ultimately, damaging it.
Some industrial compressors come with a 75% duty cycle and some with a 100% duty cycle. This allows the compressor to be continuously used without overheating it or compromising its longevity.
Therefore, investing in a compressor that has more duty cycle is a much better choice.
Compressed air is hot, and hot air contains water vapor and moisture. The harder a compressor works, the hotter it will get, and as a result, there are problems with moisture accumulation. When the hot air is used before it gets cooled down, it forms condensation residues in the airlines.
Sometimes this problem is solved by replacing the smaller tank with a larger tank. The best possible option to prevent moisture problems is allowing the air to cool down before using it. In an industrial unit, this problem is eliminated due to the use of refrigerated air-dryers which not only cools the hot air but removes moisture content.
Use of silica gel by desiccant dryers also removes moisture effectively. The silica gel can be quickly replenished from time to time.
A good plumbing system in a compressor is a necessity. This, however, is not the same as draining water from the tank. Although draining the tank extends the life expectancy, it does not affect the airlines in any way. Therefore, know the difference between draining the tank and dealing with moisture problems in the airlines.
Life expectancy of the air compressor
The size of the compressor does not correlate with the life expectancy of the compressor. An economical industrial unit will give out earlier than a higher quality consumer grade one. When it comes to air compressors, there is only a slight difference in pricing between a compressor with long life expectancy and a shorter one.
In this respect, you can choose not to buy a compressor unless the salesperson or the dealer can tell you the life expectancy.
ASME parts et al
A well-built compressor that is of good quality will have parts that comply with the codes and standards set by ASME, which stands for American Society of Mechanical Engineers. Always look for these certified parts while buying an air compressor. This ensures that you get the best deal for your money and make the purchase an investment.
Some European designs are of very high quality. So, if you’re looking for alternative models, they are a good option. But generally, stay away from cheap Chinese manufacturers. They can be deceivingly alike in appearance, but there is nothing to be said about their performance, quality or longevity.
A great source of information is the owner’s manual that comes with the compressor. It will provide you with all the required details on the specifics, use, and maintenance, etc.