Whether you are a contractor, a heavy mechanic, a car technician or a serious amateur, you need an air compressor that keeps running. Check out this air compressor purchase guide and find out more about what to look for in your next air compressor.

The horsepower rating should be a good way to determine the size of the compressor you need, but unfortunately, marketers have been exaggerating horsepower ratings to help large stores sell more compressors. The rating of industrial compressors is usually accurate, which is why industrial 5 HP compressors are typically three times more expensive and expensive than the overrated hobby 5 HP compressors.

One way to tell if you are looking at actual HP or an overrated HP is to look at the power needed to run HP. A true 5 HP motor will consume approximately 24 amps of current on a 220 volt circuit (single phase supply), while most overrated amateur compressors will operate in a normal 15 A 110 volt circuit. Run about 2 actual horsepower. If you can plug it into a normal wall plug, its maximum HP cannot exceed 2.

The rated power of a gas engine is different from the rated power of a motor. For example, to produce as much air as an industrial 5 hp electric compressor, a 10 hp gas compressor is required.

Air pressure (PSI)
Air pressure is usually measured in pounds per square inch or PSI. Most pneumatic tools require 90 PSI to function properly, but to keep the tool at 90 PSI, you need a compressor with a higher cut-off pressure. Many industrial compressors are two-stage, which means they will establish a cut-off pressure in both stages. The first phase reached approximately 90 PSI and the second phase reached 175 PSI. Most hobby and smaller commercial compressors are single-stage and shut down at 125 – 135 PSI. It sounds like air pressure is high, but they usually start at about 100 PSI, and you may not have enough pressure on the tool when you consider the pressure drop due to hose and connector limitations. .

Cubic feet per minute (CFM)
Pneumatic tools require a certain amount of air to keep running. The amount of air produced by the compressor is measured in cubic feet per minute (CFM). You may see multiple CFM ratings at different pressures on the compressor. Occasionally, you will find that the CFM has a high displacement, which is a rating based on the size and speed of the compressor pump. This rating is of little significance because it is at zero pressure and does not take into account the pump's inefficiency. On small compressors, the most important CFM rating is 90 PSI, as this is the pressure required to run most tools. CFM ratings tend to be exaggerated like HP ratings, but in the case of 90 PSI, you should get every real HP 3-4 CFM.

Air compressor box size
Is size important? The answer is not too much. The compressor box does not generate air. It only stores air. It's important to have a pump and motor large enough, because if you want to produce as much air as you want, you won't run out of air no matter how small the tank is.

However, in some cases, if you only use air intermittently, you can save money by purchasing a small compressor with a large water tank. For example, if you have a compressor that produces 10 CFM continuously, you can remove the 20 CFM from the tank in half the time. If you need to use a 1-inch impact wrench (about 20 CFM) intermittently and use a small compressor with a large gas tank, there may be enough air in the tank to do the job. This way, the compressor can generate pressure until you need to repeat the job.

On the other hand, if you want to run the tool stably (such as a sander), it is important to generate as much air as possible. Smaller water tanks are easier to carry and can reach pressure faster, while compressors with large water tanks do not start and stop frequently, and air cooling is better. A large water tank does not mean that the compressor will run less; it will start and stop at a lower frequency, but the running time will be the same as the smaller tank time. Tank sizes are typically measured in US gallons.


Single and two stage air compressors
A single stage compressor has one or more cylinders, each pumping air directly into the tank. The two-stage compressor has at least two cylinders, and air is pumped from one cylinder to the other and then into the tank. A typical two-stage, two-cylinder pump passes air through a cooling tube at a speed of about 90 PSI and into a smaller second-stage cylinder, and finally enters the tank at a pressure of 175 PSI. The main reason for buying a two-stage compressor is if you need high pressure, but not many applications require high pressure. Therefore, you may not need a two-stage compressor.

Some people think that two-stage compressors have better quality than single-stage compressors, but this is not necessarily correct. I have seen the quality of single-stage and two-stage compressors. However, most industrial compressors of 5 hp and larger are two-stage, so they are more true because they are manufactured to industry standards, so they are more suitable, not because they are two-stage. For most applications, it is best to use a high quality single stage compressor instead of a low cost, large storage two stage compressor.

Direct drive or belt drive, less oil or less oil
Both belt drive and direct drive versions have some very good compressors. The problem is that most of the direct-drive compressors you'll see are designed to reduce the cost of large stores, which of course means low quality.

The direct drive compressor is directly connected to the motor shaft, so the speed is the same as the motor, which typically rotates 1,725 ​​or 3,450 RPM. There are some exceptions, but most good-quality direct-drive compressors have lower rotational speeds. Engineers designed some compressors to double the speed of rotation, allowing them to extract more air and keep prices low for large stores. However, life expectancy has dropped to about a quarter of the low-speed compressors, and the noise is almost unbearable. A small number of manufacturers are still producing smaller RPM direct-drive compressors that are ideal for woodworking finishing and other applications that require portability and quietness.

Some direct drive compressors are oil free, which is necessary in certain applications such as breathing air or aeration. If their RPM is low and the design is correct, they will last a long time, but it is best not to use a high speed oil-free compressor. Some European manufacturers produce some high quality direct drive oil lubricated compressors. They are compact and portable, making them ideal for the construction industry. In recent years, there have been many Chinese versions of these European compressors. Just like other tools copied, some are good and some are not.

If you don't need to carry a compressor with you, or if you need more airflow, then the best option is a belt-driven oil-lubricated compressor. You still have to be careful because some high-speed aluminum pumps have a very low life and are inexpensive, just like some direct-drive compressors. The main advantage of a belt driven compressor is that the pump rotates much more slowly than the motor, which makes it more efficient in reducing wear.

Most belt-driven compressors are splash-lubricated, which means they have an oil immersion at the bottom of the connecting rod to splash the oil into the compressor crankcase. As long as there is enough oil in the crankcase, the splash-lubricated pump should last a long time. For the best value, buy a belt-driven oil-lubricated compressor (unless you need to carry it with you) and stay away from the noisy high-speed oil-free compressor.

Duty cycle
It is important to understand the duty cycle before purchasing a compressor. The duty cycle is expressed as a percentage and tells you how many minutes the compressor will run in 10 minutes. For example, a low quality amateur compressor typically has a 50% duty cycle, which means that the compressor should not run for more than 5 minutes in any 10 minute period. If the duty cycle is exceeded, the pump will become overheated and will not function properly. Most industrial compressors have a duty cycle of at least 75%, and some even up to 100%, which means you can run them continuously.

A better work cycle can help you. Both you and your friends must use a 10 CFM compressor to paint the barn. If your compressor has a 65% duty cycle and its duty cycle is only 50%, then your paint shed area will increase by 30% in one day. Assuming you need 10 CFM for both guns, your friends can only paint for 5 minutes and then have to wait 5 minutes. And you can paint for 6 1/2 minutes, just wait 3 1/2 minutes before you start painting. Increased by 30%.


In another example, an overloaded 5 HP compressor with a 100% duty cycle actually produces more continuous air than a heavy duty 7.5 HP compressor with a 75% duty cycle. Often, you can spend more money to get a compressor with a longer duty cycle for even greater value.

Moisture problem
When the air is compressed, the air becomes hot, and when the air cools, the water will condense out. The harder your compressor works, the hotter it becomes, and the more water problems you have. Sometimes the best way to solve the moisture problem is to buy a bigger compressor so that the compressor doesn't have to work hard and get hot. Moisture in the air can also cause you to experience inconsistent moisture problems. A common moisture collector provides a low point for water collection. The system is designed to collect water droplets instead of water vapor, so if the air is hot, it will carry away moisture and then the water vapor will condense in the hose. The trick is to make the air as cool as possible before entering the sump, because if it cools after the sump, it will condense more water. In body shop or industrial applications, refrigerated air dryers are often used to cool the air and remove moisture.

A desiccant dryer is another method of removing moisture. They use silica gel to absorb moisture from the air and can be replaced or heated when saturated. If your system is small, you can use a well thought out piping system. Copper or aluminum tubes do not rust and have good heat dissipation. If you use a larger size, it will work like a larger radiator and will cool the air better.

Remember to install the sump at the end of the pipe and away from the compressor as close as possible to the equipment.

A common misconception is that you should drain the tank frequently to reduce the water in the air line. It is important to drain the tank to prevent rust, but the water at the bottom of the tank is less likely to enter the air line. In fact, if you fill the tank with half of cold water, the air will cool, the moisture will condense, and there will be dry air in the tubing. This does not mean that you should pour water into the tank, but you should do everything you can to cool the air before discharging it into the trap.

Life expectancy
What if you save 10-20% by purchasing a compressor at a local large store or buying a cheaper industrial compressor, and the life of this compressor is only half that of another compressor? Are you really saving money? Some brands of contractor compressors have 10 times the life of other brands of compressors, and the price difference is usually small.

Do not purchase a compressor unless the dealer can tell you the expected life. If he doesn't know, ask him to call the manufacturer. As a rule of thumb, for a better or larger compressor, you can save $2-3 in the long run for every dollar spent. In addition to purchasing high quality compressors for extended life, you can also purchase larger compressors. For example, if the 10 hp compressor is the minimum size required for your application, buy a 15 hp compressor because it runs about 2/3 of it and will stay much cooler, so the service life may be It is twice as big. When purchasing a compressor, consider how much it costs to live an hourly life expectancy.

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