Air Compressors General Information & FAQ
Air Compressors General Information
What you need to know about Air Compressors
How Air Compressors Work
There's a lot to know about air compressor's general operation and maintenance. If you're in the market to buy an air compressor, being informed on the facts and things to consider is more important than anything for finding the best air compressor that best fits your needs.
Whether you are a doctor, mechanic, wood carpenter, handyman or homeowner, "do-it-yourself" guides are usually the best way to go for someone busy enough and getting things done to spend hours surfing the internet through manuals in search of the information they need.
This article is here to make it easier for everyone. The facts and suggestions below come from the experience, consumer reviews, and knowledge of many others who have done the work to get info on air compressor topics.
A quick explanation of what air compressors are and how they work would be the best place to start:
The Basics:
The way air compressors work is compared to how human lungs work when you inflate a balloon or blow out a flame. Yes, air compressors draw in and hold air and so does the human lungs. But you can't take the comparison any further than that.
Air compressors pull regular air into their cylinders at atmospheric pressure. But once the air is in the cylinder, it is immediately compressed by a piston in a compression chamber to the desired, higher pressure. This process of compressing the air creates heat, because of the increase of air friction. After being compressed, the air must be cooled by the compressor's cooler (sometimes called an after coolers, or radiator), which removes most of the heat caused by compression. From the after-cooler, the compressed, cooled air is passed into the compressor's tank, or receiver tank to be stored until it's used.
Receiver tanks come in many sizes (depending on the demands of the job). The compressed air is stored up to make sure that the air pressure leaving the tank is as consistent as possible. Most pneumatic tools (air tools) will demand different levels of performance from an air compressor at different times during use. Storing a large volume of compressed air in the receiver tank smooths out the peaks and valleys of the tool's demand. A pressure switch helps the compressor maintain constant air pressure as the air leaves the hose.
The compressor's pressure switch controls the amount of compressed air stored in the receiver tank. Cooled and compressed air is added to the receiver tank until the tank reaches its maximum pressure, at which point the compressor stops compressing air. Some compressors completely shut off during this phase.
The compressor will start compressing air again when the pressure in its tank drops below its preset minimum pressure. Maximum pressure and minimum pressure are sometimes called the cut-out pressure and cut-in pressure.
The compressed air then leaves the receiver tank through the compressor's air hose, and exits the system through the air tool or air chuck. A regulator on the compressor is used to set the pressure of the air leaving the compressor. Pressure should be set to match the requirements of the tool(s) being used.
Important thing to consider is that because water and moisture can be hard on tools and tool parts, some compressors are equipped with in-line air dryers, and many compressors can be upgraded to include dryers. These dryers take the water out of the compressed air as it leaves the tank, before the air is passed to the air hose. This is especially true of large, industrial-sized air compressors and compressors used for painting (to keep water out of the paint mixture).
Air Compressors also have a valve safety, used for releasing pressure from the tank in an emergency, or simply to empty the tank of air.
And that is it basically. So now for a recap:
A pressure switch keeps the pressure the same as the air leaves the compressor.
Compressed air in the compression cylinder is hot and must be cooled with an aftercooler.
A regulator is used to adjust the pressure setting of the compressor.
The compressed and cooled air is stored in a receiver tank.
Air compressors draw in regular air and pressurize it with a piston.
The pressurized air is often dried with an air dryer of some kind before it leaves the compressor and enters the tool.
A Couple Technical Things
These are called direct coupled units. Others are built so that the motor shaft drives a belt which in turn drives the compressor, and are called belt driven units. Most reviews and opinions lean toward belt driven air compressors, because they tend to be more quiet and efficient at higher motor speeds. There are a few variations when it comes to air compressor design. Most of these differences are more-or-less unimportant for most air compressor users, but we'll mention one here that's worth mentioning. Some air compressors are built with their motor shafts directly connected to their compression cylinders.
Here are just a few terms you might run into when shopping for or reading about air compressors:
Air Pressure Range
For air tools, this is the pressure range at which they should be used. Example: The air pressure range for a Porter-Cable FN250B Air Nailer is 70 PSI to 120 PSI.
CFM/SCFM
Cubic Feet per Minute/Standard Cubic Feet per Minute. These terms can refer to at least two things when it comes to air compressors:
The volume of air that an air compressor delivers over time (minutes)
The volume of air that an air tool consumes when in use (its demand).
The "Standard" in SCFM refers to standard conditions of atmospheric pressure, temperature, and humidity at which tools are sometimes tested, but either term can be used for most purposes.
For air compressors, CFM is displayed along with pressure (PSI) because the volume of air being delivered depends on the pressure it is being released at. Increasing the air pressure usually means that less air will be delivered, or less CFM.
Duty Cycle
For air compressors that need a rest period between working, duty cycle refers to the amount of time that an air compressor needs to rest and is usually written as a percentage.
Air compressors have been used in the industry for well over 100 years because Air as a resource is safe, flexible, clean and convenient. The Compressed Air Machines have evolved into highly reliable pieces of equipment that are almost indispensable in many of the applications they serve.
The Different Types of Air Compressors
Reciprocating or Piston compressors are the most common machines available on the market. They are positive displacement compressors and can be found in ranges from fractional to very high horsepowers. Positive displacement air compressors work by filling an air chamber with air and then reducing the chamber’s volume (Reciprocating, Rotary Screw and Rotary Sliding Vane are all positive displacement compressors). Reciprocating compressors work in a very similar manner as does as internal combustion engine but basically in a reverse process. They have cylinders, pistons, crankshafts, valves and housing blocks.
Rotary Screw Compressors work on the principle of air filling the void between two helical mated screws and their housing. As the two helical screws are turned, the volume is reduced resulting in an increase of air pressure. Most rotary screw compressors inject oil into the bearing and compression area. The reasons are for cooling, lubrication and creating a seal between screws and the housing wall to reduce internal leakage. After the compression cycle, the oil and air must be separated before the air can be used by the air system.
Rotary Sliding Vane Compressors like Reciprocating and Rotary Screw compressors are positive displacement compressors. The compressor pump consists primarily of a rotor, stator, and 8 blades. The slotted rotor is eccentrically arranged within the stator providing a crescent shaped swept area between the intake and exhaust ports. As the rotor turns a single revolution, compression is achieved as the volume goes from a maximum at the intake ports to a minimum at the exhaust port. The vanes are forced outward from within the rotor slots and held against the stator wall by rotational acceleration. Oil is injected into the air intake and along the stator walls to cool the air, lubricate the bearings and vanes, and provide a seal between the vanes and the stator wall. After the compression cycle, the oil and air must be separated before the air can be transferred to the air system.
Centrifugal Compressors are not positive displacement compressors like the Reciprocating, Screw or Vane Compressors. They use very high speed spinning impellers (up to 60,000 rpm) to accelerate the air then diffuser to decelerate the air. This process, called dynamic compression, uses velocity to cause an increase in pressure. In most Centrifugal compressors, there are several of these impeller/diffuser combinations. Typically, these machines have intercoolers between each stage to cool the air as well as remove 100% of the condensate to avoid impeller damage due to erosion.
As well as other types that we did not mention.
An air compressor is a device that converts power (usually from an electric motor, a diesel engine or a gasoline engine) into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts. There are numerous methods of air compression, divided into either positive-displacement or negative-displacement types.
Air Compressor Types According to Specific Criteria
- According to the design and principle of operation
- Reciprocating compressor
- Rotary compressor
- According to the number of stages
- Single stage compressor
- Multi stage compressor
- According to the pressure limits
- Low pressure compressors
- Medium pressure compressors
- High pressure compressors
- Super high pressure compressors
- According to the capacity
- Low capacity compressors
- Medium capacity compressors
- High capacity compressors
- According to the method of cooling
- Air cooled compressor
- Water cooled compressor