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Air Compressors

At its simplest, an air compressor is a piece of equipment that creates and moves pressurized air. Air under pressure provides great force, which can be used to power many different kinds of tools, applications, and industries.

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What is an air compressor?

An air compressor is a device that creates and moves pressurized air at various speeds to enable jobs, applications, and functions to be performed faster, more efficiently, more easily, and at lower costs. The devices are used in a broad range of major industries such as manufacturing, semiconductors, food and beverage, healthcare, home appliances, energy, and oil and gas.

Pressured air can provide great force that can be used to catalyze and energize many different kinds of applications for a broad array of beneficial purposes.

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Types of Air Compressors

In the compressed air industry, one of the most important dynamics is the use of oil-injected and oil-free air compressors. Each has advantages depending on the applications. Both are major markets. A key difference is that oil-free compressors are not as susceptible to contamination as oil-injected compressors, the latter of which are appropriate for industries with less-stringent air purity requirements.

  • Oil-Free Air Compressors: These compressors that do not use any lubrication in the compressor chamber. The specific oil-free compressor you need depends on the air purity level you need to achieve.
    • There is Class 0 oil-free air which differs from “technically oil-free air.” Class 0 oil-free compressors can guarantee 100 percent oil-free air. Though priced higher, these compressors are much safer for sensitive applications. By contrast, “technically oil-free” compressors are likely to allow contamination because they are lubricated compressors housing oil-removing filters.
  • Oil-Injected Air Compressors: These devices use oil to lubricate the air compression chamber, lubricate parts, and seal in the air. These are the most common types of compressors, have lower initial and maintenance costs, and longer life expectancies. 
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Air Compressor Technologies

Rotary Screw: Rotary screw air compressors use rotary-type, circular movement to compress the air. Capable of running continuously – they don’t need to be shut on and off -- these are one of the most popular types of compressors in many industries.

  • Air comes in through the inlet valve and gets funneled through twin helical screws, known as rotors, that pressurize the air. As the screws turn, the volume decreases and air pressure increases.
  • Rotary screw compressors are used when large volumes of high-pressure air are necessary.

Piston (Reciprocating) Compressor: The conventional piston compressor, one of the most widely used, has five key parts: a crankshaft, connecting rod, piston, cylinder, and valve head. The valve head holds thin metal flaps, the inlet and discharge valves, at the apex of the cylinder. One is mounted underneath and the other above the valve plate. As the piston moves down, a vacuum is created above it, allowing outside air at atmospheric pressure to push open the inlet valve and fill the area above the piston. As the piston moves up, the air above it compresses, holds the inlet valve shut and pushes the discharge valve open. The air moves from the discharge port to the tank. With each stroke, more air enters the tank and the pressure rises.

Scroll Compressor: This type of air compressor, usually oil-free, works in a circular motion. A single spiral-shaped rotor oscillates against a similar fixed spiral. As these spirals move against each other, the cavity trapping air between them becomes progressively smaller. This decrease in volume forces the fixed volume of intake air to increase in pressure. When the orbiting spiral moves, air is drawn in and becomes captured in one of the air pockets where it gradually compresses while moving towards the center. The compression cycle continues for 2.5 turns, which gives almost-constant and pulsation-free air flow. This is a relatively silent and vibration-free process, which of course is desirable.

Centrifugal Compressor: For this compressor type, air gets drawn into the center of a rotating impeller with radial blades by centrifugal force. This movement of air raises pressure and spawns kinetic energy. Before the air is led into the center of the impeller, the kinetic energy turns into pressure by passing through a diffuser.

Tooth Compressors: The compression element in a tooth compressor consists of two rotors that rotate in opposite directions inside a compression chamber. During the intake phase, air is drawn into the compression chamber until the rotors block the inlet. Next, the drawn-in air is compressed in the compression chamber, which shrinks as the rotors rotate. One of the rotors blocks the outlet port during compression, while the inlet is open to draw in new air into the opposite section of the compression chamber. When one of the rotors opens the outlet port, the compressed air is forced out of the compression chamber.

Vane Compressor: The vane compressor, commonly oil-lubricated compressors, are usually manufactured with special cast alloys. The rotary vane compressor contains a cylindrical rotor placed inside a cavity or housing. The rotor also has a few grooves or slots where vanes are placed. The rotor is intentionally located where it almost comes into contact with the housing in which it is encased. Through use of centrifugal force, this off-center placement allows the vanes to be pushed out, allowing air to be trapped between them. This air then decreases volume and gets pressurized by the rotor’s rotating motion.

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Air Compressor Design

Compressors can be either fixed-speed or variable-speed drive (VSD). The latter automatically changes its motor speed to the air demand. By contrast, fixed speed compressors function at either full throttle on or off.

  • Fixed-Speed Compressors: Fixed speed compressors run at a continuous same speed and tend to be efficient when operating at 100 percent of their capacity, meaning when the motor is running and compressed air is being manufactured. It’s obvious when a fixed-speed compressor inefficiently runs and this can easily be measured when the unit unloads and stops making air. Before the motor comes to a complete stop, it will continue to run while no air is being produced in order to protect the motor from too many start/stop cycles. 
  • Variable Speed Drive (VSD) Compressors: Variable speed drive (VSD) technology work differently. They turn the motor at the appropriate speed in relation to the amount of air required within a given plant or facility. As the demand for air increases, so does the speed of the motor. By contrast, if the demand decreases, the motor will automatically slow and only use the required energy to provide appropriate flow. VSD air compressors match the output to the demand needed. They sense the amount of flow being used and alter speed accordingly. On slower production days, breaks throughout the day, or during second and third shifts, the VSD technology is especially useful because it reduces electrical waste and saves money.

Air compressors are either single-stage or two-stage devices. The primary difference is the number of times air gets compressed between the inlet valve and the nozzle:

  • Single-stage compressor: With this type of compressor, air is sucked into a cylinder and trapped, then compressed in a single stroke with a piston. The air is only compressed once, once then travels to a storage tank.
  • Two-stage compressor: A two-stage compressor squeezes air twice, thereby achieving higher pressures than single-stage units. Although the air gets funneled into a cylinder and is compressed, it isn’t then sent to a storage tank. Rather, the air travel to a smaller piston for a second stroke and then is sent to a storage tank.

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