Working Of An Air Compressor - SKengineers

 

WHAT IS AN AIR-COMPRESSOR?

Air compressor -

Air compressor supplies air into a nail gun -

An air compressor is a pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces more and more air into a storage tank, increasing the pressure. When the tank's pressure reaches its engineered upper limit, the air compressor shuts off. The compressed air, then, is held in the tank until called into use. The energy contained in the compressed air can be used for a variety of applications, utilizing the kinetic energy of the air as it is released and the tank depressurizes. When tank pressure reaches its lower limit, the air compressor turns on again and re-pressurizes the tank. An air compressor must be differentiated from a pump because it works for any gas/air, while pumps work on a liquid.

How Does an Air Compressor Work?

Air compressors work by forcing atmospheric air under pressure to create potential energy that can be stored in a tank for later use. Just like an open balloon, the pressure builds up when the compressed air is deliberately released, converting the potential energy into usable kinetic energy. From there, this energy transfer can be harnessed to power various pneumatic tools.

Industrial air compressors work similarly to combustion engines. Generally, air compressor operation requires a pump cylinder, piston and crankshaft to transfer energy for a wide variety of tasks. These basic components can help supply air for filling up objects like tires or inflatable pool toys, or they can supply power for operating tools such as drills, nail guns, grinders, sanders and spray guns.

From impact wrenches to AC units, many versatile air-powered tools and machines are responsible for the comfort, shelter, automation and efficiency of everyday life. The compressors themselves are more compact and lightweight than other centralized power sources. They are also long-lasting, require less maintenance and are easier to move than other old-fashioned machinery.

Classification -

Compressors can be classified according to the pressure delivered -

 

Low-pressure air compressors (LPACs), which have a discharge pressure of 150 pounds per square inch (10 bar) or less.

Medium-pressure compressors which have a discharge pressure of 151 to 1,000 pounds per square inch (10.4 to 68.9 bar).

High-pressure air compressors (HPACs), which have a discharge pressure above 1,000 pounds per square inch (69 bar).

They can also be classified according to the design and principle of operation -

Single-stage reciprocating compressor

Multi-stage reciprocating compressor

Compound compressor

Single stage rotary-screw compressor

Two-stage rotary screw compressor

Rotary vane pump

Scroll compressor

Centrifugal (roto-dynamic or turbo) compressor -

In another way of classification, there are two main types of air-compressors: oil-injected and oil-less. The oil-less (or Oil-Free) system has more technical development and delivers air of better quality for critical applications where oil contamination cannot be tolerated (such as food and medicine production) although it is more expensive, louder and lasts for less time than oil-lubed pumps.

There are numerous methods of air compression, divided into either positive-displacement or roto-dynamic types.

Positive displacement compressors -

Positive-displacement compressors work by forcing air in a chamber whose volume is decreased to compress the air. Once the maximum pressure is reached, a port or valve opens and air is discharged into the outlet system from the compression chamber. Common types of positive displacement compressors are -

Types of Positive Displacement Air Compressors -

Because positive displacement is the more common type of air compression method, there is a large variety of positive displacement air compressors. However, each one works differently. Some are better for industrial use, and others are suitable for home projects and smaller applications. Here are some of the different types of positive displacement air compressors:

How a rotary screw air compressor works –

The Rotary Screw Air Compressor -

In many heavy-duty industrial applications, a piston compressor just doesn’t cut it. For the higher pressures needed for complex pneumatic and high-powered tools, professionals tend to choose rotary screw air compressors.

While the piston air compressor uses pulsation and the alternating nature of the piston mechanic, a rotary screw compressor is continuous. A pair of rotors mesh together to pull air in and compress it as it moves through a spiral. The rotary motion moves the air through a chamber and ejects it. Fast rotational rates can minimize leakage.

Many compressor types experience some shaking, which can damage the equipment and requires you to take measure to minimize vibrations. In contrast, most rotary screw compressors operate smoothly for even, vibration-free performance.

Rotary screw compressors can vary widely, with rates from 10 CFM to those in the 4-5 figure range. Control schemes include:

Stop/start -

This approach either provides power to the motor, or it does not, according to the application.

Load/unload -

The compressor is powered continuously, with a slide valve that reduces the tank’s capacity when a specific compression demand is met. This scheme is common in factory environments, and if it involves a stop timer, it is called a dual-control scheme.

Modulation -

Modulation also uses a sliding valve to adjust pressure by throttling/closing the inlet valve, matching the compressor’s capacity to the demand. These adjustments are less effective on rotary screw compressors than other types. Even when set to a 0 capacity, the compressor would still consume about 70 percent of its full power load. Still, modulation is applicable for operations in which frequently stopping the compressor is not an option.

Variable displacement -

This control scheme adjusts the volume of air that is pulled into the compressor. In rotary screw compressors, this method may be used alongside modulating inlet valves to improve efficiency and pressure control accuracy.

Variable speed -

Variable speed is an efficient way to control a rotary compressor’s capacity, though it may respond differently with different types of air compressors. It varies the speed of the motor, which affects the output. This equipment tends to be more delicate than other designs, so it may not suit particularly hot or dusty work environments.

Here’s a more technical look at how oil-injected rotary screw compressors work -

Atmospheric air enters the compressor at the inlet valve.

The air travels through the pressure control line to the regulator valve, a process that sets the system’s air pressure.

Air then enters the compressor, where it combines with oil into a mist.

Air travels the length of the two internal screws as they rotate in opposite directions.

The screw motion creates a vacuum, trapping and compressing air in the space between the screws.

The pressurized air is forced through the output and into the primary oil separator tank while still combined with the oil as a mist.

The centrifugal force inside the tank causes most oil molecules to form into droplets and collect at the bottom as reusable oil.

Air then enters a secondary separation filter where more oil is removed, purifying the air further.

The oil-free air exits the system, where it is stored in a tank or used immediately in a connected pneumatic tool or machinery.

 How a rotary vane air compressor works -

Rotary vane: A rotary vane compressor or vacuum pump has a similar principle to a rotary screw. With a rotary vane, a motor is placed off-centre inside a rounded cavity. The engine has blades with automatically adjusting arms. As the arms approach the air input, they are elongated, creating a large air cavity. As the motor spins, moving air with it, the arms approach the output and get smaller, creating a smaller space between the vanes and the round casing, which compresses the air. Vane-operated rotors are small and easy to use, making them great for homeowners and contractors.

Because of the similarity between rotary vane and rotary screw compressors, here’s a technical description of how a rotary vane air compressor functions for comparison -

Atmospheric air enters through the inlet valve and travels into the compressor.

Vanes are mounted on the internal spinning rotor, which is placed off-centre inside the cavity.

Arms with self-adjusting length divide the space, creating several cavities of varying size.

Air fills the cavity and travels around, following the rotor’s rotation.

As the cavity gets smaller, air pressure builds up and compresses the air.

The pressurized air is then forced through the compressor’s output.

 how a reciprocating/piston air compressor works -

Reciprocating/piston: In a reciprocating air compressor, a rotor’s spin forces a piston to move up and down. When the piston goes down, freestanding air is pulled into a chamber. Then, the air is compressed and forced back outward as the piston rises back up. Some compressors, called single-stage compressors, use only one piston. Others, called two-stage compressors, use two pistons and are able to pressurize more air. The reciprocating type of air compressor is one of the most common.

Technical illustration of a two-stage air compressor -

Technical illustration of a portable single-stage air compressor -

Piston-type: air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons. They use one-way valves to guide air into and out of a chamber whose base consists of a moving piston. When the piston is on its down stroke, it draws air into the chamber. When it is on its up stroke, the charge of air is forced out and into a storage tank. Piston compressors generally fall into two basic categories, single-stage and two-stage. Single stage compressors usually fall into the fractional through 5 horsepower range. Two-stage compressors normally fall into the 5 through 30 horsepower range. Two-stage compressors provide greater efficiency than their single-stage counterparts. For this reason, these compressors are the most common units within the small business community. The capacities for both single-stage and two-stage compressors is generally provided in horsepower (HP), Standard Cubic feet per Minute (SCFM)* and Pounds per Square Inch (PSI). *To a lesser extent, some compressors are rated in Actual Cubic Feet per Minute (ACFM). Still others are rated in Cubic Feet per Minute (CFM). Using CFM to rate a compressor is incorrect because it represents a flow rate that is independent of a pressure reference. i.e. 20 CFM at 60 PSI.

Rotary screw compressors: use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, whose volume is decreased as the screws turn. Rotary screw compressors can be single-stage or two-stage.

Vane compressors: use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume. This type of compressor delivers a fixed volume of air at high pressures.

Roto-dynamic or turbo compressors -

Roto-Dynamic air compressors include centrifugal compressors and axial compressors. In these types, a rotating component imparts its kinetic energy to the air which is eventually converted into pressure energy. These use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air, which pressurizes it.

Piston Air Compressor Functionality -

So, how does an air compressor get air? For those that use pistons, it involves two parts: Raising the pressure and reducing the volume of the air. Most compressors use reciprocating piston technology.

An air compressor typically uses -

An electric or gas motor

An inlet and a discharge valve to draw in and release air

A pump to compress the air

A storage tank -

 The compressor draws in air and creates a vacuum to reduce its volume. The vacuum pushes the air out of the chamber and into its storage tank. Once the storage tank reaches its maximum air pressure, the compressor turns off. This process is called the duty cycle. The compressor will turn back on when the pressure drops below a specific number.

Air compressors don’t need to have storage tanks, and a few of the smaller options forego them in favor of portability.

How Lubrication Works in Air Compressors: Oil-Flooded vs. Oil-Free

One of the most important things to know about air compressor maintenance is how lubrication works. When you’re looking at oil pumps, you’re dealing with two categories:

Oil-lubricated pumps: In this design, oil splashes onto walls and bearings within the cylinder. This method is also called oil-flooded lubrication and tends to be more durable. A piston ring is a piece of metal on the piston that helps to create the seal inside the combustion chamber. This ring can help keep oil out of the compressed air, but sometimes it can still seep into the tank.

Oil-free pumps: Oil-free pumps receive special lasting lubrication that eliminates the need for oil. Oil-free pumps are an excellent option in many industries where contamination is not an option, like breweries, food production and pharmaceutical manufacturing. They ensure that no oil contaminates the air they use in their process or product.

Oil-flooded pumps are somewhat of a mixed bag. For power tools that need lubrication, the presence of oil in the air stream can be beneficial. For tools that need oil, inline sources can distribute oil in even amounts. On the other hand, many tools can stop working correctly when even minute amounts of oil are present in the air stream.

For painting or woodworking, oil can interrupt the entire process. It may keep coatings from drying or finishing evenly. Airborne oil can even corrupt the surface of wood projects.

Thankfully, there are tools to prevent oil from entering the tank, like air-line filters and oil separators. Still, when oil-free air is critical to operation, oil-free compressors and their permanent lubrication is the best option.

Air Compressor Power Ratings: What Is CFM?

When we talk about power in an air compressor, we usually speak in terms of horsepower, but there are many other ways to determine how much pressure a machine can provide. We use cubic feet per minute (CFM) to discuss the rate and volume by which a machine compresses the air. But the rate at which outside air enters the cylinder is influenced by heat, humidity and wind in the surrounding atmosphere.

To consider these internal and external factors, manufacturers use standard cubic feet per minute (SCFM), which combines CFM with those outside factors of pressure and humidity.

Another rating that you might see is displacement CFM, which looks at the efficiency of the compressor pump. It pulls information from the revolutions per minute (RPM) of the motor and the volume of air that the cylinder can displace. This number is more of a theoretical measurement, while you can also measure CFM in terms of delivered air, or how much is actually discharged. This number is called CFM FAD, which stands for free air delivery, and is useful for measuring delivery to certain tools.