Wednesday, August 31

Back to Compressed Air School Part 3: Air Compressor Mechanics

Now that we have gone over the history of air compressors and some basics of compressed air, let’s dive deeper into the specific types and functions!

Lubricated Air Compressors

Lubricated air compressors simply mean that there is oil within the compression chamber that keeps the mechanical elements running smoothly. There are a few different types that fall under this category

Rotary Screw

This positive displacement compressor utilizes a pair of helical screws, generally referred to as the male and female rotors, to drive air into the chamber by rotating in opposite directions. Air gets trapped between these rotors and builds up pressure while also shrinking the chamber to force the compression of the air. Oil is one way to keep the screws operating smoothly. These are lower maintenance and have internal cooling mechanisms.

Rotary Vane

Utilizing similar methodology as rotary screw compressors, rotary vane compressors utilize adjustable vanes located on a drive shaft. The drive shaft is off centered so that the blades can create an airtight chamber as they push against the chamber walls by rotating and retracting to reduce the volume of the air and compress it.

Reciprocating/Piston

These positive displacement compressors utilize a series of one-way valves to push air into a chamber where a piston moving at a constant motion then compresses it. Oil is required to keep the valves and piston moving. These have more moving parts, so they require more maintenance.

Single-Stage vs. Multi-Stage

Reciprocating air compressors can either be single-stage or dual/multi-stage machines. The smaller or portable reciprocating compressors will generally be single-stage which means that air is compressed from atmospheric pressure to discharge pressure in only 1 step. This one step consists of the piston compressing the air in one stroke, which is one full rotation of the crankshaft to power the piston. Dual- or multi-stage compressors are what most industrial and commercial compressors are. This means that the air goes through another step of compression before being transferred to an air receiver tank and ultimately discharged. After the air is compressed to an intermediate pressure, it is then cooled and compressed again to a higher pressure until the final discharge pressure is achieved. These compressors have higher capabilities when it comes to air production and PSI.

Oil-Free Air Compressors

In comparison to lubricated air compressors, oil-free compressors mean there is no oil in the compression chamber of the system. This is possible because there is no mechanical contact within the chamber that would need lubrication to continue to run smoothly. These compressors are used when air quality is kept to a higher standard, and they need to minimize the risk of contamination. There are also various kinds of compressors that fall under this category

Rotary Screw

Rotary screw compressors can also be run oil-free where air quality standards are high and there needs to be low risk of contamination between the rotors pushing the air through. These compressors often use water to lubricate the screws and ensure efficient operation. Industries that may use oil-free rotary screw compressors include medicine, food, electronics, and biology.

Centrifugal

Centrifugal compressors are dynamic compressors that have what is considered a radial design. Air is drawn into the middle of a rotating impeller with radial blades and is then pushed out to the perimeter by centrifugal forces. As pressure rises, kinetic energy is created which is converted into pressure. These can also be single- or multi- stage machines.

Rotary Tooth

Rotary tooth compressors are types of displacement compressors that have two rotors that move in opposite directions similarly to rotary screw compressors. As the teeth rotate they create compression pockets that pressurize the air to be discharged through the outlet port.

Oil-Less Air Compressors

Oil-less compressors (or, oil-free compressors) simply have no oil in the machine at all.

Scroll

Scroll compressors are smaller positive displacement machines that utilize two smaller spirals (scrolls) that work in unison to compress the air. One scroll stays stationary while the other moves inside of the stationary scroll by a crank shaft. This motion causes air to compress. These are generally reliable, quiet machines. You will frequently observe these machines in clean air applications such as medical facilities and laboratories.

Compressor Control Methods

Due to the heavy-duty industrial applications these various compressors provide, there is a wide variety of sizes and control methods that our industry offers. The following are control methods that can be used depending on the specific application of your system.

Stop/Start

This control is exactly how it sounds, either power is being provided to the motor in full force, or it is not.

Load/Unload

In this control, the compressor is continuously being powered but there is a valve that can reduce the tank’s capacity when compression demands vary. When a compressor is fully loaded, it operates at full capacity until the unload pressure is reached. The compressor then switches to unload where no compressing happens and the compressor vents its internal pressure, also known as blowing down. During the unload period, demand of the plant is maintained through the air stored in the receiver(s).

Modulation

Modulation also uses a valve like the one used in load/unload. In this method, the valve is used to adjust internal pressure by throttling or closing the inlet valve based on the demand. This method is used in applications where frequently stopping the compressor is not an option, but it is a less effective method for rotary screws than others.

Variable Displacement Control

Variable displacement adjusts the volume of the air as it is being pulled into the chamber by opening valves in the pump that limit the amount the air end is used for compression. This is generally an efficient control method for double-acting reciprocating compressors, as well as some rotary screw designs, however, some rotary screw compressors will sacrifice efficiencies when using this method.

Variable Speed Control

Variable speed is considered to be very efficient however, it may respond differently with different types of compressors. This control uses a frequency drive to vary the speed on the motor and air end to control the compressor’s capacity. This allows for the machine to provide an almost proportional flow to power ratio at partial load. However, when operating at full load, variable speed can be less efficient than a fixed speed machine as fixed speed compressors have been designed to operate within specific speeds without any significant changes. The equipment used in variable speed controls also is more susceptible to environmental pollutants.

Industrial air compressors vary in so many ways and there are so many options. While you now have a better understanding of the basic kinds, mechanics, and controls we still recommend contacting one of our experts before purchasing a machine to ensure proper application to your facility.

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Our comprehensive product and service solutions keep you running 24/7.

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Our compressed air experts look forward to meeting you to discuss your equipment and support needs. Please contact us directly at (262) 695-7000 with any questions or to schedule service for your system moving forward.

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