About Single-Acting Reciprocating Air Compressors

One thing that is found in virtually every industrial environment is an air compressor. Some uses for the compressed air generated are: powering pneumatic tools, packaging, automation equipment, conveyors, controls systems, and various others. Pneumatic tools are favored because they tend to be smaller and more lightweight than electric tools, offer infinitely variable speed and torque, and can be safer than the hazards associated with electrical devices. In order to power these devices, compressed air must be generated.

There are two main categories of air compressors: positive-displacement and dynamic. In a positive-displacement type, a given quantity of air is trapped in a compression chamber. The volume of which it occupies is mechanically reduced (squished), causing a corresponding rise in pressure. Of the positive-displacement variety they are broken down further into two more categories: reciprocating and rotary.

A reciprocating compressor works like a bicycle pump. A piston reduces the volume occupied by the air or gas, compressing it into a higher pressure. There are two types of reciprocating compressors, single or double-acting. Single-acting compressors are the most common and are available up to 30HP at 200 psig.

Their small size and weight allow them to be installed near the point of use and avoid lengthy piping runs. Additionally, the single-acting reciprocating compressors do not need a separate cooling system. All of this leads to much simpler maintenance procedures, making the single-acting reciprocating compressors one of the easiest to maintain.

There are some disadvantages to this style of compressor. Rings have a tendency to wear out over time, if they’re not replaced as needed this can lead to lubricant carry-over into the air supply. These styles of compressor are relatively loud and comparatively cost more to operate than many other types. Because of this, they’re not designed for applications and processes that have a heavy-duty cycle of 70-90%. The single-acting reciprocating compressor should be used in installations where it’s only going to run 50% or less of the time.

At EXAIR we’re committed to providing you with the point of use products that’ll use your compressed air as efficiently and safely as possible. Feel free to reach out to an Application Engineer to discuss how we can help you improve in your processes.

Tyler Daniel

Application Engineer

E-mail: TylerDaniel@EXAIR.com

Twitter: @EXAIR_TD

Image courtesy of Compressor1 via Creative Commons License

About Air Compressors: Single-Acting Reciprocating Type

My colleague, Tyler Daniel, wrote a blog “Intelligent Compressed Air: Double-Acting Reciprocating Compressor”, and I wanted to extend that conversation to a close cousin, the single-acting reciprocating compressor.   As you see in the chart below, this type of compressor falls within the same family under the category of positive displacement compressors.

Positive displacement compressors increase air pressure by reducing air volume within a confined space.  The reciprocating type of air compressor uses a motor that turns a crank which pushes a piston inside a cylinder; like the engine in your car.  In a basic cycle, an intake valve opens to allow the ambient air into the cylinder, the gas gets trapped, and once it is compressed by the piston, the exhaust valve opens to discharge the compressed volume into a tank.  This method of compression happens for both the single and double-acting reciprocating compressors.  With a single-acting compressor, the air is compressed only on the up-stroke of the piston inside the cylinder.  A single-acting compressor will have an operating efficiency between 22 – 24 kW/100 cfm of air.  This type of air compressor is the most common and least expensive within the reciprocating family. 

Piston goes down: air draws in. Piston goes up: air is pushed out.

To explore the internals a bit closer, a mechanical linkage, or connecting rod, is attached to a piston and a crankshaft.  For every rotation of a motor, the piston will move up and down.  Air is being drawn into the cylinder and then compressed.  The volume of the cylinders, the number of cylinders, and the rotations per minute will determine the amount of compressed air that can be produced.  The advantages with reciprocating compressors are that they can produce high pressure, compress different types of gases, and have a cheap and rugged design.  The disadvantages would be high vibration and noise levels as well as being oversized as compared to capacity. 

There are different types of single-acting reciprocating compressors; single stage, dual stage, and multistage. The single stage uses one compression cycle to generate a pressure; generally for lower air pressures near 125 PSIG (8.6 bar). The dual stage will allow the pressure from the first cylinder to go into a smaller second cylinder. This dual compression will give a higher pressure up to 175 PSIG (12 bar). Multistage uses the same principle for multiple cylinders for much higher pressures up to 6,000 PSIG (414 bar); for applications like compressed air tanks used in SCUBA diving. They have options like air-cooled, intercooled, flooded type and oil-less. Single-acting reciprocating compressors have a wide range of uses and applications.

No matter the type of air compressor that you use, they are very expensive to use.  Air compressors are considered to be the fourth utility within a manufacturing plant.  To help use it efficiently and safely, EXAIR offers a range of products to clean, cool, blow, clean, conserve, and convey.  This would include our Super Air Knives, Super Air Nozzles, Safety Air Guns, Cabinet Coolers, and much more.  If you want to save energy, increase safety, and cut overhead costs, you can contact an Application Engineer at EXAIR.  We will be happy to help. 

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Types of Compressor image courtesy of the Compressed Air Challenge

Compressor internals image courtesy of h080Creative Commons License.

Compressed Air Purity Classes & ISO 8573-1. What Does it Mean for You?

The compressed air coming directly from your air compressor will usually require further treatment & preparation before it can be used. It’ll contain particulate matter, moisture, and hydrocarbons that the intake filter won’t remove…remember, it’s there to protect the compressor itself against damage from larger particulate. Smaller particulate and other contaminants that can affect air operated products & tools will still need to be addressed, after compression. The degree to which this additional treatment is necessary is dictated by what you’re using your compressed air for.

ISO 8573-1:2010 – Compressed air – Part 1: Contaminants and Purity Classes quantifies the quality of the air according to three properties, into different classes:

Per the descriptions above, here are the criteria by which compressed air purity is classified in these three categories. Certain applications can call for different classes for these three categories (more on that in a minute).
  • Maximum particle size & concentration of solid contaminants. These can come from rust on the inside of the distribution piping, particulate generated by wear of air system components, and atmospheric contamination that the compressor’s intake filter doesn’t catch.
  • Maximum pressure dew point. No matter where your compressor is located, the air it pulls in contains some amount of water vapor. Dew point is the temperature at which it will condense at a given pressure. As long as the compressed air temperature is above that dew point, there won’t be any water (in liquid form) in it.
  • Maximum oil content. This most often is due to carryover from oil lubricated compressors, but can come from atmospheric oil (or other hydrocarbon) vapor drawn into the compressor’s intake.

So…what does this mean to you, relating to your use of compressed air? Well, it largely comes down to the nature of your application. Whatever is in your compressed air supply will be in contact with whatever the air comes in contact with. If a machinist is using a Safety Air Gun to blow chips & coolant from machined parts, they’re not going to be particularly concerned with this specification from a regulatory standpoint. If those parts are going straight from the machine shop to a paint booth, they’re certainly going to want to use air that’s free of particulate, moisture, and oil. All of those things will, quite noticeably, affect the quality of the painted finish. Filter Separators and Oil Removal Filters installed at the point of use will take care of that. A case could be made for a purity specification and regular testing of their compressed air, but this really just falls under the confines of good engineering practice.

Compressed air use in applications where it can come in contact with food or beverages intended for consumption (by people AND animals, according to the Federal Food, Drug, and Cosmetic Act) is considered a critical factor for cleanliness. They reference guidelines from the British Compressed Air Society (BCAS) to specify purity classes for both direct and indirect contact with food and beverage products:

Direct contact requires testing and compliance to Class 2:2:1 per the above table means:

  • Particulate Class 2 – particle concentration, by particle size, in concentrations no greater than:
    • 400,000 particles sized 0.1-0.5 microns, per cubic meter
    • 6,000 particles sized 0.5-1.0 microns, per cubic meter
    • 100 particles sizes 1.0-5.0 microns, per cubic meter
  • Maximum pressure dew point Class 2 – vapor pressure dew point must be less than 40°F (40°C) at the maximum pressure of the compressed air system.
  • Oil content Class 1 – concentration must be less that 0.001 milligrams per cubic meter

Examples of direct contact applicable to the use of EXAIR Engineered Compressed Air Products include blowing air for cooling, moisture removal, coating layer distribution, etc., of unpackaged food product.

EXAIR Stainless Steel Super Air Knives are popular in food processing applications (left to right): removing excess moisture prior to flash freezing of fish filets, preventing clumping while packaging shredded cheese, and (my personal favorite) ensuring a consistent and even glazing of fresh, delicious doughnuts.

Line Vac Air Operated Conveyors and Vortex Tubes are also used in direct contact applications in the food industry:

316SS Threaded Line Vac conveys bulk grain in a distillery (left). Vortex Tube rapidly sets melted chocolate in a mold (right).

Indirect contact is slightly (but JUST slightly) less restrictive: those are Class 2.4.2. Particulate and oil content classes remain the same, but dew point can be as high as 37°F (3°C). This is where the air the air is coming into contact not with the consumable product itself, but, for example, the packaging or container:

Atomizing Spray Nozzles rinse bottles prior to labeling (left), 1″ Flat Super Air Nozzle blows off label to ensure proper scanning by sensor (center), Line Vac conveys canned goods (right).

EXAIR Corporation is committed to helping you get the most out of our products – and your compressed air system. If you have questions, I can talk about compressed air all day – and oftentimes I do! Let’s talk.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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ISO 8573-1 Chart by Compressed Air Best Practice.

Air Compressor Motors and Controls, Working Together.

One of the most important aspect of an efficient compressed air delivery system is effective utilization of compressor controls. The proper use of compressor controls is critical to any efficient compressor system operation. In order to reduce operating costs, compressor controls strategies need to be developed starting with minimizing the discharge pressure. This should be set as low as possible to keep energy costs to a minimum.

The compressor system is designed with maximum air demand in mind. During periods of lower demand compressor controls are used to coordinate a reduction in output that matches the demand. There are six primary types of individual compressor controls:

  1. Start/Stop – This is the most basic control. The start/stop function will turn off the motor in response to a pressure signal.
  2. Load/Unload – The motor will run continuously, but the compressor unloads when a set pressure is reached. The compressor will then reload at a specified minimum pressure setting.
  3. Modulating – Restricts the air coming into the compressor to reduce compressor output to a specified minimum. This is also known as throttling or capacity control.
  4. Dual/Auto Dual – On small reciprocating compressors, this control allows the selection of either Start/Stop or Load/Unload.
  5. Variable Displacement – Gradually reduces the compressor displacement without reducing inlet pressure.
  6. Variable Speed – Controls the compressor capacity by adjusting the speed of the electric motor.

All of these controls then control the compressor motors and they have several different starting methods.

There are several types of modern motor starters:

Full Voltage Starters: The original, and simplest method.  These are similar in theory to the old knife switches, but the operator’s hands aren’t right on the connecting switch.  Full line voltage comes in, and amperage can peak at up to 8 times full load (normal operating) amperage during startup.  This can result in voltage dips…not only in the facility itself, but in the neighborhood.  Remember how the lights always dim in those movies when they throw the switch on the electric chair?  It’s kind of like that.

Reduced Voltage Starters: These are electro-mechanical starters.  Full line voltage is reduced, commonly to 50% initially, and steps up, usually in three increments, back to full.  This keeps the current from jumping so drastically during startup, and reduces the stress on mechanical components…like the motor shaft, bearings, and coupling to the compressor.

Solid State (or “Soft”) Starters: Like the Reduced Voltage types, these reduce the full line voltage coming in as well, but instead of increasing incrementally, they gradually and evenly increase the power to bring the motor to full speed over a set period of time.  They also are beneficial because of the reduced stress on mechanical components.

The Application Engineering team at EXAIR Corporation prides ourselves on our expertise of not only point-of-use compressed air application & products, but a good deal of overall system knowledge as well.  If you have questions about your compressed air system, give us a call.

Jordan Shouse
Application Engineer

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Compressor Photo Credits to Bryan Lee, Creative Commons License