Sliding Vane Compressors: How they Work

When it comes to air compressors there are many different types to choose from. Each compressor performs differently and have their own benefits and downfalls. In this case we will be taking a look a Sliding Vane Compressor which is a positive-displacement compressor. Positive-displacement type compressors have a given amount of air or gas that gets trapped in a compression chamber. From there the volume of that air is mechanically reduced causing an increase in the pressure. Sliding vane compressors use a circular stator that is housed in a cylindrical rotor; the rotor contains radially positioned slots where the vanes reside. These vanes are what create the compression in the “cells”.

Diagram of a sliding vane compressor

The inlet port on the system is positioned in a way that allows the air flow into each cell, optimizing the amount of air that each cell can hold. Once the air enters the system the cell size is reduced down farther and farther as rotation continues and each vane is pushed back into its original slot in the rotor. Compression will continue until each cell reaches the discharge port. One of the more common forms of a sliding vane compressor is the lubricant injected variety. These compressors inject a lubricant into the chamber to lubricate the walls and the vanes; this removes the heat of compression, as well as provide a seal on the cell. These air compressors are generally sold in a 10 – 200 HP range with capacities running between 40 – 800 acfm.

Advantages of a lubricant injected sliding-vane compressor include:

  • Compact size
  • Relatively low purchase cost
  • Vibration-free operation does not require special foundations
  • Routine maintenance includes lubricant and filter changes

Some of the disadvantages that come with this type of compressor:

  • Less efficient than the rotary screw type
  • Lubricant carryover into the delivered air will require proper maintenance of an oil-removal filtration system
  • Will require periodic lubricant changes

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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Photo Credit to Compressed Air Challenge Handbook

Piping and Instrumentation diagrams (P&ID)

When it comes to drawings and diagrams to map out a process system, the piping and instrumentation diagrams (P&ID) are a great way to situate and find components.  They use different symbols to represent the type of products, the layout in the system, installation, and process flow.  These standard symbols are created by ANSI or ISO.  They are used in electrical, hydraulic, and pneumatic processes.  Since EXAIR has been manufacturing Intelligent Compressed Air Products since 1983, I will cover some pneumatic symbols and the process flow in this blog.

A colleague, Russ Bowman, wrote an article about “Knowing Your Symbols Is Key To Understanding Your Drawings”.  As a reference, air compressors are the start of your pneumatic system, and there are different types as represented by the symbols below.

The one on the left can be used for any air compressor. The others denote specific types of air compressor (from left:) Centrifugal, Diaphragm, Piston, Rotary, and Screw.

Air compressors are considered the fourth utility in industries because they use so much electricity; and they are inefficient.  So, you need to use the compressed air as efficiently as possible.  As a typical pneumatic system, the air compressors, receiver tanks and compressed air dryers would be on the supply side.  The distribution system, or piping, connects the supply side to the demand side.  This symbol is represented by a simple line.  The demand side will have many different types of pneumatic devices.  Since there are so many, ANSI or ISO has created some common types of equipment.  But if there isn’t a symbol created to represent that part, the idea is to draw a basic shape and mark it.

From top left, and then down: Automatic Drain Filter Separator, Pressure Regulator, and Super Air Knife

As an example, if I were to do a P&ID diagram of the EXAIR Super Air Knife Kit; it would look like the above diagram.  The kit will include the Super Air Knife with an Automatic Drain Filter Separator and a Pressure Regulator.  The Filter Separator is a diamond shape and since it has an Automatic Drain, a triangle is placed at the bottom.  Filter Separators are used to clean the compressed air and keep the Super Air Knife clean.  The Automatic Drain will discard water and oil from the filter bowl when it accumulates over a float.  The next item is the pressure regulator which is represented by a rectangle with an adjustment knob to “dial in” the desired blowing force.  And at the end, we drew a rectangle, which does represent a Super Air Knife, as marked.

Using the P&ID diagram for the process flow is also important.  You noticed that the Filter Separator will come before the Pressure Regulator.  This is significant when installing this system.  Remember the statement above about “using your compressed air as efficiently as possible”?  Inefficiencies come from two basic areas; pressure drop and overusing your compressed air.  Pressure drop is based on velocity.  The lower the velocity, the lower the pressure drop.  If the Filter Separator is placed after the Pressure Regulator, the lower pressure will increase the velocity.  Since air expands at lower pressure, the volume of air will increase.  And since the area of the compressed air pipe is the same, the velocity will have to increase.   For the second part with overusing compressed air, the Pressure Regulator will help.  You want to use the lowest amount of air pressure as possible for the Super Air Knife to “do the job”.  The lower air pressure will use less compressed air in your operation.

EXAIR products are engineered to be safe, efficient, and effective in your compressed air system.  If you need help to place them in your P&ID diagrams, an Application Engineer can help you.  It is important to have the pneumatic devices in the proper place, and if you want to efficiently use your compressed air, you can use EXAIR products for your blow-off devices.

John Ball
Application Engineer

Email: johnball@exair.com
Twitter: @EXAIR_jb

Compressed Air and Pneumatic Systems

Compressed Air Pipe

Compressed air is used to operate pneumatic systems in a facility, and it can be segregated into three main sections; the supply side, the demand side, and the distribution system.  The supply side is the air compressor, after-cooler, dryer, and receiver tank that produce and treat the compressed air.  They are generally found in a compressor room.  The demand side is a collection of devices that will use the compressed air to do “work”.  These pneumatic components are generally scattered throughout the facility.  To connect the supply side to the demand side, a distribution system is required.  Distribution systems are pipes or tubes which carry compressed air from the air compressor to the pneumatic devices.  The three sections have to work together to make an effective and efficient system.

Compressed air is a clean utility that is used in many different ways, and it is much safer than electrical or hydraulic systems.  But most people think that compressed air is free, and it is most certainly not.  Because of the cost, compressed air is considered to be a fourth utility in manufacturing plants.  For an electrical motor to reduce a volume of air by compressing it, it takes roughly 1 horsepower (746 watts) to compress 4 cubic feet (113L) of air every minute to 125 PSI (8.5 bar).  With almost every manufacturing plant in the world utilizing air compressors larger than 1 horsepower, the amount of energy needed is extraordinary.

Let’s determine the energy cost to operate an air compressor by Equation 1:

Equation 1:

Cost = hp * 0.746 * hours * rate / (motor efficiency)

where:

Cost – US$

hp – horsepower of motor

0.746 – conversion KW/hp

hours – running time

rate – cost for electricity, US$/KWh

motor efficiency – average for an electric motor is 95%.

As an example, a manufacturing plant operates a 100 HP air compressor in their facility.  The cycle time for the air compressor is roughly 60%.  To calculate the hours of running time per year, I used 250 days/year at 16 hours/day.  So operating hours equal 250 * 16 * 0.60 = 2,400 hours per year.  The electrical rate for this facility is $0.10/KWh. With these factors, the annual cost to run the air compressor can be calculated by Equation 1:

Cost = 100hp * 0.746 KW/hp * 2,400hr * $0.10/KWh / 0.95 = $18,846 per year in electrical costs.

Filters and Regulator

If we look at the point-of-use or demand side, the compressed air is generally conditioned to be used to run and control the pneumatic system.  The basic units include filters, regulators, and lubricators.  The filters are used to remove any oil, water, vapor, and pipe scale to keep your pneumatic system clean.  They fall into different types and categories depending on the cleanliness level required.

Filter Separators are more of a coarse filtration which will capture liquid water, oil, and particulate.  The Oil Removal Filters are more of a fine filtration which can capture particles down to 0.03 micron.  They are also designed to “coalesce” the small liquid particles into larger droplets for gravity removal.  One other group is for removing oil vapor and smell.  This type of filter uses activated charcoal to adsorb the vapor for food and pharmaceutical industries.  Filters should be placed upstream of regulators.

Pressure Regulators change the pressure downstream for safety and control.  Pneumatic devices need both flow and pressure to work correctly.  The lubricator, which is placed after the Regulator, helps to add clean oil in a compressed air line.  Air tools, cylinders, and valves use the oil to keep seals from wearing with dynamic functions.  Once the compressed air is “ready” for use, then it is ready to do many applications.

For EXAIR, we manufacture products that use the compressed air safely, efficiently, and effectively.  EXAIR likes to use the 5-C’s; Coat, Clean, Cool, Convey and Conserve.  We have products that can do each part with 16 different product lines.  EXAIR has been manufacturing Intelligent Compressed Air Products since 1983.  Compressed air is an expensive system to operate pneumatic systems; but, with EXAIR products, you can save yourself much money.  If you need alternative ways to decrease electrical cost, improve safety, and increase productivity when using compressed air, an Application Engineer at EXAIR will be happy to help you.

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

Intelligent Compressed Air: Rotary Air Compressors

Air Compressor
Air Compressor and Storage Tanks

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, control 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. In a dynamic compressor, velocity energy is imparted to continuously flowing air by a means of impellers rotating at a very high speed. The velocity energy is then converted into pressure energy. We’ve discussed the different styles of air compressors here on the EXAIR Blog in the past. Today I’d like to highlight the rotary compressors, one of the positive-displacement types of compressors.

Positive-displacement compressors are broken into two categories: reciprocating and rotary. The rotary compressors are available in lubricant-injected or lubricant-free varieties. Both styles utilize two inter-meshing rotors that have an inlet port at one end and a discharge port at the other. Air flows through the inlet port and is trapped between the lobes and the stator. As the rotation continues, the point inter-meshing begins to move along the length of the rotors. This reduces the space that is occupied by the air, resulting in an increase in pressure.

In the lubricant-injected varieties, the compression chamber is lubricated between the inter-meshing rotors and bearings. This lubricant protects the inter-meshing rotors and associated bearings. It eliminates most of the heat caused by compression and acts as a seal between the meshing rotors and between the rotor and stator. Some advantages of the lubricant-injected rotary compressor include a compact size, relatively low initial cost, vibration free operation, and simple routine maintenance (replacing lubricant and filter changes). Some drawbacks to this style of compressor include lower efficiency when compared with water-cooled reciprocating compressors, lubricant carry over must be removed from the air supply with a coalescing filter, and varying efficiency depending on the control mode used.

In the lubricant-free varieties, the inter-meshing rotors have very tight tolerances and are not allowed to touch. Since there is no fluid to remove the heat of compression, they typically have two stages of compression with an inter-cooler between and an after cooler after the second stage. Lubricant-free compressors are beneficial as they supply clean, oil-free compressed air. They are, however, more expensive and less efficient to operate than the lubricant-injected variety.

Each of these compressors can deliver air to your Intelligent Compressed Air Products. If you’re looking to reduce your compressed air consumption and increase the safety of your processes contact an EXAIR Application Engineer today. We’ll be happy to discuss the options with you and make sure you’re getting the most out of your compressed air usage.

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@EXAIR.com
Twitter: @EXAIR_TD