Compressor Control – A Way to Match Supply to Demand

Rarely does the compressed air demand match the supply of the compressor system. To keep the generation costs down and the system efficiency as high as possible Compressor Controls are utilized to maximize the system performance, taking into account system dynamics and storage. I will touch on several methods briefly, and leave the reader to delve deeper into any type of interest.

air compressor

  • Start/Stop – Most basic control –  to turn the compressor motor on and off, in response to a pressure signal (for reciprocating and rotary type compressors)
  • Load/Unload – Keeps the motor turning continuously, but unloads the compressor when a pressure level is achieved.  When the pressure drops to a set level, the compressor reloads (for reciprocating, rotary screw, and centrifugal type)
  • Modulating – Restricts the air coming into the compressor, as a way to reduce the compressor output to a specified minimum, at which point the compressor is unloaded (for lubricant-injected rotary screw and centrifugal)
  • Dual/Auto Dual – Dual Control has the ability to select between Start/Stop and Load /Unload control modes.  Automatic Dual Control adds the feature of an over-run timer, so that the motor is stopped after a certain period of time without a demand.
  • Variable Displacement (Slide Valve, Spiral Valve or Turn Valve) – Allows for gradual reduction of the compressor displacement while keeping the inlet pressure constant (for rotary screw)
  • Variable Displacement (Step Control Valves or Poppet Valves) – Similar effect as above, but instead of a gradual reduction, the change is step like (for lubricant injected rotary types)
  • Variable Speed – Use of a variable frequency AC drive or by switched reluctance DC drive to vary the speed of the motor turning the compressor. The speed at which the motor turns effects the output of the system.

In summary – the primary functions of the Compressor Controls are to match supply to demand, save energy, and protect the compressor (from overheating, over-pressure situations, and excessive amperage draw.) Other functions include safety (protecting the plant and personnel), and provide diagnostic information, related to maintenance and operation warnings.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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When to Use a Receiver Tank for a Compressed Air Application

Recently, I worked with a production engineer at a Tier 1 supplier for the auto industry.  An upcoming project was in the works to install a new line to produce headlight lenses.  As a part of the process, there was to be a “De-static / Blow-off” station, where a shuttle system would bring a pair of the parts to a station where they would be blown off and any static removed prior to being transferred to a painting fixture and sent off for painting.  For best results, the lenses were to be dust and lint free and have no static charge, ensuring a perfect paint result.

The customer installed a pair of 18″ Gen4 Super Ion Air Knives, to provide coverage of the widest 16″ lens assembly, that were staged in pairs.

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The Super Ion Air Knife Kit, and Everything that is Included.

The customer was limited in compressed air supply volume in the area of the plant where this process was to occur. 50 SCFM of 80 PSIG was the expected air availability at peak use times, which posed a problem –  the Super Ion Air Knives would need up to 105 SCFM if operated at 80 PSIG.  A further review of the design parameters for the process revealed that the system needed to blow air for only 4 seconds and would be off for 25 seconds to meet the target throughput.

This scenario lends itself perfectly to the use of a Receiver Tank.  Running all of the design numbers into the calculations, showed that the 60 Gallon Receiver Tank we offer, would allow for a 20 second run-time, and require 13.1 seconds to refill.  These figures were well within the requires times, and would allow for the system to work as needed, without having to do anything to the compressed air supply system.

receiver_tank
60 Gallon Receiver Tank

The moral of the story is – if you have a process that is intermittent, and the times for and between blow-off, drying, or cooling allows, a Receiver Tank can be used to allow you to get the most of your available compressed air system.

Note – Lee Evans wrote an easy to follow blog that details the principle and calculations of Receiver Tanks, and it is worth your time to read here.

If you would like to talk about any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Rotary Scroll-Type Compressor

Over the last few months, my EXAIR colleagues and I have blogged about several different types of air compressor types including single and double acting reciprocating, rotary screw and sliding vane air compressors. You can click on the links above to check those out. Today, I will review the basics of the rotary scroll-type compressor.

The rotary scroll type compressor falls under the positive displacement-type, the same as the other types previously discussed.  A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced.  When a given amount of air occupies a smaller volume, the pressure of the air increases.

Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber. The rotary scroll uses two inter-meshing scrolls, that are spiral in shape. One of the scrolls is fixed, and does not move (in red).  The other scroll (in black) has an “orbit” type of motion, relative to the fixed scroll. In the below simulation, air would be drawn in from the left, and as it flows clockwise through the scroll, the area is reduced until the air is discharged at a high pressure at the center.

Two_moving_spirals_scroll_pump
How it Works- A fixed scroll (red), and an ‘orbiting’ scroll (black) work to compress the air

It is of note that the flow from start to finish is continuous, providing air delivery that is steady in pressure and flow, with little or no pulsation.

There is no metal to metal sliding contact, so lubrication is not needed.  A drawback to an oil free operation is that oil lubrication tends to reduce the heat of compression and without it, the efficiency of scroll compressors is less than that of lubricated types.

The advantages of the rotary scroll type compressor include:

  • Comes as a complete package
  • Comparatively efficient operation
  • Can be lubricant-free
  • Quiet operation
  • Air cooled

The main disadvantage:

  • A limited range of capacities is available, with low output flows

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Rotary Scroll GIF:  used from of Public Domain

Oil Removal Filters – Keeping Compressed Air Clean

Compressed air filters help to keep the air clean and condensate free to protect equipment from dust, dirt, pipe scale, oil and water. Even though the compressed air system will typically have a main dryer, additional treatment is often necessary. For this discussion, we will focus on the oil removal process and filter type.

After the compressed air has passed through a particulate filter, the dirt, dust and water droplets have been removed.  Oil that is present is much smaller in size, and mostly passes though the particulate filter.  The installation of a coalescing filter will provide for the removal of the majority of the fine oil aerosols that remain. The coalescing filter works differently than the particulate filters. The compressed air flows from inside to outside through the coalescing filter media. The term ‘coalesce’ means to ‘come together’ or ‘form one mass.’  The process of coalescing filtration is a continuous process where the small aerosols of oil come in contact with fibers of the filter media. As other aerosols are collected, they will join up and ‘come together’ and grow to become an oil droplet, on the downstream or outside surface of the media.  Gravity will then cause the droplet to drain away and fall off the filter element.

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Example of a 0.03 Micron Coalescing Oil Removal Filter

Some important information to keep in mind –

  • Change the filter regularly, not just when the differential pressures exceeds recommended limits, typically 5 PSI
  • Coalescing filters will remove solids too, at a higher capture rate due to the fine level of filtration, using a pre-filter for solids will extend the life
  • Oil free compressors do not provide oil free air, as the atmospheric air drawn in for compression contains oil vapors that will cool and condense in the compressed air system.

If you would like to talk about oil removal filters or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Discharge of Air Through an Orifice

My Application Engineer colleagues and I frequently use a handy table, called Discharge of Air Through an Orifice. It is a useful tool to estimate the air flow through an orifice, a leak in a compressed air system, or through a drilled pipe (a series of orifices.) Various tables and online calculators are available. As an engineer, I always want to know the ‘science’ behind such tables, so I can best utilize the data in the manner it was intended.

DischargeThroughAnOrifice

The table is frequently found with values for pressures less than 20 PSI gauge pressure, and those values follow the standard adiabatic formula and will not be reviewed here.  The higher air pressures typically found in compressed air operations are of interest to us.

For air pressures above 15 PSI gauge the discharge is calculated using by the approximate formula as proposed by S.A. Moss. The earliest reference to the work of S.A. Moss goes back to a paper from 1906.  The equation for use in this table is-EquationWhere:
Equation Variables

For the numbers published in the table above, the values were set as follows-

                  C = 1.0,      p1 = gauge pressure + 14.7 lbs/sq. in,    and T1 = 530 °R (same as 70 °F)

The equation calculates the weight of air in lbs per second, and if we divide the result by 0.07494 lbs / cu ft (the density of dry air at 70°F and 14.7 lbs / sq. in. absolute atmospheric pressure) and then multiply by 60 seconds, we get the useful rate of Cubic Feet per Minute.

The table is based on 100% coefficient of flow (C = 1.0)  For well rounded orifices, the use of C = 0.97 is recommended, and for very sharp edges, a value of C = 0.61 can be used.

The table is a handy tool, and an example of how we use it would be to compare the compressed air consumption of a customer configured drilled pipe in comparison to that of the EXAIR Super Air Knife.  Please check out the blog written recently covering an example of this process.

If you would like to talk about the discharge of air through an orifice or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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About Sliding Vane Air Compressors

Over the last few months, my EXAIR colleagues have blogged about several different types of air compressor types including single and double acting reciprocating and rotary screw. (You can select the links above to check those out.) Today I will review the basics of the sliding vane type, specifically the oil/lubricant injected sliding vane compressor.

The lubricant injected sliding vane compressor falls under the positive displacement-type, the same as the other types previously discussed.  A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced.  When a given amount of air occupies a smaller volume, the pressure of the air increases.

Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber.  The single and double acting reciprocating use a piston that cycles up and down to reduce the compression chamber size. The rotary screw uses two inter-meshing rotors, where the compression chamber volume reduces as the air approaches the discharge end.  For the lubricant sliding vane type, the basic design is shown below.

Sliding Vane2
Air enters from the right, and as the compression chamber volume reduces due to counterclockwise rotation, the pressure increases until the air discharges to the left

The compressor consist of an external housing or stator, and the internal circular rotor, which is eccentrically offset.  The rotor has radially positioned (and occasionally offset) slots in which vanes reside.  As the rotor rotates, the centrifugal forces on the vanes cause them to move outwards and contact the inner surface of the stator bore.  This creates the compression areas, formed by the vanes, rotor surface and the stator bore.  Because the rotor is eccentrically offset, the volume of the compression area reduces as the distance between the rotor surface and the stator reduces.  As the rotor turns counterclockwise, the vanes are pushed back into the rotor slots, all the while in contact with the stator surface.  The shrinking of the compression area leads to the increase in air pressure.

Oil is injected into compression chamber to act as a lubricant, to assist is sealing, and to help to remove some of the heat of compression.

The advantages of the lubricant sliding vane compressor type is very similar to the lubricant injected rotary screw.  A few key advantages include:

  • Compact size
  • Relatively low initial cost
  • Vibration free operation- no special foundation needed
  • Routine maintenance includes basic lubricant and filter changes

A few of the disadvantages include:

  • Lubricant gets into the compressed air stream, requires an air/lubricant separation system
  • Requires periodic lubricant change and disposal
  • Less efficient than rotary screw type
  • Not as flexible as rotary screw in terms of capacity control in meeting changing demands

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Diagram:  used from Compressed Air Challenge Handbook

What Makes A Compressed Air System “Complete”?

It’s a good question.  When do you know that your compressed air system is complete?  And, really, when do you know, with confidence, that it is ready for use?

A typical compressed air system. Image courtesy of Compressed Air Challenge.

Any compressed air system has the basic components shown above.  A compressed air source, a receiver, dryer, filter, and end points of use.   But, what do all these terms mean?

A compressor or compressed air source, is just as it sounds.  It is the device which supplies air (or another gas) at an increased pressure.  This increase in pressure is accomplished through a reduction in volume, and this conversion is achieved through compressing the air.  So, the compressor, well, compresses (the air).

A control receiver (wet receiver) is the storage vessel or tank placed immediately after the compressor.  This tank is referred to as a “wet” receiver because the air has not yet been dried, thus it is “wet”.  This tank helps to cool the compressed air by having a large surface area, and reduces pulsations in the compressed air flow which occur naturally.

The dryer, like the compressor, is just as the name implies.  This device dries the compressed air, removing liquid from the compressed air system.  Prior to this device the air is full of moisture which can damage downstream components and devices.  After drying, the air is almost ready for use.

To be truly ready for use, the compressed air must also be clean.  Dirt and particulates must be removed from the compressed air so that they do not cause damage to the system and the devices which connect to the system.  This task is accomplished through the filter, after which the system is almost ready for use.

To really be ready for use, the system must have a continuous system pressure and flow.  End-use devices are specified to perform with a required compressed air supply, and when this supply is compromised, performance is as well.  This is where the dry receiver comes into play.  The dry receiver is provides pneumatic capacitance for the system, alleviating pressure changes with varying demand loads.  The dry receiver helps to maintain constant pressure and flow.

In addition to this, the diagram above shows an optional device – a pressure/flow control valve.  A flow control valve will regulate the volume (flow) of compressed air in a system in response to changes in flow (or pressure).  These devices further stabilize the compressed air system, providing increased reliability in the supply of compressed air for end user devices.

Now, at long last, the system is ready for use.  But, what will it do?  What are the points of use?

Points of use in a compressed air system are referred to by their end use.  These are the components around which the entire system is built.  This can be a pneumatic drill, an impact wrench, a blow off nozzle, a pneumatic pump, or any other device which requires compressed air to operate.

If your end use devices are for coating, cleaning, cooling, conveying or static elimination, EXAIR Application Engineers can help with engineered solutions to maximize the efficiency and use of your compressed air.  After placing so much effort into creating a proper system, having engineered solutions is a must.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE