A Review of Centrifugal Air Compressors

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, sliding vane and rotary-scroll air compressors. You can click on the links above to check those out. Today, we will examine centrifugal air compressors.

The types of compressors that we have looked at to date have been of the Positive Displacement type.  For this type, an amount of air is drawn in and trapped in the compression area, and the volume in which it is held is mechanically reduced, resulting is rise in pressure as it approaches the discharge point.

types of compressors

The centrifugal air compressors fall under the Dynamic type. A dynamic compressor operates through the principle that a continuous flow of air has its velocity raised in an impeller rotating at a relatively high speed (can exceed 50,000 rpm.) The air has an increase in its kinetic energy (due to the rise in velocity) and then the kinetic energy is transformed to pressure energy in a diffuser and/or a volute chamber. The volute is a curved funnel that increases in area as it approaches the discharge port. The volute converts the kinetic energy into pressure by reducing speed while increasing pressure. About one half of the energy is developed in the impeller and the other half in the diffuser and volute.

Centrifugal Compressor
Centrifugal Compressor Components

The most common centrifugal air compressor has two to four stages to generate pressures of 100 to 150 PSIG.  A water cooled inter-cooler and separator between each stage removes condensation and cools the air prior to entering the next stage.

Some advantages of the Centrifugal Air Compressor-

  • Comes completely packaged fort plant air up to 1500 hp
  • As size increases, relative initial costs decrease
  • Provides lubricant-free air
  • No special foundation required

A few disadvantages-

  • Higher initial investment costs
  • Has specialized maintenance requirements
  • Requires unloading for operation at reduced operational capacities

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 air compressors 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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The Decibel

The decibel is a unit of measurement that relates the ratio of a physical value to another value and is expressed on a logarithmic scale.  The common symbol for decibel is dB.  The decibel is used as a measure for many parameters in science and engineering such as acoustics (sound), electronics (power levels) and control theory.

The decibel originates from methods used to express performance and loss in telegraph and telephone circuits.  The term ‘bel’ was coined in honor of Alexander Graham Bell, and the decibel, being 1/10th of a bel was established.

For most of us, the decibel is the familiar term relating to how loud a sound is.

With sound, the sound pressure is typically what is measured and is the local pressure deviation from the base or equilibrium atmospheric pressure, caused by a sound wave. In air, the sound pressure can be measured by a standard microphone, and is measured in pascals (Pa.)

To get to the common decibel reading we are familiar with, a little mathematics comes into play.

Capture

  • where Lp is the Sound Level in dB, prms is the measured sound pressure, and pref is the standard sound reference pressure of 20 micropascals.
  • The prms is what is measured by a microphone

Below are some representative sounds and the decibel rating – Note that sounds that are above 85 dB can cause hearing issues, and proper protection should be taken.Decibel Scale Still Photo

Some other interesting blogs about sound for you take a look at-

Measuring and Adding Sounds

Sound Power Level and Sound Pressure

Super Air Knife Math – When 72  + 72 = 75

If you would like to talk about sound 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-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

EXAIR Industrial Housekeeping Vacuums – A Look at the Advantages Over Electric Vacuums

EXAIR‘s line of Industrial Housekeeping Products consist of liquid and dry vacuums that utilize compressed air to operate. They do not have electric motors which most ‘all purpose’ vacuums rely on.  In industrial settings with harsh conditions and rugged use, the Industrial Housekeeping Products are ideally suited for the everyday wear and tear.                               6091   6095.jpg

The heart of the EXAIR liquid type vacuums, like the Reversible Drum Vac, the High Lift Reversible Drum Vac, and the same range in the Chip Trappers is shown here. Installing onto an any closed head, 30, 55, or 110 gallon drum creates a high powered vacuum.  It can fill a 55 gallon drum in less than 2 minutes.  Then, with the turn of a knob, the the same pump quickly empties the drum. Useful for refilling coolant sumps, vacuuming floor spills, or transferring contaminated liquids to filtration tanks in minutes.  Flow rate can be controlled with the knob.  The Chip Trapper has an internal filtration system to separate the chips from the coolant, allowing for disposal of the solids, and for pumping of the clean coolant back into the sump.

rdvfam
5, 30, 55, and 110 Gallon Reversible Drum Vacs – A size to Meet Most Any Requirement

Also available are the dry/solid type vacuum systems, the Chip Vac, Heavy Duty Dry Vac and the Heavy Duty HEPA Vac which use the vacuum units shown below.6093                                                                    6097

The EXAIR Chip Vac and Heavy Duty Dry Vac is a powerful industrial vacuum for use on wet or dry chips. Both are for use with 30, 55, or 110 gallon drums, and the Chip Vac can also be used on a 5 gallon drum. The Heavy Duty HEPA Vac has the added benefit of the HEPA quality filter system.  Filtration per IEST-RP-CC-007 to 99.97% minimum filtration at the 0.3 micron level ensures performance meets the needed criteria.

chipvac_family
5, 30, 55, and 110 Gallon Chip Vacs

When you look at the vacuum producing units, you’ll notice there is not an electrical cord.  Because the EXAIR Industrial Housekeeping Products utilize compressed air for operation, they do not have the same issues with wear, breakdown and failures as electrical type vacuums. There are no moving parts in the vacuum units to wear out or break.  They are maintenance free, just supply clean, dry air, and they will operate for a long time. For the the liquid type systems, there is a built in pressure/vacuum relief device and auto safety shut-off to prevent overfilling.

There are no motors and bearings to wear out and no impellers to clog.  The Chip Vac is 50% quieter than a comparable electric vacuum.  Safety is a top priority, and the use of the unit on wet floor and spills is not an issue.

If you would like to talk about Industrial Housekeeping products 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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Threaded Line Vacs: Low Cost Conveyor Uses Ordinary Pipe!

EXAIR’s Threaded Line Vac air operated conveyor takes an ordinary pipe and converts it into a powerful conveying system.  A fast, low cost way to convey items such as pellets, scrap trim, bulk solids, chips, paper, small parts, sawdust, granules, and so much more. The Threaded Line Vac attached easily to plumbing pipe couplers, like PVC or iron, making it easy to build a complete system with parts readily available from your local supply house or hardware store.

The Threaded Line Vac are ideally suited for conveying large volumes of materials over long distances.  Like the standard, smooth bore ended Line Vacs, the unit works by ejecting a small amount of compressed air to produce a vacuum suction on the inlet side and a high output flow from the outlet side.  By using a pressure regulator to control the compressed air supply pressure, the rate of conveyance can be controlled to match the application needs, while minimizing the compressed air usage.

3inTrLVbckls_prHR1670x574

Models are available from 3/8 NPT to 3 NPT with materials of construction of aluminum and types 313 and 316 stainless steels (excellent corrosion resistance.)  High temperature versions of the stainless steel Threaded Line Vacs are available as well, for temperatures up to 900°F.

For those processes requiring the highest rates of conveyance over the longest distances and/or the most abrasive of materials, the Heavy Duty Threaded Line Vac is the right choice.  Designed with the most rugged industrial processes and applications in mind, the special hardened alloy construction helps prevent premature wear that could occur with standard aluminum or stainless steel models, under the harshest of conditions.  The performance has been boosted, to be able to convey more material, over longer distances and higher vertical rises.

HDLV

To select the right model, information regarding these criteria is helpful-

  • Size of parts and bulk density of material being conveyed
  • What size hose, tube, or pipe is desired
  • Target conveyance rate
  • Distance of conveyance, both horizontal and vertical legs
  • Preferred material of construction

For another look, please see the video available that provides more details about the Line Vac family.

If you would like to talk about Line Vacs 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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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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