Tag: air compressor

Intelligent Compressed Air: Single Acting Reciprocating Air Compressors

Published on by Russ BowmanLeave a comment

Of all the types of air compressors on the market, you can’t beat the single acting reciprocating air compressor for simplicity:

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

This simplicity is key to a couple of major advantages:

  • Price: they can cost 20-40% less than a similar rated (but more efficient) rotary screw model, up to about 5HP sizes.  This makes them great choices for home hobbyists and small industrial or commercial settings.
  • High pressure: It’s common to see reciprocating compressors that are capable of generating up to 3,000 psig.  Because the power is transmitted in the same direction as the fluid flow, they can handle the mechanical stresses necessary for this much better than other types of air compressors, which may need special modifications for that kind of performance.
  • Durability: out of necessity, their construction is very robust and rugged.  A good regimen of preventive maintenance will keep them running for a good, long time.  Speaking of which…
  • Maintenance (preventive): if you change your car’s oil and brake pads yourself, you have most of the know-how – and tools – to perform regular upkeep on a reciprocating air compressor.  There’s really not that much to them:

    The internals of a single acting reciprocating compressor.

Those advantages are buffered, though, by certain drawbacks:

  • Efficiency, part 1: The real work (compressing the air) only happens on the upstroke.  They’re less efficient than their dual acting counterparts, which compress on the downstroke too.
  • Efficiency, part 2: As size increases, efficiency decreases.  As stated above, smaller sizes usually cost appreciably less than more efficient (rotary screw, vane, centrifugal, etc.) types, but as you approach 25HP or higher, the cost difference just isn’t there, and the benefits of those other types start to weigh heavier in the decision.
  •  Maintenance (corrective):  Whereas they’re easy to maintain, if/when something does break, the parts (robust and rugged as they are) can get pretty pricey.
  • Noise: No way around it; these things are LOUD.  Most of the time, you’ll find them in a remote area of the facility, and/or in their own (usually sound-insulated) room.
  • High temperature:  When air is compressed, the temperature rises due to all the friction of those molecules getting shoved together…that’s going to happen with any air compressor.  All the metal moving parts in constant contact with each other, in a reciprocating model, add even more heat.
  • Oil in the air: If you’re moving a piston back & forth in a cylinder, you have to keep it lubed properly, which means you have oil adjacent to the air chamber.  Which means, no matter how well it’s built, you’re likely going to have oil IN the air chamber.

All that said, the benefits certainly do sell a good number of these compressors, quite often into situations where it just wouldn’t make sense to use any other type.  If you’re in the market for an air compressor,  you’ll want to find a local reputable air compressor dealer, and discuss your needs with them.  If those needs entail the use of engineered compressed air products, though, please feel free to give me a call to discuss.  We can make sure you’re going to ask your compressor folks the right questions.

Russ Bowman
Application Engineer
EXAIR Corporation
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Compressed Air Uses In Industry

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Air Compressor

There are so many uses for compressed air in industry that it would be difficult to list every one of them as the list would be exhaustive.  Some of the uses are the tools used in production lines, assembly & robotic cells, painting, chemical processing, hospitals, construction, woodworking and aerospace.

It is considered as important as water, electricity, petroleum based fuels and often referred to as the fourth utility in industry. The great advantage of compressed air is the high ratio of power to weight or power to volume. In comparison to an electric motor compressed air powered equipment is smoother.  Also compressed air powered equipment generally requires less maintenance, is more reliable and economical than electric motor powered tools.  In addition they are considered on the whole as safer than electric powered devices.

Even amusement parks have used compressed air in some capacity in the operation of thrill rides like roller coasters or to enhance the “wow factor” of certain attractions. Compressed air can be found in your dentist’s office where it is used to operate drills and other equipment. You will find compressed air in the tires on your car, motorcycle and bicycles. Essentially, if you think about it, compressed air is being used nearly everywhere.

Here at EXAIR, we manufacture Intelligent Compressed Air Products to help improve the efficiency in a wide variety of industrial operations. Whether you are looking to coat a surface with an atomized mist of liquid, conserve compressed air use and energy, cool an electrical enclosure, convey parts or bulk material from one location to another or clean a conveyor belt or web, chances are we have a product that will fit your specific need.

Atomizing nozzle
Atomizing Nozzles Can Apply Even Coatings
Super Air Amplifier
Air Amplifiers pull in a large volume of ambient air to increase air flow volume and are great for cooling applications!
Heavy Duty Threaded Line Vac
For conveying heavy or abrasive products the Heavy Duty Threaded Line Vacs have male NPT Threads to make permanent and rigid installation into a piping system a breeze.

If you would like to discuss quiet, efficient compressed air products, I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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Intelligent Compressed Air: Sliding-Vane Compressors

Published on by Tyler DanielLeave a comment

If you’re an active reader of the EXAIR blog, you’ve seen several posts over the last year about the various different types of air compressors. From the positive-displacement style of compressors (Rotary Scroll, Rotary Screw, Single and Double Acting Reciprocating Compressors,) as well as a review of a dynamic style (Centrifugal Compressors). In this blog, I’ll be discussing another of the positive-displacement variety: The Sliding-Vane Compressor.

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

In positive-displacement type compressors, a given quantity of air or gas is trapped in a compression chamber. The volume of this air is then mechanically reduced, causing an increase in pressure. A sliding-vane compressor will consist of a circular stator that is housed in a cylindrical rotor. The rotor then has radially positioned slots where the vanes reside. While the rotor turns on its axis, the vanes will slide out and contact the bore of the stator wall. This creates compression in these “cells”. An inlet port is positioned to allow the air flow into each cell, allowing the cells to reach their maximum volume before reaching the discharge port. After passing by the inlet port, the size of the cell is reduced as rotation continues and each vane is then pushed back into its original slot in the rotor.  Compression will continue until the cell reaches the discharge port. The most common form of sliding-vane compressor is the lubricant injected variety. In these compressors, a lubricant is injected into the compression chamber to act as a lubricant between the vanes and the stator wall, remove the heat of compression, as well as to provide a seal. Lubricant injected sliding-vane compressors are generally sold in the range of 10-200 HP, with capacities ranging from 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

With the host of different options in compressor types available on the market, EXAIR recommends talking to a reputable air compressor dealer in your area to help determine the most suitable setup based on your requirements. Once your system is up and running, be sure to contact an EXAIR Application Engineer to make sure you’re using that compressed air efficiently and intelligently!

Tyler Daniel

Application Engineer

E-mail: TylerDaniel@exair.com

Twitter: @EXAIR_TD

Diagram:  used from Compressed Air Challenge Handbook

CFM, ICFM, ACFM, SCFM: Why so many volumetric flow rates?

Published on by John BallLeave a comment
Air Compressor

Flow rate is the quantity of material that is moved per unit of time.  Generally, the quantity of material can be expressed as a mass or a volume.  For example, mass flow rates are in units of pounds per minute or kilograms per hour.  Volumetric flow rates are stated in cubic feet per minute or liters per hour.  The trick begins when volumetric flow rates are used for a compressible gas.  In this blog, I will go over the various acronyms and the reasons behind them.

What acronyms will be covered?

CFM – Cubic Feet per Minute

SCFM – Standard Cubic Feet per Minute

ACFM – Actual Cubic Feet per Minute

ICFM – Inlet Cubic Feet per Minute

The volumetric component of the flow rate is CFM or Cubic Feet per Minute.  This term is commonly used for rating air compressors.  From history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of cylinder.  With the volume of the compression chamber and the rotations per minute of the motor, RPM, they could calculate the volumetric air flows.  As conditions change like altitude, temperature, and relative humidity, the value of CFM changes.  To better clarify these conditions, compressor manufacturers decided to add terms with definition.  (For your information, air compressors still use CFM as a unit of air flow, but now this is defined at standard temperature and pressure).

The first letter in front of CFM above now defines the conditions in which the volumetric air flow is being measured.  This is important for comparing pneumatic components or for properly sizing pneumatic systems. Volume is measured with three areas: temperature, pressure, and relative humidity.  We can see this in the Ideal Gas Law: P * V = n * R * T or Equation 1:

V = n * R * T / P

V – Volume

n – Number of molecules of gas

R – Universal Gas Constant

T – Absolute Temperature

P – Absolute Pressure

The volume of air can change in reference to pressure, temperature, and the number of molecules.  Where is the relative humidity?  This would be referenced in the “n” term.  The more water vapor, or higher RH value, the less molecules of air is in a given volume.

SCFM is the most commonly used term, and it can be the most confusing.  The idea of this volumetric air flow is to set a reference point for comparisons.  So, no matter the pressure, temperature, or relative humidity, the volumetric air flows can be compared to each other at that reference point.  There have been many debates about an appropriate standard temperature and pressure, or STP.  But as long as you use the same reference point, then you can still compare the results.  In this blog, I will be using the Compressed Air and Gas Institute, CAGI, reference where the “Standard” condition is at 14.5 PSIA, 68 deg. F, and 0% RH.  Since we have a reference point, we still need to know the actual conditions for comparison.  It is like having a location of a restaurant as a reference, but if you do not know your current location, you cannot reach it.   Similarly, we are “moving” the air from its actual condition to a reference or “Standard” condition.  We will need to know where the air began in order to reach that reference point.  We will talk more about this later in this blog.

ACFM is the volumetric air flow under actual conditions.  This is actually the “true” flow rate.  Even though this term is hardly used, there are reasons why we will need to know this value.  We can size an air compressor that is not at “Standard” conditions, and we can use this value to calculate velocity and pressure drop in a system.  We can correlate between SCFM and ACFM with Equation 2:

ACFM = SCFM * [Pstd / (Pact – Psat Φ)] * (Tact / Tstd)

Where:

ACFM = Actual Cubic Feet per Minute
SCFM = Standard Cubic Feet per Minute
Pstd = standard absolute air pressure (psia)
Pact = absolute pressure at the actual level (psia)
Psat = saturation pressure at the actual temperature (psi)
Φ = Actual relative humidity
Tact = Actual ambient air temperature (oR)
Tstd = Standard temperature (oR)

ICFM is one of the newest terms in the history of air compressors.  This is where devices are added to the inlet of an air compressor, affecting the flow conditions.  If you have a blower on the inlet of an air compressor, the volumetric flow rate changes as the pressure and temperature rises at the “Inlet”.  If a filter is used, then the pressure drop will decrease the incoming pressure at the “Inlet”.  These devices that affect the volumetric flow rate for an air compressor should be considered.  The equation to relate the ACFM to ICFM is with Equation 3:

ICFM = ACFM * (Pact / Pf) * (Tf / Tact)

Where:

ICFM = Inlet Cubic Feet Per Minute

Pf  = Pressure after filter or inlet equipment (PSIA)

Tf = Temperature after filter or inlet equipment (°R)

Examples of these different types of flow rates can be found here in this EXAIR blog by Tyler Daniel.

To expand on my explanation above about SCFM and ACFM, a technical question comes up about the pressure when using SCFM.  The reference point of 14.5 PSIA is in the definition of SCFM.  Remember, this is only a reference point.  The starting location is actually required.  This would be the ACFM value where the air values are true and actual.  As an example, two air nozzles are rated for 60 SCFM.  An EXAIR Super Air Nozzle, model 1106, is cataloged at 80 PSIG, and a competitor is cataloged at 60 PSIG.  By comparison, they look like they use the same amount of compressed air, but actually they do not.  To simplify Equation 2, we can compare the two nozzles at the same temperature and RH at 68 Deg. F and 0% RH respectively.  This equation can be reduced to Equation 4:

ACFM = SCFM * 14.5 / (P + 14.5)

@60 PSIG Competitor:

ACFM = 60 SCFM * 14.5 PSIA/ (60 PSIG + 14.5 PSIA)

= 11.7 ACFM

@80 PSIG EXAIR Super Air Nozzle:

ACFM = 60 SCFM * 14.5 PSIA / (80 PSIG + 14.5PSIA)

= 9.2 ACFM

Even though the SCFM is the same amount, you are actually using 21% more air with the competitive nozzle that was reported at 60 PSIG.  So, when it comes to rating compressed air products or air compressors, always ask the conditions of pressure, temperature and RH.  The more you know about volumetric flow rates, the better decision that you can make.  If you need help, you can always contact our application engineers at EXAIR.

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