Intelligent Compressed Air: What is an Air Compressor?

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Example of the supply side of a compressed air system

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.

types of compressors

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.

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. These are the types of compressors that would be commonly found in your garage. The double-acting reciprocating compressor is much like its single-acting brethren, only it uses both sides of the piston and cylinder for air compression. This doubles the capacity of the compressor for a given cylinder size. They are much more efficient than single-acting compressors, but are more expensive and do require a more specialized installation and maintenance.

Rotary compressors are available in lubricant-injected or lubicrant-free varieties. These types of compressors use 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 intermeshing 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 compressors, the compression chamber is lubricated between the intermeshing rotors and bearings. This takes away the heat of compression and also acts as a seal. In the lubricant-free varieties, the intermeshing 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 intercooler 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.

On the other side of the coin, we have the dynamic compressors. These are comprised of two main categories: axial and centrifugal. These types of compressors raise the pressure of air or gas by imparting velocity energy and converting it to pressure energy. In a centrifugal air compressor, air continuously flows and is accelerated by an impeller. This impeller can rotate at speeds that exceed 50,000 rpm. Centrifugal air compressors are generally much larger and can accommodate flow ranges of 500-100,000 CFM. They also provide lubricant-free air.

Axial compressors are used for situations that require lower pressure but high flow rates. They do not change the direction of the gas, it enters and exits the compressor in an axial direction. It is accelerated and then diffused which creates the increase in pressure. A common application that would be served by this type of compressor is to compress the air intake of gas turbines. They have a relatively high peak efficiency, however their large overall size and weight as well as the high starting power requirements pose some disadvantages.

Just as you can find a wide variety of makes and models of automobiles, the same can be said for air compressors. The size, type, and features will be dictated by the types of applications that you’ll be needing the compressed air for in your facility. A quick chat with your local air compressor supplier will help you to determine which type is most suitable for you.

Of course, any of these types of compressors can be used to supply air to your engineered Intelligent Compressed Air Products. If you have an application in your facility that could benefit from an engineered solution, give us a call. An Application Engineer would be happy to discuss your options with you and see to it that you’re getting the most out of your compressed air!

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

 

Images Courtesy of  the Compressed Air Challenge and thomasjackson1345 Creative Commons.

About Rotary Screw Air Compressors

Recently, EXAIR Application Engineers have written blogs about reciprocating type air compressors: Single Acting (by Lee Evans) and Dual Acting (by John Ball.) Today, I would like to introduce you, dear EXAIR blog reader, to another type: the Rotary Screw Air Compressor.

Like a reciprocating compressor, a rotary screw design uses a motor to turn a drive shaft. Where the reciprocating models use cams to move pistons back & forth to draw in air, compress it, and push it out under pressure, a rotary screw compressor’s drive shaft turns a screw (that looks an awful lot like a great big drill bit) whose threads are intermeshed with another counter-rotating screw. It draws air in at one end of the screw, and as it is forced through the decreasing spaces formed by the meshing threads, it’s compressed until it exits into the compressed air system.

Rotary Screw Air Compressor…how it works.

So…what are the pros & cons of rotary screw compressors?

Pros:

*Efficiency.  With no “down-stroke,” all the energy of the shaft rotation is used to compress air.

*Quiet operation.  Obviously, a simple shaft rotating makes a lot less noise than pistons going up & down inside cylinders.

*Higher volume, lower energy cost.  Again, with no “down-stroke,” the moving parts are always compressing air instead of spending half their time returning to the position where they’re ready to compress more air

*Suitable for continuous operation.  The process of compression is one smooth, continuous motion.

*Availability of most efficient control of output via a variable frequency drive motor.

*They operate on the exact same principle as a supercharger on a high performance sports car (not a “pro” strictly speaking from an operation sense, but pretty cool nonetheless.)

Cons:

*Purchase cost.  They tend to run a little more expensive than a similarly rated reciprocating compressor.  Or more than a little, depending on options that can lower operating costs.  Actually, this is only a “con” if you ignore the fact that, if you shop right, you do indeed get what you pay for.

*Not ideal for intermittent loads.  Stopping & starting a rotary screw compressor might be about the worst thing you can do to it.  Except for slacking on maintenance.  And speaking of which:

*Degree of maintenance.  Most maintenance on a reciprocating compressor is fairly straightforward (think “put the new part in the same way the old one came out.”)  Working on a rotary screw compressor often involves reassembly & alignment of internal parts to precision tolerances…something better suited to the professionals, and they don’t work cheap.

Like anything else, there are important factors to take under consideration when deciding which type of air compressor is most suitable for your needs.  At EXAIR, we always recommend consulting a reputable air compressor dealer in your area, helping them fully understand your needs, and selecting the one that fits your operation and budget.

Russ Bowman
Application Engineer
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Air: What is it?

Air Balloons

What is Air? Air is an invisible gas that supports life on earth. Dry air is made from a mixture of 78% Nitrogen, 21% Oxygen, and 1% of remaining gases like carbon dioxide and other inert gases.  Ambient air contains an average of 1% water vapor, and it has a density of 0.0749 Lbs./cubic foot (1.22 Kg/cubic meter) at standard conditions.  Air that surrounds us does not have a smell, color, or taste, but it is considered a fluid as it follows the rules of fluid dynamics. But unlike liquids, gases like air are compressible.  Once we discovered the potential of compressing the surrounding air, we were able to advance many technologies.

Bellows

Guess when the earliest air compressor was used?  Believe it or not, it was when we started to breathe air.  Our diaphragms are like compressors.  It pulls and pushes the air in and out of our lungs.  We can generate up to 1.2 PSI (80 mbar) of air pressure.  During the iron age, hotter fires were required for smelting.  Around 1500 B.C., a new type of air compressor was created, called a bellows.  You probably seen them hanging by the fireplaces.  It is a hand-held device with a flexible bag that you squeeze together to compress the air.  The high stream of air was able to get higher temperature fires to melt metals.

Then we started to move into the industrial era.  Air compressors were used in mining industries to move air into deep caverns and shafts.  Then as the manufacturing technologies advanced, the requirements for higher air pressures were needed.  The stored energy created by compressing the air allowed us to develop better pneumatic systems for manufacturing, automation, and construction.  I do not know what the future holds in compressed air systems, but I am excited to find out.

Since air is a gas, it will follow the basic rules of the ideal gas law;

PV = nRT  (Equation 1)

P – Pressure

V – Volume

n – Amount of gas in moles

R – Universal Gas Constant

T – Temperature

If we express the equation in an isothermal process (same temperature), we can see how the volume and pressure are related.  The equation for two different states of a gas can be written as follows:

P1 * V1 = P2 * V2  (Equation 2)

P1 – Pressure at initial state 1

V1 – Volume at initial state 1

P2 – Pressure at changed state 2

V2 – Volume at changed state 2

If we solve for P2, we have:

P2 = (P1 * V1)/V2  (Equation 3)

In looking at Equation 3, if the volume, V2, gets smaller, the pressure, P2, gets higher.  This is the idea behind how air compressors work.  They decrease the volume inside a chamber to increase the pressure of the air.  Most industrial compressors will compress the air to about 125 PSI (8.5 bar).  A PSI is a pound of force over a square inch.  For metric pressure, a bar is a kg of force over a square centimeter.  So, at 125 PSI, there will be 125 pounds of force over a 1” X 1” square.  This amount of potential energy is very useful to do work for pneumatic equipment.  To simplify the system, the air gets compressed, stored as energy, released as work and is ready to be used again in the cycle.

Air Compressor

Compressed air is a clean utility that is used in many different applications.  It is much safer than electrical or hydraulic systems.  Since air is all around us, it is an abundant commodity for air compressors to use.  But because of the compressibility factor of air, much energy is required to create enough pressure in a typical system.  It takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet of air (113L) to 125 PSI (8.5 bar) every minute.  With almost every manufacturing plant in the world utilizing compressed air in one form or another, the amount of energy used to compress air is extraordinary.  So, utilizing compressed air as efficiently as possible is mandatory.  Air is free, but making compressed air is expensive

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR.

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

 

Picture: Hot Air Rises by Paul VanDerWerf. Creative Commons Attribution 2.0 Generic.

Picture: Bellows by Joanna Bourne. Creative Commons Attribution 2.0 Generic.

Picture: Air Compressor by Chris Bartle. Creative Commons Attribution 2.0 Generic.

Intelligent Compressed Air: Membrane Dryers – What are they and How Do they Work?

Recently we have blogged about Compressed Air Dryers and the different types of systems.  We have reviewed the Desiccant and Refrigerant types of dryers, and today I will discuss the basics of  the Membrane type of dryers.

All atmospheric air that a compressed air system takes in contains water vapor, which is naturally present in the air.  At 75°F and 75% relative humidity, 20 gallons of water will enter a typical 25 hp compressor in a 24 hour period of operation.  When the the air is compressed, the water becomes concentrated and because the air is heated due to the compression, the water remains in vapor form.  Warmer air is able to hold more water vapor, and generally an increase in temperature of 20°F results in a doubling of amount of moisture the air can hold. The problem is that further downstream in the system, the air cools, and the vapor begins to condense into water droplets. To avoid this issue, a dryer is used.

Membrane Dryers are the newest type of compressed air dryer. Membranes are commonly used to separate gases, such as removing nitrogen from air. The membrane consists of a group of hollow fiber tubes.  The tubes are designed so that water vapor will permeate and pass through the membrane walls faster than the air.  The dry air continues on through the tubes and discharges into the downstream air system. A small amount of ‘sweep’ air is taken from the dry air to purge and remove the water vapor from inside the dryer that has passed through the membrane tubes.

Membrane Dryer
Typical Membrane Dryer Arrangement

Resultant dew points of 40°F are typical, and dew points down to -40°F are possible but require the use of more purge air, resulting in less final dry compressed air discharging to the system.

The typical advantages of Membrane Dryers are-

  1.  Low installation and operating costs
  2.  Can be installed outdoors
  3.  Can be used in hazardous locations
  4.  No moving parts

There are a few disadvantages to consider-

  1. Limited to low capacity systems
  2. High purge air losses (as high as 15-20% to achieve lowest pressure dew points
  3. Membrane can be fouled by lubricants and other contaminants, a coalescing type filter is required before the membrane dryer.

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Product, 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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Membrane Dryer Schematic – From Compressed Air Challenge, Best Practices for Compressed Air Systems, Second Edition

 

 

 

About Double Acting Reciprocating Air Compressors

My colleague, Lee Evans, wrote a blog “About Single Acting Reciprocating Compressors”, and I wanted to extend that conversation to a more efficient relative, the double 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.

Compressor Types

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.  The double acting compressor compresses the air on both the up-stroke and the down-stroke of the piston, doubling the capacity of a given cylinder size.  This “double” compression cycle is what makes this type of air compressor very efficient.  A single acting compressor will have an operating efficiency between 22 – 24 kW/100 cfm of air while the double acting compressor has an operating efficiency between 15 – 16 kW/100 cfm.  Therefore, electricity cost is less with a double-acting reciprocating air compressor to make the same amount of compressed air.

To explore the internals a bit closer, the mechanical linkage used to move the piston is slightly different as well as the additional intake and exhaust valves.   Instead of the connecting rod being attached directly to the piston as seen inside a single acting compressor, a crosshead is added between the compression piston and the connecting rod (view picture below).  The rod that connects the crosshead to the compression piston can be sealed to keep the cylinder completely encapsulated.  For every rotation of the electric motor, the air is being compressed twice.  With the added heat of compression, the double acting compressors are generally water-cooled.  Also, with the added mechanism between the crank and the piston, the rotational speeds are typically less.  Because of the larger size, water jackets, and added parts, the initial cost is more expensive than the single acting compressor, but the efficiency is much higher.

Double Acting Reciprocating Air Compressor

Double acting compressors are generally designed for rugged 100% continuous operations.  Dubbed the work horse of the compressor family, they are also known for their long service life.  They are commonly used in high pressure services in multistage styles and can come in lubricated and non-lubricated configurations.   With the dual compression, slow speed and inter-cooling, it makes this type of air compressor very proficient in making compressed air.

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

 

Photos:  used from Compressed Air Challenge Handbook

Estimating the Cost of Compressed Air Systems Leaks

Leaks in a compressed air system can waste thousands of dollars of electricity per year. In fact, in many plants, the leakage can account for up to 30% of the total operational cost of the compressor. Some of the most common areas where you might find a leak would be at connection joints like valves, unions, couplings, fittings, etc. This not only wastes energy but it can also cause the compressed air system to lose pressure which reduces the end use product’s performance, like an air operated actuator being unable to close a valve, for instance.

One way to estimate how much leakage a system has is to turn off all of the point-of-use devices / pneumatic tools, then start the compressor and record the average time it takes for the compressor to cycle on and off. The total percentage of leakage can be calculated as follows:

Percentage = [(T x 100) / (T + t)]

T = on time in minutes
t = off time in minutes

The percentage of compressor capacity that is lost should be under 10% for a system that is properly maintained.

Another method to calculate the amount of leakage in a system is by using a downstream pressure gauge from a receiver tank. You would need to know the total volume in the system at this point though to accurately estimate the leakage. As the compressor starts to cycle on,  you want to allow the system to reach the nominal operating pressure for the process and record the length of time it takes for the pressure to drop to a lower level. As stated above, any leakage more than 10% shows that improvements could be made in the system.

Formula:

(V x (P1 – P2) / T x 14.7) x 1.25

V= Volumetric Flow (CFM)
P1 = Operating Pressure (PSIG)
P2 =  Lower Pressure (PSIG)
T = Time (minutes)
14.7 = Atmospheric Pressure
1.25 = correction factor to figure the amount of leakage as the pressure drops in the system

Now that we’ve covered how to estimate the amount of leakage there might be in a system, we can now look at the cost of a leak. For this example, we will consider a leak point to be the equivalent to a 1/16″ diameter hole.

A 1/16″ diameter hole is going to flow close to 3.8 SCFM @ 80 PSIG supply pressure. An industrial sized air compressor uses about 1 horsepower of energy to make roughly 4 SCFM of compressed air. Many plants know their actual energy costs but if not, a reasonable average to use is $0.25/1,000 SCF generated.

Calculation :

3.8 SCFM (consumed) x 60 minutes x $ 0.25 divided by 1,000 SCF

= $ 0.06 per hour
= $ 0.48 per 8 hour work shift
= $ 2.40 per 5-day work week
= $ 124.80 per year (based on 52 weeks)

As you can see, that’s a lot of money and energy being lost to just one small leak. More than likely, this wouldn’t be the only leak in the system so it wouldn’t take long for the cost to quickly add up for several leaks of this size.

If you’d like to discuss how EXAIR products can help identify and locate costly leaks in your compressed air system, please contact one of our application engineers at 800-903-9247.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

 

 

 

 

 

About Single Acting Reciprocating Compressors

Whether you’re new to the field of compressed air, an experienced technician, or just in the market for a new compressor, you may find yourself coming into contact with various compressor types.  Within the world of compressed air supply there are two types of compressors: positive displacement and dynamic.  These two compressor types branch off into several different variations, as shown in the chart below.

Compressor types

Positive displacement compressors increase air pressure by reducing air volume within a confined space.  In a positive displacement compressor mechanical linkage is used to reduce the volume of air (the fluid), which results in a change to the air pressure.  To think of it another way, the energy which is used to displace the air volume is converted into an increase in air pressure.

Dynamic compressors, on the other hand, utilize an increase in air velocity to cause a change in air pressure.  For a dynamic compressor, the fluid (air) is accelerated to a high velocity through a rotor or impeller.  The kinetic energy of the air is then converted to an increased potential energy/static pressure by slowing the flow through a diffuser.  The air at the outlet of the diffuser is the compressed air which is used to perform work.

The internals of a single acting reciprocating compressor.

Within this vast field of compressed air generation, one of the most common types of compressors is the single acting reciprocating compressor.  The term “single acting” refers to manner in which the cylinder inside of the compressor motor interacts with the working fluid (the air).  When the fluid (air) acts only on one side of the piston, the motor is referred to as “single acting”.  This type of motor relies on the load of the motor, a flywheel, springs, other cylinders, or some other device/momentum to return the piston back to its original location.

Single acting compressors can be air-cooled or water cooled, lubricated or non-lubricated, and packaged to provide a wide range of pressure and flow capacities.  Because of this adaptability, single acting compressors are quite common and serve a variety of industrial needs.

No matter the type of compressor on the system’s supply side, having engineered products on the demand side improves overall system performance and efficiency.  If you’d like to discuss engineered solutions for your compressed air system, EXAIR Application Engineers are ready and waiting.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

 

Compressor internals image courtesy of h080, Creative Commons License.