Choosing the Right Industrial Air Compressor: Four Factors that Affect Compressor Sizing

For many Plant Managers and Engineers, selecting the right-sized industrial air compressor can be a challenging task.  Compressed air is an energy source that is rarely measured or tracked in existing industrial facilities, so reliable data on this subject is often unavailable.  Frequently, this information must be cobbled together from the stories and anecdotes of plant personnel.  While these observations are well-intentioned, they usually provide little benefit to an engineer that is sizing a new air compressor.

When designing and building a brand-new facility, selecting the right-sized industrial air compressor can be even more daunting.  Without an existing baseline from which to start, engineers must meticulously tally the air demand of every pneumatic component to be installed, apply a usage factor for the expected duty cycle, and hope the end result is an accurate representation of reality.

Both of these scenarios are fraught with uncertainty, and many engineers intentionally “pad” their estimates to avoid selecting a compressor that is too small.  This often leads to the installation of oversized and improperly selected air compressors.

It has been a huge problem in Texas where a business only needed 50hp, but someone says ‘ I’m just going to buy 100hp because it will be more than I need.’ But, that is a really bad idea because the machine will run at half-capacity all the time, so it is not the way it is designed to run and it is going to use more power than a 50 horsepower running at 100% capacity.

~ Brad Bonnecaze, Sullair of Houston

In spite of these uncertainties, there is good news for folks tasked with selecting and purchasing a new industrial air compressor.  By carefully considering the following questions, engineers can increase the likelihood they will choose the right-sized unit for their next project.

How will the compressed air be used?

As with any industrial system, the end-use application is the most important factor in determining air compressor sizing.  Each application is different, and carries a unique set of considerations.

Some applications require a steady, predictable supply of compressed air with little variation (e.g.- process air for chemical manufacturing).  In these scenarios, it is usually best to install a compressor with a full load capacity that closely matches actual air usage.  Little consideration for part load efficiency is needed, and extra capacity is not required.  The best strategy in this case is to achieve optimal performance at full load, and ensure the process is never under-supplied with compressed air.

By contrast, other applications require highly variant supplies of compressed air.  These processes need air compressors that can fully support the largest anticipated demand, but also operate efficiently in part load conditions.  For example, an industrial machine shop might have several machining centers that are used at different times.  Air usage will depend on which combination of machines is utilized for a particular job.  In these scenarios, compressors with efficient part-load operation are often beneficial.  In some cases, the installation of multiple compressors for staggered operation is a viable approach.

What are the expected ambient conditions?

Ambient conditions are always an important factor in industrial air compressor sizing and selection.  Not only do equipment sub-components need to be rated for operation in the expected conditions, but air compressor output capacity is also affected.  In high heat, ambient air is less dense because its molecules are farther apart from each other.  As such, an air compressor must work harder in order to deliver the same output as compared to colder temperatures.  Likewise, high humidity also reduces air compressor capacity.  Water molecules displace air molecules on humid days, so compressors must intake a greater volume of ambient air to produce the same output.

Consideration must be given to these factors when selecting a new compressor.  The full load capacity of an industrial air compressor can be reduced by 10% or more due to high heat and humidity.  Engineers must pay careful attention to ensure they size new compressors for the worst-case conditions expected at the site.

Are there any intermittent demands?

Applications that use compressed air are often divided into two primary categories.  Dynamic applications require a steady supply of compressed air at all times.   A common example of a dynamic application is a sandblast cabinet.  When the sandblast nozzle is engaged, an uninterrupted supply of compressed air is needed.  The only acceptable interruption of this flow occurs when the operator releases the nozzle trigger.

Conversely, intermittent applications require a fixed burst of compressed for a short time, with a break between cycles that allows the system to recover.  An example of an intermittent application is a pneumatic cylinder that actuates periodically.  A fixed volume of compressed air is required to push the piston through its stroke, but no other compressed air is needed until the next cycle.

When sizing a new air compressor, engineers should be aware of how each compressed air application fits into these categories.  Often, intermittent demands can be supplemented by installing air receiver tanks at the point-of-use.  This strategy can sometimes reduce the amount of required compressor horsepower.  Talk to a trusted and competent compressed air professional for more information on employing air receivers to supplement intermittent demands.

What is the compressed air quality requirement?

Some critical applications require the compressed air supply to be extremely dry and free of contaminants.  In these instances, regenerative dryers must be utilized to achieve ultra-low dew points in the compressed air stream.

While regenerative dryers are useful for this purpose, they reduce the overall volume of compressed air that is available for the application.  When sizing a new industrial air compressor, engineers must incorporate regenerative dryer purge losses into their demand estimates to ensure an adequate volume of air is supplied at the point-of-use.

When designing a compressed air system, start from the point-of-use and move backward from there.  First, determine the flow, pressure and air quality that is required at the application.  Many folks try to pick the air compressor first…but, the best strategy is to start at the end, then add up all of the variables (such as demand, dryer purge losses and leaks) before selecting an air compressor that can handle the job. 

~Steve Mahaffey, Sullair of Houston

Wrapping Up…

Though the general concepts of mechanically-compressed air have been around for hundreds of years, in the recent years, industrial air compressor technology has evolved rapidly in order to keep up with constantly changing demands and needs for compressed air.

Thus, searching for the right industrial air compressor system with the right size for your facility or your next job can be tricky. But now you’re equipped with the tools and knowledge necessary to make an educated decision on which compressor is a perfect fit your business.

An Industrial Air Compressor Maintenance Checklist: 5 Areas Your Technician Should Check

Industrial Air Compressor

In most industrial facilities, an industrial air compressor is an important utility.  It is used for power, process and control throughout all aspects of operation, and is essential for keeping production up and running.  However, unlike other utilities, compressed air is usually generated onsite with user-owned equipment.

While most companies do not produce their own electricity, heat or water, they often own and operate an on-site compressed air system.  Because this utility is vital to a facility’s operation, a thorough preventative maintenance program should be implemented to minimize unexpected downtime resulting from compressor failure.

Every industrial air compressor is configured with a few common components that must be checked and serviced regularly.  A working knowledge of the following five items will enable industrial compressed air users to better understand and maintain their equipment.

Filters

Filters are necessary components in all the mechanical systems of an industrial air compressor.  They are installed in the air circuit, lubricant circuit and control circuit to protect against contamination.   Each filter serves a unique and important purpose, and must be cleaned or replaced based on the manufacturer’s recommended intervals and environmental conditions.

Additionally, there are several filters that require regular attention when it comes to lubricated rotary screw compressors.

Air Inlet Filter:

Removes contaminants from inlet air before it enters the compressor.

Lubricant Filter:

Cleans the industrial air compressor’s lubricant – protecting precision metal parts from damage caused by solid contaminants in the oil circuit.

Air/Oil Separator:

Because lubricated rotary screw compressors operate with a flooded compression chamber, an air/oil mixture is discharged from the compressor airend.  Before usable compressed air can be discharged from the compressor package, the air and lubricant must be separated.  The air/oil separator serves this purpose, and is equipped with a replaceable coalescing filter element that should be changed regularly.

Control Line Filters: 

Many industrial air compressors are equipped with pneumatic lines for controlling and sensing various functions during unit operation.  Proper maintenance of control line filters helps to ensure the air compressor is responding appropriately to changes in system demand.

Coolers

Effective removal and dissipation of heat is an ongoing battle in every industrial air compressor.  The heat of compression must continually be expelled to prevent unsafe operating conditions and machine failure.  In light of this fact, compressor coolers must be regularly inspected and cleaned during routine maintenance.

Air-cooled rotary screw compressors are often equipped with a heat exchanger for the lubricant, and a separate exchanger (aftercooler) for the discharge air stream.  Over time, these coolers will become clogged with dust and other contaminants from the ambient environment.

Water-cooled compressors are usually configured similarly.  However, they use water as a cooling media instead of air.  Cleaning and maintaining water-cooled heat exchangers can be trickier than their air-cooled counterparts, as these units are more subject to corrosion, clogging and leaks.

Additionally, many centrifugal and multi-stage industrial air compressors include inter-stage coolers to reduce temperature between stages of compression.  These coolers must also be checked regularly to identify potential problem areas.

Lubricant

Air compressor lubricants serve a multitude of functions.  In oil-flooded compressors, lubricants are used to remove the heat of compression, seal clearances in the compression chamber, and lubricate bearings.  In compressor motors and gearboxes, separate lubricants are used to reduce friction between components for increased equipment longevity.

Regardless of their purpose, all industrial air compressor lubricants must be selected and maintained properly.  This is one of the most critical maintenance areas for operators of industrial compressed air systems.  Users that neglect to monitor and maintain the condition of their compressor lubricant can cause irreparable damage to their compressed air systems.

Regular oil sampling and analysis can turn a good compressor PM program into a great one.  By analyzing the chemical properties of a compressor’s lubricant, we can spot potential problems before they occur.  At Sullair of Houston, our standard oil sample report highlights many aspects of lubricant condition – like particle count, water content and total acid number.  This allows us to make informed and insightful recommendations to our customers about how to better maintain their equipment.  ~ George Saez, Service Manager Sullair of Houston

Controls

Controls are an often misunderstood and overlooked system in industrial air compressors.  However, their impact on the reliability, longevity and efficiency of industrial air systems cannot be understated.

In general, the controls of an industrial air compressor enable it to respond appropriately to changes in system demand.  For example, when system pressure drops, an air compressor should respond by increasing output to offset demand.  When pressure rises, an air compressor should respond by decreasing output to prevent over-pressuring the system.

There are many compressor control types available today – each with a unique set of characteristics and benefits.  However, regardless of type, all compressor control systems consist of several mechanical and electrical components that must be incorporated into the unit PM program.  Potential maintenance items include valves, solenoids, regulators, switches, transmitters, tubing, etc.   If one of these items requires service, the air compressor can operate unpredictably.

Call a trusted compressor service provider for help troubleshooting and repairing control problems.

Drains

Most industrial air compressor systems are equipped with several condensate drains.  Often, there are drains installed on compressors, filters, dryers and receiver tanks.  When functioning properly, these drain valves expel liquid water to protect downstream equipment from potential corrosion or damage.  However, if not regularly checked and maintained, a compressed air condensate drain can fail – having a detrimental impact on system performance and reliability.

A drain valve that is stuck in the open position can be a nuisance.  It creates an air leak that reduces system efficiency, or even causes overall pressure to sag.

A drain valve that is stuck in the closed position can often be more problematic – sending liquid slugs of water downstream to overwhelm dryers, filters and pneumatic equipment in the plant.

Many pneumatic control valves used in chemical process and refining applications cost thousands of dollars.  Even a small slug of water can ruin them, and replacement costs are very high.  Protecting these components from liquid water is critical – and a simple, inexpensive maintenance program for compressor condensate drains can help to prevent major failures down the road.  ~ Steve Mahaffey, Sullair of Houston