Industrial Air Compressor Treatment Basics

Air Treatment Basics

In previous installments of this blog, we have discussed not only industrial air compressor basics and their application, but also sizing and upkeep in industrial settings. While this subject is of great importance, and will be continually covered in future articles, consideration should also be given to other components of the air system that affect the usefulness of compressed air as an industrial energy source.

All industrial air compressors have the same basic principle of operation – they draw in ambient air and increase pressure by reducing the empty space between molecules. By doing this, the compressor not only increases air pressure, but also increases the concentration of any other substances that are present. Foreign materials that are drawn in with the ambient air (like water vapor, dust, fumes or aerosols) are concentrated at higher levels when the air is compressed and often can be detrimental to pneumatic processes and machinery.

For this reason, many industrial compressed air systems are equipped with dedicated equipment to clean and dry the compressed air stream. This equipment ensures that compressed air quality is suitable for its end use. Compressed air dryers, filters and condensate management systems all perform this function, and are often as important to an air system as the compressor itself. Equipment of this type is commonly referred to as “Air Treatment.”

Types of Contamination

There are three primary types of compressed air contaminants as described by the International Standards Organization in their specification ISO 8573.1. These contaminant types are: water (or water vapor), solid particulate and oil. In this article, we will discuss the presence of water in compressed air systems, and the equipment that is commonly used to remove it.

Water

Water is a contaminant that is always present in compressed air systems. The concentration of ambient humidity during the compression process, combined with a sharp rise in temperature due to the heat of compression, causes all industrial compressors to discharge air at 100% saturation. The water vapor in compressed air will begin condensing into liquid as it travels through the pipe and cools. Because liquid water can corrode metal components and damage expensive industrial equipment, it is often removed from compressed air immediately after it leaves the compressor to protect the rest of the air system from its harmful effects.

Liquid water can be removed by moisture separators that utilize centrifugal motion to separate water droplets from the air stream. This liquid is then expelled from the system using a drain valve. However, water vapor cannot be removed by a moisture separator. Water vapor is often considered a contaminant in compressed air, because it has the potential to condense into liquid in the future. In order to effectively remove water vapor, a dedicated compressed air dryer must be utilized. Several
dryer technologies are used throughout the industry for this purpose – each with its own advantages and limitations. The most common types are Refrigerated Dryers and Regenerative Dryers.

Refrigerated Dryers

Refrigerated Dryers are commonly used in industrial and commercial settings, and often provide an affordable and effective solution for removing water from a compressed air system. They operate as their name suggests – by using a refrigeration circuit to decrease the temperature of the compressed air. When air temperature is decreased, its capacity to hold water vapor is also decreased. Thus, when air passes through a refrigerated dryer, a significant portion of its laden water vapor is condensed into liquid. This liquid water is then collected in a moisture separator, and expelled through a drain valve.

These dryers are commonly used because they provide an adequate solution for many industrial facilities at relatively low cost. However, there are some limitations to the application of refrigerated dryers. For example, refrigerated dryers cool the air in order to remove moisture. Therefore, their lowest pressure dewpoint is limited by the temperature at which water will freeze. Accordingly, refrigerated dryers are generally rated to produce pressure dewpoints around +38°F to avoid freezing. Similarly, the performance of refrigerated dryers can be greatly affected by ambient conditions. High ambient heat and poor ventilation exert a heavier workload on refrigeration systems, and cause the performance of refrigerated dryers to decline. These limitations must be accounted for when refrigerated dryers are sized and selected for a given application.

Regenerative Dryers

Regenerative Dryers pass compressed air through a media bed to remove water vapor. These units employ the properties of certain chemicals that have a high affinity to attract water molecules. Common chemicals used in regenerative dryer media beds are activated alumina, molecular sieve and silica gel. Regenerative dryers are capable of drying air much more thoroughly than refrigerated dryers, and can produce pressure dewpoints as low as -100°F.

These units are often used in applications where compressed air quality is of the utmost importance. Chemical manufacturing, oil & gas refining, medical applications and power plant systems often include regenerative air dryers. Additionally, regenerative air dryers are commonly used in outdoor applications where extremely high or low temperatures are possible, as they are less sensitive to ambient conditions than refrigerated dryers.

While regenerative dryers produce lower pressure dewpoints than refrigerated dryers, and are generally more robust, they come with a unique set of considerations. First, regenerative dryers are more expensive to own and operate than refrigerated dryers. They not only carry a higher initial purchase price, but also reduce the overall capacity and efficiency of the systems in which they are installed.

Normally, regenerative dryers are configured with two media beds. At any given time, one bed is drying, while the other bed is regenerating (or purging the moisture that has been picked up during the drying process.) During regeneration, the dryer actually consumes compressed air. For this reason, regenerative dryers must be selected carefully, as to not under supply a given application. Additionally, regenerative dryers decrease overall system efficiency by using compressed air for purging.

In Conclusion

Various technologies can be utilized to reduce the amount of air that these dryers use for regeneration (such as external heaters or blowers). However, each added feature will increase the initial purchase price of the dryer, so end users often employ these technologies only when the long-term efficiency gains outweigh the increase in initial cost.

In future articles, we will explore how additional air treatment equipment is used to combat the other forms of compressed air system contamination (particulate and oil). Additionally, we will cover the basics of air treatment sizing and selection.

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.