Strategies to optimize compressed air systems

In most plants, no compressed air means no production. Keeping the air system up and running is normally the responsibility of the maintenance crew. When more capacity is needed, purchasing steps in and sends a new compressor, based on which supplier is offering the best deal. But is new equipment really necessary?

01/01/2009


In most plants, no compressed air means no production. Keeping the air system up and running is normally the responsibility of the maintenance crew. When more capacity is needed, purchasing steps in and sends a new compressor, based on which supplier is offering the best deal.

But is new equipment really necessary? Is the existing system really operating at full capacity? What about what goes on in the plant outside the compressor room %%MDASSML%% could that be having an impact on system performance? All of these areas should be addressed and revisited on a routine basis to ensure that a facility’s compressed air system is fully operational and providing the best value.

Analyze the system to avoid wastes

Begin with a thorough system analysis to determine pressure profiles and actual compressed air demand. Simply measuring amperage is not enough. Equipment like data loggers, opti-couplers, vacuum transducers, kilowatt meters, flow meters and pressure transducers monitor and record information on load/unload signals, inlet pressure signals, power and demand flow.

These tools should remain in place during normal operations and during down time for up to 10 days. This method provides a clear picture of the facility’s unique air usage profile.

Identifying and repairing leaks is one of the very first steps to improving air system efficiency %%MDASSML%% and every facility has leaks. Studies indicate that approximately 35% of all compressed air produced %%MDASSML%% more than a third %%MDASSML%% is lost through leaks. Ultrasonic leak detection devices and services are available and relatively inexpensive to facilitate leak detection efforts.

Next, identify unnecessary and wasteful practices. One common example is using compressed air to “blow-off” an area or work surface. Using compressed air for this purpose is a serious OSHA violation when the compressed air is over 30 psig. Even small pieces of debris flying through the air at 110 psig can cause injury.

Finally, do not run the compressed air system at unnecessarily high pressures, as the resulting elevated energy consumption results in higher operating costs.

Component review

Piping is a frequently overlooked component - especially in older facilities. Piping can be inadequate for the new demands of recent expansions or upgrades and should be evaluated for proper diameter and good condition.

The interior surface of both black iron and galvanized iron piping will rust over time, causing a rough surface that catches contaminants. Often within just a few years, iron piping develops build-up, releasing rust and scale into the compressed air. These contaminants not only degrade air quality, but significantly reduce the effective internal pipe diameter creating unwanted pressure drops and velocity problems.

Other materials for piping include stainless steel, copper and aluminum. Stainless-steel piping will not rust and will provide good air quality, but the materials are costly. Copper provides excellent value for the investment and ideal delivery characteristics with minimal pressure drop. However, recent developments in modular piping products also offer simple, easy installation with excellent long-term performance. Plus, modular aluminum can be easily reconfigured during expansions or facility reorganizations.

Appropriately sized air receiver tanks add significantly to overall system efficiency. Receivers should be sized to meet high volume demand events.

For facilities that have stable usage patterns, a “wet” tank alone is often sufficient. Through centrifugal force, these tanks provide the first stage of moisture separation to help maintain compressed air quality. However, their primary function is storing and delivering compressed air to help meet periods of peak demand and to prevent excessive compressor cycling. For wet tanks acting as a system’s sole storage device, a good rule of thumb is to allow for more than 5 gallons per cfm produced by the “trim” compressor (the compressor that cuts on to handle peak demand).

In facilities with sudden spikes in demand, systems should include both a wet tank and a “dry” tank. The wet tank should be located as close to the compressor as possible, and the dry tank should be located downstream of air treatment equipment to provide users with a large volume of air. Wet tanks are generally sized at 1 gallon per cfm of each cycling compressor. No sizing rule appropriately applies to dry tanks; they should be sized according to the customer’s demand.

Equipment for clean air

Hot compressed air holds a lot of moisture vapor pulled in from atmospheric air. As it moves through the system, the air cools and moisture vapor condenses into liquid. This liquid can cause rust and corrosion in pipes, products and tools, and as a result lead to more frequent replacement of tools, increased maintenance and higher production costs.

Each application’s unique requirements will determine the appropriate compressed air dew point and in most cases dictate the type of dryer needed. Refrigerated and desiccant dryers are the two basic types of compressed air dryers.

Refrigerated dryers cool air and condense the resulting water vapors. These dryers are efficient and can be used in most applications. Desiccant dryers use desiccant material to adsorb moisture and provide exceptionally dry air. Dessicant dryers are more costly than refrigerated dryers to purchase and operate, and for that reason should only be used at points that require dew points of less than 35 F.

In addition to holding moisture vapor, ambient air contains various contaminants. These contaminants %%MDASSML%% dirt, dust, oil, carbon monoxide and chemical pollutants %%MDASSML%% are drawn into working compressors, then compressed and intensified.

Like the level of dryness needed, each application also determines the level of in-line filtration required. Place coarser particulate filters before the dryer to take up the bulk liquid or particulate loading. Finer filters for removing small particulate, aerosols and vapors go after the dryer. Routinely checking and replacing filter elements is critical to proper operation. Review the manufacturer’s recommendations. This will help eliminate pressure drop across the system and maintain effective filter performance.

Systems should also include high-quality, automatic drain traps. If the contaminants filtered and separated from tanks and filters are not removed, they will find their way back into the system.

Planning, designing and purchasing

If current clean air treatment equipment is not performing to expectations, one should consider installing an air main charging system before investing in new equipment. This economical device controls the air velocity during system pressurization so that dryers and filters are not overloaded when the compressor is started %%MDASSML%% for the first time or after several days of inactivity.

Master controllers, as opposed to more simplistic cascade controllers, use a single control signal to operate multi-compressor systems within a narrow pressure band to offer optimum energy savings while also reducing losses to artificial demand and leaks. Precise pressure control is important, because for every 2 psi reduction in operating pressure, users save 1% in energy costs. Plus, master controllers balance load hours to reduce maintenance as well as select the most efficient compressor combination to meet the demand, instead of just turning on the next available unit. Some controllers also provide remote access to real-time system data, manage large groups of compressors, or alert service technicians of problems with the system.

There is no shortage of quality compressed air system equipment in today’s market. The latest advances, including variable frequency drive technology and computer-based master system controllers, may help to drastically improve air system reliability and overall productivity. However, equipment upgrades and system improvements are only as effective as the overall system design, and they require a solid compressed air delivery “infrastructure” to perform at peak efficiency and deliver maximum reliability.

A collaborative and consultative selling process is more likely to produce an efficient and reliable system. Representatives selling quality equipment will work to understand each facility’s specific operational and application requirements and can clearly explain product features and relate benefits to specific needs. One size fits all solutions should be evaluated with caution.

This is the full text of the article cut short in the December 2008 issue due to a production error.


Author Information
Wayne Perry is a technical director for Kaeser Compressors. He can be reached at 540-898-5500 or e-mail him at wayne.perry@kaeser.com .




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