Compressed Air

The significant impact of compressed air systems on workflow and product quality is often overlooked.


Choose the right dryer for your needs. Dryer performance is stated in terms of specific conditions (ambient temperature, compressed air inlet pressure, and compressed air temperature).

Refrigerated dryers are the most common. They employ a refrigeration system to lower the compressed air temperature well below the ambient temperature. This condenses the moisture vapor into liquid that can be drained out of the system. This lowers the “pressure dew point” of the compressed air, and as long as the compressed air does not cool below this new dew point, any remaining moisture will remain in vapor form. Refrigerated dryers are designed to produce dew points between 35 F and 50 F at rated conditions.

High temperature refrigerated dryers are similar to standard refrigerated dryers but include an aftercooler and are primarily used with piston compressors as the air must be pre-cooled prior to entering the dryer. These are usually designed to achieve 50 F dew points at rated conditions.

Desiccant dryers operate very differently from refrigerated dryers. They work by directing the compressed airflow across a bed of desiccant material that adsorbs moisture vapor out of the air. Desiccant dryers are used to produce dew points as low as –100 F and are recommended when air quality requirements are extremely high.

Filtration

Filters are categorized based on the contaminants they are designed to capture. They may be designed to capture more than one type of contaminant.

Moisture separators are designed to mechanically separate liquid water and oil from the air stream. Particle filters are designed to capture dirt, dust, etc. but may remove some water and oil mists.

Coalescing oil filters are finer filters designed to remove oil aerosols/mists and fine particles. These are usually placed after a refrigerated dryer.

Vapor adsorbers are designed for eliminating oil vapors only and should be placed after all other filters and dryers. Staging filters in the system provide more effective filtration, lower pressure drop at each filter, and longer filter life. Some systems have differential pressure gauges, liquid level indicators, and/or built-in drains.

Proper filter maintenance will minimize pressure drop and ensure good air quality. Failure to change filters will guarantee higher pressure drop and greatly reduce filter performance.

Condensate Drains/Traps

The drain trap is a critical but often overlooked component of the compressed air system. Drains remove liquid contaminants from the system. If the filtered and separated contaminants are not removed from tanks, refrigerated dryers, and filters, they build up and find their way back into the air system. Liquid accumulation in tanks will gradually eliminate the air storage capacity in the tank. This will cause periods of inadequate airflow/pressure, and could cause a reciprocating compressor to exceed its duty cycle and overheat.

There are several types of drains:

• Manual drain

• Timed drain

• Automatic demand drain — a mechanical or electric device that activates when the liquid level reaches a certain point inside the drain.

Piping and Distribution

Pressure drop. Restrictions in airflow create air turbulence that results in a reduced system pressure. This occurs in many components, including the dryer, filters, valves, and piping. The degree depends on the choice of material and pipe size. Pressure drop can be greatly reduced with proper system design and maintenance, but there will always be some. Be sure to account for the total pressure drop when selecting the compressor’s operating pressure.

Pipe size has a major impact on system performance. Pressure drop changes exponentially with pipe diameter. Bigger is better. Look ahead when planning a system and allow for business growth. It is time-consuming and expensive to install a larger distribution system later. There are standard charts published that provide guidelines. Check with a compressed air specialist or call the Compressed Air and Gas Institute (www.cagi.org).

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