Properly Assess Compressed Air Demand

A careful analysis can lead to substantial savings and more.

By William Scales, Scales Industrial Technologies Inc.

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Where a single system must provide process, packaging and instrument air, use the highest quality air required. Many process plants instead rely on two systems — one for "plant" air, which may not be dried, and the other, generally at a much higher quality, for instrument air or some processes. Many plants will use dry nitrogen as a backup for the instrument air system. This is especially important when short-duration power interruptions occur and minimum pressures must be maintained for all instruments and valve actuators.

Breathing air. Air supplied to respirators, hoods and helmets and to special breathing air systems must satisfy U.S. Occupational Safety and Health Administration (OSHA) standard 1910:13d. It requires drying, filtration and treatment to meet specific levels, including for carbon monoxide, with an alarm system. Compressed breathing air must comply with the requirements for Type 1 – Grade D breathing air as described in American National Standards Institute (ANSI) Compressed Gas Association (CGA) Commodity Specification for air ANSI/CGA-G7.1. The OSHA standard references this specification and is essentially consistent with ANSI. Breathing air also must be tested for contaminants such as methane, nitrogen oxide, nitrous oxide, halogenated hydrocarbons and other hydrocarbons. Grossly contaminated or oxygen deficient air may not be able to be purified to levels acceptable for breathing.

The CGA Standard Commodity Specification G7.1, Grade D, commonly is specified for plant breathing air systems. At a minimum, this air must conform to OSHA standard 1910.134 (revision effective April 1998) or, in Canada, Canadian Standards Association standard CAN3-Z180.1-M85. Check to ensure compliance with all local standards.

COMPRESSOR CONSIDERATIONS
Many companies use lifecycle costing derived from compressed air data and bid response forms. These forms help in judging among possible options, including air-cooled, water-cooled and various types of compressors and ancillary equipment. The data forms could include: package power, pressure, flow, inlet conditions, dew point for dryers, cooling water or ventilating requirements, service life and necessary maintenance. Purchase decisions often consider actual energy and maintenance costs for a 10-year period (Figure 2) as well as equipment reliability.

For more information on the types of compressors, see "Don't Err with Air Compressors," and "Dare to Compare Air Compressors."

The inlet air to the compressor should be as cool, dry and clean as possible. Clean is defined as a minimum amount of dust and foreign matter. The air cannot contain contaminating gases such as ammonia, chlorine, sulfur, carbon monoxide, etc., that can affect the compressor, piping, process or pneumatic system components. Compressed air must be free of corrosive contaminants and hazardous gases.

For lubricant-free positive-displacement-type compressors (screw and reciprocating), lowering the inlet air temperature 5°F will improve compressor performance approximately 1%. Where practical, consider outside air for many applications. If the inlet piping takes air from a remote location, such as outdoors or from conditioned plant air, increase the pipe one size for every ten feet of length. Do not place the intake point near cooling tower or exhaust fan discharge, as this could contaminate the inlet air to the compressor. If not supplied with the compressor, install an inlet vacuum gauge to monitor the condition of the intake filter.

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