Vital signs

This article explains how measuring certain key variables can help you determine the overall health of your compressed air system.

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By Tom Taranto

PlantServices.com

Compressed air systems hold one of the keys to greater productivity, efficiency and profitability in your facility. The Compressed Air Challenge (CAC) is a voluntary collaboration of industrial users; manufacturers, distributors and their associations; facility operating personnel and their associations; consultants; state research and development agencies; energy efficiency organizations; and utilities. This group has one purpose in mind—helping you improve the performance of your compressed air system.

The CAC has developed two levels of training for plant engineers: Fundamentals of Compressed Air Systems and Advanced Management of Compressed Air Systems. This is the second in a series of articles that introduces some key points of CAC training.

The process of baselining compressed air system performance involves taking measurements to determine the effectiveness and efficiency of your compressed air system to meet productive air demands efficiently. Measurements of pressure, flow, power and energy consumption are the “vital signs” of a compressed air system.

 Measurements can help you understand how the compressed air system is   operating.
 The information gained can be used to identify opportunities for system    improvements.
 Data logging system performance can help you better understand how to optimize your  system.
 Make sure you have the right tools for the job, know how to use them, understand their  limitations, and know how to interpret the data being produced.

The resultant information will help you understand the business impact of compressed air system operations: namely its effect on productivity, product quality and the cost of compressed air per unit of production.
Compressed air system performance can be measured in many ways using several techniques and types of equipment. When gathering data, e.g., pressure, flow, power, and energy consumption, it’s important to remember that the ultimate goal is to reduce operating costs.

Measurements can be spot checks of performance during operating shifts. Although spot checks can provide some answers, they don’t produce a complete picture. Since compressed air systems are dynamic in nature, with frequent changes in operation, data logging performance will provide a more accurate baseline. Data logging documents  the interactions occurring with changes in demand and the resultant supply side response.

The informational objectives below relate to the stability of system performance and energy consumed. The baseline measurement should document current performance and identify opportunities for improvement.

• Record the plant’s air demand profile through airflow measurements made during normal  plant operations.
• Create a pressure profile to establish the current compressor control interactions, with  particular attention to compressors operating in unloaded, or partially loaded states.
• Develop a power profile relating delivered airflow and power consumed.
• Evaluate overall supply-side efficiency by measuring total compressor energy   consumption, delivered airflow to the system and energy cost.
• Determine the pressure drop through treatment equipment and evaluate the effect of  increasing or decreasing airflow.
• Identify peak transient air demands by evaluating compressor control response and  considering the potential benefit of engineered air storage.
• Measure pressure gradient across distribution system piping and document the effect of  changing air demand.
• Profile dynamic air use at high-volume intermittent demands and consider potential air  storage opportunities.
• Evaluate perceived high-pressure air demands and measure applied air pressure at the  point of use.

Informational objectives may vary from system to system and will depend on the defined scope of study. However, measured data must be consistent with the information desired. Select appropriate locations to measure flow, pressure and power.

Several key pressure measurements may be made. On the supply side, for example, measure at each compressor, upstream and downstream of air treatment equipment, such as dryers and filters. The demand side might include pressure measurement at various points in the distribution system, at critical end uses, at perceived high-pressure air demands and high-volume intermittent air demands.

Airflow measurement at each system supply point will identify the plant air demand profile. Flow measurement can further help identify the existence of intermittent demands and transient events. Flow and pressure measurements at high-volume intermittent air demands can help determine the potential for application of secondary storage.

Power measurement at each compressor provides insight into the response of compressor controls to changing compressed air demand. Totaling power consumed during a particular time interval provides a measure of energy consumption.

Compressed air flow and energy consumed ultimately determine the cost of producing compressed air. Knowing this allows you to assess the cost of normal operations and waste. This provides the baseline for justifying improvements in the compressed air system and verification of results.

The tools required range from simple gauges and meters for spot check measurements to transmitters and data loggers for trending and dynamic analysis. When selecting tools, consider both accuracy and repeatability. Accuracy is the ability of a particular device to measure to a known value or standard. Repeatability is its ability to achieve the same measured value consistently.