Energy Efficiency Reference/Industrial/Compressed Air/Walkthrough Checklist

Compressed Air: Walkthrough Checklist
This section provides a checklist of things to look for while at the plant. It provides an overview of the major compressor savings opportunities. It is designed to help assure that major savings opportunities are addressed, and to help identify areas on which to focus more detailed analysis.

During a site evaluation, it is important to record gauge readings on air compressors, other equipment, and at other locations where it can be obtained throughout the plant. Many air compressors include airflow and pressure gauges, while other equipment often includes pressure gauges, making it possible to measure pressure drop across the oil separator and dryer. Look for high pressure drops through equipment and lines. Look for intentional pressure reductions, such as with valves and pressure regulators. More detailed aids for recording and analyzing compressor data are provided in later chapters.

See Reference & Data Collection sections for more detail on opportunities identified.

Major Savings Opportunities:
 * Large compressor serving minimal system needs in off hours (e.g. fire suppression system).
 * Install small dedicated air compressor to serve minimal after hour need such as a fire suppression system and isolate from main plant air system.
 * It is common to leave a larger compressor on continuously - 7 days a week - to serve a small use such as a dry fire suppression system. The compressor may operate for long periods at an extremely inefficient part load condition. System leaks also consume air and energy continually. It is generally cost effective to install a separate dedicated compressor for such a need and separate it from the main compressed air system to avoid losing air and energy to system leaks during non-production times.
 * Multiple compressors operation at part load.
 * When more than one compressor is operating, all but one should operate at full capacity and maximum efficiency. A final trim compressor can match output to system requirements. In the best case, the "trim" compressor can have more sophisticated controls for greater efficiency at part load operation.
 * Manually sequence multiple compressors: Set existing controls to load compressors sequentially so that unneeded compressors can be turned off manually or automatically. The downside of this option is the increase in pressure variations unless electronic controls are installed.
 * Install automatic compressor sequencing controls: Sequence compressors to avoid operating several compressors at part load. Fewer compressors will operate more efficiently at higher loads. The largest savings come from the sequencer turning unneeded compressors off.
 * Throttle controlled screw compressor operating at partial capacity for large part of time.
 * Generally: A throttle-controlled compressor should not operate below 70-80% capacity for much of the time. A throttle-controlled compressor consumes approximately 70% of its full load power when delivering no air and 85% power at half capacity.
 * Install load-unload controls for more efficient air compressor operation. Load unload control allows a compressor to operate either at full output and maximum efficiency or unloaded. Because the compressor does not turn off, the motor is not damaged. However, load-unload control requires larger receiver capacity and a significant variation in system pressure to create an acceptable cycle time. An unloaded screw compressor with the discharge pressure reduced to near atmosphere consumes approximately 17% to 26% of the energy required for the compressor to operate at full capacity. When the compressor discharge pressure drops to an intermediate pressure, such as 40-50 psig, there are fewer savings. This recommendation does not work everywhere because:
 * Some processes are too sensitive to allow the variation in pressure required.
 * The load/unload cycle can make maintenance personnel uncomfortable. It is often set to unload at low load causing the compressor to operate at inefficient part loads. Potential savings are lost. A small amount of compressed air is released to atmosphere when the compressor unloads. If the compressor cycles too often this air loss can be significant.