Compressed Air Best Practices - April 2008 - (Page 14)

| 04/08 Focus Industry PLASTICS BLOW-MOLDING | COMPRESSED AIR AUDIT OF THE MONTH Minimizing Blow-Molding Pressure Fluctuations Rate of Change automation can differentiate between a compressor failure that should be reacted to immediately and a large demand side event that should be allowed to use the available storage in order to avoid a compressor start. Stable header pressure is the first step and can only be created in a PET blow-molding system through the use of flow controllers and additional storage as previously installed in this plant. In this manner, the recommended storage can be fully utilized while minimizing the risk of a production interruption due to compressed air problems. Programming critical levels of storage will allow the automation to decide when it must react to preserve the system in virtually all conditions. The automation system must be capable of accepting designation of base load and trim compressors so that the reciprocating compressors in the system can be used in the most efficient manner. The automation platform should start, stop, load and unload all the compressors as necessary to provide the appropriate response to demand or supply changes. It is important to realize that in compressed air systems, one preventable start on a high-pressure reciprocating compressor will cost $2,000–$5,000 in electrical demand charges for that month; only automation can be attentive enough to prevent these costs. Additionally, the appropriate automation system should use communication protocol from the main automation panel to the compressor interface panels to minimize the risk of a signal error and to minimize the installation costs. This system must be capable of interfacing with the compressors and must always fall back to active local compressor controls in such a manner that the compressor maintains its current operating status. It is critical that the system not defeat or replace any safety circuits on the compressors, which are being controlled. The main panel should be located centrally in the maintenance area so that the system can be frequently monitored. The recommended automation system also provides a SCADA system for managing and monitoring efficiency. CO2 O Achieving Stable Pressure at the BlowMolding Machine After installing a bypass and storage tank, it is reasonable to expect to achieve pressure stability within +/1–2% which will allow pressure reductions of 1.5–2.0 bar in header pressure. kW Storage at the Blow-Molding Machine The next, most significant step in stabilizing the blow-pressure is to install compressed air storage at the blow-molding machine as close to the rotary union as possible. For larger volumes of air this is more important, however, this step should only be taken after the previously recommended steps to stabilize system pressure. The diagram depicts the proper arrangement of the bypass and storage if required. With a bypass and storage installed, it is reasonable to expect to achieve pressure stability within 1–2% of set-point which will allow pressure reductions of 1.5–2.0 bar in header pressure without affecting final blow-pressure levels. The energy savings will approach 5% in addition to the 10% which can be achieved by reducing header pressures based on finer control of the automation and control valves in the system. Table 3: Piping Pressure Drop 14 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - April 2008 Contents From the Editor Utility-Air News The Compressed Air Audit of the Month: Minimizing Blow-Molding Pressure Fluctuations AF Compressors: Focused on 40 Bar, Oil-Free Air for the PET Industry Real World Best Practices: PET Plants Using Boosters for High-Pressure Air Guaranteeing Audit ROI for Blow-Molders Managing Vacuum Pressure Differential Are You Taking Full Advantage of Pneumatic Valve Technology? Are You Strategic? Resources for Energy Engineers: Training Calendar & Product Picks Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - April 2008

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