Compressed Air Best Practices - May 2008 - (Page 28)

| 05/08 Compressed Air Industry AUTOMOTIVE ASSEMBLY | REAL WORLD BEST PRACTICES GUIDELINES FOR STARTING AIR FOR CENTRIFUGAL COMPRESSORS IV. Identifying the starting air compressed air demand Identifying this demand is very complex — it varies by manufacturer and basic design. It varies by accessory selection. It varies by the units overall condition and what type of maintenance it receives. To develop this section we reviewed the training material, operating/installation manuals and other published data from the four major centrifugal manufacturers used in North America. We also went back to models from the 1970s, 1980s, 1990s and current units. We interviewed manufacturer field service personnel, user operating personnel and other knowledgeable people. Buffer seal air: Of the four leading manufacturers supplying centrifugals in North America, two of them use similar versions of double labyrinth seals. These seals require air injection or “buffer air” between the two sets of seals to assure no oil migration from the shaft lubricating oil to the compression area. Reviewing various installation manuals, we found a 10 to 20 cfm demand for instrument air. Instrument air has been defined as refrigerated dryers with 5 micron filtration to .01 micron particulate filtration. “In the field, we . have observed centrifugals with more than 100 cfm of leaks in the flanges and valving.” Realizing that the larger the compressors and the seal itself, the more buffer air may be required. We have also found a more specific reference to seal air estimated flow volume. This data was confirmed by engineering and service personnel of several manufacturers. Typical flow values for buffer air: p p New — 1% of total flow Worn — 3% of total flow What else may affect the actual value of “seal buffer air”: p p Adjustment and condition of seal air regulator — normal setting at 6 psig Adjustment of bleed off valve if used These are estimated values. For any specific compressor, check with the OEM. Let’s see what this means: At a 10–20 cfm demand, we are in the 5 to 10 hp aircooled, single acting, reciprocating class with a 120 gallon air receiver or larger at 150–175 psig. The maximum flow available with a typical industrial 10 hp compressor would be about 30 cfm at 175 psig — regulated to* 10 psig (or lower) with an estimated regulator droop of 20 psig. We would have about 140–160 cfm of stored air that could be bled out of the tank to augment the compressor flow if the demand was to exceed its capacity. A typical 1,000 hp compressor delivers 5,000 cfm — 1% buffer air when new is 50 cfm demand. At this level, the 10 hp unit would be able to stay on about 2–3 minutes supplying buffer air. If the seal wears to the full 3%, this demand will go to 150 cfm — the run will last just a little over 1 minute. Of course, this is only the buffer air — demand does not include the air required for the various controls. Summary: An older centrifugal with buffer air and “worn seal” will use up to 150 cfm per 1,000 hp of instrument air at about 10 psig unless otherwise stated. If there is an adjustable bleed, there may be an additional 15–25 cfm demand. * Normal buffer air is 5–7 psig with the auto cutoff usually about 2–3 psig. The higher the pressure, the higher the flow. 28 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - May 2008 Contents From the Editor Utility-Air News The Compressed Air Audit of the Month Pneumatics Real World Best Practices Delta Industries: Doing What is Right for Customers Delta Industries: Six Truths You Need to Know about Really Putting the Customer First Power Supply Industries Resources for Energy Engineers Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - May 2008

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