Compressed Air Best Practices - May 2008 - (Page 32)

| 05/08 Compressed Air Industry AUTOMOTIVE ASSEMBLY | REAL WORLD BEST PRACTICES GUIDELINES FOR STARTING AIR FOR CENTRIFUGAL COMPRESSORS Refrigerated dryers do not do well in a “start up” situation. They have to come on and “cool down” before they can handle a load of air to dry. This can be up to one hour of operation depending on the size and model; otherwise, they will deliver “wet air” to the system. This limitation really makes them unacceptable as a source of dry air for starting air. It is interesting to note that we have observed many field starting air systems with refrigerated dryers. However, when the starting and pressurization sequence takes less than 30 minutes to 1 hour, the refrigerated dryer will probably not meet the specification. Desiccant dryers offer a -40 ºF pressure dew point or lower. They do have several drawbacks: p The bed can become saturated Producing and drying the air at 120 psig to a 35 ºF pressure dew point will remove about 89% of the water vapor. A 35 ºF PDP and 120 psig, the membrane dryer will offer the following operational system pressure dew points: Buffer air: 7 psig = @ -10 ºF PDP Control air: 80 psig = @ +25 ºF PDP Both of these operational pressure dew points should be very effective. This will require very stringent filtration (.01 micron particulate) to protect the membrane which will also deliver “clean air” to the buffer and control air. The membrane dryer has the best initial cost after a refrigerated dryer. Drying the air at a higher pressure and then using it at a lower pressure will have the effect of “Lowering the System Pressure Dew Point”. Summary: We believe the membrane dryer with proper filtration will perform as required for this application. Considering the initial price, flexibility, predictability, performance and ease of installation, this would be our dryer of choice, particularly with an internally cooled rotary screw. The next dryer of choice would be an appropriately sized heatless type. over time if the valves are not shut or leak. If this occurs, the initial start up air may be wet. p Desiccant dust can be a signifi- VI. Start up compressed air sizing recommendations by type and size of centrifugal compressors Selecting and sizing a “standard” unit for this application is made very complex because of the leakage factor that could exist. The other variables are very identifiable and a clear predictable maximum demand can be established, if required, which is both unit and site specific. To set up the following matrix for volume (cfm), we are assigning 50 cfm of leaks including condensate drains and 25 cfm of other open bleeds as a maximum demand. Installations that have such leaks and drains that would exceed this on this unit to be started should address the correction of them or at least take control steps to block them off at start up. Condensate drains can be modified (if approved by maintenance) to level activated, electric actuated such as the capacitance tube moisture sensing drain which should not be open during start up. 500 HP AIR USAGE — MAXIMUM CFM EXPECTED DOUBLE SEAL SINGLE SEAL 1000 HP DOUBLE SEAL SINGLE SEAL 1500 HP DOUBLE SEAL SINGLE SEAL 2000 HP DOUBLE SEAL SINGLE SEAL cant problem when a new dryer is received and/or a new bed of desiccant is installed. The after filter is designed to handle this but it would be very critical if the dust got into the starting air system. Heatless desiccant dryers also use about 15% of the rated flow for pure air — this too must be considered when selecting the air supply. In reality, on a short run, a well maintained heatless desiccant dryer will usually perform well. Membrane dryers — the membrane dryer also uses purge or sweep air but only proportional to the “actual flow demand” — not always the full “rated” flow. Membrane dryers offer a very solid +35 ºF pressure dew point with the ability to sometimes reach +10 of pressure dew point. They need no warm up time and there is no dust generated to worry about. 32 www.airbestpractices.com Normal flow Buffer air 5–7 psig Control air Standard pneumatic controls Miscellaneous bleeds Leaks Dryer purge air* Total value required cfm Required pressure/psig seal control valves 2500 75 2500 0 5000 150 5000 0 7500 225 7500 0 10,000 300 10,000 0 25 25 50 30 205 10 / 85 25 25 50 15 115 10 / 85 25 25 50 37 288 10 / 85 25 25 50 15 115 10 / 85 25 25 50 48 373 10 / 85 25 25 50 15 115 10 / 85 25 25 50 60 460 10 / 85 25 25 50 15 115 10 / 85 * Dryer purge air desiccant number is somewhat fixed – membrane 15-20% will be proportional to actual flow and probably average less. http://www.airbestpractices.com

Table of Contents Feed for the Digital Edition of Compressed Air Best Practices - May 2008

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.