Compressed Air Best Practices - February 2008 - (Page 12)

| 02/08 Compressed Air Industry BULK HANDLING | COMPRESSED AIR AUDIT OF THE MONTH Dense-Phase Transport Wreaks Havoc on Compressed Air System Dense-Phase Transport Systems Create Demand Peaks A number of dense-phase transport systems were in use. Comparing time stamps associated with the operation of these systems and the time stamps in my data, I was able to identify how much air was needed to cycle each of them, the frequency of their use and when they were being used simultaneously. This information would be useful later in helping to manage the demand, but for now, we had learned part of what had not been known: the peaks were significant (increasing as much as 2500+ scfm) and of short duration (60 to 90 seconds). Two compressors were running continuously to serve peaks lasting only 90 seconds. If these peaks in flow could be served from stored air, at least one compressor could be turned off. But would there be enough off-peak, surplus capacity to store from just five compressors? If so, how large would the receiver need to be and how much would pressure need to change? I collected more data; I wanted to be certain that I had recorded and could analyze the worst-case scenarios. We also needed to validate pressure requirements so we could deliver air at a pressure appropriate to needs of production. Our hope was to be able to lower pressure. Without data, all I had was another opinion, and I don’t devise engineered solutions solely on opinion. Now we had learned which production events created peaks in the flow rate that required all compressors to run, how large the peaks were, how long they lasted and how often they occurred. Because the individual events were brief, we suspected that they could be served from air stored in a tank. However, we also identified reoccurring two-hour periods when a continuous series of transport events were occurring. These periods established the highest average demand. We needed to be able to turn off a compressor during these periods. On the Supply-Side Our approach to the problem of serving large intermittent demands uses a computer application I developed called Flow Based Analysis (FBA) to compare the capacity of installed compressors (supply) with the recorded flow (demand) and to seek an equilibrium. We enter information on the supply, such as the maximum pressure capability, capacity in scfm at maximum full flow pressure (MFFP), kW at MFFP and unloaded kW, for each compressor. We also enter information on power cost. We then enter information on the demand — the actual system flow data, typically in 12-hour blocks where data was sampled at 5 second intervals. FBA will then model the behavior of the system as system parameters are manipulated. Three Areas of Flexibility First, we can manipulate the recorded value for flow. Maybe there are a few leaks or other forms of waste we can fix to suppress the demand, or we can increase the rate to reflect the addition of new production equipment. Second, we can manipulate the total receiver volume. Third, we can manipulate the pressure set points used for compressor control. The algorithms in FBA compare the capacity of compressors (supply) to the recorded flow (demand) for every entry (every 5 seconds in this case). If there is a surplus, pressure rises. If there is a deficit, pressure decays. If pressure decays to the “load” set point, the application models the starting and loading of another compressor. If pressure rises to the “unload” set point, a compressor is unloaded and stopped after a timer runs out. We can increase receiver volume and change the span between control set points. This manipulates the volume of air stored. As the volume increases, the frequency of compressors loading and unloading decreases. Perhaps, we will get to a point where one compressor (or more) shuts down and does not restart. If total receiver volume is too small, pressure will decay below an acceptable point no matter how wide the control span, and additional horsepower will be needed. Total volume cannot be too great, but the cost of the tank could be prohibitively high. The computer is actually simulating what is going to happen in the compressor room as the flow rate, receiver volume and set points are changed. The chart to the right illustrates the storage options for the system in question. It shows the minimum pressure we would be able to maintain during the worst-case event as receiver volume changes and only five compressors are allowed to run. The model reveals that even with a span of 15 psi, if less than 14,000 gallons of receiver volume is added, the worst-case series of transport events will draw down the system, causing an uncontrollable decay of pressure. CO2 O Three Ways to Meet Intermittent Demands There are three well known approaches to meeting large intermittent demands. These are: 1. Demand-side receiver(s) dedicated to the event. 2. Compressor(s) dedicated to the event. 3. A supply-side receiver. In this instance there are a number of transport systems, some are located at the farthest corners of the plant, and one in the middle. Dedicated equipment for each of them wasn’t a practical option. 12 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - February 2008 Contents From the Editor Utility-Air News February Audit of the Month: Dense-Phase Transport System Wreaks Havoc on Compressed Air System Real World Best Practices: Material Conveying with Pneumatic and Vacuum Systems Industrial Vacuum Cleaners Use Compressed Air in Portland Cement Manufacturing Processes The Numatics Air Preparation Group The Ins and Outs of Vacuum Generators Dekker Vacuum Technologies Do Your Meetings Sabotage Your Profits? Resources for Energy Engineers Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - February 2008

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