Plant Services - April 2008 - (Page 49)

EfficiEncy Compressors Go beyond plugging holes tion #1 50 scfm waste 400 scfm @ 70 psig 50 scfm waste 50 scfm waste 50 scfm waste 200 scfm Illus tration #2 70 psig 150 scfm of demand reduction 325 scfm @ 100 psig productivity 65 scfm waste 260 scfm Smoother is better 200 scfm neg roc event unload 400 scfm 100 scfm 200 scfm time unload unload p s i g event stops load unload event stops 100 psig productivity 50 scfm waste 200 scfm demand control 250 scfm @ 100 psig 150 scfm of demand reduction Figure 2.this 200 a 200negative rate of change event (green) begins, When When scfm scfm negative rate of change event (green) begins, the pressure drops. At a lower pressure, you the pressure drops. At a lower pressure, you can see what happens can add either 200 (orange) or 400 (blue) scfm units. The when you add either 100 scfm (magenta), which units. The preferred approach is200 (orange) or 400 (blue) scfm controls preferred The longer it be rate recover, the less energy the change.approach wouldtakes of change, pressure, and time the to which is 100 scfm system requires. (magenta) which controls the ROC. The longer rgy it takes to recover, the less ene the system requires. 70 psig productivity Less is more 5 scfm 297.5 scf Figure 1. Fixing leaks can provide a nonlinear result if you don’t control demand. n #2 operation, set points or controls that determine how well the equipment operates and why. Figure 3. Storing 297.5 scf and adding 5 scfm for 30 seconds, A compressor train has each unit configured with its own satisfies the event with 5 scfm instead of requiring a 75-hp filtration and drying equipment piped into a manifold in Illustration #4 compressor that’s part loaded 95% of the time. parallel with others. Compressor trains must be evaluated By storing 297.5 scf + 5 scfn for 30 seconds, carefully because it’s difficult to get the equipment to work we are able be converted into with flow (scfm or together. Trains offer particularly great savings opportuniinformation must to satisfy the eventmass 5 scfm continuously instead of requiring a 75 hp or ties because, in most cases, the compressors are only partly Headline lb/min), psig and input kilowatts under site conditions, loaded, even when pressure settings can’t be satisfied. whichmore compressor a minimum barometric pressure, should include at plus accessory equipment heavily part loaded 95% of thetime. Using flow meters to determine usage can yield inaccutemperature, relative humidity and cooling water temrate information and, by themselves, don’t show what the perature differential. compressors are doing. Air systems change constantly. In a The difference between FAD and mass flow can be as great positive rate of change, supply exceeds demand. In a negative as 18%. You’ll need to evaluate and correct performance rate of change, demand exceeds supply. In a neutral rate of based on extreme conditions to determine if the system will change, supply and demand are stable. Evaluations based on work at other ambient conditions. A great ROI project that a positive rate of change indicate the way that the compresdoesn’t provide production support on hot or humid days unload unload unload 200 scfm sors are neg roc to demand. The larger the compressors, the reacting will come back to bite you. Don’t use pressure readings on 400 scfm event stops p event s higher the flow that will be measured (Figure 2). the compressors or dryers for evaluation purposes because i The only true indication of demand is when the system this information has proved to be as much as 7% off. load g set pts 110-100 psig 100 scfm unload false is in a negative rate of change, when the pressure is falling. Many auditors neglect inlet filtration, control systems, 200 scfm event stops P3 = 115-100p loader time Measure pressure and flow simultaneously to determine thetrim/ cooling water or air-cooling systems, filtration equipment, sep control storage rate of change. Remember, only a portion of the data is use-backdryers, and drainage as cost-reduction opportunities. You 26.7 scf/psi #2 local or >2852 gallons ful for determining demand. of changewhy evaluating signals, up must evaluate potential energy throughout the system and signal When this 200 scfm negative rate This is event (green) begins, master control signal Illustration see the pressure and set points is so important, what when differentials drops. At a lower pressure, you can#5 even happensyou’re how it can affect operation of the system, including: ﬁlter when you using flowadd eitherto audit. or 400 (blue) scfm units. The meters 200 (orange) • Point of use preferred approach would be rate change, pressure, and time of refrig base P3T P4T ﬂow dryer • Dedicated storage with checked or metered recovery which is 100 scfm (magenta) which controls the ROC. The longer meter load #1 aftercooler w/fan rgy it takes to recover, the less ene the system requires. • General overhead storage FAD versus mass flow control • Peak-shaving storage Measure the power input to the compressors. Power P4 = upper range mod demand • Control storage never lies. If you can make pressure at full load and you expander 85p+0-1p demand set pts 115-105 paig The scf/psi, or storage capacitance, influences energy rearen’t pulling full-load power ratings, you’re not making quirements, the system’s rate of change, what the supply sees flow. The industry rates compressor performance based as demand, system reliability, control integrity and the workon free air delivery (FAD), psig and brake horsepower at ability of transient point-of-use applications (Figure 3). different conditions for different compressor types. This April 2008 www.PLANTSERVICES.com 49 300 scf for 30 sec 1 X hr When the supply system is properly conﬁgured, control storage is http://www.PLANTSERVICES.com

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Plant Services - April 2008

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