Plant Services - April 2008 - (Page 50)

more compressor plus accessory equipment heavily part loaded 95% of thetime. EfficiEncy Compressors Storage = power set pts 110-100 psig trim/ back #2 up base load #1 sep local signal false loader P3 = 115-100p control storage 26.7 scf/psi or >2852 gallons control master signal ﬁlter refrig dryer P3T P4T ﬂow meter aftercooler w/fan control upper range mod set pts 115-105 paig demand demand expander P4 = 85p+0-1p Figure 4. When properly configured, storage controls cycling, operating efficiency and reliability. If a 1,600-scfm base unit fails, the back-up gets a load signal @ 100 psig and with a typical cold start permissive @ 15 seconds. The required control storage = 1,600 scfm x (15/60) x atm/useful dp (14.3/15) x 7.48 gal/scf. When the supply system is properly conﬁgured, control storage is used to control unit cycling, operating eﬃciency, and back up reliability. Assuming that the base unit @ 1600 scfm fails. With The back up getting a load signal @ 100 psig and a typical cold the devil’s in distribution start you know 15seconds, the following is the formula to Once permissive @ system demand, evaluate the distribution determine the required the system has pinch points X system. Determine if control storage: 1600 scfm X (15/60)or high ful atm/use dp (14.3/15) X 7.48 gal/scf = control storage. ing expended. Adiabatic expansion trades energy for lower pressures. It’s a common bad practice for point-of-use applications to exhibit a 15 psi to 45 psi drop between the inlet to a filter/regulator and the inlet pressure to the user. At the same time, more compressors come online or new compressors are being installed to increase the supply pressure by 5 psi. It’s a matter of correcting the point of use for $100 or adding tens of thousands of dollars of supply energy. To find a potential solution without reviewing the constituents of demand will catch only about one-third of the potential reduction in operating cost (Table 1). The constituents of demand that contribute to the system’s total demand include: • Appropriate usage • Improving open blowing, transient events and users that can tolerate reduced operating pressure • Artificial demand (unregulated users, leaks and excessive pressure) • Waste (drainage, leaks, dryer purge and centrifugal blow-off) pressure differentials. When demand waste is eliminated and transient points-of-use are flatlined, most piping changes can be corrected with no additional actions because differential pressure varies as the square of the flow change. Reduce real costs Clarify to the candidate audit teams that the deliverables you expect might not be identical to the shopping list of things that they sell. Therefore, a 5% drop in flow reduces the differential by 25%. In addition, many systems have been designed as cutand-paste configurations, with piping sized with generous margins above the highest flow, often to rely on piping to serve as storage. This introduces considerable droop in the system, with a much higher pressure drop at the point of use than at the supply. The droop ratio is an important variable for determining the control profile. Expanding compressed air completely at the point of use with a minimum number of system leaks produces the best efficiency. If the system is to provide useful storage, the pressure at the expansion point must be less than the lowest supply pressure at the clean-up equipment discharge. This requires discipline to set regulators at the lowest required pressure instead of jacking them up to fix undefined pointof-use problems. In the absence of such discipline, you could install central or sector expanders (Figure 4). Use isentropic, not adiabatic, expansion. The former expands gas to a lower density with little, if any, energy be50 Knowing accurate usage and tolerating a moderate amount of waste, you can find the supply that satisfies demand. Each shift might have a different demand. Most power is logged as averages. Match actual power, including accessory power, with actual conditions. Demand charges can be 50% of the energy bill. Watch for seasonal or time-based charges for hours, weekdays and weekends. There can be from two to 24 energy bill categories that are needed to accurately determine actual and potential energy costs. Total annual operating cost includes cooling tower pumps and fans, and dryer electrical costs. Look at water cost (if any) including make up, sewer, cooling fans (if any) and treatment chemicals. Don’t forget maintenance costs, including inspection, in-house and contract labor, parts, rental, if any, including fuel and maintenance. You also The numbers Constituents Production Diaphragm pumps (6 ea) Leaks Dryer purge Drainage Artificial demand Total demand System pressure Total Power Current (scfm) 359 430 745 208 186 96 2,019 85 psig 459 Kw Proposed (scfm) 359 163 200 0 0 0 717 89 psig 158 Kw Table 1. Typical values for the constituents of demand from an audit. Without this demand assessment, we would be guessing about leak rates. April 2008 www.PLANTSERVICES.com http://www.PLANTSERVICES.com

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Plant Services - April 2008

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