Better Than a Crystal Ball

Computer modeling lets you forecast compressed air energy savings, reducing the risk of a poor decision

By Jeff Yarnall, P.E

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The average flow during the 14-day study period is 1,127 cfm, with an average power requirement of 290 kilowatts (kW), yielding an overall efficiency rating of only 3.89 cfm/kW.

The 3-hour (hr) graph (9 a.m. to noon) using 3-sec. intervals shows the purge demand of a desiccant-type regenerative air dryer. See Fig. 2. The graph indicates purge reduction controls are non-existent or malfunctioning.

The daily average demand is less than the output of the 250-horsepower (hp) (1,269-cfm) air compressor. If an extremely large receiver (more than 1,000,000 gal.) is installed, startup of the second compressor could be prevented. A receiver this large, however, is not practical.

Figure 1. The Starting Point

A system block diagram provides a snapshot of the current compressed air system setup.

An overall efficiency rating of only 3.89 cfm/kW allows room for improvement. In theory, an ideal air system would have an efficiency rating above 6 cfm/kW for all flow rates. This ideal system would have a constant cost per cubic foot of air produced and allow additional savings when demand is reduced.

The goal of computer modeling is to identify scenarios that yield an average system efficiency greater than 5 cfm/kW. How should the system configuration and automation controls be enhanced to increase system efficiency while conserving capital?

Figure 2. Existing System Demand Profile Provides Clues

 

This 3-hr pressure, power and flow profile of the existing system provides a baseline for the evaluation of "what-if" scenarios.

By proposing alternatives (1 through 4), the model will reveal whether or not the efficiency rating can be raised above the 5 cfm/kW threshold. The chosen alternatives, a narration of the modeling steps and the results are summarized below.

Alternative #1

. Do not change anything and live with current energy costs (Fig. 1 and Fig. 2).

Alternative #2.

Convert the existing compressors to load/unload control. Add a 3,800-gal. receiver and pressure flow controller. The predicted system performance is shown in Fig. 3. With a simple cascading set of pressure setpoints based on the dry receiver pressure, a time-dependent model can be designed. See the sidebar.

Figure 3. Alternative 2: Load/Unload Configuration Reveals Some Savings

 

The compressors in Alternative #2 operate load and unload as needed to satisfy the pressure signal at the dry receiver. The pressure flow controller maintains downstream air pressure in a narrow band. System efficiency improved from 3.89 cfm/kW to 4.2 cfm/kW.

Alternative #3.

This alternative involves a different capacity-control approach. The existing 250-hp compressor is replaced by a 250-hp variable-frequency-drive (VFD) controlled air compressor. The predicted system performance is shown in Fig. 4.

Figure 4. Alternative 3: New 250-Hp VFD Replaces Fixed-Speed Operation

Under Alternative #3, the header pressure varied between +/- 5 psi from the desired pressure, a deviation acceptable to plant personnel. System efficiency rose to 5.7 cfm/kW.

Alternative #4

. In the interest of preserving capital (reducing project costs), this alternative substitutes a 200-hp VFD compressor for the 250-hp unit in Alternative 3. The predicted system performance is shown in Fig. 5.

Table 1 provides a comparison of the four alternatives and lists the average power required over the two-week study period. It also indicates the predicted system efficiency if the alternative were in place. Plant personnel believe the 14-day study period is typical and a good representation of the entire year. However, the Alternative #4 pressure profile indicates the proposed system would not be able to meet system demands and that the pressure falls below acceptable limits.

 

An Excel spreadsheet and the modeling steps previously described can be used construct a detailed model of a compressed air system. System parameters can be manipulated easily and the results computed within seconds. The demand profile also can be adjusted to reflect a leak reduction program or the addition of moisture load/purge controls for the air dryer.

Additional scenarios also can be investigated. What if management would like the ability to expand production during the next three to five years? Could the proposed design meet the higher demands with the same high efficiency?

Pressure profiles

The energy results obtained from the detailed model are similar to results obtained using other methods. However when short-duration time intervals are used, pressure profiles can be plotted to reveal whether or not a particular alternative will maintain header pressure above a minimum level.

Each scenario modeled in the summary table saved energy, but decisions should not be made based on energy savings alone. The 3-hr period beginning at 9 a.m. was chosen because its profile had large swings in demand.

Alternative #2.

As shown in Fig. 3, the compressor(s) operate load and unload as needed to satisfy the pressure signal at the dry receiver. The pressure flow controller maintains downstream air pressure in a narrow band.

Alternative #3

. As shown in Fig. 4, the VFD control maintains a narrow pressure band. The pressure does rise to unload and fall to reload the 150-hp fixed speed compressor. The header pressure varied +/- 5 psi from the desired header pressure. Plant personnel agreed that fluctuations within this range would be acceptable.

Alternative #4.

As shown in Fig. 5, the system was not able to hold stable pressure levels. The pressure drops below 80 psig for several minutes. Regardless of the energy savings, this alternative should not be considered further unless demands are reduced or a large receiver with a flow controller is added.

Figure 5. Alternative 4: New 200-Hp VFD Can't Keep Up

Alternative #4 was not able to hold stable pressure levels and could not be considered unless demands could be reduced or a large receiver with a flow controller could be added.

An energy and financial analysis is needed to rank the options further. Many financial benchmarks can be used to evaluate alternatives. For this discussion, simple payback is used. See Table 2.

Table 2 indicates that Alternative #3 has the shortest payback, but Alternative #2 also has a relatively short payback and requires less capital. Further evaluation of project life expectancy, dryer enhancement, possible salvage values, tax credits, future energy costs and future changes in the demand profile also should be considered before making a final decision.

Conclusions

Using commonly available computers and software, plant personnel can calculate compressed air energy savings with a high level of confidence. Energy savings can be predicted using various tools, but more important is the ability to predict how the header pressure responds to dynamic changes in system demands.

1Watson and Scutella, "Slashing Compressed Air System Costs," Chemical Processing, June 2002; van Ormer et al., "Choose a Winning Drive Strategy," Chemical Processing, November 2002.

Yarnall is manager of the Auditing and Consulting Group of Rogers Machinery Co., Portland, Ore. Contact him via e-mail at jeff.yarnall@rogers-machinery.com.

 

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