An important part of developing the design for any project is determining its cost effectiveness. In most cases there are several design options considered. Deciding which option is best includes evaluating the life cycle cost of each.
Aeration blower upgrades may be part of a total plant upgrade and minimizing energy consumption is a critical consideration. Blower replacements are also a common Energy Conservation Measure (ECM) in cost-reduction programs.
Regardless of the reasons for blower and aeration system replacement, the designer faces challenges in determining the cost effectiveness of the design. Process demands and energy costs are complex and variable. Moreover, several methods for calculating cost effectiveness are available.
Examining Actual Blower Power Cost
Aeration blowers are almost always driven by electric motors and determining the cost of electric energy is the first challenge. This seems like a simple task, but the outward simplicity may be deceptive.
Many analyses use the average cost of electricity. This is also known as the composite rate. It is simple to determine. The designer simply divides the total monthly electric bill by the total energy used. The result is a composite of the many factors that go into actual costs for electricity as shown in Table 1.
Table 1. Sample Electric Charges
The average annual expense can be calculated as follows:
Total$/Year=kW ave ∙ 8760 hours ⁄ year ∙Average $ ⁄ kWh
The national average composite rate for industrial users is $0.07/kWh.
Large users, such as a water resource recovery facility, typically have a complex billing structure based on the utility’s cost to generate and distribute power. The variables include time of day, day of the week, and rate of usage. Power used during the day on weekdays is more expensive than power used at night or on weekends. In addition to usage charges, most utilities also impose demand charges based on the peak power draw during a month or year using a fifteen-minute running average. Using actual time-based billing rates will provide a more accurate power cost than the composite rate. The calculation is as follows:
Factor in Blower Duty Cycle
Establishing rates for electricity only provides half the data needed. In order to evaluate blower operating costs, the load profile, or duty cycle, needs to be established. Ideally the duty cycle of the blowers matches the process demand of the aeration basins.
Actual process loads rarely match design assumptions. Actual or near-term average daily flow is typically lower than the projected design load. Seasonal variations are significant for some facilities, and side streams can create additional loading.
The dominant factor in process loading and blower duty cycle is the diurnal fluctuation. (See Figure 1.) The peak load is typically twice the minimum. The loads corresponding to utility billing times can be derived from the diurnal flow pattern.
Figure 1. Typical Diurnal Loading Pattern
Since airflow and blower kW are essentially proportional to hydraulic loading the duty cycle can be established as a function of average kW.
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