Blower efficiency is a justifiable concern during the design and selection of aeration equipment. However, efficiency may not be the most important consideration in aeration blower applications. In many cases the blower with the highest efficiency will not provide the lowest energy consumption! Blower turndown is a parameter that is generally more important than efficiency in optimizing energy use.
A blower system for a municipal water resource recovery facility (WRRF) always includes multiple blowers. Regulatory requirements dictate that the system has standby capability. The system must be able to deliver design maximum air flow with the largest unit out of service.
Turndown may reference an individual blower’s operating range or the operating capability of the entire blower system. Turndown is often expressed as a percentage:
Turndown % = ((qmax – qmin) / qmax) x 100
|Turndown||=||ability to reduce air flow rate, %|
|qmax , qmin||=||maximum and minimum blower or system flow rates|
Another common way of expressing turndown is as a ratio of maximum to minimum flow – for example, a system with 6:1 turndown is equivalent to 83% turndown.
Importance of Turndown
Aeration blowers are a major concern in the typical WRRF for both process performance and for energy consumption.
In the activated sludge process, pollutants are removed by a biological process. Microorganisms use oxygen diffused into the aeration basin to metabolize organic compounds and to convert ammonia to nitrate. If the blowers can’t deliver the required air to the aeration basin the treatment process will fail.
Since aeration consumes 50% to 70% of the energy used in most WRRFs, optimizing the blowers is key to minimizing energy cost. Providing more air than necessary to the process doesn’t improve pollutant removal, but it does greatly increase operating cost. Being able to modulate the air flow rate to match but not exceed the process demand is the key to minimizing energy consumption. That means that the blower system must have sufficient turndown to match the minimum process needs.
Basis of Turndown Requirements
The hydraulic load (wastewater flow rate) and the organic load (mass of pollutants) entering a typical WRRF are constantly varying.
The most obvious variation is the daily (diurnal) fluctuation in hydraulic load. (See Figure 1.) The concentration of pollutants also varies during the day, but for most plants this isn’t as significant. Design calculations and permits usually reference a plant’s average daily flow (ADF), and this value is usually identified as the plant’s capacity. However, the flow rate is seldom exactly equal to the ADF. The ratio of daily peak flow to minimum rate is approximately 2:1.
Figure 1: Typical Diurnal Loading Pattern
The timing of diurnal flow variations generally match on-peak and off-peak electric energy rates, with peak flow coinciding with on-peak energy rates. When calculating energy costs of various alternatives some simplifying assumptions may be made:
- Average air required during on-peak rates = 115% of air required at ADF
- Average air required during off-peak rates = 85% of air required at ADF
- Highest air required, determining demand charge = 120% of air required at ADF
Municipal WRRFs are designed for a twenty-year life, and most designs include a generous allowance for increased loading due to population growth. This means that for most of the facility’s life it is operating at hydraulic loads less than the design ADF. The EPA has estimated that most WRRFs operate at 1/3 of the design ADF.
Good design practice requires that the blower system be capable of meeting the projected worst case load. This must include both maximum anticipated hydraulic loading and maximum anticipated organic loading. The assumed concentration of pollutants at the end of the twenty-year design life is usually higher than the actual concentration experienced. This further increases the difference between maximum and minimum required blower system capacity.
In some WRRFs short-term loading may dictate the maximum design air flow rate. Short-term loads include rain events. These increase hydraulic loading and flush accumulated solids from sewers, increasing organic loading. Industrial facilities are notorious for releasing slug loads with a high concentration of pollutants into the sewer system. Internal WRRF side streams from sludge treatment may also create a short-term increase in organic load.
Factors other than loading may affect the demand for air. For example, a minimum air flow may be required to maintain proper mixing of the operating aeration basins.
The combined effect of these variables is the need for a blower system with a wide operating range. The system turndown should be at least 6:1 (83%), but to optimize both energy and process performance an 8:1 turndown (88%) is preferred.
Alternate Ways to Achieve Turndown
Most individual blowers provide approximately 50% turndown (2:1). The actual turndown varies with the blower technology, control method used to modulate air flow, and the available sizes from a given manufacturer. For many applications ambient conditions place additional restrictions on the turndown for each blower. To achieve more than 50% system turndown, it is necessary to use multiple small blowers and vary the number operating as well as the flow rate for each bower.
There are many design approaches used to establish blower configurations. Using two blowers, each sized to meet 100% of the maximum air flow demand at the twenty-year design load, is not uncommon. This severely limits system flexibility and provides no opportunity for energy optimization.
With increasing energy costs and operator demands for process flexibility, most new blower systems are designed with multiple operating blowers. In small WRRFs the blowers may not include any capability for modulating air flow. The result is unnecessarily high energy cost and limited process flexibility. These systems have the advantage of low cost and simplicity, but it is more common to provide air flow rate control for each blower. Depending on the blower technology, control may be accomplished by throttling, adjustable guide vanes and diffuser vanes, or variable speed control.
A common system approach is to install three equal sized blowers, each capable of delivering 50% of the maximum air flow at design conditions. This will generally provide 75% turndown (4:1). Another common arrangement is four blowers, each sized to provide 33% of maximum air demand. This yields 83% turndown (6:1), which is adequate for some applications.
One way to achieve the preferred 8:1 turndown is to install four blowers, two capable of providing 50% of maximum flow and two sized to provide 25% of maximum flow. This system meets both the requirement for redundancy and provides high turndown.
Some designers resist providing more than two or three blowers because of potentially higher equipment and installation cost. This can be the case, but often the higher cost of multiple blowers is offset by the lower cost of each smaller unit. More importantly, this system can optimize energy use. Over the course of the life of the blower system, the initial equipment cost is much lower than the cumulative cost of twenty years of energy consumption.
Comparison of Energy Demand
The importance of turndown to energy cost can be illustrated by an example analysis of alternate systems. Note that every system is different, and the results of the example are typical but not universal. Blower size, electric power rates, control methods, and load variability all influence the comparison.
This example uses variable speed multistage centrifugal blowers. The blower size was based on a typical mid-size WRRF, and energy consumption was taken from the manufacturer’s performance curves. For comparison, a single constant speed blower, with no turndown, was also evaluated. Assumed aeration system requirements are:
- Max design air flow @ 20 years = 6,000 SCFM
- Discharge pressure at 100% design air flow = 9.0 psig
- Evaluation barometric and inlet pressure = 14.7 psia
- Evaluation inlet temperature = 68 °F
- Diffuser submergence = 17’-5” = 7.54 psig static pressure
- Typical diurnal flow variation
- Current max air demand = 1/3 of maximum design air demand
- $0.18/kWh on-peak 60 hours per week, air flow = 115% of air required at ADF
- $0.06/kWh off-peak 108 hours per week, air flow = 85% of air required at ADF
- $20.00/kW monthly demand charge, air flow= 120% of air required at ADF
An additional analysis was performed, assuming an organic load concentration at 80% of the design value. This illustrates the benefit of 8:1 turndown in a typical application. The results of the analysis show that a further 8% reduction in energy consumption can be obtained by increasing the turndown from 6:1 to 8:1. (See Table 1.)
The results of the analysis clearly show the impact of a blower system that has adequate turndown to match process loads. The comparison shows the total annual power cost for each alternative. (See Figure 2.)
Figure 2: Example Annual Power Cost
The cost of the first five years of operation, including equipment cost, was also calculated. (See Figure 3.) A five-year total was used because after that time operating loads are assumed to increase. This reduces the need for high system turndown during the remaining life of the facility.
Figure 3: Example Cumulative Five-Year Cost
The analysis used the blower system efficiency, which included the blower, the motor, and the variable frequency drive (VFD) efficiencies. This is often referred to as “wire-to-air” efficiency. System efficiency is not a constant value, but varies throughout the operating range. (See Figure 4.) However, even though the four-blower system includes a blower with lower peak efficiency than the baseline, the total energy cost is lower because of the lower minimum flow achieved. Too much air, even at high efficiency, wastes power!
Figure 4: Example Blower Efficiencies
Blowers for wastewater aeration are part of a complex treatment system. The process demand for air is constantly changing. Optimizing energy cost requires modulating the blower system air flow rate to meet the system requirements without delivering excess air to the aeration basins. Accomplishing this requires blower systems selected to maximize turndown, matching the air supply to the full range of process needs.
For more information contact Tom Jenkins, President, JenTech Inc. at email: firstname.lastname@example.org or visit www.jentechinc.com. Mr. Jenkins has texts now available in hardcopy and electronic versions titled Aeration Control and Facility Design.
To read more Aeration Blower Technology articles, visit www.blowervacuumbestpractices.com/technology/aeration-blowers.