By Stephen Horne
In recent years there have been many changes in wastewater treatment. In low pressure air systems, locally fabricated blower packages are being replaced by factory designed and built packages. Constant flow systems are being replaced by variable flow systems and manual process measurements are being replaced by automated process control systems. The industry has also seen new blower technologies such as gearless, high speed turbines and rotary-screws. All of these advancements can help to improve low pressure air system efficiency.
It is common for new system designs to include variable frequency drives. While variable frequency drives offer great flexibility, they have a high first cost and have their own efficiency drawbacks, including drop off in motor efficiencies at partial loading / speeds.
VFDs are perfectly suited for applications where the air flow is variable. However, in applications where there is a broad range of airflows, the low pressure air system can be controlled more efficiently by cycling fixed speed machines to cover base loads and using VFD machines to provide the trim load.
The most efficient low pressure air systems are a combination of fixed speed units and VFD controlled units that work in conjunction with the wastewater treatment process controls. The merger of these two control methods is known as adaptive system control. The wastewater treatment process controller communicates the air demand to the adaptive control which provides the air needed. Adaptive control is an attractive alternative because it can offer reduced investment cost, lower cost of redundancy, improved system reliability, and optimized system performance.
A typical “conventional” system will include three or more identically sized units with identical variable frequency drives. In this arrangement, several units are running in parallel with another unit for redundancy. The parallel units are often designed to share the demand and are tuned to the same speed resulting in an accumulation of inefficiencies.
In comparison, an adaptive control system is set up with three or more units. Only one or two units will include variable frequency drives and the balance of the air blowers are fixed speed machines with reduced current starting and auto-dual control. This can reduce investment cost by limiting the number of variable frequency drives needed.
The performance advantages depend on the size of units selected and effectiveness of the control scheme. The goal when sizing units is to use the minimum number of fixed speed machines to provide the base load while staying within the turndown range of the VFD unit to trim the transient system variations. The fixed speed units, set to operate at their best efficiency point, provide the best wire-to-air efficiency when considering all of the losses in power transmission. The VFD unit can be set to follow the variations in demand to optimize system performance, while still matching system demand.
The efficiency gains can be seen by examining each unit’s specific power and the combination of the units in service over the entire operating range of the system. Specific power is a ratio of total input power (KW at the input to the unit’s control system) over the produced flow rate (CFM). In low pressure air systems this is usually given as kW/100 CFM.
It is very important to consider the input kW to the control panel including all losses, not just the efficiency of the airend or the estimated power at the motor at a given design point. On VFD driven machines, input kW will vary over the entire operating range of the machine. At constant pressure, the efficiencies will drop at the lower end of the operating range; not only because of the lower airend efficiencies, but because of the losses in the motor and drives. An adaptive system will minimize running the VFDs in their low efficiency ranges.
As an example, consider a 2,000 CFM system at 7.0 PSIG (assuming standard conditions). A typical specification may require four liked sized machines with one of these being a full time backup. For the conventional setup, the units are to be sized for at least 666 CFM at 7.0 PSIG which results in a 30 hp blower. The combined average specific power of three units in operation will be 4.26 kW/100 CFM over the entire operating range of the system.
The adaptive control concept would use one 30 hp VFD driven blower package, three 20 hp mid-load fixed speed machines, and one 40 hp base-load fixed speed machine. Redundancy and backup is built into the system. The average specific performance of this system over the entire operating range is 3.68 kW/100 CFM.
Figure 1 shows a comparison of the specific power of the two systems assuming constant pressure and variable flow. At full flow, the VFD solution is essentially the same as the adaptive control. However, at lower flows the adaptive control system shows greater efficiency. If the entire performance envelope is averaged, the adaptive control system is 13% better than the VFD-only solution. Over a 10-year period, this equates to $95,000 in savings (based on 8.3 cents/kWh).
While this is an example of a small system that only considers average wire-to-air specific power for two control strategies, the potential for large savings is clear. Furthermore, many plants are sized with provision for community growth. The specified low pressure air system capacity may be much more than what is actually required. With this consideration, the plant may not see full flow conditions for many years to come, which will only further increase the operational savings provided by a high efficiency low pressure air system.
Because only one blower uses a VFD and the other blowers use dependable, reduced current starters the overall system reliability is improved. Auto-dual control (load/unload as well as on/off) for each constant speed blower provides for very close control of the flow and minimizes the number of starts per hour on the blower drive motors.
Modern wastewater treatment technology controls three cycles: DO, NH4, and NO3 in each basin individually using flow regulation valves. The process control PLC computes a variable target pressure for the blower station (i.e.; flow pressure control). The low pressure air system adaptive control receives the target pressure information and controls the individual blowers to provide maximum efficiency over a wide turn-down range (because turning blowers off is more efficient than reducing the frequency of multiple blower motor power supplies). Combined with lower initial cost, lower cost for redundancy, and greater reliability this control scheme is very appropriate for wastewater treatment plants with sophisticated automation.
About the Author: Stephen Horne is U.S. Product Manager for Kaeser Compressors’ Omega Blower line. He has over 10 years’ experience with the design and function of blower systems in wastewater aeration applications. Horne serves as Kaeser’s in-house engineer for machine modifications and system design, and he is a primary blower product and application instructor in the company’s factory certified training program. He may be contacted at [email protected].