Turndown designates the operating range of an aeration blower or a blower system – and it can often be the most important factor in determining the ability of a system to match process demand. It is also critical to the system’s energy optimization. Unfortunately, in designing blower systems and controls turndown is not always given the attention that its importance merits. Here’s a look at the critical nature of turndown in wastewater treatment plants and recommendations for ensuring adequate turndown when utilizing Positive Displacement (PD) and centrifugal blowers.
Importance of Turndown
Turndown is usually expressed as a percentage. Greater turndown means more flexibility for operators in matching blower flow rate to process demand.
In municipal wastewater treatment plants, blower system capacity usually exceeds process demand. System dDesigners must provide capacity for loading growth over a projected twenty-year life. The blower system designers must meet the process needs at projected worst-case oxygen demand plus a reasonable factor of safety. It’s common for a treatment plant to operate at one third of the designed average daily flow.
Techniques for extending turndown for centrifugal blowers include monitoring air, bearing, and lubrication system temperatures.
Normal diurnal flow variations increase needed turndown. In most municipal treatment plants peak daytime loading is twice minimum nighttime loading.
Turndown is more important than efficiency in reducing power costs. Regardless of the blower system efficiency, excess air flow wastes power. A system with limited turndown simply can’t take advantage of high efficiency blowers.
Operators need adequate turndown to maintain process performance. Nutrient removal processes for nitrogen and phosphorus require anoxic and/or anaerobic zones. Many of these processes recycle mixed liquor from the effluent end of aeration basins to anoxic or anaerobic zones at the influent end. Excess dissolved oxygen in the recycle will impede developing anoxic and anaerobic conditions. The result is process failure.
System Turndown Recommendations
It is important to note that blower system turndown is the important factor. Individual blowers typically have a turndown of 40% to 60 percent% - roughly a 2:1 ratio of maximum to minimum flow. This assumes blower air flow is modulated by using variable speed, throttling, or guide vanes. Varying the number of blowers while also modulating flow rate can provide high system turndown. If the turndown on individual blowers is 50% or more, stepless changes in air rate can be achieved when adding blowers.
The optimum design should include a minimum of 80% system turndown (5:1 max to min ratio). For most water resource recovery facilities (WRRF) this is the turndown needed to accommodate the combined effects of the difference between design and actual loading and diurnal variations. A system turndown of 88% (8:1 max to min ratio) is preferred. This provides additional flexibility for unusual demand fluctuations.
Most regulatory agencies require redundant blowers so that process performance can be maintained if there is a blower failure. Worst- case design air flow must be available with the largest blower out of service.
It’s common to use two large blowers, each sized for 100% design capacity. Multiple small blowers provide better turndown. The capital savings achieved using a few large blowers is easily offset by the wasted energy from the inability to meet low process demand.
Four blowers sized to provide 33% of design flow each will generally provide 80% turndown. Two blowers providing 25% of design capacity combined with two blowers providing 50% capacity will provide 88% turndown, meeting the preferred goal.
Positive Displacement (PD) Blowers
The turndown for PD blowers is generally limited by heat buildup. This may be either in the motor or in the blower itself, depending on the operating characteristics of the system.
If discharge pressure is constant, PD blowers create a constant torque load on the motor. Since torque is proportional to current, motor amperage is also constant. The heat generated by the motor resistance, I2R heating, isn’t reduced at lower speed, but the cooling air flow is. That means the motor temperature will rise.
Motors operated at high temperature experience insulation failure. NEMA insulation class refers to the allowable temperature rise above 40 °C (104 °F) ambient before insulation breaks down. For most ODP and TEFC motors, excess temperature occurs at rated torque and 50% of nominal speed.
Manufacturers rate motor insulation by temperature rise above allowable, but it is more convenient for operators and engineers to have actual temperature limit values. For the most common insulation classes these are:
- Class B: 80 °C rise (max = 120 °C/248 °F)
- Class F: 105 °C rise (max = 145 °C/293 °F)
- Class H: 125 °C rise (max = 165 °C/329 °F)
Most motors are rated for service at 3,300 feet ASL altitude. At higher ambient temperatures, or higher elevations, the motor must be de-rated.
The blower itself also experiences an increase in operating temperature at lower speeds. For any type of blower the temperature of the discharge air can be calculated from known parameters:
Internal air leakage from blower discharge back to the inlet, referred to as “slip”, is constant at a given pressure ratio. Mechanical losses from friction don’t change significantly with speed. The result is a decrease in blower efficiency at lower speed, creating higher discharge temperature. At lower speed there is also less air moving through the blower, so less heat is removed as illustrated in Figure 1. The heating is compounded by high inlet temperature.
Figure 1: Shown is an e Example of tTemperature iIncrease for PD bBlowers.
Two factors establish the temperature limit for PD blowers. The first is the service temperature limit for blower lubricants and seals. Operation above 250 °F will usually cause failure. The limit should be verified with the manufacturer.
The second limit is based on the temperature differential between the inlet and discharge sides of the blower. Excess temperature differential can cause warping of the blower end plates. The result is metal- to- metal contact between the rotors and end plates, causing catastrophic damage.
The variation in efficiency with speed differs between lobe- and screw- type PD blowers. Lobe- type PD blowers generally have maximum efficiency at maximum speed. Screw- type unitsPDs generally exhibit maximum efficiency near mid-range. It is common, therefore, for screw- type PDs to have better turndown than lobe- type PDs.
Advantages of Sophisticated Monitoring
Techniques for improving turndown for any blower entail replacing approximate or generalized limits with systems that allow for monitoring the conditions that limit turndown. For example, it is common for blowers to operate at discharge pressures below the design point. This reduces torque, current, and I2R heating. Furthermore, the heat dissipation capacity of motors is better at normal operating temperatures than at 40 °C. By monitoring actual motor temperature the speed can be reduced until the temperature approaches unwanted values.
Similar considerations apply to monitoring blower discharge temperature and temperature differential. Because pressure ratios and inlet air temperatures are often lower than design values the generalized 50% speed limit doesn’t accurately correlate with potential damage.
Energy savings from increased turndown can easily offset the cost of more sophisticated monitoring, particularly on large blowers. The increased protection of sophisticated systems may also prevent catastrophic failure from unusual operating condition.
It is important to consider the manufacturers’ limitations and recommendations in designing blower protection and turndown systems.
The turndown limits for centrifugal (dynamic) blowers are more complex than those for PD blowers. Turndown may be based on blower temperature, but more frequently, the minimum flow rate is determined by surge.
Surge is a pulsating air flow that occurs at high discharge pressures and low flow rates. Prolonged operation in a surge conditions usually results in catastrophic failure. This can be the result of vibration in bearings, or high blower temperature. High impeller speeds generally increase sensitivity to surge.
Blower performance curves identify a surge “point.”. Operation at flow rates lower than that point may cause surge. Variable speed blower curves usually show the surge limit line, which represents the surge point as a function of speed. The surge control line may also be shown as illustrated in. (See Figure 2. ) This represents the recommended minimum safe flow. Operating at flow rates lower than the surge control line triggers the surge control mechanism.
Figure 2: An eExample of sSurge lLimit and sSurge cControl lLines for dDynamic Bblowers.
The designer should be aware that the occurrence of surge is variable with speed, inlet air conditions (especially temperature), control method, and discharge piping configuration. There should always be a margin of safety between the surge limit line and the surge control line.
Surge Control Measures
Surge control, or protection, can be implemented in many ways, including:
- Shut down the blower.
- Open a blow-off valve to reduce pressure and increase air flow.
- Modulate the blower to increase air flow.
- Use speed, guide vanes or throttling valves may be used to modulate the blower.
One disadvantage of using blow-off valves for surge control is the reduction in discharge pressure, which may eliminate air flow to the process. A modulating blow-off valve can be used to maintain enough discharge pressure to provide air to the process while maintaining flow at a safe value.
Surge control can be optimized in several ways. Determining flow rate is more accurate than using motor amps for monitoring. Instead of using a fixed, conservative surge point, the surge line can be corrected for actual inlet conditions and operating speed. Advanced control algorithms can detect the pressure or amperage fluctuations that accompany surge, providing reliable protection without an excessive margin of safety.
Motor heating isn’t usually a problem with dynamic blowers, since the torque and current decrease with reduced blower speed and flow. The minimum speed, valve, or guide vane position is usually dictated by the reduction in discharge pressure capability and the corresponding change in the surge point.
Centrifugal blowers exhibit temperature increases at reduced flow due to reduced efficiency and reduced heat removal. Heat can damage bearings and seals and compromise lubrication. A loss of clearance between impellers and casings may result from differential thermal expansion, causing catastrophic failure.
Improving Turndown with Centrifugal Blowers
Techniques for extending turndown for centrifugal blowers include monitoring air, bearing, and lubrication system temperatures. This allows operation at reduced flow while using high temperature alarms to prevent failure.
Note that increasing maximum flow can also improve turndown and operator flexibility for both types of blowers. Increasing maximum flow rate can reduce the need to start multiple blowers to meet process demand. Monitoring motor power can identify impending overload before it occurs. System pressure and ambient temperature rarely match worst- case design values. Throttled or guide vane controlled blowers may produce higher flow rates than the design point. Variable speed blowers may be operated slightly faster than 60 hertzHz provided power draw stays within the drive’s allowable load and vibration is avoided.
The operating range of aeration blowers is limited by many factors. Having adequate turndown is more important than efficiency in minimizing power consumption.
The factors that limit turndown for PD and centrifugal blowers share similarities and also differ in many ways. Temperature is always a concern. Centrifugal blowers have additional limitations because of the potential for surge.
Regardless of blower type, control systems that accurately monitor key operating parameters can optimize turndown. This provides operator flexibility and minimum energy consumption while protecting the blower system from failure.
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