Most blower applications for wastewater treatment are for conventional activated sludge aeration. The water level is typically constant, and pressure variations are usually less than one psi. There are other applications, however, that undergo significant variations in water level. These processes present challenges, but they can be accommodated with proper blower system design.
Types of Processes
There are a variety of processes in wastewater treatment that routinely change in water level. These include most areas of the treatment facility: preliminary, secondary, and sludge treatment.
Many plants employ equalization (EQ) basins to attenuate the variations in wastewater flow from diurnal load changes and peak flows from storm events. The EQ basins start out nearly empty and are gradually filled as flow is bypassed from the main wastewater processes. Ideally the EQ basin is at maximum level at the end of the day, when flow is pumped from it to the plant. Aeration is employed to prevent septicity and to keep solids in suspension.
Sequencing Batch Reactors (SBR) are a secondary treatment process that employs variable level. From a minimum after decanting to maximum level before the settling phase often represents a 2:1 level change. SBRs can also pose a special challenge because aeration is stopped for settling and decant periods.
Waste sludge processes often employ variable depth aeration. Sludge holding or storage tanks are similar to EQ tanks – they serve to reduce slug loading to digestion. Aeration is used to prevent septicity and promote mixing. The sludge level changes as waste sludge is added or pumped to digesters.
Many treatment plants, particularly small facilities, use aerobic digestion to reduce sludge volume and stabilize the sludge. In batch operation, the aerobic digester volume changes as supernatant is decanted and sludge is withdrawn or added.
Changes in depth can affect the required air flow rate, since mixing and oxygen demand may be proportional to the wastewater or sludge volume. However, the most significant impact of variable depth processes on blowers is the change in system pressure that must be accommodated.
The discharge pressure at the blower always equals the system’s resistance to air flow, the system pressure. This consists of two components:
Static pressure is created by the submergence of the aeration diffusers in the wastewater or sludge:
pstatic = static pressure, psig
d = depth of submergence, feet
Friction losses are created by the movement of air through pipe, fittings, valves, and diffusers:
Δpf = pressure drop due to friction, psi
QS = air flow rate, SCFM
d = actual pipe inside diameter, inches
pm = mean system pressure, psia
T = air temperature, °R
Le = equivalent length of pipe and fittings, feet
Once pressure drop for one flow rate is determined it can be calculated for other flow rates:
kf = constant of proportionality for friction losses, psi/SCFM2
pstatic = static pressure, psig
pfriction = friction pressure, psig
pdes = total system pressure at design flow, psig
Qdes = design system air flow rate, SCFM
The sum of these two pressures is plotted against the air flow rate to create the system curve.
When the water level and submergence vary the result is a family of system curves. Generally plotting a system curve at maximum, minimum, and one intermediate level will provide a good idea of the variations in discharge pressure that the blower system must deal with. (See Figure 1.) It is important to remember that the blower does not determine the discharge pressure – rather, the system establishes the pressure the blower must overcome to produce air flow.
Figure 1. Typical System Curves.
Positive Displacement (PD) Blower Performance
There are two types of PD blowers in aeration applications, the older lobe type and the newer screw type units. Both deliver a fixed volume of air with every revolution, essentially independent of pressure. Controlling air flow rate requires controlling blower speed. With both types the blower efficiency changes with speed and discharge pressure. The variation in efficiency is more pronounced with screw types, since they have an internal pressure ratio that is optimized for one discharge pressure.
The discharge pressure for a PD blower inherently matches the system pressure. Consequently, the blower accommodates changing water levels without operator or control system intervention. This results in simple operation, and in early applications only PD blowers were used with variable level processes.
With PD blowers a rising water level causes an increase in discharge pressure, even if there is no control system. Because power is a function of both flow rate and pressure, the blower power draw will also increase. (See Figure 2.)
Figure 2. PD Power as a Function of Flow Rate and Pressure.
Centrifugal Blower Performance
Centrifugal blowers, also referred to as dynamic blowers, are variable volume variable pressure machines. There are three types commonly employed in aeration applications: geared single stage, multistage, and high speed gearless (commonly called turbo blowers). The details of mechanical and electrical configuration vary considerably between them, but the thermodynamics, analysis, and application considerations are essentially identical for all three types.
Centrifugal blowers exhibit performance curves that show the operating characteristics of flow vs. pressure and flow vs. power. Because air is a variable density fluid the performance curve shifts with inlet pressure, inlet air temperature, and relative humidity. Further changes in the curves result from changes in speed or from modulating inlet guide vanes or throttling valves.
All centrifugal blowers will experience unstable pulsating flow if the flow rate falls below a minimum value. This unstable flow regime is called “surge”, and it can result in mechanical damage to the blower.
The operating flow of a centrifugal blower under any specific set of conditions is determined by plotting the curve at those conditions with the system curve. The intersection of the two curves establishes the operating air flow rate, and from this the power can be determined. If the water level and static pressure increase, the intersection point shifts to lower flow, assuming the blower control system is not modulated. (See Figure 3.)
Figure 3. Change In Flow for Dynamic Blowers with Variable Level.
If a variable level application requires reduced flow at low water level it is possible to modulate the blower to achieve the reduced flow. If the blower is not modulated as the water level rises, operation may fall below the minimum surge flow. (See Figure 4.)
Figure 4. Fixed Inlet Valve Operation on Rising Water Level.
Control System Performance
In the past the potential for developing unstable flow in variable level processes led many engineers to avoid applying centrifugal blowers to these applications. With today’s technology and using good design practices the efficiency and flexibility of centrifugal blowers can be used in applications where the discharge pressure will vary dramatically.
Regardless of blower type, the control system should continuously measure the blower air flow rate. If the flow rate changes the control system should modulate the blower to restore the set flow rate.
There are a variety of techniques available for modulating centrifugal blowers. Inlet throttling and variable guide vanes are common on older systems. However, for any type of centrifugal, the most efficient method is variable speed operation, typically using a variable frequency drive (VFD).
The blower performance curve for a given set of inlet conditions shifts as the speed changes. The shift in the curve resulting from a speed change follows the affinity laws:
Na, Nc = actual and original curve speed
Qa, Qc = flow rate at actual and original curve speed
pi, pd = absolute pressure at inlet and discharge
Pa, Pc = power draw at actual and original curve speed
k = ratio of specific heats, ≈ 1.395
The intersection of the new curve with the appropriate system curve identifies the operating point. (See Figure 5.)
Figure 5. Response of Centrifugal Blower to Changes in Speed and Level.
The savings in power obtained by using variable speed instead of throttling centrifugal blowers are significant. Throttling creates a parasitic pressure drop, with the pressure ratio across the blower remaining essentially constant. By reducing the speed, the pressure ratio is changed, eliminating the losses. (See Figure 6.)
Figure 6. Power Variation for Variable Speed and Level.
Variable level processes cause changes in the discharge pressure demand for blowers. In the past this created challenges for some types of blowers and restricted some applications.
Modern control technology and systems have eliminated these concerns. The details of the design and the blower performance must be matched to the type of blower in each application. By using VFDs for controlling flow and pressure, both PD and centrifugal blowers can provide flexibility and high efficiency.
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