Blower manufacturers are the source for the most accurate information on aeration blower power consumption. This includes the impact of various control technologies on the many types of blowers used for aeration. However, system designers often need to analyze several alternatives, making reliance on input from suppliers inconvenient. An understanding of the principles of operation will also enhance the designer’s ability to assess the data received from various sources.
Blowers are volumetric flow devices. ACFM (Actual Cubic Feet per Minute) at any conditions or, FAD (Free Air Delivery), at ambient conditions are terms commonly used to measure volumetric flow rates. Blower manufacturers prefer to work in ICFM (Inlet Cubic Feet per Minute), the volumetric flow at the blower inlet connection. It is important to always define the temperature and pressure of the volumetric flow rate under consideration to avoid errors.
The aeration process demand is a mass flow rate, often expressed as SCFM (Standard Cubic Feet per Minute) at 68° F, 14.7 psia, and 36% relative humidity. It is possible to convert between standard conditions and actual conditions:
ACFM = Actual volumetric flow rate, ft3/min
SCFM = Flow rate at standard conditions, ft3/min
Ta = Actual air temperature, °R
pa = Actual air pressure, psia
This formula ignores relative humidity. For most process calculations, the resulting error is negligible. If relative humidity is significant, for example in evaluating power guarantee, the calculation becomes:
pb = barometric pressure, psia
RH = relative humidity, decimal
psat = saturation vapor pressure, psia (function of temperature)
Blower discharge pressure also has an impact on blower power. For most aeration processes, 80% to 90% of the discharge pressure is static pressure resulting from diffuser submergence. The second factor in system pressure, is the friction resulting from air moving through piping and diffusers. The total pressure can be calculated and plotted against the flow rate, creating the system curve:
ptotal = Total discharge pressure, psig
d = Depth of water at top of diffuser, ft
kf = Constant of proportionality for friction, psi/SCFM2
Q = Air flow rate, SCFM
If an operating point of flow and pressure is known, either from calculation or measurement, the constant kf can be calculated:
Pactual = Discharge pressure, psig
Qactual = Air flow rate, SCFM
The system curve is necessary to evaluate blower performance. The intersection of the system curve with the blower performance curve identifies the operating flow rate, pressure, and power requirement. Regardless of the evaluation method used, it is important to identify the conditions of the air at the blower system inlet and discharge. The necessary parameters include:
- barometric pressure
- inlet pressure (barometric minus losses)
- inlet temperature
- relative humidity
- flow rate (SCFM preferred)
- discharge pressure
For any blower, the power can be calculated using:
Pwa = wire-to-air power, kW
Qs = flow rate, SCFM
Ti = inlet air temperature, °R
ηwa = wire-to-air efficiency, decimal (includes blower, motor, and VFD)
X = adiabatic factor, dimensionless
pd, pi = discharge and inlet pressure, psia
k = ratio of heat capacity = Cp/Cv, dimensionless
In many evaluations, the cost of electricity is taken as a composite (average) rate. This is obtained by dividing the total cost for a month by the total power used in a month. The annual cost determination is straightforward:
The composite rate may not accurately reflect actual operating cost, typically composed of several components:
- On-Peak rate, typically 12 hours per weekday
- Off-Peak rate 12 hours per weekday and 24 hours for weekends (lower than on-peak)
- Peak demand charge for highest 15-minute average per month
Each of these usually represents 1/3 of total power cost. The air-flow rates and power cost for each component can be estimated if the average daily air flow, Qave, is known:
The total annual electricity cost is the sum of On-Peak, Off-Peak, and Demand costs.
Blower performance is often referenced to efficiency, and design-point efficiency is sometimes used to compare blowers. However, the end user pays for energy and power, not efficiency. The total power across the expected operating range should be used for comparisons.
Positive Displacement (PD) Blowers
Positive Displacement (PD) blowers move a fixed volume of air from inlet to discharge for every revolution of the blower shaft. For positive displacement blowers at a given speed, the performance “curve” is quite linear. The total blower volumetric air-flow rate will not change substantially with discharge pressure, but power will increase with increasing pressure.
There are two types of PD blowers commonly used in wastewater treatment. The lobe type PD (often referred to as a “Roots” blower) has been available for 150 years. The newer type screw PD blowers provide excellent efficiency and broad operating ranges, albeit at a higher capital cost than lobe types.
Regardless of type, modulating the PD’s air flow rate requires varying the blower speed. The most practical technique for doing this is the variable frequency drive (VFD).
PD blower performance may be provided in various formats. Manufacturer’s formulas and characteristics may be available, although this information is often considered proprietary. It is more common to provide tabulated data indicating flow and power at various speeds and pressures. See Figure 1. Linear interpolation can be used to determine performance at operating points not included in the table. The “forecast” function in spreadsheet programs is a useful tool for interpolation.
Figure 1: Sample PD Tabulated Data
Graphical representation of PD performance is also useful in estimating performance for variable speed PD blowers. See Figure 2. It should be noted neither flow, or pressure performance, has a zero intercept. This is a result of the blower’s internal leakage and mechanical friction. Using speed ratio alone will produce significant inaccuracies. The conventional formulas for calculating lobe type PD blower performance illustrates the non-zero intercept:
N = rotational speed, rpm (Nslip is the speed required to offset internal leakage)
CFR = blower displacement, cubic feet per revolution
bhp = blower shaft power, brake hp
FHP = friction horsepower, bhp
Figure 2: Sample PD Graphical Data
Dynamic blowers, commonly called centrifugal blowers, have more complex performance characteristics than PD blowers. Centrifugal blowers convert the kinetic energy of the rotating impeller to kinetic energy in the air stream. In the diffuser section, and in the volute, some of this kinetic energy (i.e. velocity pressure) is converted to potential energy (i.e. static pressure). Unlike PD blowers, centrifugal blower flow-rate and discharge pressure are influenced by the density of inlet air, and by the process system’s resistance to flow.
There are three types of centrifugal blowers used in wastewater treatment.
- Multistage blowers achieve required discharge pressure by successive compression in each stage.
- Geared single stage blowers achieve required discharge pressure by high impeller rotational speed. Standard motors are connected to the blower by speed increasing gear boxes.
- High speed gearless blowers, commonly called turbo blowers, achieve required discharge pressure by high rotational speed. The impellers are mounted directly on special motors, operating at several thousand rpm, and are powered through VFDs with output frequencies of several hundred Hz.
There are a variety of techniques used to modulate the flow rate of centrifugal blowers:
- Inlet throttling, typically with a butterfly valve, creates a pressure drop at the blower inlet. This method is inexpensive, but also has the lowest energy efficiency. Throttling can be used on any type of centrifugal blower, but it is common only on multistage blowers.
- Guide vanes, both on inlet and discharge, are generally found on geared single-stage blowers. Guide vanes modify the blower performance by changing the pressure vs. flow characteristics. Inlet guide-vanes pre-rotate the air, reducing the kinetic energy transferred. Variable discharge diffuser vanes modify the conversion of velocity pressure to static pressure. Both are more efficient than throttling, but introduce a loss in efficiency at low flows by creating a pressure drop.
- Variable speed, typically using VFDs, can be used with any centrifugal blower and is always used with turbo blowers. This is both the most efficient and most expensive control method. Varying blower-speed shifts the blower performance curve down and to the left.
Determining the performance of a centrifugal blower first requires calculating the impact on the performance curve of differences between the air density used to develop the manufacturer’s curves, and the actual site density. Then, a family of curves is developed from the new curve, identifying the performance with several levels of modulation. Finally, the system curve is superimposed on these curves to identify the flow rate.
For inlet throttling, creating a modulated curve requires calculating the pressure drop through the throttling valve at several flow rates. Next, calculate the discharge pressure using the new inlet pressure and the unthrottled pressure ratio. The pressure drop through the valve is determined from Cv, this in turn is a function of the valve design and position:
Δpv = pressure drop across the valve, psi
Qs = flow rate, SCFM
Cv = valve coefficient from manufacturer’s data, dimensionless
SG = specific gravity, dimensionless, (1.0 for air)
Tu = upstream absolute air temperature, °R
pu = upstream absolute air pressure, psia
Inlet guide vane operation is more complex, and the manufacturer usually supplies a set of curves showing performance at several vane positions. These curves need adjustment for changes in inlet density. The system curve can then be superimposed to determine performance. See Figure 3.
Figure 3: Example Inlet Guide Vane Control
Centrifugal blowers controlled with VFDs don’t follow simple linear relationships like PD blowers. Regardless of the type of centrifugal, the affinity laws must be used to create the modulated performance curves: See Figure 4.
Qa,c = actual and curve volumetric flow rate, ICFM
Na,c = actual and curve rotational speed, rpm
Xa,c = actual and curve adiabatic factors, dimensionless
Pa,c = actual and curve blower power, hp
pd,i = inlet and discharge pressure, psia
Figure 4: Variable Speed Centrifugal
Calculations for optimizing blower efficiency for aeration systems always involve the blower characteristics, the control system’s effect on the blower performance, and the process system requirements. By using the proper calculation methodology, blower-system energy consumption for various blowers and control schemes can be estimated. This permits the designer to evaluate the life-cycle cost for several alternative designs. The result will be a system economically meeting the end user’s objectives.
For more information contact Tom Jenkins, President, JenTech Inc. at email: email@example.com or visit www.jentechinc.com. Mr. Jenkins has texts now available in hardcopy and electronic versions titled Aeration Control and Facility Design at www.e-wef.org/store.
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