energy conservation by optimizing aeration systems by tom jenkins, p.e. dresser roots, inc. vikram...
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Energy Conservation by Optimizing Aeration Systems
By
Tom Jenkins, P.E.Dresser Roots, Inc.
Vikram M. Pattarkine, Ph.D.Brinjac Engineering, Inc
Michael K. Stenstrom, Ph.D., P.E.C & EE Dept, UCLA
Outline• Overview
• Types of Aeration Systems, Terminology and Relative Efficiency
• Operational Issues – Cleaning and Avoiding Power Loss through Fouling and Scaling
• Blower Overview and Optimization
• Conclusion
Types of Aeration Systems• Mechanical or surface aerators
– High speed – 900 to 1200 RPM, no gear boxes, easy to install, high heat loss, spray issues, low efficiency
– Low speed – gear boxes to reduce RPM to 30 to 60, long lead time to install, high heat loss, spray issues, medium efficiency
Types of Aeration Systems• Diffused or subsurface aerators
– Coarse bubble – ¼ to ½ inch orifices, low efficiency, low maintenance, low to ultra-low efficiency
– Fine bubble or fine pore – millimeter to sub-millimeter orifices or porous media, highest efficiency, significant cleaning and maintenance issues
Types of Aeration Systems• Combined systems
– Jets, turbines, and aspirating devices – generally two prime movers such as a blower and a motor-gearbox, low efficiency, generally not used for new applications unless there are special concerns or needs
Terminology
• Efficiency– Standard oxygen transfer efficiency (SOTE) (percent
oxygen transferred)– Standard oxygen transfer rate (SOTR) (mass
transferred per unit time)– Standard aeration efficiency (SAE) (mass transferred
per unit time per unit power)
• All “standard” terminologies defined for clean water such as tap water (secondary process effluent is never suitable for clean water testing)
Terminology
• Process Conditions (OTE, OTR, AE)– Adjustment formulas based upon driving force,
temperature, barometric pressure, water quality, saturation concentration, etc.
– Driving force and water quality the most significant
– Driving force = (DOS – DO)/DOS
– Water quality – alpha factor, 0 to 1 !– Total correction can result in process water transfer of
only 30 to 80% of clean water transfer
ASCE/EWRI Standards• Clean Water Oxygen Transfer Standard
– 1984, 1991 and 2006.
• Process Water Testing Guidelines– 1996
These two documents are quite useful in defining aeration performance, and create a “level playing” field to evaluate aeration systems and facilitate low bid or life-cycle purchase evaluations – use them!
Energy Approximations (wire power)Aerator
Type SAE
lbO2/hp-h (kgO2/kW-h)
Low SRT AE
at 2 mg/L DO
High SRT AE
At 2 mg/L DO
High Speed
1.5–2.2 (0.9–1.3) 0.7–1.4 (0.4-0.8)
Low
Speed 2.5–3.5 (1.5–2.1) 1.2-2.5 (0.7–1.5)
Turbine 2-3 (1.2-1.8) 0.6-0.9
(0.4-0.6)
0.9-1.4
(0.6-0.8)
Coarse Bubble
1-2.5 (0.6 –1.5) 0.5 – 1.2
(0.3-0.7)
0.6–1.6
(0.4-0.9)
Fine
Pore 6–8 (3.6–4.8) 1.2-1.6
(0.7–1.0)
3.3-4.4
(2–2.6)
Approximations – use only as a guideline – transfer efficiency will depend on site specific conditions
Most Common Systems Today• Municipal Treatment Plants – fine pore
systems:– Discs, ceramic, plastic and membranes– Tubes, membranes– Panels and strips
• Municipal HPO-AS Systems – Low speed mechanical – Some new impeller designs to improve
efficiency
Most Common Systems Today• Lagoons, ditches, industrial systems
sometimes are best designed with alternative aeration systems due to extremely high oxygen uptake rates, odd geometries, heat loss considerations, requirements for wet installation or wet maintenance
Fine Pore Aeration Systems
• Why fine “pore” and not fine “bubble” ???– Fine bubbles can be created by turbines and
other mechanical devices. – Fine pore systems create bubbles by passing
air through pores or orifices
• Generally the best design choice for energy conservation, but there are issues and problems to avoid
Some Example Systems• Ceramic domes – legacy system
• Ceramic discs – popular today
• Membrane discs – maybe most popular at present
• Membrane tubes – popular today
• Membrane panels and strips – popular today, and among the most energy efficient
Ceramic Domes
Ceramic Discs
Membrane and Plastic Discs
EPDM
PVC
Ceramic
EPDM
Plastic
Tubes
Panels and Strips
Efficiency Varies
• Key to overall transfer efficiency is the air flow per unit area of diffuser surface and the number of diffusers used
• More diffusers and more area creates efficiencies that are at the upper part of the fine pore range
• Few diffusers and high flow per diffusers will provide only low efficiency, at the low end of the range or even approximating lower efficiency devices
Fouling and Scaling
• Fine pore diffusers invariably undergo fouling, scaling and material changes that reduce efficiency
• Some type of routine maintenance program is always required: otherwise, efficiency declines to values that may be so low that they don’t justify the capital investment
• Also, back pressure may built which may prevent plant operation
Our Database of Full-Scale Results
• More than 20 years of observations of ~ 35 plants
• Ceramic discs, ceramic domes, membrane discs, membrane and plastic tubes, panels and strips
• New (< 1 month), Used (< 24 months) and Old (> 24 months) and cleaned
• Cleaning – tank top hosing, brushing, acid washing
Our Database of Full-Scale Results
• Process operation matters • Conventional, low MCRT or sludge age –
lowest efficiency• Long MCRT, nitrifying, good efficiency• Long MCRT, nitrifying, denitrifying, best
efficiency• Flow per unit area of diffuser and tank
surface is more important to defining performance that the generic diffuser type or material
Efficiency per process type
0.4
0.8
1.2
1.6
S
OT
E /
(
%/f
t)
3.0
3.8
4.6
5.4
S
OT
E /
(%/m
)
U SED
U SED
U SED
U SED
N EWN EW
N EW
N EW
O LD
O LD
U SED U SED
N EW
N EW
C LEAN ED
O LD
O LD
O LD
N EW
N EW
U SED
U SED
U SED
C LEAN ED
C LEAN ED
U SED
U SEDU SED
C onventiona lC onventiona lC onventiona lC onventiona lC onventiona lC onventiona lC onventiona lC onventiona l N itrify ing on lyN itrify ing on lyN itrify ing on lyN itrify ing on lyN itrify ing on lyN itrify ing on lyN itrify ing on lyN itrify ing on ly N itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tionN itrifica tion/D enitrifica tion
2 4 6M C R T (d) 13 15 17 19 21M C R T (d) 10 14 18 22M C R T (d)
CONVENTIONAL N-ONLY NDN
3.75%/m
4.30%/m
4.60%/m
S
OT
E/Z
(%
/ft)
S
OT
E/Z
(%/m
)
NEW & CLEANED
<24 mo.
>24 mo.
Transfer Efficiency• Here-to-fore, transfer efficiency measurements
required an expert using an off-gas analyzer, a few days of time, and thousands of dollars in fees
• A real-time off-gas oxygen transfer efficiency analyzer has been developed by the UCLA-Southern California Edison Team, with California Energy Commission Funding, and the design is in the public domain
• It is described in detail during the technical sessions
0 4 8 1 2 1 6 2 0 2 4 2 8
m o n t h s i n o p e r a t i o n
0
0 . 4
0 . 8
1 . 2
1 . 6
2
2 . 4
po
we
r w
as
te /
cle
an
ing
co
st
po
we
r /
init
ial
po
we
r
1 0
1 0
1 0
1 0
1 0
2 0
2 0
2 0
2 0
2 0
2 0
2 0
2 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
3 0
4 0
4 0
4 0
4 0
4 0
4 0
Economics of Fouling
Summary• Fine pore systems generally, but not always
offer the best energy conservation• Fine pore systems require a dedication to
maintenance; otherwise, select different alternatives
• Reputable manufactures have valuable experience with piping and assembly – Listen to them!
• The consultant or process engineer must define the efficiency – Require this information from them!
Blowers
• All fine pore diffuser systems, coarse bubble systems and most combined aeration systems require blowers – they are an indispensable part of the system
• The next section describes blower types and guides for selection
Positive Displacement (PD)
Constant flow at constant speed
Pressure varies with load
Most common <200 hp
Energy Conservation:Most Common Blower Types
Energy Conservation:Most Common Blower Types
Multistage Centrifugal
Variable flow
Approx. Constant Pressure
Most common 100 < hp > 750
Single Stage Centrifugal
Variable flow
Pressure varies with load
High efficiency
Most common > 500 hp
Energy Conservation:Most Common Blower Types
Energy Conservation:Uncommon Blower Types
Regenerative
Very High Speed Centrifugal
New proprietary technology
High efficiency
Limited size range
Characteristics similar to PD
Limited to small flows and low pressures
Energy Conservation:Blower System Design Considerations
Provide lots of turndown capability
Use multiple smaller blowers
Select blowers for current requirements
Evaluate energy over range of actual near term operating condition
Energy Conservation:Blower System Design Considerations
Minimize system pressure
Most Open Valve Control for automatic
controls
Keep diffuser drop leg valves open for manual
control
Minimize diffuser pressure drop – orifice size,
diffuser configuration, clean diffusers
Energy Conservation:Blower System Design Considerations
Use automatic DO control
20% to 50% energy reduction
Newer technology IS reliable
Energy Conservation:Blower System Design Considerations
Use efficient blower control
Use technology appropriate to blower system
Integrate with basin controls
Energy Conservation:Blower Upgrades / Revamps
Collect Actual Operating Data on Process:Dissolved Oxygen (DO) Concentration
Air Flow Rates
Dissolved Oxygen (DO) Concentration
System Pressure
Minimum, Maximum, and Average for Typical Operation
Compare to Design Conditions
Energy Conservation:Blower Upgrades / Revamps
Collect Actual Operating Data on Blowers:
Number of Units Operating
Blower Power (kW) or Amps
Minimum, Maximum, and Average for Typical
Operation
Frequency of Manual Adjustments
Compare to Design Conditions
Energy Conservation:Blower Upgrades / Revamps
All Blower TypesProvide proper maintenance – filters, seals, diffuser
cleaning
Change to energy efficient motors
Add smaller blowers to achieve turndown
Combine air use for other functions (Post-Aeration,
Channel Aeration, etc.)
Update Controls
Energy Conservation:Blower Upgrades / Revamps
PD BlowersChange sheaves to optimize capacity
Multistage Centrifugal BlowersChange impellers to match actual
conditions
Single Stage Centrifugal BlowersChange impellers to match actual conditions
Add Inlet Guide Vanes and/or Variable Discharge Diffuser Vanes
Energy Conservation:Control System Techniques
All Blower Types
Automatic DO Control to match air rates to process demand
Use MOV Control to minimize pressure
Automatic starting and stopping of blowers
Parallel control instead of cascade control
Design Control System for Reasonable Payback – 2 to 5
years
Include Process Improvement in Evaluation
Energy Conservation:Control System Techniques
PD BlowersUse VFDs (Variable Frequency Drives) to modulate air flow
Multistage Centrifugal BlowersVFDs to modulate air flow (with appropriate curves)
Automatically controlled inlet throttling to modulate flow and improve turndown
Single Stage Centrifugal BlowersInlet Guide Vanes and Variable Discharge Diffusers to
modulate flow and improve turndown
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