Aeration is achieved by getting the right amount of oxygen into the water for the capacity of the water to handle it. How best to achieve that complicated, multi-variable outcome?
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You can't control me! Throttling blowers and with valves and VFDs
1. You Can't Throttle Me!
Brian Gongol
DJ Gongol & Associates, Inc.
January 26, 2022
LONM/NWOD Snowball Conference
Kearney, Nebraska
2. Ben Franklin on aeration
"Beware of little expenses;
a small leak will sink a great ship."
3. Aeration uses 50% to 70% of WWTP energy
Can't really treat without it
Best bet is to conserve through efficiency
Move just enough air to achieve treatment
17. 4. Temperature
How hot or cold is the available air?
How hot or cold is the receiving water?
18. Must size for the worst-case scenario
Hot input air at low density
going into warm water
that doesn't hold DO very well
19. But pushing too much air is wasteful
If you're using 10% too much, then that's 5% to 7% of
plant energy going to waste
Wasted energy means higher operating costs
20. Is your budget too big to spend?
If so, please see me after this presentation
25. Think of blower operation like riding a bike
Sometimes you have to shift gears to go uphill,
but you'd be crazy to pedal going downhill
26. Option #1: Valve throttling
Simple
Inefficient
Manual throttling requires multivariate operator
judgment
Limited by butterfly valve control range
27. Option #2: Speed throttling
More complex: Requires integrating controls
Temperature-based feedback
Automation can account for changes in air temperature
and water temperature
Limited by blower turndown potential
May deliver electrical savings
28. Big temperature swings complicate throttling
Air temperatures swing quickly
Water temperatures change slowly
Manual throttling can be labor-intensive
29. I was told there would be no math
Here's the Streeter-Phelps equation
for working out dissolved oxygen concentrations:
Ss - O = [kd/(ko - kd)] Du [e-(kd/v)x
- e-(ko/v)x
] + (Ss - O0) e-(ko/v)x
34. Step 1: Energy audit
How much energy is being used?
How much does it cost?
Are any cost changes ahead?
35. Step 2.a.: System audit
Are plant loads as designed?
Has the process changed?
Is the treatment sufficient?
Is the system harmonized with local climate conditions?
36. Step 2.b.: Breaking out the power bill
What is the utility rate structure in effect?
Are prices fixed and flat?
Do prices vary with consumption (block pricing)?
Do prices vary with demand?
Do prices vary with time of day?
Do penalties or surcharges apply to high loads?
37. Step 3: Site audit
What do we know about performance data?
What data do we have on blowers, motors, and
controls?
What site conditions constrain our options?
38. Step 4: Equipment audit
Is the right equipment in place to meet system needs?
39. Step Pre-5: A question
Can you be rewarded, recognized, or promoted
for improving processes and saving money?
44. Automatic controls cost money up-front
Capital expense must be justified
Energy savings are the main bucket of value
Even with cheap power, process optimization makes
sense
Automatic controls can save on labor costs (or simply
take dumb work off the to-do list)
Payback periods under 5 years are common
45. What makes a VFD applicable?
Positive-displacement blowers may benefit from VFD
controls
Centrifugal blowers (including multi-stage and turbo)
obey the same affinity laws as pumps
46. Affinity laws
Reductions in speed have magnified results in
reductions in power
VFDs only effective with at least 1 psi rise to surge
47. Rise to surge
Same condition, but
the lower option
offers far more useful
range to the VFD
48. Effects of temperature change
Slight reduction in
speed as VFD adapts to
temperature change
HP Change From
193.91 to 173.48
Reduction of 20.43 HP
49. Other benefits
VFDs can reduce inrush
current
Adding a PLC opens up
integration with SCADA
PLCs can support algorithm-
based flow control
Sensor-based operation does
away with manually hunting
the best condition
Can't do any of the above
with valve-based throttling
53. Do you know how much 1 horsepower costs?
1 hp = 0.7457 kilowatts
0.7457 kW for 24 hours over 365 days = 6,532 kWh
At $0.10 per kWh, 1 hp costs $653.20 per year
54. Lower horsepower means less electricity
1 hp equals 0.7457 kilowatts
200 hp equals 149.14 kilowatts
Dropping to 188 hp (75° condition) uses 140.91 kilowatts
Dropping to 176 hp (50° condition) uses 131.24 kilowatts
Dropping to 166 hp (25° condition) uses 123.79 kilowatts
55. Less electricity means lower costs
Normal daily temperature variations would put this one
blower in the range to vary by 10 kilowatts just between
high and low daily temperatures
56. Less electricity means lower costs
Normal daily temperature variations would put this one
blower in the range to vary by 10 kilowatts just between
high and low daily temperatures
If you can save just 10 kilowatts for 12 hours of each
day, that's 43,800 kilowatts of energy saved per year
57. Less electricity means lower costs
Normal daily temperature variations would put this one
blower in the range to vary by 10 kilowatts just between
high and low daily temperatures
If you can save just 10 kilowatts for 12 hours of each
day, that's 43,800 kilowatts of energy saved per year
Then, scale that up to the potential savings across
seasonal variations that could be saving 25 to 30
kilowatts for months at a time
58. Less electricity means lower costs
Normal daily temperature variations would put this one
blower in the range to vary by 10 kilowatts just between
high and low daily temperatures
If you can save just 10 kilowatts for 12 hours of each day,
that's 43,800 kilowatts of energy saved per year
Then, scale that up to the potential savings across
seasonal variations that could be saving 25 to 30
kilowatts for months at a time
It's not hard to achieve tens of thousands of dollars in
savings by automating speeds on a single blower
59. Questions?
Thank you for your time
and attention
This presentation will be
available through
gongol.net/presentations
Brian Gongol
DJ Gongol & Associates
brian@gongol.net
515-223-4144
@djgongol on LinkedIn,
Facebook, and Twitter
60. Sources
Skew-T chart from the National Weather Service:
https://www.spc.noaa.gov/exper/soundings/22012512_OBS/
Water temperature vs DO graph:
https://www.usgs.gov/special-topic/water-science-school/science/dissolved-oxygen-and-
water
Kearney air temperature records:
https://www.weather.gov/wrh/Climate?wfo=gid
Kearney forecast graph:
https://forecast.weather.gov/MapClick.php?lat=40.7008&lon=-
99.0846&lg=english&&FcstType=graphical&menu=1
Streeter-Phelps equation:
http://ponce.sdsu.edu/onlinedostph.html
Graphs provided courtesy of Hoffman & Lamson/Gardner-Denver
Blower photos provided courtesy of Hoffman & Lamson/Gardner-Denver