You can't control me! Throttling blowers and with valves and VFDs
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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
- 4. One size does not fit all
- 5. One size does not fit all Even at the same plant, inputs and outputs will vary over the course of a year
- 6. That's because density varies with temperature As temperature goes down, density goes up
- 7. Are you good at weather forecasting?
- 8. Are you good at weather forecasting? You don't actually have to read a "Skew-T" chart to get the main point
- 9. Warm fronts are squishy
- 10. Cold fronts have sharp edges
- 11. Warm fronts ride over the top Lower density causes them to float
- 12. Cold fronts wedge under from below Greater density causes them to sink
- 13. Blowers are selected on four dimensions
- 14. 1. Flow How much air is needed?
- 15. 2. Location Where is the air being obtained? Where is the blower itself being placed?
- 16. 3. Pressure How hard must the air be pushed?
- 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
- 21. Cold weather Air is denser Water holds more dissolved oxygen Less air needed
- 22. Warm weather Air is less dense Water holds less dissolved oxygen More air needed
- 23. Efficiency means modulating airflow Matching the air supply to the process needs prevents energy from going to waste
- 24. Think of blower operation like riding a bike
- 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
- 31. It just isn't as easy as "Summer" and "Winter" Here's a case of both within a 48-hour period
- 32. Throttling is threefold Process adaptation Motor protection Blower protection (rise to surge)
- 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?
- 40. Step 5: Survey the incentives
- 42. State energy programs: naseo.org
- 43. Local utility rebates (Or just net savings for the municipal water & power utility)
- 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
- 50. Sample condition
- 51. Cost savings Design condition at 95°F: 200 hp At 75°F: 188 hp At 50°F: 176 hp At 25°F: 166 hp
- 52. Do you know how much 1 horsepower costs?
- 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