1. Energy Saving Measures - 1
Tom Jenkins
JenTech Inc.
6789 N. Elm Tree Road
Milwaukee, WI 53217

2. Understanding Blower Systems, Dissolved Oxygen, and Aeration Process Controls and How they Affect Energy Costs
Blower Types and Characteristics
Energy Impact of Blower Controls
Evaluating the Savings of DO Control
Influence of Advanced Control Strategies

3. Useful Formulae For all Blowers Ignoring Relative Humidity:
RH = relative humidity, decimal
PVa = saturated vapor pressure of water at actual temperature, psi
Ta = actual air temperature, °F
pa = actual air pressure, psia
pb = barometric pressure, psia
Ignoring Relative Humidity:
Q= air flow SCFM
effmotor= motor efficiency, decimal
PF = motor Power Factor, decimal
I = current, Amps
V = Voltage

4. Curve Adjustments For Variable Speed Centrifugal Blowers
For PD Blowers
Q = volumetric air flow rate, ICFM
Disp = blower displacement, Cubic Feet per Revolution
N = blower rotational speed, rpm
Slip = slip corrected for actual operating conditions, rpm
bhp = blower shaft power required, horsepower
Fg = gas power constant from manufacturer (typically )
ΔPb = total pressure rise across blower, psi
FP = friction power corrected for actual operating conditions, horsepower
Curve Adjustments For Variable Speed Centrifugal Blowers
Q1 , Q2= air flow at original and new operating speed, ICFM
P1 , P2=gauge pressure at original and new operating speed, psig
p1 , p2= power at original and new operating speed, horsepower
N1 , N2= original and new operating speed, rpm

5. Useful Formulae ΔP = pressure drop through valve, psi
Q = air flow rate, SCFM
Cv = valve flow coefficient from manufacturer’s data
SG = specific gravity of gas, dimensionless (air = 1.0)
Tu = upstream temperature, °R
Pu = upstream pressure, psia

6. Sample Problem 1 Blower Power
A 250 hp direct coupled positive displacement blower is operating at a constant speed of 1170 rpm. The measured discharge pressure is 7.5 psig. An aeration study has determined that the average air flow required by the aeration basins at current loadings is 3,900 CFM. It is expected that the current plant load will begin to increase after three years, gradually rising to design loads in 10 years.
Will it be cost effective to replace the current starter with a Variable Frequency Drive?
Assume the following:
Blower Displacement = 4.6 CFR
Blower Slip = 56 rpm
Constant Fg =
Friction Horsepower = 24
Motor Efficiency = 95%
Power Cost = $0.07/kWh
VFD Installed Cost = $29,000 each

7. Sample Problem 1 Blower Power
Step 1: ????

8. Sample Problem 1 Blower Power
Step 1: Determine actual air flow from the blower

9. Sample Problem 1 Blower Power
Step 1: Determine actual air flow from the blower

10. Sample Problem 1 Blower Power
Step 2: ????

11. Sample Problem 1 Blower Power
Step 2: Determine Blower Power Required

12. Sample Problem 1 Blower Power
Step 2: Determine Blower Power Required

13. Sample Problem 1 Blower Power
Step 2: Alternate Blower Performance

14. Sample Problem 1 Blower Power
Step 2: Alternate Blower Performance

15. Sample Problem 1 Blower Power
Step 2: Alternate Blower Performance

16. Sample Problem 1 Blower Power
Step 3: ????

17. Sample Problem 1 Blower Power
Step 3: Determine Electrical Power

18. Sample Problem 1 Blower Power
Step 3: Determine Electrical Power

19. Sample Problem 1 Blower Power
Step 4: ????

20. Sample Problem 1 Blower Power
Step 4: Calculate New rpm

21. Sample Problem 1 Blower Power
Step 4: Calculate New rpm

22. Sample Problem 1 Blower Power
Step 4: Calculate New rpm
This is > 50% of initial speed of 1170 rpm - OK

23. Sample Problem 1 Blower Power
Step 5: ????

24. Sample Problem 1 Blower Power
Step 5: Calculate New Blower Power and Electrical Power

25. Sample Problem 1 Blower Power
Step 5: Calculate New Blower Power and Electrical Power

26. Sample Problem 1 Blower Power
Step 5: Calculate New Blower Power and Electrical Power

27. Sample Problem 1 Blower Power
Step 5: Alternate New Blower Power

28. Sample Problem 1 Blower Power
Step 5: Alternate New Blower Power

29. Sample Problem 1 Blower Power
Step 6: ????

30. Sample Problem 1 Blower Power
Step 6: Calculate Electrical Power Savings and Payback

31. Sample Problem 1 Blower Power
Step 6: Calculate Electrical Power Savings and Payback

32. Sample Problem 1 Blower Power
Step 6: Calculate Electrical Power Savings and Payback OK

33. Sample Problem 1 Blower Power
Additional Considerations:
Multiple installed blowers, one running
Two blowers = 3 year payback
Three blowers = 4.5 year payback
Utility rebates and funding incentives
$50/hp = $37,500 rebate for three
2.6 year payback
Could convert to belt drive
Sheave change for new flow
Inexpensive
Can’t automate or respond to upsets

34. Sample Problem 2 Aeration Requirements
A municipal WWTP has three square (62’ x 62‘) complete mix aeration tanks operating in parallel. Each tank has 225 fine pore diffusers with characteristics shown in Slide 16. The actual average DO concentration is 4.5 ppm, but 2.0 ppm is adequate to insure proper treatment. The tanks are operating at mixing limit of 0.12 CFM/sq ft. (1,380 CFM). Current hydraulic retention time is longer than design requirements for process performance.
Saturation DO is 10.0 ppm, The density of ambient air is lb/cu ft and contains 23.0% O2 by weight (20.9% O2 by volume).
Will taking one of the three tanks out of service result in lower energy requirements?
Hint: Refer to Slide 36 from Session 1

35. Sample Problem 2 Aeration Requirements
Step 1: ????

36. Sample Problem 2 Aeration Requirements
Step 1: Determine air flow per diffuser and tank area

37. Sample Problem 2 Aeration Requirements
Step 1: Determine air flow per diffuser and tank area

38. Sample Problem 2 Aeration Requirements
Step 1: Determine air flow per diffuser and tank area

39. Sample Problem 2 Aeration Requirements
Step 2: ????

40. Sample Problem 2 Aeration Requirements
Step 2: Determine Oxygen Transfer Rate with 3 Tanks

41. Sample Problem 2 Aeration Requirements
Step 2: Determine Oxygen Transfer Rate with 3 Tanks
Oxygen Transfer Rate = 43 kg/hr w/ 3 Tanks
Note: Can be done analytically but beyond scope of this presentation

42. Sample Problem 2 Aeration Requirements
Step 3: ????

43. Sample Problem 2 Aeration Requirements
Step 3: Determine Oxygen Transfer Rate with 2 Tanks

44. Sample Problem 2 Aeration Requirements
Step 3: Determine Oxygen Transfer Rate with 2 Tanks
w/ 2 Tanks each one must remove 50% more

45. Sample Problem 2 Aeration Requirements
Step 4: ????

46. Sample Problem 2 Aeration Requirements
Step 4: Determine air flow per diffuser to get OTR with 2 Tanks at 2.0 ppm DO

47. Sample Problem 2 Aeration Requirements
Step 4: Determine air flow per diffuser to get OTR with 2 Tanks at 2.0 ppm DO
w/ 2 Tanks air flow per diffuser is 2.5 SCFM

48. Sample Problem 2 Aeration Requirements
Step 5: ????

49. Sample Problem 2 Aeration Requirements
Step 5: Calculate New Total Air Flow and Compare with Mixing Requirement

50. Sample Problem 2 Aeration Requirements
Step 5: Calculate New Total Air Flow and Compare with Mixing Requirement

51. Sample Problem 2 Aeration Requirements
Step 5: Calculate New Total Air Flow and Compare with Mixing Requirement
Air flow for Process is > Mixing Air Flow - OK

52. Sample Problem 2 Aeration Requirements
Step 6: ????

53. Sample Problem 2 Aeration Requirements
Step 6: Calculate Savings

54. Sample Problem 2 Aeration Requirements
Step 6: Calculate Savings

55. Sample Problem 2 Aeration Requirements
Step 6: Calculate Savings

56. Sample Problem 2 Aeration Requirements
Additional Considerations:
Blowers Must Have Adequate Turndown
Verify Pressure Increase is Minimal
Process Considerations Must Be Met
Hydraulic Residence Time
Peak Flow and Rain Events
Tank Out of Service Must be Protected
Diffuser and Piping Protected from Sunlight
Tank Freezing
Tank Floating

57. Aeration Process Control: DO and Blowers
Questions and Answers