Energy Saving Measures - 1 Tom Jenkins JenTech Inc. 6789 N. Elm Tree Road Milwaukee, WI 53217 414-352-5713 tom.jenkins.pe@gmail.com

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

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

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 0.00436) 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

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

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 = 0.00436 Friction Horsepower = 24 Motor Efficiency = 95% Power Cost = $0.07/kWh VFD Installed Cost = $29,000 each

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

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

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

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

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

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

Sample Problem 1 Blower Power Step 2: Alternate Blower Performance

Sample Problem 1 Blower Power Step 2: Alternate Blower Performance

Sample Problem 1 Blower Power Step 2: Alternate Blower Performance

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

Sample Problem 1 Blower Power Step 3: Determine Electrical Power

Sample Problem 1 Blower Power Step 3: Determine Electrical Power

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

Sample Problem 1 Blower Power Step 4: Calculate New rpm

Sample Problem 1 Blower Power Step 4: Calculate New rpm

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

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

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

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

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

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

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

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

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

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

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

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

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 0.075 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Sample Problem 2 Aeration Requirements Step 6: Calculate Savings

Sample Problem 2 Aeration Requirements Step 6: Calculate Savings

Sample Problem 2 Aeration Requirements Step 6: Calculate Savings

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

Aeration Process Control: DO and Blowers Questions and Answers