Water tools

Diffused aeration: air flow, diffuser count and blower power

The chain runs from the oxygen the process needs, through the transfer efficiency the diffusers achieve in your water at your depth, to the air the blowers must move and the power they draw. Manufacturer curve values are inputs.

Process oxygen demand (average)kg O₂/d

Peak factor1.2 to 1.5.

Side water depthm

Diffuser height above floorm

Alpha factor0.25 to 0.45 for BOD-only processes; higher with nitrification.

Fouling factor F1.0 new, about 0.8 fouled.

Clean-water SOTE per metre of submergence%/mFrom the manufacturer curve at the design air flux; fine-bubble discs about 6.

Air flow per diffuser at average loadm³/h

Diffuser dynamic wet pressure at peak fluxkPa

Pressure factor Ψ1.0 new, about 1.3 fouled.

Air line head losskPa

Blower efficiency

Inlet air temperature°C

Inlet absolute pressurekPaSite atmospheric pressure minus filter and silencer losses.

Blowers

Blower shaft power at peak

359kW

Blower shaft power at average

276kW

Discharge pressure (gauge)

53.9kPa

Energy per day (average)

6,636kWh/d

Specific energy

0.77kWh/kg O₂

Air and diffusers

Air flow at peak

16,829m³/h

Air flow at average

12,946m³/h

Number of diffusers

3,808

Transfer efficiency in process water

9.9%

Standard oxygen transfer rate, peak

468kg O₂/h

Submergence

4.70m

How it works

Transfer efficiency in process water from the clean-water curve value at the design air flux, the submergence and the alpha and fouling factors:

α F S O T E = α F \frac{S O T E}{Z} Z

(1)

Oxygen that must pass through the blowers, and the air it rides in (air at 1.225 kg/m³ and 23% oxygen by mass):

W_{O_{2}} = \frac{S O T R}{α F S O T E} A F R = \frac{W_{O_{2}}}{ρ_{a i r} y_{O_{2}}} N_{D} = \frac{A F R_{p e a k}}{A F R_{d i f f}}

(2)

Discharge pressure is submergence (10 m of water = 101.325 kPa) plus line loss plus the diffuser wet pressure, multiplied by Ψ when fouled. Shaft power from adiabatic compression, excluding motor losses:

p_{d} = ρ g Z + h_{L} + Ψ D W P

(3)

P = \frac{W_{a i r} R T_{i n}}{29.7 n e} [(p_{a b s} / p_{i n})^{n} - 1] n = 0.283

(4)

where

$α$: process water to clean water transfer ratio
$F$: fouling factor, 1 when new
$S O T E / Z$: standard transfer efficiency per metre of submergence, %/m
$Z$: diffuser submergence, m
$S O T R$: standard oxygen transfer rate, kg O₂/h
$W_{O_{2}}$: oxygen through the blowers, kg O₂/h
$A F R$: air flow, m³/h
$ρ_{a i r}$: air density, 1.225 kg/m³
$y_{O_{2}}$: mass fraction of oxygen in air, 0.23
$N_{D}$: number of diffusers
$A F R_{d i f f}$: air flow per diffuser, m³/h
$p_{d}$: blower discharge pressure, kPa gauge
$ρ g Z$: hydrostatic head, kPa
$h_{L}$: air line head loss, kPa
$Ψ$: pressure factor on the diffuser, 1 when new
$D W P$: diffuser dynamic wet pressure, kPa
$P$: blower shaft power, kW
$W_{a i r}$: air mass flow, kg/s
$R$: gas constant, 8.314 J/mol·K
$T_{i n}$: inlet air temperature, K
$e$: blower efficiency
$p_{a b s}, p_{i n}$: absolute discharge and inlet pressure, kPa

Fouling lowers efficiency (F) and raises head loss (Ψ); with a fixed diffuser count the per-diffuser flux rises and the curve value should be re-read. Keep the flux inside the manufacturer's guaranteed range, including purge flows.

Diffused aeration: air flow, diffuser count and blower power

How it works

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