General Rules

Mixer Installations and Positioning

in Wastewater Treatment Plants
2 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Table of Contents

1 General comments ............................................................................................ 7

2 Nomenclature and Symbology ........................................................................ 9

2.1 List of Abbreviations ..................................................................................................... 9
2.2 Definitions ................................................................................................................... 10

3 Type of Tanks .................................................................................................. 11

3.1 Round tanks ............................................................................................................... 11
3.2 Rectangular tanks ....................................................................................................... 12
3.3 Race track shaped tanks ............................................................................................ 13
3.4 Serpentine tanks ......................................................................................................... 14
Calculation of the flow Path .....................................................................................................14

4 Relations .......................................................................................................... 15
4.1 Maximum propeller diameter depending on tank dimensions ...................................... 15
Determination of the maximum propeller diameter ...................................................................15
4.2 Minimum tank volume depending on mixing capacity.................................................. 16
Minimum size of mixing tanks ..................................................................................................16

5 Minimum Submergence.................................................................................. 17
5.1 Minimum submergence for mixer ................................................................................ 17
5.2 Minimum Submergence for flow booster ..................................................................... 18
5.3 Propeller protection with Vortex-Shield ....................................................................... 19

6 Positioning in non-aerated tanks .................................................................. 20

6.1 Clearances for mixer ................................................................................................... 20
Clearance to side walls............................................................................................................20
Clearance to tank bottom ........................................................................................................20
Clearance to rear wall .............................................................................................................20
6.2 Clearances for flow booster ........................................................................................ 21
Clearance to side walls............................................................................................................21
Clearance to tank bottom ........................................................................................................21
Clearance to rear wall .............................................................................................................21
6.3 Vertical view positioning for flow booster..................................................................... 22
6.4 Vertical view positioning for mixer ............................................................................... 23
Mixer directed downwards .......................................................................................................24
Mixer directed upwards ...........................................................................................................25
6.5 Plan view positioning for mixer.................................................................................... 26
Mixer directed upwards and downwards ..................................................................................26
6.6 Positioning in square tanks ......................................................................................... 27

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 3
Table of Contents

One unit installed ....................................................................................................................27

One unit installed ....................................................................................................................28
Two units installed...................................................................................................................29
Three units installed ................................................................................................................30
6.7 Positioning in rectangular tanks .................................................................................. 31
One unit installed in rectangular tank (1 < l 1/w1 ≤ 3) .................................................................31
One unit installed in rectangular tank (1 < l 1/w1 < 2) .................................................................32
One unit installed in long rectangular tank (3 < l 1/w1 <= 5) .......................................................33
Two units installed in rectangular tank (1 < l1/w1 <= 3) .............................................................34
Three units installed in rectangular tank (1 < l 1/w1 <= 3) ...........................................................35
6.8 Positioning in round tanks ........................................................................................... 36
One unit installed (classic method) ..........................................................................................36
Two units installed (classic method) ........................................................................................37
Three units installed (classic method) ......................................................................................38
Liquid circulation mixer ............................................................................................................39
Liquid circulation flow booster ..................................................................................................40

7 Further Rules ................................................................................................... 41

7.1 Optimum dimensional ratios in oxidation ditches......................................................... 41
7.2 Flow booster in oxidation ditches ................................................................................ 42
7.3 Positioning of flow booster in non-aerated oxidation ditches ....................................... 43
With deflection walls ................................................................................................................43
Without deflection walls ...........................................................................................................44
7.4 Contiguous mixers in oxidation ditches ....................................................................... 45
7.5 Positioning of flow booster in donut tanks ................................................................... 46
One unit installed ....................................................................................................................46
Two or more units installed ......................................................................................................47
7.6 Contiguous mixers in other tanks ................................................................................ 48
7.7 Filling grade of flow booster in a channel .................................................................... 49
7.8 Distance to objects ..................................................................................................... 50
7.9 Tank flows influence ................................................................................................... 51
Flow direction ..........................................................................................................................51
Improvement of retention time .................................................................................................52
7.10 Positioning in aerated tanks with Disc Diffuser System (DDS) .................................... 53
Mixers in tanks with full tank floor DDS coverage .....................................................................53
Mixers in tanks with partial tank floor DDS coverage ................................................................54
Flow booster in tanks with full tank floor DDS coverage ...........................................................55
Flow booster in tanks with partial tank floor DDS coverage ......................................................56
Flow booster in round tanks equipped with disc diffuser system ...............................................57

4 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Table of Contents

7.11 Spiral flows created by flow booster horizontal flow .................................................... 58

7.12 Positioning of flow booster in oxidation ditches with DDS ........................................... 59
With deflection walls ................................................................................................................59
Without deflection walls ...........................................................................................................60
7.13 Flow booster in donut tanks equipped with DDS ......................................................... 61
One unit installed ....................................................................................................................61
Two or more units installed ......................................................................................................62
7.14 Oxidation ditches with surface aerators (Brushes) ...................................................... 63
7.15 Storm water tanks ....................................................................................................... 64
7.16 Other positioning examples for flow booster ............................................................... 65
Rectangular tanks with ratio 2 < l1/w1 < 3 .................................................................................65
Rectangular tanks with ratio 3 < l1/w1 < 5 .................................................................................66

8 Notes ................................................................................................................ 67

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 5
 General comments

1 General comments

This document is only for the ABS internal use.

It is allowed to give single pages to customers.

NOTE
The recommended positioning is based on a proper inlet
and outlet design without taking any other built in
equipment into account as it is shown in the drawings.

1. Obtain information about the process and application including solids type, dry
matter content (DMC), tank or basin dimensions, flow through tank or basin, inlet
and outlet locations, diffuser and obstruction locations.
2. Select a mixer that will provide the dynamic response needed for the process and
application.
3. Select mixer location and position to provide maximum mixer performance and
create smoothest possible running condition.
All three of the above steps are equally important and need to be executed correctly in
order to get a reliable submersible mixer installation. Often times step three does not
get the attention it needs which can lead to poor mixing results. Proper positioning will
result in a well-mixed tank that has minimal losses and an even distribution of shear
forces and velocities throughout the entire tank.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 7
General comments

The following is a list of general location recommendations that apply to submersible

mixers.
■ To achieve maximum performance the flow approaching the mixer should produce
minimum disturbance. This is accomplished by even distribution of the
approaching flow around the mixer, restricting air from entering the propeller area
and prevents vortexes. To prevent vortexes in the propeller area the approaching
velocities should not be excessive.
■ Restrictions or obstructions close to the propeller should be avoided.
■ The mixer’s jet flow could transmit unbalanced loads to the propeller. Obstacles in
front of the propeller may cause reflected waves that can result in unbalanced
loads on the propeller.
■ Minimum submergence levels and proper mixer orientation should be observed.
Vortexes cause unbalanced loads on the mixer propeller and shock loading on the
drive train.
■ Locating the mixer propeller no closer than the minimum distance a propeller
blade tip should be located from a tank sidewall or floor prevents unbalanced
propeller loads.
■ Mixers used in combination with aeration equipment, such as in oxidation ditches,
should be isolated from the aeration system. It is recommended, that the suction
side and the discharge side be kept clear of. Give guidelines or refer where
guidelines are located.
■ If the mixer location is restricted, that they must be located close to each other,
special positioning should be used to prevent the two jet streams from merging
into one, which reduces the overall mixing performance of the mixers.
■ The mixer’s output jet stream should be pointed in the direction of the tank flow to
obtain optimum mixing performance.
The following pages contain general rules and recommendations for submersible
mixers and positioning that should help to find good solutions for the most mixer
applications which will help in providing good installation.

8 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Nomenclature and Symbology

2 Nomenclature and Symbology

2.1 List of Abbreviations
h1 [m] Liquid depth
h2 [m] Clearance between mixer centerline and tank floor
h2a [m] Clearance between mixer centerline to DDS
h3 [m] Liquid level above the propeller tip (Submergence)
h4 [m] Clearance between mixer propeller tip and tank floor
h4a [m] Clearance between mixer propeller tip to DDS
h5 [m] Liquid level above the mixer centerline
ld [m] Distance of propeller from the rear wall
la [m] Distance of propeller to DDS
d2 [m] Propeller diameter
AP [m2] Propeller cross area
aW [m] Clearance between propeller and side wall
aP [m] Clearance between contiguous propeller tips
D [m] Outer tank diameter
d1 [m] Inner tank diameter (Liquid)
di [m] Inner tank diameter (Donut tank)
L [m] Total tank length including wall
W [m] Total tank width including wall
l1 [m] Inner tank length (Liquid)
w1 [m] Inner tank width (Liquid)
lC [m] Length of the straight channel (Oxidation ditch)
wC [m] Channel width (Oxidation ditch and donut tank)
R [m] Radius of the round tank
AC [m2] Channel cross area
V [m3] Tank Volume (Liquid)
n [-] No. of mixers or flow boosters
Q [m3/s] Mixing capacity
FP [N] Propeller thrust

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 9
Nomenclature and Symbology

2.2 Definitions

mixer = ABS submersible mixer type XRW, RW

Characterized by a propeller speed which amounts to > 150 RPM and a propeller
diameter between 200 - 900mm.

flow booster = ABS flow booster type XSB, SB

Characterized by a propeller speed which amounts to < 150 RPM and a propeller
diameter which is > 900mm.

Fig. 1 ABS submersible mixer RW Fig. 2 ABS submersible mixer XRW

Fig. 3 ABS flow booster SB Fig. 4 ABS flow booster XSB

10 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Type of Tanks

3 Type of Tanks
3.1 Round tanks

Fig. 5 Round tank Fig. 6 Donut shaped tank

Round tanks (Fig. 5) mixed by circulation (see pages 36, 37, 38 and 46, 47). These
tanks are typically used for biological, buffer, equalization and sludge storage tanks.
Donut shaped tanks (Fig. 6) are used as oxidation ditches for aerobic, anoxic and
anaerobic processes. Often the inner round tank is used as contact-, degassing- or
anoxic zone.
These tanks are very efficient as mixing or horizontal flow tanks, because very low
power consumption is needed for the different mixing processes.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 11
Type of Tanks

3.2 Rectangular tanks

Fig. 7 Rectangular tank

Fig. 8 no deflection walls Fig. 9 centric deflection walls Fig. 10 eccentric deflection walls

Rectangular tanks (Fig. 6) are used as mixing tanks or oxidation ditches (Fig. 7, 8, 9)
for aerobic, anoxic and anaerobic processes. These tanks are relatively inefficient as
horizontal flow tanks due to the high hydraulic losses caused by the sharp corners.
Therefore the required mixing power, for the different mixing processes, is also high.
Rectangular tanks with centric deflection walls (Fig. 8) minimize the required mixing
power.
Rectangular tanks with eccentric deflection walls (Fig. 9), slightly increases the
required mixing power compared to the tank in Fig. 8.
The given rules on page 43 and 59 are applicable to flow booster installed in
rectangular tanks with deflection walls (Fig. 8, 8).
Special rules are applicable for flow booster installed in rectangular tanks without
deflection walls (Fig. 7) (see pages 44 and 60).

12 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Type of Tanks

3.3 Race track shaped tanks

Fig. 11 no deflection walls Fig. 12 centric deflection walls Fig. 13 eccentric deflection walls

Race track shaped tanks are used as oxidation ditches (Fig. 10, 11, 12) for aerobic,
anoxic and anaerobic processes. These tanks are relatively efficient as horizontal flow
tanks, due to the low hydraulic losses caused by the rounded curves. Therefore the
required mixing power, for the different mixing processes, is also low.
In race track shaped tanks, without deflection walls, (Fig. 10), the required mixing
power is higher than the required power for tanks with deflection walls (Fig. 11 and 12).
In race track shaped tanks, with centric deflection walls, (Fig. 10), minimizes the
required mixing power.
In race track shaped tanks, with eccentric deflection walls (Fig. 11) slightly increases
the required mixing power compared to the tank in Fig. 10.
Rules as shown on page 43 and 59 are applicable to flow booster in race track shaped
tanks, without and with deflection walls, (Fig. 10, 11, 12).

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 13
Type of Tanks

3.4 Serpentine tanks

Fig. 14 Serpentine tank

Fig. 15 centric deflection walls Fig. 16 eccentric deflection walls

Serpentine tanks are used as oxidation ditches (Fig. 13, 14, 15) for aerobic, anoxic and
anaerobic processes. These tanks are very efficient as horizontal flow tanks, due to the
low hydraulic losses caused by the rounded curves and the large outer bend. Therefore
the required mixing power, for the different mixing processes, is also low.
In serpentine shaped tanks, without deflection walls (Fig. 13), the required mixing
power is higher than the required power for tanks with deflection walls (Fig. 14 and 15).
In serpentine shaped tanks, with centric deflection walls, (Fig. 14), minimizes the
required mixing power.
In serpentine shaped tanks, with eccentric deflection walls, (Fig. 15), the required
mixing power slightly increases compare to the tank in Fig. 14.
Rules as shown on page 43 and 59 are applicable to flow booster in Serpentine
shaped tanks, without and with deflection walls, (Fig. 13, 14, 15).

Calculation of the flow Path

The flow path can be calculated with the given formula below.
Liquid Volume
Flow Path 
h1  wc

14 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Relations

4 Relations
4.1 Maximum propeller diameter depending on tank dimensions

3500

3000
Propeller diameter [d2 in mm]

2500

2000
flow booster
1500

1000

mixer
500

0
0 2 4 6 8 10 12 14 16
Tank width [w1 in m] or diameter [d1 in m]

Fig. 17 Diagram maximum propeller diameter depending on tank dimensions

Determination of the maximum propeller diameter

The propeller’s task is to convert mechanical energy, discharged into the tank, into
kinetic flow energy (turbulence).
The resulting irregular unsteady flow exposes the propeller to extreme hydraulic forces
during operation. This is especially valid for mixed tanks. These forces are transferred
via the mixer to the drive train and the entire installation.
In order to guarantee smooth running and trouble-free operation of the mixer, the
maximum propeller size can be determined by means of the above diagram.

NOTE
The graph is valid for clean water and liquids similar to water.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 15
Relations

4.2 Minimum tank volume depending on mixing capacity

10000
Tank volume [V in m³]

1000

100

10
0 0,4 0,8 1,2 1,6 2 2,4 2,8 3,2 3,6 4 4,4 4,8 5,2 5,6 6 6,4
Mixing capacity [Q in m³/s]

Fig. 18 Diagram tank volume depending on mixing capacity

Minimum size of mixing tanks

The propeller’s task is to convert mechanical energy, discharged into the tank, into
kinetic flow energy (turbulence).
Due to the irregular unsteady flow resulting from this, the mixing propeller is exposed to
extreme hydraulic forces during operation. This is especially valid for mixed tanks.
These forces are transferred via the mixer to the drive train and the entire installation.
In order to guarantee a smooth running and trouble-free operation of the mixer, the
maximum mixer size can be determined by means of the above diagram.
The graph is valid for clean water and liquids similar to water.

16 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Minimum Submergence

5 Minimum Submergence
(To avoid vortex formation for mixer and flow booster)

5.1 Minimum submergence for mixer

Mixer
Propeller Minimum*
Diameter Submergence
Model h3 (mm)
d2 (mm)
RW 200 200 400
h3
h5 XRW 210 210 400

h1 RW 280 280 450

XRW/RW 300 300 500

h2 XRW/RW 400 400 700
h4
XRW/RW 480 480 500

XRW/RW 650 650 900

Fig. 19 Minimum submergence for mixer
XRW/RW 750 750 750

XRW/RW 900 900 1500

NOTE
Measured values in clean water with motionless surface.
*Guideline value: Minimum submergence can be lower

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 17
Minimum Submergence

5.2 Minimum Submergence for flow booster

Flow booster Propeller Minimum*

Model Diameter Submergence
d2 (mm) h3 (mm)
h3 XSB/SB 900 900 500
h5
SB 1200 1200 650

XSB 1400 1400 750

h1
XSB/SB 1600 1600 900

h2 XSB/SB 1800 1800 1000

h4 XSB/SB 2000 2000 1100

XSB/SB 2200 2200 1200

Fig. 20 Minimum submergence for flow booster XSB/SB 2500 2500 1350

XSB 2750 2750 1500

h3 = 0.75 d2 + 0.25 m

NOTE
Measured values in non aerated track tanks with swirls on the surface.
*Guideline value: Minimum submergence can be lower

Minimum submergence levels should be maintained. Vortexes cause unbalanced loads

on the mixer propeller.
The risk for Vortex formation is high in water like fluids such as activated sludge if the
liquid surface above the propeller is very smooth. The more movement there is on the
surface, the lower the risk of vortex formation. In viscous liquids like sludge’s or paper
pulp a vortex will not form as easily as it does in water like fluids. The minimum
submergence, h3, of the mixer propeller strongly depends on the flow conditions in the
tank. The minimum submergence given, in the above dimensional drawing for flow
booster, is a general recommendation and should be used for safe reliable flow booster
operation.
For special cases, for example, if a mixer or flow booster is installed in a tank where
the liquid level changes, for lower submergence levels or in very shallow tanks a vortex
grid should be installed on the liquid surface. Please contact our local mixer experts for
assistance and guidance.

18 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Minimum Submergence

5.3 Propeller protection with Vortex-Shield

XRW210
XRW/RW 300
XRW/RW 400
XRW/RW 650
Fig. 21 Mixer with Vortex-Shield

Mixer
Propeller Minimum*
Diameter Submergence
Model h3 (mm)
d2 (mm)
h3 RW 200 200 -
h5
XRW 210 210 200

RW 280 280 -
h1
XRW/RW 300 300 250
h2
h4 XRW/RW 400 400 250

XRW/RW 480 480 -

XRW/RW 650 650 300

Fig. 22 Minimum submergence for mixer

XRW/RW 750 750 -
with Vortex-Shield
XRW/RW 900 900 -

NOTE
Measured values in non aerated track tanks with swirls on the surface.
*Guideline value: Minimum submergence can be lower

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 19
Positioning in non-aerated tanks

6 Positioning in non-aerated tanks

6.1 Clearances for mixer
Clearance to side walls
For good mixing efficiency the clearance
aw between the mixer propeller tip and the
side wall should be at least 0.5 times the
aw ≥ 0.5 d2 propeller diameter d2.

NOTE
This rule is valid for aerated
aw tanks as well.

Fig. 22 Clearance to side walls mixer

Clearance to tank bottom

In shallow tanks, in tanks with changing
liquid levels or in circulation ditches the
clearance h4 between the mixer propeller
tip and the bottom should be at least 1 time
the propeller diameter d2.

h4 ≥ d2 NOTE
h4
The bottom clearance h4 for
mixer is automatically achieved
by installing the mixer’s guide rail
stop as shown in the mixer
Fig. 23 Clearance to tank bottom mixer typical installation drawings.

Clearance to rear wall

To prevent unbalanced propeller loads, the
ld ≥ 1.5 d2 clearance ld between the mixer propeller
and the rear wall should be at least 1.5
times the propeller diameter d2.

NOTE
This clearance is automatically
achieved when using the
standard ABS guide rail system.
This rule is valid for aerated
ld tanks as well.

Fig. 24 Clearance to rear wall mixer

20 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

6.2 Clearances for flow booster

Clearance to side walls

aw ≥ 0.3m

For good mixing efficiency the clearance a w

between the flow booster propeller tip and
the side wall should be at least 0.30m (1 ft.).

aw NOTE
This rule is valid for aerated tanks
as well.

Fig. 25 Clearance to side wall flow booster

Clearance to tank bottom

In shallow tanks or in circulation ditches the
0.05m ≤ h4 ≤ 0.25m clearance h4 between the flow booster
propeller tip and the bottom should be
between 0.05 m (2 in.) and 0.25 m (10 in.).
For different installation methods the
distance of the flow booster propeller tip h4
should be 0.05m to 0.25 times the liquid
depth h1.

NOTE
The bottom clearance h4, for
h4 mixers is automatically achieved
by installing the flow booster on
the concrete pedestal.

Fig. 26 Clearance to tank bottom flow booster This rule is valid for aerated tanks
as well.

Clearance to rear wall

In shallow tanks (h1 < 3 d2) the clearance ld
ld ≥ 1 to 1.5 d2m between the flow booster propeller and the
rear wall should be at least 1.5 times the
propeller diameter d2.
In deep tanks (h1 ≥ 3 d2) the clearance ld
between the flow booster propeller and the
rear wall should be at least 1.0 times the
propeller diameter d2.

NOTE
This rule is valid for aerated
tanks as well.
ld

Fig. 27 Clearance to rear wall flow booster

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 21
Positioning in non-aerated tanks

6.3 Vertical view positioning for flow booster

0.05 m ≤ h4 ≤ 0.25 h1

For different installation methods the

distance of the flow booster propeller
tip h4 should be 0.05 m to 0.25 times
h1 the liquid depth h1.

h4

Fig. 28 Vertical view position for flow booster

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
This is a general recommendation for all mixed tanks, liquid circulation tanks and
oxidation ditches, where flow booster are used.

22 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

6.4 Vertical view positioning for mixer

h2 = 0.25 to 0.33 h1

To generate the best mixing

performance in blending and
h1
suspension processes the mixer
should be vertically located as shown
in the illustration.
h2

Fig. 29 Vertical view position for mixer

NOTE
The dimension h2 is automatically achieved by installing the mixer’s guide rail
stop as shown in the mixer typical installation drawings.
This is a general recommendation for all mixed tanks where mixers are used.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 23
Positioning in non-aerated tanks

Mixer directed downwards

d1 (w1)

h2 = 0.25 to 0.33 h1
α = 22° to 30°

min.
3 d2
h1
α

h2 d2

Fig. 30 Mixer directed downwards

To incorporate solids that float or for deep tank the mixer can be directed downwards
by using the vertical angle adjustment bracket. If the liquid depth h1 is deeper than the
tank diameter d1 or the tank width w1 the vertical angle adjustment bracket must be
used. The inclination α of the mixer should be either 22° or 30°.
To generate the best mixing performance in Blending and Suspension processes the
mixer should be vertically located as shown in the illustration above.
The plan view positioning must in accordance with the recommendations on page 26.

NOTE
This is a general recommendation for all mixed tanks where mixers are used.

24 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

Mixer directed upwards

d1

h2 = 0.33 to 0.5 h1
α = 22° to 30°

h1
α

h2

Fig. 31 Mixer directed upwards

To prevent floating crust especially on sludge tanks the mixer must be directed
upwards by using the vertical angle adjustment bracket. The inclination α of the mixer
should be either 22° or 30°.
To generate the best mixing performance in Blending and Suspension processes, the
mixer should be vertically located as shown in the illustration above.
The plan view positioning must in accordance with the recommendations on page 26.

NOTE
This is a general recommendation for all mixed tanks where mixers are used.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 25
Positioning in non-aerated tanks

6.5 Plan view positioning for mixer

Mixer directed upwards and downwards

0.3 w1

7 – 10°

d1 w1

7 – 10°

Fig. 32 Mixer position in round and square tanks 0.3 w1

In round tanks the mixer must be positioned as shown above, i.e. with 7° - 10°
orientation left of tank centerline.
In square or rectangular tanks the mixer should be positioned as shown above, i.e. at
0.3 times of w1 from the tank corner and with mixer centerline oriented as shown.

26 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

6.6 Positioning in square tanks

One unit installed

0.3 w1 0.3 w1
ld ≥ 1.5 d2 ld ≥ 1 to 1.5 d2

w1 w1

mixer flow booster

0.3 w1 0.3 w1
Fig. 33 Positioning in square tank

The mixer and flow booster must be positioned as shown above, i.e. at 0.3 times w1
from the tank corner with the centerline oriented as shown.
To prevent unbalanced propeller loads, the minimum distance ld behind the propeller
should be as shown in the illustration above.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with recommendations on page 21.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 27
Positioning in non-aerated tanks

One unit installed

0.4 w1 0.6 w1

d2

w1

mixer and flow booster

Fig. 34 Positioning in square tank (1 unit)

In square tanks, where the tank width w1 is less than 5 times of the propeller diameter
d2, the mixer should be positioned as shown in the illustration above.
The mixer and flow booster can be positioned as shown above, i.e. at 0.4 (0.6) times of
w1 from the tank corner and with centerline oriented as shown.
To prevent unbalanced propeller loads the clearance l d between the mixer propeller
and the rear wall should be at least 1.5 times the propeller diameter d2, in accordance
with the recommendations on page 20.
The clearance ld between the flow booster propeller and the rear wall should in
accordance with the recommendations on page 21.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

28 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

Two units installed

0.3 w1 0.3 w1

0.3 w1
0.3 w1

w1 w1

0.3 w1

0.3 w1
mixer flow booster
0.3 w1 0.3 w1

Fig. 35 Positioning in square tank (2 units)

The mixer and flow booster must be positioned as shown above, i.e. at 0.3 times w1
from the tank corner with the centerline oriented as shown.
The mixer should be installed at different liquid levels.
To prevent unbalanced propeller loads the clearance l d between the mixer propeller
and the rear wall should be at least 1.5 times the propeller diameter d 2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 29
Positioning in non-aerated tanks

Three units installed

l1

0.1 w1
mixer and flow booster

0.2 w1
0.3 w1
0.4 w1

w1

0.5 l1
0.6 l1

Fig. 36 Positioning in rectangular tank (3 units)

To prevent unbalanced propeller loads, the minimum distance behind the propeller
should be as shown in the above illustration.
The mixer and flow booster must be positioned as shown above, i.e. at 0.1, 0.4, 0.6
times w1 from the tank corner and with centerline oriented as shown.
The mixer should be installed at different liquid levels.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should in
accordance with the recommendations on page 21.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

30 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

6.7 Positioning in rectangular tanks

One unit installed in rectangular tank (1 < l1/w1 ≤ 3)

l1 l1
0.3 l1 0.3 l1

mixer flow booster

w1

0.3 l1 ld ≥ 1.5 d2 0.3 l1 ld ≥ 1 to 1.5 d2

Fig. 37 Positioning in rectangular tank (l1/w1 1 to 3)

The mixer and flow booster must be positioned as shown above, i.e. at 0.3 times l1
from the tank corner with the centerline oriented as shown.
To prevent unbalanced propeller loads, the minimum distance behind the propeller
should be as shown in the above illustration.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 31
Positioning in non-aerated tanks

One unit installed in rectangular tank (1 < l1/w1 < 2)

mixer and flow booster

0.6 w1

w1
0.4 w1

l1
Fig. 38 Positioning in rectangular tank (1 < l1/w1 < 2)

If the ratio l1/w1 is < 2 the mixer and flow booster can also be positioned as shown
above, i.e. at 0.4 (0.6) times the tank width w1 from the tank corner and with centerline
oriented as shown.
To prevent unbalanced propeller loads, the minimum distance behind the propeller
should be as shown in the above illustration.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 in accordance with recommendation on page 20.
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

32 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

One unit installed in long rectangular tank (3 < l1/w1 <= 5)

ld ≥ 1 to 1.5 d2
0.4 w1

mixer and flow booster

w1

approx. 0.4 l1

l1
Fig. 39 Positioning in long rectangular tank (l1/w1 3 to 5)

Min. d2 ≥ w1/8 (Mixer and Flow booster)

Max. d2 ≤ 0.15 w1 (Mixer)
Max. d2 ≤ 0.2 w1 (Flow booster)

In case of rectangular tank with ratio l1/w1 between 3 to 5, the minimum propeller
diameter for both mixer and flow booster must be greater than or equal to w1/8.
The maximum propeller diameter has to be less than 0.15 times w 1 for mixer and 0.2
times w1 for flow booster.
To prevent unbalanced propeller loads the clearance l d between the mixer propeller
and the rear wall should be at least 1.5 times the propeller diameter d 2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 33
Positioning in non-aerated tanks

Two units installed in rectangular tank (1 < l1/w1 <= 3)

0.3 l1
mixer and flow booster

0.25 w1
w1
0.25 w1

0.3 l1

l1
Fig. 40 Positioning of 2 units in a rectangular tank

The mixer and flow booster must be positioned as shown above, i.e. at 0.3 times the
tank length l1 from the tank corner and with centerline oriented as shown.
To prevent unbalanced propeller loads, the minimum distance behind the propeller
should be as shown in the above illustration.
To prevent unbalanced propeller loads the clearance l d between the mixer propeller
and the rear wall should be at least 1.5 times the propeller diameter d2 (see page 20).
The mixer should be installed on different liquid levels.
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

34 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

Three units installed in rectangular tank (1 < l1/w1 <= 3)

l1

0.1 w1
mixer and flow booster

0.2 w1
0.3 w1
0.4 w1

w1

0.5 l1
0.6 l1

Fig. 41 Positioning of 3 units in a rectangular tank

The mixer and flow booster must be positioned as shown above, i.e. at 0.3 times the
tank length l1 from the tank corner and with centerline oriented as shown.
To prevent unbalanced propeller loads, the minimum distance behind the propeller
should be as shown in the above illustration.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The mixer should be installed on different liquid levels.
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 35
Positioning in non-aerated tanks

6.8 Positioning in round tanks

One unit installed (classic method)

ld ≥ 1.5 d2 ld ≥ 1 to 1.5 d2

7 – 10° 7 – 10°

mixer flow booster

Fig. 42 Mixer positioning in round tanks

The mixer or the flow booster must be positioned as shown above, i.e. with 7° - 10°
orientation right of the tank centerline to prevent unbalanced propeller loads, the
minimum distance behind the propeller should be as shown in the above illustration.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

36 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

Two units installed (classic method)

7 – 10°

7 – 10°

mixer and flow booster

Fig. 43 Mixer positioning in round tanks (2 units installed)

The mixer and the flow booster must be positioned as shown above, i.e. with 7° - 10°
orientation right of the tank centerline, to prevent unbalanced propeller loads.
The mixer should be installed on different liquid levels.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 37
Positioning in non-aerated tanks

Three units installed (classic method)

7 – 10°

7 – 10°

7 – 10°

mixer and flow booster

Fig. 44 Mixer positioning in round tanks (3 units installed)

The mixer and the flow booster must be positioned as shown above, i.e. with 7° - 10°
orientation right of the tank centerline, to prevent unbalanced propeller loads.
The mixer should be installed on different liquid levels.
The clearance ld between the mixer propeller and the rear wall should be at least 1.5
times the propeller diameter d2 (see page 20).
The clearance ld between the flow booster propeller and the rear wall should be in
accordance with the recommendations on page 21.
The bottom clearance of the mixer propeller centerline h 2 should be 0.25 to 0.33 of the
liquid depth h1.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

38 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Positioning in non-aerated tanks

Liquid circulation mixer

d1 0.2 d1 0.3 d1

mixer
Vortex

15 – 20°

h1 d1

h2

Bottom cone

Fig. 45 Liquid circulation in round tanks

In round tanks the mixer must be positioned at 0.2 d 1 from the tank wall with mixer
centerline oriented at 15° to 20° as shown above.
The circulating flow is one of the easiest flow patterns to generate. This flow pattern
provides, especially with higher viscosity liquids with higher viscosity or high solid
content, a poor homogenization (because the whole tank content is in rotation).
It is however an effective method to mix liquids with low solid concentration. But pay
attention, that heavy solid particles can settle in the center of the tank.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 39
Positioning in non-aerated tanks

Liquid circulation flow booster

0.3 R

15 – 20°

Fig. 46 Liquid circulation in round tanks

When round tanks are equipped with a center crossing catwalk, the flow booster must
be positioned at 0.3 R from the beginning of catwalk with flow booster centerline
oriented at 15° to 20° as shown above.
Depending on the tank diameter d1, water depth h1 and number of mixers the
positioning and the orientation of the mixers are decided case by case.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

40 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7 Further Rules
7.1 Optimum dimensional ratios in oxidation ditches

wC

wC

lC
Fig. 47 Optimum ratio between channel length and width

lC ≥ 5 to 5.5 wC

0.8 h1 2 h1

h1 h1

Fig. 48 Optimum ratio between channel width and liquid height

0.8 ≤ h1 ≤ 2 h1

The above described dimensional ratios are optimum for a new oxidation ditch design.
Existing oxidation ditches equipped with surface brush aerators normally exceed the
above ratios.

NOTE
For refurbishment with flow booster and DDS, the DDS supplier has to be
contacted for an acceptable positioning of equipment.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 41
Further Rules

7.2 Flow booster in oxidation ditches

≥ wC

Horizontal
velocity
distribution in
the curve

≥ 2 wC

Fig. 49 Avoiding high channel-turbulence

The above illustration shows that the flow booster should not be located directly behind
the curve of the channel, because the change of the flow direction in this area together
with different flow velocity fluctuations will cause high turbulence. If this turbulence
extends into the operating area of the propeller it will cause excessive vibration. In
addition, it will cause high forces and stress on the flow booster installation and drive
train. This could result in reduced life time and increased breakdowns.
Minimum rear distance between the flow booster propeller and the end of the curve
must be at least equal to the channel width wC.
Minimum front distance between the flow booster propeller and the beginning of the
curve must be at least equal to the channel width 2 wC.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

42 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.3 Positioning of flow booster in non-aerated oxidation ditches

With deflection walls

≥ wC ≥ 5d2

wC

≥ 2 wC ≤ 0.5 lC

lC

Fig. 50 Flow booster in a non-aerated oxidation ditch with deflection walls

For trouble free operation in oxidation ditches with deflection walls the flow booster
should be located as shown on the above illustration. Positioning the flow booster in
this manner will prevent unbalanced propeller loads caused by flow turbulence.
The following rules must be applied:
■ Minimum rear distance to be kept between the flow booster propeller and the end
of the curve must be at least equal to the channel width w C.
■ Minimum front distance to be kept between the flow booster propeller and the
beginning of the curve must be at least equal to the channel width 2 w C.
■ Optimum positioning of the flow booster in the channel to achieve the most
uniform mixing and flow generation in the middle of the straight channel length, i.e.
0.5 lC.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.
These rules are valid for serpentine tanks (Multi-channel) as well.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 43
Further Rules

Without deflection walls

≥ wC ≥ 12d2

wC

≥ wC

lC ≥ 18d2
Fig. 51 Flow booster in a non-aerated oxidation ditch without deflection walls

wC ≥ 0.15 lC
h1 > 2d2; min. 3 m

To achieve the calculated horizontal flow velocity in a rectangular tank without

deflection walls the minimum requirement of the tank geometries is shown in the above
illustration.
The following rules must be applied:
■ The rear distance to be kept between the flow booster propellers and the end of
the turn must be at least equal to the channel width w C.
■ The front distance to be kept between the flow booster propellers and the
beginning of the turn must be at least equal to 12 times of the propeller diameter
d2.
■ The straight length lC should be at least > 18 times of the propeller diameter d 2.
■ The liquid depth h1 has to be > 2d2, at least > 3m.
■ The cannel width wC has to be at least > 0.15 of the straight length lC.
If these geometrical ratios are not given, the propeller jet will be reflected in the turn
and a large amount of kinetic energy will be destroyed. The result is that the calculated
horizontal flow will not be generated.

44 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.4 Contiguous mixers in oxidation ditches

aP

aP

Fig. 52 Contiguous mixers in oxidation ditches

aP ≥ 0.5 d2 (for flow booster)

aP ≥ d2 (for mixer)

The following rules must be applied:

■ If multiple mixer must be installed in a tank, the spacing a P between the propeller
tips should be at least equal to the propeller diameter d 2.
■ If multiple flow boosters are installed in a tank, the spacing aP between the
propeller tips should be at least equal to 0.5 the propeller diameter d 2.
Where this is not possible, experience has shown that a short wall between the flow
boosters can be used.

NOTE
This rule is valid for aerated tanks as well.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 45
Further Rules

7.5 Positioning of flow booster in donut tanks

One unit installed

0.5 wC

15 – 20°

R
wC

Fig. 53 One flow booster in a donut tank

When donut shaped tanks are equipped with a center crossing catwalk, the flow
booster has to be positioned as shown above, i.e. in the center of the channel width
centerline oriented at 15° to 20° as shown above.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

46 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

Two or more units installed

0.25 wC

0.50 wC 15 – 20°

0.25 wC

R
wC

Fig. 54 Two or more flow booster in a donut tank

When donut shaped tanks are equipped with a center crossing catwalk, the flow
booster have to be positioned as shown above, i.e. at 0.25, 0.5, 0.25 times wC from the
beginning of catwalk with flow booster centerline oriented at 15° to 20° as shown
above.
The above illustration is referring to a couple of flow booster to be installed. Depending
on the channel width wC, water depth h1 and number of mixers installed the positioning
and the orientation of the mixers must be decided on a case by case basis.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 47
Further Rules

7.6 Contiguous mixers in other tanks

≥ 25°

Fig. 55 Mixers installed close together

Some times, due to practical reasons mixers have to be installed very close together as
shown in the above illustration.
Special positioning as shown above should be used to prevent the two jet streams from
merging into one.
In round tanks the mixer should be orientated at an angle of ≥ 25° as shown on the
above illustration above.
In square and rectangular tanks one mixer should be directed perpendicular to the wall
and the other one with at an angle of ≥ 25° as shown on the illustration above.
The distance between the different guide rails should be > 1.5 d 2.

NOTE
Due to the loss of mixing performance the mixing grade should be at least 15%
higher than the standard.

48 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.7 Filling grade of flow booster in a channel

AC

AP AP AP

Fig. 56 Filling grade in a channel

AP · n ≤ 0.4 · AC

When flow booster are installed parallel to each other in an oxidation ditch the propeller
cross section area AP should not occupy more than 40% of the liquid channel cross
section AC.

NOTE
This rule is valid for Aerated tanks as well.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 49
Further Rules

7.8 Distance to objects

≥ 10 d2
Fig. 57 Minimum distance to objects

Walls, obstacles and built in equipment downstream of a mixer should be at a distance

of at least 10 times the propeller diameter d 2.

50 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.9 Tank flows influence

Flow direction

Inlet Outlet

Inlet Outlet

Fig. 58 Influences on the tank-flow

When the mixer is to suspend solids in tanks with large flow-through velocities ≥ 0.10
m/s, the propeller jet must be directed in the opposite direction to the flow.
With low flow-through velocities < 0.10 m/s, the propeller jet can directed either in
direction of flow without negatively effecting the mixing performance.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 51
Further Rules

Improvement of retention time

Short circuit Improved retention time

Fig. 59 Improvement of the retention time

Mixers can be located in a tank to prevent short circuiting and to improve the retention
time of the liquid within the tank.
The correct mixer position is dependant on the location of the inlet and outlet.

52 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.10 Positioning in aerated tanks with Disc Diffuser System (DDS)

Mixers in tanks with full tank floor DDS coverage

h1

h2a

Fig. 60 Mixer in tank with full floor DDS coverage

h2a ≥ 3d2

When the entire tank floor is covered with the Disc Diffuser System (DDS), the mixer
and aeration system shall not operate together. In this configuration, either the mixer is
switched on and the aeration system is switched off, or vice versa.
In the event that both systems are required to operate simultaneously, refer to page 54
for recommendations.
To avoid damage to any component of the Disc Diffuser System the mixer needs to
have a distance h2a between its centerline and the diffusers surface equal at least 3
times the propeller diameter d2.

NOTE
During the aeration time the mixer has to be switched off.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 53
Further Rules

Mixers in tanks with partial tank floor DDS coverage

h1

h2a w1

1.5 d2 la

0.3 l1 2d2

l1
Fig. 61 Mixer in tank with partial floor DDS coverage

la = 2.5 d2 + 2.5m

When the mixer is to be operated in a mixed tank together with the Disc diffuser
system, the mixer must be vertically positioned closer to the bottom i.e. h 2a = 1 to 3 d2.
The front clearance la between the mixer propeller and the disc diffuser system should
be given by the above formula.
To assure trouble free operation the diffuser air pipe mounting brackets must be
designed and supplied in robust SS material. It is recommended that each air pipe
element be equipped with at least 1 SS mounting bracket.
To prevent unbalanced propeller loads caused by air and flow turbulence, the minimum
distance behind the propeller should be 1.5 times the propeller diameter d 2.

54 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

Flow booster in tanks with full tank floor DDS coverage

h1

h4a

Fig. 62 Flow booster in tank with full floor DDS coverage

h4a ≥ 0.5 d2

When the entire tank floor is covered with the Disc Diffuser System (DSS), the flow
booster and aeration system shall not operate together.
In this configuration, either the flow booster is switched on and the aeration system is
switched off, or vice versa.
In the event that both systems are required to operate simultaneously, refer to page 56
for recommendations.
To avoid damage to any component of the Disc Diffuser System the flow booster must
be positioned with a distance h 4a between the propeller tip and the diffusers surface
equal at lest 0.5 the propeller diameter d 2.

NOTE
During the aeration time the flow booster has to be switched off.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 55
Further Rules

Flow booster in tanks with partial tank floor DDS coverage

h1
w1
h4a

2d2 la

0.3 l1 d2

l1
Fig. 63 Flow booster in tank with partial floor DDS coverage

la = 2.5 d2 + 2.5m

When the flow booster is to be operated in a mixed tank together with a Disc diffuser
system, the flow booster must be vertically positioned closer to the bottom.
The front clearance la between the flow booster propeller and the disc diffuser system
should be given by the above formula.
To assure trouble free operation the diffuser air pipe mounting brackets must be
designed and supplied in robust SS material. It is recommended that each 6 m air pipe
element be equipped with at least 2 SS mounting bracket.
To prevent unbalanced propeller loads caused by air and flow turbulence, the
clearance ld between the flow booster propeller and the rear wall should be as
recommended on page 21.

NOTE
The bottom clearance, h4a, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
The high concrete pedestal should be not used in such applications.
For different installation methods the distance of the flow booster propeller tip h4a
should be 0.05m (2 in) to 0.25 times the liquid depth h1.

56 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

Flow booster in round tanks equipped with disc diffuser system

0.3 R = 15 – 20°

=
h1 2/3 R

h1

h4a

Fig. 64 Flow booster in round tank with partial DDS coverage

When round tanks are equipped with a center crossing catwalk, the flow booster has to
be positioned as shown above, i.e. at 0.15, 0.3 times R from the beginning of catwalk
with flow booster centerline oriented at 15° to 20° as shown above.
For round tanks equipped with one, two or more flow booster the following rules must
be applied.
The front clearance between the flow booster propeller and the disc diffuser system
should be at least equal to 2/3 R.
The rear clearance between the flow booster propeller and the first diffuser row must
be equal to the water depth h 1.
Depending on the tank diameter d1, water depth h1 and number of mixers the
positioning and the orientation of these are decided case by case.

NOTE
The bottom clearance h4a, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4a
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 57
Further Rules

7.11 Spiral flows created by flow booster horizontal flow

Without Flow booster
With Flow booster

Fig. 65 Spiral flows in a tank with DDS

The spiral flows in the DDS are caused by the presence of uncovered areas of the tank
bottom. The spiral flows cause an increase in the air bubble rise velocity causing a
reduction in oxygen transfer due to the reduced retention time of the bubbles. In
general, when flow booster are operating in Oxidation Ditches with DDS, the negative
effect of spiral flows is amplified by the horizontal flow created by the flow booster.
Vortexes and backflow due to the air rising barrier, often occur.

58 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.12 Positioning of flow booster in oxidation ditches with DDS

With deflection walls

≥ wC ≥ wC/n ≥ wC/n or ≥ h1

wC

≥ 5d2

≥ 2wC ≥ h1 ≥ wC/n ≥ wC

lC

Fig. 66 Multiple flow booster in oxidation ditch with DDS

For trouble-free operation in oxidation ditches with Disc Diffuser Systems, the flow
booster should be located as shown the above illustration.
Positioning the flow booster in this manner will prevent unbalanced propeller loads
caused by air and flow turbulence.
Minimum rear distance to be kept between the flow booster propeller and the end of
the curve must be at least equal to the channel width wC.
Minimum front distance to be kept between the flow booster propeller and the
beginning of the curve must be at least equal to the channel width 2 wC.
Optimum positioning of the flow booster in the channel to achieve the most uniform
mixing and flow generation in the middle of the straight channel length, i.e. 0.5 lC.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.
These rules are valid for serpentine tanks (Multi-channel) as well.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 59
Further Rules

Without deflection walls

≥ wC ≥ 12d2
≥ wC/n

wC

≥ 5d2

≥ 12d2 ≥ h1 ≥ wC

LC ≥ 18d2

Fig. 67 Multiple flow booster in oxidation ditch without deflection walls and DDS

For trouble-free operation in oxidation ditches with Disc Diffuser Systems, the flow
booster should be located as shown the above illustration.
Positioning the flow booster in this manner will prevent unbalanced propeller loads
caused by air and flow turbulence.
To achieve the calculated horizontal flow velocity in a rectangular tank without
deflection walls the minimum requirement of the tank geometry is shown in the above
illustration.
The following rules can be applied:
■ Minimum rear distance to be kept between the flow booster propeller and the end
of the turn must be at least equal to the channel width w C.
■ Minimum front distance to be kept between the flow booster propellers and the
beginning of the turn must be at least equal to 12 times the propeller diameter d 2.
■ The straight length lC should be at least ≥ 18 times of the propeller diameter d 2.
■ The liquid depth h1 must be at least ≥ 5d2. At least > 3m.
■ The cannel width wC has to be at least 15% of the straight length lC.

NOTE
If these geometrical ratios are not followed, the propeller jet will be reflected in the
turn and a large amount of kinetic energy will be destroyed. The result is that the
calculated horizontal flow will not be generated.

60 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.13 Flow booster in donut tanks equipped with DDS

One unit installed

0.5 wC 15 – 20°

0.5 wC
h1
2/3 R

wC

0.5 wC

Fig. 68 Single flow booster in donut tank

For trouble free operation in donut shaped tanks with Disk Diffuser Systems, the flow
booster should be located as shown on the above illustration.
When donut shaped tanks are equipped with a center crossing catwalk, the flow
booster has to be positioned as shown above, i.e. at 0.5 times wC from the beginning of
catwalk with flow booster centerline oriented at 15° to 20° as shown above. This
positioning will prevent unbalanced propeller loads caused by air and flow turbulence.
Minimum rear distance to be kept between the flow booster propeller and the last disc
diffuser row must be at least equal to the water depth h 1.
Minimum front distance to be kept between the flow booster propeller and the first disc
diffuser row must be at least equal to the channel width wC
The above distances h1 and wC must be referenced to the centerline of the channel.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 61
Further Rules

Two or more units installed

0.25 wC
15 – 20°

0.50 wC
0.5 wC
0.25 wC 2/3 R

h1

wC

0.5 wC

Fig. 69 Multiple flow booster in donut tank

For a trouble free operation in donut shaped tanks with Disk Diffuser Systems, the flow
booster should be located as shown on the above illustration.
When donut shaped tanks are equipped with a center crossing catwalk, the flow
booster have to be positioned as shown above, i.e. at 0.25, 0,5 times w C from the
beginning of catwalk with flow booster centerline oriented at 15° to 20° as shown
above. This positioning will prevent unbalanced propeller loads caused by air and flow
turbulence.
Minimum rear distance to be kept between the flow booster propeller and the last disc
diffuser row must be at least equal to the water depth h 1.
Minimum front distance to be kept between the flow booster propeller and the first disc
diffuser row must be at least equal to the channel width wC.
The above distances h1 and wC must be referenced to the centerline of the channel.

NOTE
Because the velocity increases from the inner wall to the outer wall the outer flow
booster should have a smaller propeller diameter, than the inner flow booster.

62 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.14 Oxidation ditches with surface aerators (Brushes)

h1 h1

Water depth < 4 m Water depth ≥ 4 m

Fig. 70 Oxidation ditches with surface aerators

The flow booster has to be placed outside the flow and air action zone of the surface
aerator.
If the liquid depth is > 4.0 m the flow booster can be installed downstream of the
aeration rotor.
If the liquid depth is < 4.0 m the flow booster should be installed upstream of the
aeration rotor.

NOTE
The bottom clearance h4, for flow booster is automatically achieved by installing
the flow booster on the concrete pedestal.
For different installation methods the distance of the flow booster propeller tip h4
should be 0.05 m (2 in) to 0.25 times the liquid depth h1.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 63
Further Rules

7.15 Storm water tanks

1
15°

30° 2

4 30°

15°

d1

Fig. 71 Storm water tank with multiple mixers

A storm overflow tank - holding tank - or settlement tank fills during a heavy rainfall.
Rainwater from storms contains waste such as sludge, solids, leaves, sand, etc.
Many of these materials start to decompose in a short time period resulting in foul
odors. Periodic cleaning of the tank is required to prevent this from happening.
Installing a mixer will suspend the waste material in the tank so they can be taken away
with the tank effluent water to a treatment plant.
A precondition for optimum cleaning of the tank is correct mixer size and positioning.
Some times aeration is also required.
The following rules can be applied:
■ Depending on the tank size 1, 2, 3 or 4 mixers may be used.
■ The mixers must be installed step by step in the sequence 1 to 4 as shown above
mentioned sequence 1 to 4.

64 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Further Rules

7.16 Other positioning examples for flow booster

Rectangular tanks with ratio 2 < l1/w1 < 3
1 to 1.5 d2

d2

w1

l1
Fig. 72 Other positioning example for flow booster

Rectangular tanks with an l1/w1 Ratio between 2 and 3 can be equipped with flow
booster as shown in the above illustration.
In shallow tanks (w1 or d1 < 3d2) the clearance ld between the flow booster propeller
and the rear wall should be at least 1.5 times the propeller diameter d 2.
In deep tanks (w1 or d1 > 3d2) the clearance ld between the flow booster propeller and
the rear wall should be at least 1.0 times the propeller diameter d 2.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 65
Further Rules

Rectangular tanks with ratio 3 < l1/w1 < 5

≥ 5 d2 1 to 1.5 d2

d2

w1

l1
Fig. 73 Other positioning example for flow booster

Rectangular tanks with an l1/w1 Ratio between 3 and 5 can be equipped with flow
booster as shown in the above illustration.
In shallow tanks (w1 or d1 < 3d2) the clearance ld between the flow booster propeller
and the rear wall should be at least 1.5 times the propeller diameter d 2.
In deep tanks (w1 or d1 > 3d2) the clearance ld between the flow booster propeller and
the rear wall should be at least 1.0 times the propeller diameter d2.
When flow boosters are installed in series as shown in the above illustration, they
should have a minimum distance of 5d 2 between each other.

66 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Notes

8 Notes

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 67
Notes

68 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Notes

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 69
Notes

70 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Notes

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 71
Notes

72 General Rules for mixer and flow booster Installations Edition 7, 31.10.16
 Notes

www.sulzer.com

E10092 en 5.2011, Copyright © Sulzer Pumps

This brochure is a general product presentation. It does not provide a warranty or guarantee of any kind. Please, contact us for a description of the warranties and guarantees
offered with our products. Directions for use and safety will be given separately. All information herein is subject to chang e without notice.

Edition 7, 31.10.16 General Rules for mixer and flow booster Installations 73