2.3.1 Sizing of Flash Mixer

Design Criteria

Governing parameters in the design of Flash Mixer are Velocity Gradient (G) and the Detention Time (t). Varying, values of G and t have been recommended in the literature. The values recommended for the design of Flash Mixer along with their reference and the values adopted are given below:


Design Parameter
Ref 1
Ref 2
Ref 4
Ref 5
Ref 6
Adopted
Velocity Gradient (G)

500 – 1000 1/s
600 – 1000 1/s
300 or More
500 - 600
1000
Detention Time (t)
10 - 20 Sec
60 - 120 Sec
1 - 10 Sec
30 - 60 Sec
20 -30 Sec
10 Sec
Power Requirement
2 – 5 Kw /M3/Min


1-3 Watt/M3/Hr
4-10 Kw/M3/Sec
10.9 Kw/M3/Sec
Depth /Dia Ratio


0.5 - 1.1
0.2 to 0.4

1.0

1. Water Supply & Sewerage by E.W Steel & Terence J.McGhee (P-232)
2. Hand Book of Environmental Engineering Calculations by C. C. Lee & Shun Dar Lin (P - 1.363)
3. Environmental Engineering, A Design Approach by ArcadioP.Sincero & Gregoria A. Sincero (P 267)
4. Introduction to Environmental Engineering by Davis & Cornell (P203 – 204)
5. Water Supply Engineering by BC. Punmia, Ashok jain & Arun Jain (P – 277)
6. Water Supply by A.C Twort, F. M. law (P – 252)

In addition Ref (3) recommends the following G & t values:

Detention Time (t) – Sec
Velocity Gradient (G) - 1/Sec
0.5
4000
10 – 20
1500
20 – 30
950
30 – 40
850
40 -130
750

Design

Design Flow = 18.9 MGD
= 35.0 Cusecs
Detention Time = 10 Sec
Volume = 350 Cft
Average Depth = 8.0 Ft
Area = 43.75 Sft
Diameter = 7.5 Ft
Depth /Dia Ratio = 7.5/8
= 0.94
Provide Flash Mixer Tank 7.5 Ft diameter and 8 Ft deep fitted with vertical shaft mounted high speed Agitator Mixer.

2.3.2 Sizing of Rapid Mixer

Temperature = 20 0C ( 68 0F)
Absolute Viscosity (µ) = 0.00098 N-s/m2
= 2.050 x 10-5 lb.s/ft2
Design

Dynamic Viscosity (µ) =2.050 x 10-5 lb.s/ft2
Volume of Flash Mixer (V) =350 Cft
Velocity Gradient (G) =1000 1/s

Power of Agitator (P) = G2 µV
=(1000)2x2.050x10-5x 350
= 7175 Ft lb / s
= 7175 / 550
= 13.05 hp
= 13.05 x 0.746
= 9.73 Kw
Efficiency of Motor Power to Water Power = 0.9
Motor Power of the Mixer = 9.73/0.9
= 10.81 Kw

Provide 1 No Mixer with output power of 10.81 Kw at rotational speed of 110 - 175 Rev / Min.