كيفية تصميم خزانات المزج الكيميائى الافقية/ Horizontal-Shaft Paddle Flocculator

Flocculation

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Horizontal-Shaft Paddle Flocculator

Specifications

Flocculators work much like flash mix chambers, though the water is agitated at a slower rate (a lower velocity gradient) and the detention time is greater.

Here, we will calculate the tank volume, tank dimensions, and power requirements for a horizontal-shaft paddle flocculator like the one shown below.

Chambered flocculation basin.

Specifications:

Rectangular tank
Depth: 7-16 ft
Width: 10-50 ft.
Length: 2 x width
Minimum number of stages: 2
Minimum number of channels: 2
Mixing by horizontal shaft paddles
Variable speed motors for tapering mixing
Shaft rotational speed: 60-100 rev/hr
Paddle area: <10-20% of tank cross section
Peripheral paddle speed: 0.5-2 ft/sec
Detention time: 30-60 min
Flow through velocity: 0.5-1.5 ft/min
Velocity gradient: 30-60 sec-1
Mixing opportunity parameter: 104-105

Summary of Calculations

First we will determine the flocculator's dimensions, as follows:

Decide on the number of channels.

Calculate the flow in one channel.

Calculate the volume of one channel.

Assume a depth.

Calculate the width.

Calculate the length.

Then we will check the flow through velocity:

Calculate the cross-sectional area of one channel.

Calculate the velocity in one channel.

Determine whether the flow through velocity is acceptable.

Finally, we will calculate the power requirements:

Calculate the volume of the entire flocculator.

Calculate water horsepower for the flocculator.

Calculate electric horsepower for the flocculator.

Estimate power costs for the flocculator.

Tank Volume

The tank volume is calculated just as it was for the flash mix chamber.

However, the flocculation basin is typically divided into two or more channels, as shown below.

Two channel flocculator.

Since the water must be divided between the various channels in the flocculator, the flow is similarly divided.

To find the flow in one channel, we use the following formula:

Qc = Q/n

Where:

Qc = flow in one channel
Q = total flow
n = number of channels

In our example calculations, we will use the same treatment plant which we used for the flash mix chamber calculations.

The flow of this plant, as you will remember, is 2 cfs.

We want to determine the volume of one channel of a two-channeled horizontal-shaft paddle flocculator.

The detention time is given as 30 minutes.

First we find the flow in one channel:

Qc = 2 cfs/2

Qc = 1 cfs

Then we determine the volume of the channel:

V = (1 cfs) (30 min) (60 sec/min)

V = 1,800 ft.3

When calculating the volume, we used the same formula used for the flash mix chamber, but added in a conversion since detention time was given in minutes rather than seconds.

Tank Dimensions

The tank dimensions of the flocculator channel are calculated in the same manner that was used for the flash mix chamber.

However, since the flocculator is a rectangle rather than a cylinder, the formulas used are different.

First we choose a depth within the specified range.

We will use a depth of 10 feet here.

The flocculator specifications note that the length should be equal to twice the width.

So we can find the width using the following formula:

Calculating width.

Where:

W = width, ft
V = volume, ft3
d = depth, ft

In the case of our example, the width is calculated as follows:

Calculations.

The length can be calculated using the following formula:

L = 2 W

Where:

L = length, ft
W = width, ft

So the length of our basin would be:
L = 2 (9.5 ft)

L = 19 ft

Check Velocity

The velocity of water flowing through the flocculation basin must be within a very specific range, designed to gently mix the water without breaking apart the floc.

In the case of our basin, the specifications list the flow through velocity as between 0.5 and 1.5 ft/min.

To check whether the flow through velocity of the basin we've designed is within these limits, we used the following formula:

v = 60 Qc/Ax

Where:
v = velocity, ft/min
Qc = flow in one channel
60 = conversion from seconds to minutes
Ax = cross-sectional area, ft2, calculated as follows:
Ax = W d

In our example, first we calculate the cross-sectional area of one of our channels:
Ax = (9.5 ft) (10 ft)

Ax = 95 ft2

Then we calculate the velocity of water flowing through the channel:

v = 60 (1 cfs)/(95 ft2)

v = 0.63 ft/min

This velocity is within the acceptable range, so we can use the tank dimensions calculated above to design our flocculation basin.

If the velocity had been too high, we would have assumed a greater number of channels and then chosen new dimensions.

If the velocity was too low, we would have repeated the calculations using a smaller depth or only one channel.

Power Requirements

The power requirements are calculated just as they were for the flash mix chamber, except that you must be sure to calculate the power requirements for the entire flocculator rather than for just one channel.

We will assume a water temperature of 60°F and a velocity gradient of 45 sec.-1

Our calculations are shown below:

First, the volume of the entire flocculator is calculated:

Flocculator volume = (Channel volume) (Number of channels)

Flocculator volume = (1,800 ft3) (2)

Flocculator volume = 3,600 ft3

Next, the water horsepower is calculated:

Calculations.

Then the electrical horsepower is calculated:

Calculations.

Finally, the cost is calculated:

Cost = (17.9) (0.376) ($0.05)

Cost = $0.34

Notice that the cost to run the flocculator is much less than the cost to run the flash mix chamber.

This smaller cost is due to the much less vigorous mixing occurring in the flocculator.

Conclusions

Our plant requires a two channel, horizontal-shaft paddle flocculator with a total volume of 3,600 ft.3

Each channel will have a volume of 1,800 ft3, a depth of 10 ft, a width of 9.5 ft, and a length of 19 ft.

The flow through velocity in each channel will be 0.63 ft/sec.

The power cost for the entire flocculator will be $0.34 per day.

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كيفية تصميم خزانات المزج الكيميائى الافقية/ Horizontal-Shaft Paddle Flocculator Chambe10

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كيفية تصميم خزانات المزج الكيميائى الافقية/ Horizontal-Shaft Paddle Flocculator Flocch10

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