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CPE3002 Design > Agitators > Flashcards

Agitators Flashcards

1
Q

What is the equation for Reynold’s number when looking at an agitator?

A

u=N

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2
Q

What is the equation for power required by an agitator?

A
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3
Q

What is the equation for flow number for an agitator?

A
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4
Q

What is the equation for Froude number, and at what value would you expect to seea vortex?

A

A number much greater than 1.

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5
Q

What is the equation relating flow rate with the efficiency of an agitator?

A
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6
Q

What is average shear rate proportional to?

A

N

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7
Q

What is the maximum shear rate proportional to?

A

ND

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8
Q

What is the equation for blending time?

A
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9
Q

Give 3 alternatives to using baffles.

A
  1. Use a square tank
  2. Place the impeller non-centre
  3. Place the impeller at an angle
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10
Q

Give 4 reasons and examples of why we use agitators

A
  1. Circulation, eg. enhancement of heat exchange between the fluid and the container boundary
  2. Suspension, eg. ensuring solids remain suspended in a slurry to keep the mixture homogenous
  3. Disaggregation, eg. breaking up large particles into smaller ones to obtain better dispersion
  4. Mixing, eg. blending of two miscible liquids
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11
Q

Describe the agitator placement heuristics for a flat-bottomed tank.

A
  1. Tank has 4 baffles each with a width of a tenth of the tank diameter
  2. Impeller is raised a distance equal to a third of the tank diameter from the bottom of the tank
  3. The impeller diameter is equal to a third of the tank diameter
  4. The tank diameter is equal to the depth of the liquid
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12
Q

Describe the agitator placement heuristics for a round-bottomed tank.

A
  1. The distance from the bottom of the dome to the bottom of the impeller is equal to a sixth of the distance from the bottom of the dome to the liquid level (or a third if the intention is to keep solids suspended)
  2. The ratio of the impeller diameter to the tank diameter should be between 0.3 and 0.6
  3. The width of the baffle is equal to a twelfth of the tank diameter
  4. The height of the impeller is equal to an eigth of its diameter
  5. The baffle length must be greater than the tank diameter
  6. The baflles must be offset from the side of the tank by a distance equal to a sixth of the width of the baffle
  7. The distance between the bottom of the impeller and where the dome begins must be equal to half of the impeller diameter
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13
Q

Explain the function of a draft tube

A

They are often used in conjunction with an axial impeller to promote and direct the suction and discharge stream, which is valuable in deep tanks especially when suspending solids, dispersing gases or pumping bubbles. Its diameter will be slightly larger than the impeller and can be up to as long as the diameter of the impeller.

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14
Q

Describe and give some simple dimension heuristics for a turbine impeller

A

Flat disc with rectangular blades attached. The length of the blade is typically equal to a quarter of the impeller diameter. The width of the blade is typically equal to a fifth of the impeller diameter. They give a predominently radial flow and can have up to 16 blades, but typically have 6 or 8.

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15
Q

Give a description of a propeller impeller

A

It is similar to a turbine impeller but with a pitched blade, that promotes a more axial flow.

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16
Q

Give a description of a paddle impeller

A

One large, flat blade (sometimes 4 blades) placed low down in the tank. Usually used for more viscous fluids as turbulent flow is unlikely to be achieved. It is common to place more than one in the same shaft, at different distances from the bottom of the tank

17
Q

Give a description of an anchor impeller

A

It goes close to the bottom of the tank and scoops round to follow either wall quite a way up. They move very slowly and are used for non-newtonion or high viscosity fluids. The edge of the impeller moves close to the vessel wass to provide shear and aid heat transfer

18
Q

Give an alternative to using an agitator

A

Could use external circulation by means of a pump. It is a good idea to have these magnetically sealed to prevent material entering the casing. This is more common in the nuclear industry.

19
Q

What must the speed of the impeller be to suspend solids successfully?

A

The horizontal velocity at the bottom of the tank must be greater than the settling velocity of the solid - higher concentrations require more power.

20
Q

Describe how to disperse gases with an impeller successfully

A

Control the gas holdup by drawing the bubbles down. Have an impeller speed high enough so that the impeller does not become ‘flooded’ with bubbles.

21
Q

What is the suggested tip speed and power for a CSTR with a jacket?

A

3-4 m/s

0.1 kW/m3

22
Q

Suggest a suitable tip speed and impeller power for liquid-liquid mixing

A

5 m/s

1 kW/m3

CPE3002 Design (6 decks)

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