# Mechanical Operations Questions and Answers – Jet Mixers

This set of Mechanical Operations Multiple Choice Questions & Answers (MCQs) focuses on “Jet Mixers”.

1. A jet mixer depends on which of the following forces?
a) Gravitational force
b) Centrifugal force
c) Centripetal force
d) Magnetic force

Explanation: A jet mixer has nozzles within the tank, which create high velocity jets, thereby creating a strong centrifuging force. This centrifugal force enables efficient mixing. A major difference between centrifugal and centripetal force is that, the former acts radially outwards from the axis, while the latter acts radially inwards.

2. A jet mixer ideally mixes which of the following?
a) Liquids and solids
b) Slurries with viscosity below 1 Pa s
c) Slurries and liquids below 1000 cP
d) Liquids and slurries above 1000 cP

Explanation: When a mixer is used, we need a minimum of two substances to be mixed with each other. When a slurry is mixed with a liquid, either a jet mixer can be used, or a mechanical agitator can be used. The choice among the two, is dependent on the value of viscosity. If the viscosity is below 1000cP, a jet mixer is used, else a mechanical agitator is used.

3. What is the usual size of the diameter of the particles, that are fed to a jet mixer?
a) 20-mm
b) 25-mm
c) 55-mm
d) 60-mm

Explanation: The primary nozzles of the jet mixers are prone to clogging. If the particles have a diameter greater than that of the nozzles, the nozzles get blocked. In order to avoid this, the largest particle fed, should have a diameter smaller than that of the nozzles’. Considering the usual nozzle size in jet mixers, the appropriate diameter of particles that can be fed is 55-mm.

4. Which of the following does not require violent agitation?
a) Neutralization
b) Flash mixing
c) Disinfection
d) Pigment blending

Explanation: Flash mixing, disinfection and pigment blending typically require 10-30s of agitation and fall under the category of violent agitation. Neutralization on the other hand, requires mild agitation for a period of 3-60 minutes, therefore, it is the odd one out.

5. Which of the following agitation methods in a jet mixer is best suited for pH control?
a) Mild agitation
b) Violent agitation
c) Any mode of agitation can be appropriate for pH control
d) Medium agitation

Explanation: In a jet, different agitation methods are used for different applications. Mild agitation is usually used for neutralizing action. Violent agitation is used for pigment blending or disinfection. Medium agitation typically takes place for a time of 3 seconds to 3 minutes and is optimum for pH control.
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6. Which of the following is true about gas liquid contacting with respect to jet mixers?
a) Regular agitators are more effective than jet mixers for gas liquid contacting
b) Jet mixer allows us to use the same power with less gas usage, or, same gas usage with less power consumption
c) Great bubble surface formed by jet mixers, reduces the efficiency of gas liquid contacting
d) Less bubble surface formed by jet mixers, increases the efficiency of gas liquid contacting

Explanation: Great bubble surface and shear in case of jet mixers, increases the efficiency of gas liquid contacting. Jet mixers are more effective than regular mechanical agitators for gas liquid contacting.

7. Which of the following is not true about jet mixers?
a) Jet mixers can be present at different levels in a vessel
b) High installation cost
c) High efficiency
d) Long life

Explanation: Some of the few advantages of jet mixers are that, they have a low installation cost and a low capital cost. Jet mixers are usually anchored at the bottom, however, they can also be fixated on the walls of the vessel or top of the vessel. Therefore, jet mixers can be present at different levels. Jet mixers also have a longer life compared to regular mixers.

8. Which among the following is/are a component of a jet mixer?
a) Gears and shaft
b) Gears
c) Centrifugal pump
d) Centrifugal pump and motor

Explanation: A jet mixer has a centrifugal pump in order to give rise to the centrifugal force, needed for mixing. Unlike a mechanical agitator, a jet mixer does not have any moving parts like shaft and gear. A mechanical agitator uses a motor but a jet mixer does not.

9. Can tanks have multiple jet mixers?
a) Yes, multiple jet mixers can simply turn over the contents at a desired rate
b) No, there is no such arrangement of multiple jet mixers in a tank
c) Yes, jet mixers can be present but, only at different levels
d) Yes, jet mixers can be present either at different levels or at the sides

Explanation: Tall tanks employ multiple jet mixers are under certain circumstances. They can be at different levels, or installed side entering. A single jet mixer is enough for simply turning over the contents at a desired rate.

10. Select the incorrect statement.
a) Jet mixers use less energy than mechanical agitators
b) Jet mixers can mix contents of a tank even if it is not completely full, unlike a mechanical agitator
c) Shear increases efficiency of jet mixers
d) Baffles are a necessity when you use jet mixers

Explanation: Jet mixers use less energy than mechanical agitators do in case of gas-liquid mixing and suspensions. Baffles are not a necessity in jet mixers however, they can increase efficiency. Shear encourages intense mixing and thereby, increases the efficiency of mixing.

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