Chapter 2- The Principles of Air Flow, Air Pressure and Air Filtration

Question 1

Define Velocity pressure

Answer 1

Velocity pressure is the kinetic pressure in the direction of flow necessary to cause a fluid at rest to flow at a given velocity. Velocity pressure is the total pressure minus the static pressure

Question 2

Define static pressure

Answer 2

static pressure is the potential pressure exerted in all directions by the fluid. For a fluid in motion it is measured in the direction normal to the direction of flow. It has the potential to either burst or collapse a duct or other enclosure.

Question 3

What does Q,V and A represent in the formula Q=VA

Answer 3

Q is the quantity of air usually referred to in cubic feet per minute (CFM) V is the speed of flow or velocity of the air usually referred to in feet per minute (FPM) A is the area of the conduit through which the air flows usually ductwork expressed in feet.

Question 4

What is a Pitot tube?

Answer 4

A pitot tube is a device used to measure the velocity pressure of an airstream by simultaneously measuring its static and total pressure.

Question 5

Define pressure drop

Answer 5

Pressure drop is the resistance of a device to the flow of a fluid through it. The pressure drop of a filter is a measure of its resistance to airflow through it.

Question 6

What are the three categories of air filters?

Answer 6

Mechanical air filters
Electrostatically charged Filter Media
Electronic Air Cleaners

Question 7

Describe a mechanical air filter

Answer 7

Mechanical air filters capture particles on the filter media. The particle has to collide with the fibers which make up the filter media. Once it collides it has to adhere to the fiber

Question 8

Describe electrostatically charged filter media

Answer 8

Electrostatically charged filter media increases initial filtration efficiency without affecting resistance to airflow. Particles pick up an electrical charge as they pass through the air. These particles in turn tend to stick to the filter fibers

Question 9

Describe electronic air cleaners

Answer 9

Electronic air cleaners are two stage are externally powered devices that impose a charge on airborne particles in the ionizer section. The charged particles are then collected on oppositely charged plates in the collector section.

Question 10

What are the four processes responsible for the capture of particles in a mechanical filter

Answer 10

Impingement
Interception
Diffusion
Straining

Question 11

Define Impingement

Answer 11

Impingement is the process in which particles are removed from an air stream because of their inertia. The air moves in a straight line colliding with the filter fiber or surface to which it may become attached.

Question 12

Define interception

Answer 12

Interception occurs when a particle follows the air stream but still comes in contact with the fiber as it passes around it

Question 13

Define diffusion

Answer 13

Diffusion explains the capture of very small particles at lower air velocities. Minute particles tend to move in erratic paths described as Brownian Motion

Question 14

Define straining

Answer 14

Straining occurs when the smallest dimension of a particle is greater then the distance between adjoining filter media fibers

Question 15

Define van der Waals forces

Answer 15

van der Waals is the force of attraction between molecules

Question 16

Define dipole moment

Answer 16

Dipole moment happens when there is a greater concentration of electrons at one point or end of a molecule than the other.

Question 17

Describe two disruptive forces

Answer 17

The flow of air through the filter

System vibration may dislodge the particles

Question 18

define efficiency of a filter

Answer 18

Efficiency of a filter is the ability to remove the greatest amount of particles

Question 19

Define pressure drop of a filter

Answer 19

Pressure drop is the difference between the inlet and outlet pressure of the filter created by the resistance of the filter. The greater the resistanc the greater the energy required to overcome it. The filter with the lowest pressure drop is preferred

Question 20

Define filter capacity

Answer 20

Capacity is the amount of air the filter can handle. the volume of air per unit of time that a clean filter can handle at a specific pressure drop.

Question 21

Define Electrostatic attraction (passive)

Answer 21

Electrostatic attraction (passive) is when the filter media becomes Electrostatically charged by the flow of air over the filter. the smaller the fibers the stronger the electrostatic forces will be.

Question 22

Define electrostatic attraction (active)

Answer 22

electrostatic attraction (active) is when synthetic filet fibers can be actively charged during the manufacturer to be either positive or negative

Question 23

What are the three ways to charge a filter

Answer 23

Triboelectric charging
Corona charging
Charging by induction

Question 24

Define triboelectric charge

Answer 24

Triboelectrically charged material results from rubbing together dissimilar polymers

Question 25

Define Corona charged material

Answer 25

Corona charged material involves exposing fibers of filter material to an electrode designed to create high voltage either positive or negative. The ions generated on the electrode are collected on the surface creating the static charge