Air Pressure (module 4)

Question 1

What is pressure?

Answer 1

• ratio of force to the area over which that force is distributed
• force per unit area applied in a direction perpendicular to the surface of an object.

Question 2

What is the symbol for pressure?

Answer 2

p or P

Question 3

What is the equation for pressure?

Answer 3

• force/unit area
• P=F/A

Question 4

what is the equation for energy?

Answer 4

• force x distance moved
• E= F x L

Question 5

What is the equation for air pressure?

Answer 5

• energy/volume
• P= E/V
• P= E/AL

Question 6

How does air move?

Answer 6

From high to low pressure

Question 7

What is absolute pressure?

Answer 7

Gauge pressure plus barometric or atmospheric pressure.

Question 8

When is absolute pressure 0?

Answer 8

can be zero only in a perfect vacuum

Question 9

What is atmospheric pressure?

Answer 9

the pressure due to the weight of the atmosphere (air and water vapor) on the earth’s surface.

Question 10

What is the average atmospheric pressure at sea level on a column?

Answer 10

14.7 pounds per square inch (psia)= 1 atm

Question 11

What is standard air?

Answer 11

14.7 psia at 70 °F
Density = 0.075 lbm/ft3 = 1.202 kg/m3

Question 12

What is the baseline for absolute pressure?

Answer 12

407 in h2O absolute = 0 in h2O w.g.

Question 13

What is 14.7 psia in gage pressure?

Answer 13

0 psig

Question 14

What are the comparisons in gage pressure?

Answer 14

• If p > 1 atm then pg > 0 (positive)
• If p < 1 atm then pg < 0 (negative)
• If p = 1 atm then pg = 0

Question 15

What are the units for pressure?

Answer 15

PSI

Question 16

Why do we use inches of water (water column) for ventilation systems?

Answer 16

More accurate and shows smaller differences and there’s low pressure in ventilation systems

Question 17

What level of pressure is there in ventilation?

Answer 17

Usually low

Question 18

Atmospheric pressure equation

Answer 18

407 h2o= 0 in h2o gage (w.g.)

Question 19

T or F: Thermal energy of molecular motion is kinetic (internally), but observed externally as potential

Answer 19

True

Question 20

What is potential energy?

Answer 20

Static pressure

Question 21

T or F: Static pressure (psig) can be +ve or -ve (above or below atmospheric pressure)

Answer 21

True

Question 22

What is static pressure?

Answer 22

• associated with the potential energy
• omnidirectional
• atmospheric pressure at sea level static pressure

Question 23

what is the symbol for static pressure?

Answer 23

SP

Question 24

what is velocity pressure?

Answer 24

• associated with the kinetic energy of the moving airstream and is exerted only in the direction of flow
• ex: blowing on your hand

Question 25

What is the symbol for velocity pressure?

Answer 25

VP

Question 26

T or F: Velocity pressure can only be positive?

Answer 26

True

Question 27

What is total pressure?

Answer 27

- reflects the combined static and velocity components - exerted in the direction of the flow

Question 28

What is the equation that connects TP, SP, and VP?

Answer 28

TP= SP + VP

Question 29

What is the abbreviation for total pressure?

Answer 29

TP

Question 30

What is an example of static pressure?

Answer 30

- Put your finger over a small hole in your mouth and blow - Feel static pressure against your cheeks and finger

Question 31

How is pressure measured?

Answer 31

- manometer - static pressure is higher inside the tire than in the atmosphere

Question 32

What is velocity pressure (dynamic pressure)?

Answer 32

- pressure we feel from moving air - ex: Put your hand an inch in front of your mouth and blow. Feel pressure of moving air against your hand - air moves in one direction

Question 33

what happens if we pop a hole in a tire?

Answer 33

- If we pop a little hole in the tire, the air will all exit because Ptire >> Patm and Air naturally moves from high to low P - Sp goes down, Vp is high since the SP difference between the tire and atmosphere is large - Vp decreases until Vp=0 and Sp=SPatm - eventually balenced

Question 34

How do we measure velocity pressure?

Answer 34

- manometer - put pressure tap facing stream of moving air

Question 35

What is the velocity equation?

Answer 35

- V= 4005 X √VP - pressure units: inches of water in w.g - velocity units: ft/min

Question 36

What is the Velocity pressure equation? (VP)

Answer 36

VP = (1⁄2density) * (V^2)

Question 37

What are the 5 components of a Local exhaust ventialtion system? (LEV)

Answer 37

- Hood - duct - filter - fan - stack