Air Pressure (module 4) Flashcards
What is pressure?
- 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.
What is the symbol for pressure?
p or P
What is the equation for pressure?
- force/unit area
- P=F/A
what is the equation for energy?
- force x distance moved
- E= F x L
What is the equation for air pressure?
- energy/volume
- P= E/V
- P= E/AL
How does air move?
From high to low pressure
What is absolute pressure?
Gauge pressure plus barometric or atmospheric pressure.
When is absolute pressure 0?
can be zero only in a perfect vacuum
What is atmospheric pressure?
the pressure due to the weight of the atmosphere (air and water vapor) on the earth’s surface.
What is the average atmospheric pressure at sea level on a column?
14.7 pounds per square inch (psia)= 1 atm
What is standard air?
14.7 psia at 70 °F
Density = 0.075 lbm/ft3 = 1.202 kg/m3
What is the baseline for absolute pressure?
407 in h2O absolute = 0 in h2O w.g.
What is 14.7 psia in gage pressure?
0 psig
What are the comparisons in gage pressure?
- If p > 1 atm then pg > 0 (positive)
- If p < 1 atm then pg < 0 (negative)
- If p = 1 atm then pg = 0
What are the units for pressure?
PSI
Why do we use inches of water (water column) for ventilation systems?
More accurate and shows smaller differences and there’s low pressure in ventilation systems
What level of pressure is there in ventilation?
Usually low
Atmospheric pressure equation
407 h2o= 0 in h2o gage (w.g.)
T or F: Thermal energy of molecular motion is kinetic (internally), but observed externally as potential
True
What is potential energy?
Static pressure
T or F: Static pressure (psig) can be +ve or -ve (above or below atmospheric pressure)
True
What is static pressure?
- associated with the potential energy
- omnidirectional
- atmospheric pressure at sea level static pressure
what is the symbol for static pressure?
SP
what is velocity pressure?
- associated with the kinetic energy of the moving airstream and is exerted only in the direction of flow
- ex: blowing on your hand
What is the symbol for velocity pressure?
VP
T or F: Velocity pressure can only be positive?
True
What is total pressure?
- reflects the combined static and velocity components
- exerted in the direction of the flow
What is the equation that connects TP, SP, and VP?
TP= SP + VP
What is the abbreviation for total pressure?
TP
What is an example of static pressure?
- Put your finger over a small hole in your mouth and blow
- Feel static pressure against your cheeks and finger
How is pressure measured?
- manometer
- static pressure is higher inside the tire than in the atmosphere
What is velocity pressure (dynamic pressure)?
- 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
what happens if we pop a hole in a tire?
- If we pop a little hole in the tire, the air will all exit because Ptire»_space; 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
How do we measure velocity pressure?
- manometer
- put pressure tap facing stream of moving air
What is the velocity equation?
- V= 4005 X √VP
- pressure units: inches of water in w.g
- velocity units: ft/min
What is the Velocity pressure equation? (VP)
VP = (1⁄2density) * (V^2)
What are the 5 components of a Local exhaust ventialtion system? (LEV)
- Hood
- duct
- filter
- fan
- stack
