Air Flow Flashcards
List three (3) advantages of having the blower set for continuous operation.
Improved air filtration.
Increased building comfort.
Reduces cold areas in the building.
A duct system that is designed for a heating only system will be of sufficient size if you wanted to add cooling to system.
TRUE OR FALSE ?
FALSE
Why is it so important to balance a residential HVAC system?
If a system is not balanced properly, the end result will be cold areas in the building.
Which of these two instruments, the Inclined manometer or
U-tube manometer are most accurate?
The Inclined Manometer is most accurate.
A Velometer cannot be used to measure static pressures over what water column?
10” WC
What are two (2) basic types of Anemometers?
Thermal and Rotating vane.
What is the atmospheric pressure at sea level?
14.7 PSIA
What is static pressure?
Air will exert pressure on the walls of the vessel in which it is confined.
What is total pressure in a duct system?
The total pressure is the sum of the static and velocity pressures.
What causes dynamic pressure losses in a duct system?
Dynamic losses are the result of changes in direction and velocity of air flow. They occur whenever an airstream makes turns, diverges, converges, narrows, widens, enters, exits, or passes dampers, gates, coils, filters, and sound attenuators.
What is the Air Flow formula?
(Velocity)
(Velocity pressure)
Air Velocity = 4005 X square root of Velocity Pressure. (VP)
Velocity Pressure = (V/4005) squared.
Velocity divided by 4005 and then squared.
Example: Velocity Pressure is 0.06” wc what is our velocity? (fpm)
- 06” square root = 0.244948974
- 244948974 X 4005 = 981 fpm
Example: 4. If our system has a velocity of 1750 fpm, what is the velocity pressure?
1750 fpm divided by 4005 = 0.436953807
Square 0.436953807= 0.191 Velocity pressure (VP)
What is aspect ratio?
Duct sizing
Aspect ratio is the ratio between the width and height of a duct.
Cant exceed the 4:1 ratio.
Example: a piece of duct is 24” X 6”
24” divided by 6” = 4
So the duct meets the requirements of the 4:1 ratio.
What is Equivalent length?
Equivalent length is the comparison of the calculated pressure drop through a fitting to the pressure drop through a straight length of duct of the same size.
Example: one 6” 90 degree elbow is equivalent to 10 feet of 6” pipe.
What is the maximum recommended aspect ratio for rectangular duct?
4:1
Avoid using duct with an aspect ratio greater than 4:1.
Because it will increase friction and it cost more to fabricate.
How is the radius ratio of an elbow calculated?
The radius ratio is calculated by using the centre line radius ( one-half the width of the elbow + the throat radius ) of the elbow and dividing it by the width of the elbow.
Centre line radius + throat radius divided by the width of the elbow.
Example: throats 5” and the width is 10” so half of 10 is 5.
5+5=10
10 divided by 10 = 1
The radius ratio for a 10” X 10” elbow with a 5” throat
Is 1
