Duct Design Fundamentals Flashcards
Define the following terms:
Total Pressure
Velocity Pressure
Static Pressure
Total Pressure: the sum of the velocity pressure and static pressure
Velocity Pressure: the force exerted in the duct system due to the relative speed of the air
Static Pressure: the force exerted in all directions when air pressure is either elevated above or lowered below the atmospheric pressure
Which of the following affects duct friction loss (choose all that apply):
duct size
duct length
thickness of duct wrap
air velocity
duct construction material
fitting type
duct size, duct length, air velocity, duct construction material, fitting type
True or False? A fan begins to convert static pressure into velocity pressure in the first few feet of supply duct.
False - the fan converts velocity pressure into static pressure in the first few feet of duct
A bowling ball sitting still at the top of a hill contains what type of energy?
If this same ball is rolling downhill it contains mostly what type of energy?
Potential Energy
Kinetic Energy
Explain Bernoulli’s law in simple terms:
for any change in velocity there is a corresponding and inverse change in static pressure.
Calculate the velocity of 2250 cfm of air flowing through a each a round duct and a square duct with the same cross-sectional area, 1.5 sq ft.
V = Q/A 2,250/1.5 = 1,500 fpm
Which shape duct (round or square has the highest pressure loss per unit length? Explain why
Square - Square duct has a higher perimeter to area ratio
What is the maximum allowable airflow through a duct with a cross sectional area of 1.5 square ft. and a maximum velocity of 1,000 feet per minute.
Q=VA 1,0001.5 = 1,500 cam
Increasing the aspect ratio of a duct causes which of the following to occur? (choose all that apply)
a. ) the ratio of perimeter to area increases
b. ) the weight of the duct increases
c. ) the duct heat gain increases
d. ) the duct cross-sectional area increases
e. ) the fabrication cost is increased
f. ) the friction loss increases
a.), b.), c.), e.), f.)
Given a design friction rate of 0.08 in wg/100 ft EL for a smooth sheet metal supply duct, what is the equivalent friction rate for flexible, vinyl-coated ductwork with helical wire core?
Table 4 in appendix: multiplier of 3.2 = .26 in wg
The phenomenon of system effect is caused by which of the following conditions? (choose all that apply)
undersized duct system
an oversized fan
an elbow located directly at the fan outlet
turning vanes in an elbow
a very short supply duct
a very long supply duct
an elbow located directly at the fan outlet, a very short supply duct
