Duct Sizing Flashcards
The _________ is the total amount of air that the fan must deliver.
total supply air quantity
Define the term block load cfm.
The air source cfm that meets the maximum simultaneous cooling load for the zones. This is often less than the sum of the largest zone cams because they usually do not occur at the same time or day of the year.
True or False? Flex duct should be used as much as possible because it is easier and less expensive to install.
Explain your answer
False. Flex duct has over three times the friction loss rate of smooth sheet metal and should be used in 8 ft or less lengths to minimize total friction loss.
List the advantages and disadvantages of using lined ductwork:
Advantages:
- attenuates fan noise (quieter)
- eliminates need for external duct insulation
Disadvantages:
- increases duct friction loss
- difficult to clean, may harbor microorganisms if allowed to get dirty and damp
Why should you provide several feet of straight duct immediately after the fan?
Air is very turbulent leaving the fan and needs time to establish a uniform velocity profile.
When fittings are located too close to the fan outlet it creates an additional friction loss value known as system effect
Is it good design practice to downsize trunk duct sections after each branch take-off to save money. Why?
No. Transition fittings cost more to fabricate and install than straight duct sections. You should attempt, whenever possible, to maintain at least one of the duct dimensions when transitioning from one duct size to another and should only downsize when the velocity drops below the recommended value.
Describe the recommended procedure for sizing the first section of the duct downstream of the fan.
The first duct section is sized based on an assumed velocity. Recommended design velocity values can be found in ASHRAE material.
True or False? Ceiling plenums should be used for return air on all floors of a building whenever possible. Why?
False. Return air plenums should not be used on top floors adjacent to roofs due to the heat gain from the roof being entrained in the return air.
Under certain conditions the temperature of the structural beams or other metals located in the plenum may be colder than the dew point of the return air. Under these conditions, condensation will form on these surfaces causing stained ceiling tiles and possibly wet surfaces that can harbor growth of microorganisms.
The sum of all pressure losses external to the HVAC unit is referred to as the: _______
a. ) duct friction loss
b. ) total friction loss
c. ) external static pressure
d. ) velocity pressure
e. ) total static pressure
f. ) total fitting loss
external static pressure
Which of the following best describes a duct system wth an operating static pressure of 3.25 in wg?
high velocity
SMACNA Pressure Class 3
high pressure
low velocity
SMACNA Pressure Class 4
SMACNA Pressure Class 4
The sum of all pressure losses from the duct system plus any internal unit losses such as coils, filters, etc. is referred to as the:
a. ) duct friction loss
b. ) total friction loss
c. ) external static pressure
d. ) velocity pressure
e. ) total static pressure
f. ) total fitting loss
e.) total static pressure
True or False? The equal friction method of duct design means that all duct sections have equal friction loss.
False. Equal friction method use the assumption that all duct sections are sized using an equal friction rate. Variations in section friction losses are balanced by downsizing the shorter runs or by installing balancing dampers.
True or False? The equal friction method should not be used to design return air duct systems.
False. The equal friction rate is generally the preferred method for sizing return duct systems.
Which of the following are advantages of the equal friction method as compared to the static regain method?
duct system is self-balancing
lower friction loss
easier to design
results smaller ducts
reduced fan energy requirements
reduced installation costs
easier to design
results in smaller ducts
reduced installation costs
What are the advantages of the modified equal friction method of duct design?
By downsizing the shorter duct runs with lower friction loss there is less of a requirement to “choke” the airflow using balancing dampers. This also results in smaller ducts, which saves money.
