Ducting PPT Flashcards
Air obeys which two gas laws?
Charles and Boyles
Moving air prevents ____________
Stratification
Air settling at different heights according to temperature is described as?
Stratification
What components have an effect on stratification?
Thermostats and ducting outlets
Factors for calculating air changes (6)
- # occupants
- use of space
- dry-bulb temps
- RH
- fresh air admitted by infiltration
- efficiency of unit
Name four types of duct systems:
Radial
Extended Plenum
Reducing Trunk
Perimeter loop
Name a few non-metal duct types
Fiberglass
Urethane
Plastic
Sheetrock
Textile
Ducts made of fibreglass or asbestos should be ____________
plastic-coated
Which types of ducts must be replaced when installing a new furnace?
Asbestos
Asbestos is classified as a _________
carcinogen
Removal of asbestos shall be done only by who?
abatement companies
Duct size transitions should be tapered:
- Reductions: __” for every 4” in length.
- Enlargements: 1” for every 4 - __” in length
1”
4-7”
Aspect ratio of ducting shall be :
1:1
Diffusers spread air more uniformly than registers. True or False?
True
Grilles are non-adjustable fixtures that cover return duct inlets. True or False?
True
Registers do what?
Deliver air to a room
Dampers control air flow; name 3 types
- multiple-blade damper
- butterfly damper
- split-damper
Most dampers have automatic controls that open or closed based on the __________
thermostat
Power devices for dampers include 4 types; name them
- electric motors
- pneumatic motors
- hydraulic motors
- solenoids
In a pneumatic system, air lines are controlled by the?
thermostat
Pneumatic systems are most commonly used in what applications?
large commercial and industrial systems
What are the two types of fire dampers?
static and dynamic
What are the three rating classifications for fire dampers?
Class A
Class B
Class C
Before installing ducts, check local ____ damper regulations
fire
Turbulence can be caused by abrupt changes in _______ of airflow
direction
_____-______ elbows minimize turbulence
long-radius
You can minimize turbulence in short-radius elbows by using elbows with built-in ________ _____
directional vanes
Never install duct at the end of ?
The plenum
Some things to consider when designing ducting (4)
- air handler central
- short runs
- minimize fittings
- avoid obstacles
Before sizing ducts, the technician must know what?
Heating and cooling loads
Which mode requires more CFM and why?
Cooling due to moist coil and dense air must be overcame
heat rises
Airflow through a blower depends on what two factors?
External static pressure and blower speed
What is ASP?
Available static pressure
- amount of friction or resistance used to design longest duct run
What is DPL?
Device pressure losses
- pressure losses in system caused by devices other than ducts
Friction losses are expressed in what units?
inches of water column
Where do friction losses occur?
pressure drops inside the duct resulting from air contacting the surfaces
To determine duct size, what are two tools you may use?
- airflow friction chart
- duct calculator
Name some variables to consider when using an airflow friction chart?
duct diameters
airflow rates
pressure drops/friction losses
air velocities
What is the total pressure drop allowed?
0.5” w.c between supply and return air duct
Device that blocks infiltration of outside air through an opening?
Air curtain?
Ducting sections are connected with what type of cleats/fasteners?
S fasteners and D cleats
Flexible duct connections reduce?
vibration/noise
Where are duct connections assembled?
in the field
Round ducting sections are connected with?
self tapping sheet metal screws
Round duct requires more or less clearance than square/rectangular duct?
More
Insulation and acoustic lining can be applied to the inside or outside of the duct. True or False?
True
How is interior insulation secured to the duct?
tabs and washers, glue, or both
Interior duct insulation is usually coated with what?
fibreglass
One thing to be aware of when using interior acoustic lining for a duct is what?
reduces cross sectional area of the duct
Ductboard can be fabricated easily using special?
knives
How are sections of ductboard secured?
taping and stapling
Flexible ducting should be stretched to prevent duct collapsing. True or False
True
Flexible ducting has low initial costs and does not transmit noise well. True or False
True
