Duct Design Exercise Flashcards
Describe Static pressure
Air pressure in the duct used for selecting fans
Describe Velocity pressure
pressure generated by the velocity and weight of the air, used to measure air flow
Describe Total pressure
It is used to find velocity pressure. Static pressure + velocity pressure = total pressure
What can occur if air density is not taken into acount?
Many systems will have very low air flow
As air is heated or humidified, its specific volume _________ and its density ________
increases, decreases
When are density corrections not required?
If the temperature is between 40F and 100F and up to 1000ft elevation
What is important when ductwork is designed?
Cooling/Heat load and the desired supply air temperature
What is total pressure? formula?
Sum of velocity pressure and static pressure
TP=VP+SP
Describe how velocity and static behave in a duct and explain why velocity pressure needs to be calculated?
Static is exerted in all directions and the velocity is exerted only in the direction of airflow
Because it must be measured perpendicular and parallel to airflow while static pressure is pushing in the direction of air flow
What causes total pressure to drop?
Drops along with airflow because of frictional and turbulence losses
What is total pressure used for?
To find the velocity.
_________ pressure determines the actual mechanical energy that must be supplied to the system
Total pressure
Name and describe 5 designations of ducts in your own words
- Supply air: supplies conditioned air
- Return air: returns conditioned air
- Fresh air: supplies outdoor air
- Exhaust air: exhausts discharge air outdoors from toilets/kitchens/etc
- Mixed air: mixes outdoor air and return air
What is a plenum?
Main part of the supply and return duct system
Name three air terminal devices
- Diffuser
- Grille
- Register
What is the difference between a register and a grille
A register is a grille equipped with a volume control damper
What material is most commonly used in comfort air conditioning systems?
Galvanized steel’ secondly aluminum
Why is stainless steel used for kitchen exhausts?
Moisture, Laden air, and fume exhaust and prevents corrosion
What is the biggest drawback of using non-metallic duct?
Its fire classification
Where are flex ducts used?
Locations where space is restricted
What is a major disadvantage to using flex duct and what should be done to prevent this problem?
Friction loss
Make runs short and stretched as tight as possible
Which duct design is most efficient and why?
Round duct because it offers the least resistance.
What duct design is most commonly seen in residential and commercial applications and why is it preferred?
Rectangular duct is used due to space limitations and ease of install in ceilings/between joists. The aspect ratio (width to height) is also low
Name 5 advantages of using round duct
- Lower pressure drops
- Less surface area requiring less insulation
- Available in longer lengths, spiral locks add rigidity, less leaks
- Higher acoustic performance
- Healthier indoor environment due to lack of edges
What aspect ratio is best for square duct?
2-3 is ideal in trading off added duct cost of material and fan energy savings
A customer complains that every time the AC comes on the noise is overpowering. What could cause this in a rectangular duct system?
High aspect ratio can transmit excessive noise if not properly supported; duct bang
Which duct has the greatest cross-sectional area and a minimum contact surface?
Round duct
What must you take into account if you are planning on changing the shape of a section of duct work? why?
You must consider the duct pressure drop of each configuration and use the “equivalent diameter” to ensure you’re not negatively impacting the airflow
What is the friction rate of a well designed duct system?
0.1’’ per 100ft of duct length
What tool can be used to determine equal friction?
Duct slide rule, Duct calculator, Friction rate chart
Name 3 advantages to using the equal friction method?
- Simplicity
- Reduces air velocities in direction of airflow, which reduces noise
- Best method for CAV systems
What must be installed in the system when using the equal friction method and why?
Balancing dampers must be installed to balance the system
What system is not recommended to use the equal friction method and why?
VAV systems due to terminal units requiring pressure independent control
Name 4 causes of friction loss in a duct design
1.
2.
3.
4.
Name 3 causes of duct fitting dynamic losses?
- Change in air direction via elbows, fittings, take-offs
- Restrictions or obstructions; dampers, filters, coils
- Changes in air velocity due to changes in duct size
Name two institutions responsible for setting standards for ducting?
- ASHRAE
- SMACNA
What must be taken into account when adding elbows and fittings? why?
Equivalent length numbers of fittings are needed to calculate pressure drops
What happens to an evaporator coil when condensation occurs?
higher drop at the same velocity
Give examples where large pressure drops may occur?
coils, heat exchangers, filters
What is ESP and what is it used for?
external static pressure is created downstream of the fan and created by duct losses and pressures the fan must overcome. Calculated by engineer
What should be done post install?
Balancing the air. Air-flow sometimes exceeds design flow
What happens to velocity, total, and static pressures as the air moves further away from the fan outlet?
- Velocity decreases and static increases
- Total pressure is the sum of the static and velocity
- as velocity pressure has decreased, static increased
What can be done to achieve 100% effective duct length?
Straight run from the blower long enough to achieve uniform velocity - no fittings near fan outlet
Name 2 common causes of system effect on the fan and how to fix these problems on the supply side?
- uneven air flow to fan inlet
- six and three rule - spinning air at inlet
- use straight duct with no elbow within 24”
What can be done to return air to improve system effectiveness?
Length four times the diameter of the inlet and straight
What is the recommended length of an extended plenum system and what happens when you go longer?
limited to 24’. Longer creates pressure build up near end
It is not recommended to put a takeoff where on an extended plenum system?
end cap of the plenum
How close should the last run be from the end cap?
branch run starting collars 24 in. from the end caps.
what should be done if the system is longer than 24”
reducing plenum duct system
At what point should the duct be reduced?
When air velocity lost to the branch run exceeds 50%
What is the advantage of using a reducing trunk system?
maintains uniform pressure and air velocity
Where is radial system commonly used?
attics, crawl spaces, slabs and used with up-flow/down-flow/horizontal furnaces
What is the limitation of a perimeter system?
limited to facilities built on slab in cold climate; perimeter designed to create an insulator and prevent heat loss
What must the pressure be within a space?
Balanced in order to maintain neutral air pressure
What is the best method in a R/A for best pressure and comfort? why?
Each room having a return; avoids pressurization, pressure imbalances, quite
downside: cost and friction losses increase blower requirements
What is recommended when installing a central return system? why?
Transfer grilles or jumper ducts
air in must = air out
What problem does adding carpet to a room have on airflow?
seals the room and decreases air flow; install transfer grill
As a general rule, what is the smallest duct diameter for a jumper duct?
6”
On what type of systems are turning vanes used?
low velocity systems
Name and describe two damper types
Parallel: full open/closed
Opposed: Good for vol control
Where are fire dampers located? why?
Point of duct penetration to room; retains integrity and fire rating of wall
Describe a fusible links operation and its rating
165f-286f
melts once temperature limit and shuts damper
What are diffusers designed to do?
Supply air in various directions and mix conditioned air with present air in space
Why are grilled typically not used in the supply side?
Their inability to control the air.
Whats the difference between a grille and a register?
adjustable deflectors/dampers to restrict air flow supplied/returned/exhausted.
When does noise become an issue and what should try to be maintained?
Usually 85-90CFM is an issue
maintain by setting up zone system correctly
What is meant by throw?
Horizontal distance from a diffuser at a specified velocity.
What happens as air leaves a diffuser?
As soon as this air leaves the outlet, it begins to attract secondary air already existing in the room
What is the aspiration ratio and why is it important?
Total volume/Primary air volume
High aspiration good as it shows most air is kept in motion v s stagnant
What shall be considered when locating an RA grille?
Ensure it is not located in primary air stream which can mess up return air supply temp
Considerations for duct system sin bars, kitchens, lavatories, dining rooms, clubs, etc
located near or at the ceiling level to collect the warm air “build-up,” odors, smoke, and fumes
What determines gauge of duct work?
greater of duct’s dimension determines the sheet thickness (gauge)
Whose guidelines shall be followed when determining ducts correct gauge?
SMACNA
Noise complaints can be a result of to thin a metal causeing it to bang or creek. Residential often use a higher gauge to save money
What does SMACNA stand for?
Sheet metal and air conditioning contractors national association
Which gauge is thinner? 16 or 22?
22
What is the advantage to an aspect ratio close to one?
lowest sheet metal and insulation/installations costs
How many cfm per ton of cooling and how many cfm per ton for a heat pump?
400cfm
450-500cfm
Recommended hanger spacing of horizontal duct?
< 4’ - every 8’
4-10’ - every 6’
< 10’ - every 4’
How much energy is lost on a standard duct system?
25 to 40% of heating or cooling energy
Where are ducts most efficient?
Ducts in a conditioned space are the most energy-efficient ducts.
What should be done with duct work run in an unconditioned space such as an attic or crawl space?
They should be insulated. Heat gains and losses are minimized and go directly to the conditioned space
What can leaky duct joints cause?
Leaky supply causes depressurization of the structure
and leaky return causes pressurization
What problems can occur if the ducting is configured poorly?
Twisting/blockages/collapsing
What should be kept at minimum and why?
Reducing the number of bends and turns
The evaporator coil is freezing up, name some air movement problems that would cause this to happen?
Ductwork too small
restrictions
no return air
plugged filter
plugged coils
fan not working
What should be in each room?
One return duct
How is a building divided when zoning is considered?
Zoning is a practice of dividing a building into distinct thermal zones, which have similar heating and cooling requirements.
interior zone
exterior zone
What is the purpose of zoning?
Independent control of temperature and humidity
What is the 24”rule?
24” of straight plenum before any fittings
Plenum length is restricted to?
150’
Electric duct heaters require ho such plenum length before they can be installed?
48”
What type of fitting should be used?
Long and radius fittings instead of short and mitred
When should ducting be insulated?
15F TD between inside of ductwork and outside is when insulation shall be considered?
Two problems caused by leaky ductwork?
lack of airflow
increased costs of runtime
What is an acceptable range of noise for the HVAC industry?
NC values for different types of buildings range from 30 to 40 decibels
5 sources of noise in a ducted system?
- duct borne noise - flow of air
- radiate equipment noise - noise from equipment
- break in noise - equipment noise elsewhere
- break-out noise - transmits through wall
- terminal end noise - air flowing through register
VAV drives should have what?
VFD
fast acting reliefs may be used but not nearly as effective
What can be done to reduce equipment noise?
vibration insulators
insulate mech room
flexible fabric connections
Procedures when using flexible ducting ?
Use only at terminal ends. Completely stretched and restricted to a 6ft length
What can be done to reduce noise at diffusers?
acoustical flex between duct and diffuser
What can be a cause of regenerated noise in a medium and high velocity system?
turbulence produces noise
What does turbulence cause in a system?
Turbulence increases the noise levels and system operating costs.
5 things to look at when testing systems?
- air flow balanced
- total supply as designed
- total return + total exhaust = total supply
- ducts, plenum, air handler tight
- static pressure correct
Name 5 of 10 things to do when balancing
- inspect system
- open all dampers
- velocities at each outlet
- measure free grill area
- air flow volume at each outlet
What device uses a pitot tube and what is it used for?
Inclined manometer to record pressure readings
