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Term 3 - Ductwork > Duct Design Exercise > Flashcards

Duct Design Exercise Flashcards

1
Q

Describe Static pressure

A

Air pressure in the duct used for selecting fans

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2
Q

Describe Velocity pressure

A

pressure generated by the velocity and weight of the air, used to measure air flow

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3
Q

Describe Total pressure

A

It is used to find velocity pressure. Static pressure + velocity pressure = total pressure

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4
Q

What can occur if air density is not taken into acount?

A

Many systems will have very low air flow

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5
Q

As air is heated or humidified, its specific volume _________ and its density ________

A

increases, decreases

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6
Q

When are density corrections not required?

A

If the temperature is between 40F and 100F and up to 1000ft elevation

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7
Q

What is important when ductwork is designed?

A

Cooling/Heat load and the desired supply air temperature

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8
Q

What is total pressure? formula?

A

Sum of velocity pressure and static pressure

TP=VP+SP

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9
Q

Describe how velocity and static behave in a duct and explain why velocity pressure needs to be calculated?

A

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

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10
Q

What causes total pressure to drop?

A

Drops along with airflow because of frictional and turbulence losses

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11
Q

What is total pressure used for?

A

To find the velocity.

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12
Q

_________ pressure determines the actual mechanical energy that must be supplied to the system

A

Total pressure

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13
Q

Name and describe 5 designations of ducts in your own words

A
  1. Supply air: supplies conditioned air
  2. Return air: returns conditioned air
  3. Fresh air: supplies outdoor air
  4. Exhaust air: exhausts discharge air outdoors from toilets/kitchens/etc
  5. Mixed air: mixes outdoor air and return air
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14
Q

What is a plenum?

A

Main part of the supply and return duct system

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15
Q

Name three air terminal devices

A
  1. Diffuser
  2. Grille
  3. Register
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16
Q

What is the difference between a register and a grille

A

A register is a grille equipped with a volume control damper

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17
Q

What material is most commonly used in comfort air conditioning systems?

A

Galvanized steel’ secondly aluminum

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18
Q

Why is stainless steel used for kitchen exhausts?

A

Moisture, Laden air, and fume exhaust and prevents corrosion

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19
Q

What is the biggest drawback of using non-metallic duct?

A

Its fire classification

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20
Q

Where are flex ducts used?

A

Locations where space is restricted

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21
Q

What is a major disadvantage to using flex duct and what should be done to prevent this problem?

A

Friction loss

Make runs short and stretched as tight as possible

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22
Q

Which duct design is most efficient and why?

A

Round duct because it offers the least resistance.

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23
Q

What duct design is most commonly seen in residential and commercial applications and why is it preferred?

A

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

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24
Q

Name 5 advantages of using round duct

A
  1. Lower pressure drops
  2. Less surface area requiring less insulation
  3. Available in longer lengths, spiral locks add rigidity, less leaks
  4. Higher acoustic performance
  5. Healthier indoor environment due to lack of edges
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25
Q

What aspect ratio is best for square duct?

A

2-3 is ideal in trading off added duct cost of material and fan energy savings

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26
Q

A customer complains that every time the AC comes on the noise is overpowering. What could cause this in a rectangular duct system?

A

High aspect ratio can transmit excessive noise if not properly supported; duct bang

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27
Q

Which duct has the greatest cross-sectional area and a minimum contact surface?

A

Round duct

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28
Q

What must you take into account if you are planning on changing the shape of a section of duct work? why?

A

You must consider the duct pressure drop of each configuration and use the “equivalent diameter” to ensure you’re not negatively impacting the airflow

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29
Q

What is the friction rate of a well designed duct system?

A

0.1’’ per 100ft of duct length

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30
Q

What tool can be used to determine equal friction?

A

Duct slide rule, Duct calculator, Friction rate chart

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31
Q

Name 3 advantages to using the equal friction method?

A
  1. Simplicity
  2. Reduces air velocities in direction of airflow, which reduces noise
  3. Best method for CAV systems
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32
Q

What must be installed in the system when using the equal friction method and why?

A

Balancing dampers must be installed to balance the system

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33
Q

What system is not recommended to use the equal friction method and why?

A

VAV systems due to terminal units requiring pressure independent control

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34
Q

Name 4 causes of friction loss in a duct design

A

1.
2.
3.
4.

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35
Q

Name 3 causes of duct fitting dynamic losses?

A
  1. Change in air direction via elbows, fittings, take-offs
  2. Restrictions or obstructions; dampers, filters, coils
  3. Changes in air velocity due to changes in duct size
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36
Q

Name two institutions responsible for setting standards for ducting?

A
  1. ASHRAE
  2. SMACNA
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37
Q

What must be taken into account when adding elbows and fittings? why?

A

Equivalent length numbers of fittings are needed to calculate pressure drops

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38
Q

What happens to an evaporator coil when condensation occurs?

A

higher drop at the same velocity

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39
Q

Give examples where large pressure drops may occur?

A

coils, heat exchangers, filters

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40
Q

What is ESP and what is it used for?

A

external static pressure is created downstream of the fan and created by duct losses and pressures the fan must overcome. Calculated by engineer

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41
Q

What should be done post install?

A

Balancing the air. Air-flow sometimes exceeds design flow

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42
Q

What happens to velocity, total, and static pressures as the air moves further away from the fan outlet?

A
  1. Velocity decreases and static increases
  2. Total pressure is the sum of the static and velocity
  3. as velocity pressure has decreased, static increased
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43
Q

What can be done to achieve 100% effective duct length?

A

Straight run from the blower long enough to achieve uniform velocity - no fittings near fan outlet

44
Q

Name 2 common causes of system effect on the fan and how to fix these problems on the supply side?

A
  1. uneven air flow to fan inlet
    - six and three rule
  2. spinning air at inlet
    - use straight duct with no elbow within 24”
45
Q

What can be done to return air to improve system effectiveness?

A

Length four times the diameter of the inlet and straight

46
Q

What is the recommended length of an extended plenum system and what happens when you go longer?

A

limited to 24’. Longer creates pressure build up near end

47
Q

It is not recommended to put a takeoff where on an extended plenum system?

A

end cap of the plenum

48
Q

How close should the last run be from the end cap?

A

branch run starting collars 24 in. from the end caps.

49
Q

what should be done if the system is longer than 24”

A

reducing plenum duct system

50
Q

At what point should the duct be reduced?

A

When air velocity lost to the branch run exceeds 50%

51
Q

What is the advantage of using a reducing trunk system?

A

maintains uniform pressure and air velocity

52
Q

Where is radial system commonly used?

A

attics, crawl spaces, slabs and used with up-flow/down-flow/horizontal furnaces

53
Q

What is the limitation of a perimeter system?

A

limited to facilities built on slab in cold climate; perimeter designed to create an insulator and prevent heat loss

54
Q

What must the pressure be within a space?

A

Balanced in order to maintain neutral air pressure

55
Q

What is the best method in a R/A for best pressure and comfort? why?

A

Each room having a return; avoids pressurization, pressure imbalances, quite

downside: cost and friction losses increase blower requirements

56
Q

What is recommended when installing a central return system? why?

A

Transfer grilles or jumper ducts

air in must = air out

57
Q

What problem does adding carpet to a room have on airflow?

A

seals the room and decreases air flow; install transfer grill

58
Q

As a general rule, what is the smallest duct diameter for a jumper duct?

A

6”

59
Q

On what type of systems are turning vanes used?

A

low velocity systems

60
Q

Name and describe two damper types

A

Parallel: full open/closed

Opposed: Good for vol control

61
Q

Where are fire dampers located? why?

A

Point of duct penetration to room; retains integrity and fire rating of wall

62
Q

Describe a fusible links operation and its rating

A

165f-286f

melts once temperature limit and shuts damper

63
Q

What are diffusers designed to do?

A

Supply air in various directions and mix conditioned air with present air in space

64
Q

Why are grilled typically not used in the supply side?

A

Their inability to control the air.

65
Q

Whats the difference between a grille and a register?

A

adjustable deflectors/dampers to restrict air flow supplied/returned/exhausted.

66
Q

When does noise become an issue and what should try to be maintained?

A

Usually 85-90CFM is an issue

maintain by setting up zone system correctly

67
Q

What is meant by throw?

A

Horizontal distance from a diffuser at a specified velocity.

68
Q

What happens as air leaves a diffuser?

A

As soon as this air leaves the outlet, it begins to attract secondary air already existing in the room

69
Q

What is the aspiration ratio and why is it important?

A

Total volume/Primary air volume

High aspiration good as it shows most air is kept in motion v s stagnant

70
Q

What shall be considered when locating an RA grille?

A

Ensure it is not located in primary air stream which can mess up return air supply temp

71
Q

Considerations for duct system sin bars, kitchens, lavatories, dining rooms, clubs, etc

A

located near or at the ceiling level to collect the warm air “build-up,” odors, smoke, and fumes

72
Q

What determines gauge of duct work?

A

greater of duct’s dimension determines the sheet thickness (gauge)

73
Q

Whose guidelines shall be followed when determining ducts correct gauge?

A

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

74
Q

What does SMACNA stand for?

A

Sheet metal and air conditioning contractors national association

75
Q

Which gauge is thinner? 16 or 22?

A

22

76
Q

What is the advantage to an aspect ratio close to one?

A

lowest sheet metal and insulation/installations costs

77
Q

How many cfm per ton of cooling and how many cfm per ton for a heat pump?

A

400cfm

450-500cfm

78
Q

Recommended hanger spacing of horizontal duct?

A

< 4’ - every 8’

4-10’ - every 6’

< 10’ - every 4’

79
Q

How much energy is lost on a standard duct system?

A

25 to 40% of heating or cooling energy

80
Q

Where are ducts most efficient?

A

Ducts in a conditioned space are the most energy-efficient ducts.

81
Q

What should be done with duct work run in an unconditioned space such as an attic or crawl space?

A

They should be insulated. Heat gains and losses are minimized and go directly to the conditioned space

82
Q

What can leaky duct joints cause?

A

Leaky supply causes depressurization of the structure

and leaky return causes pressurization

83
Q

What problems can occur if the ducting is configured poorly?

A

Twisting/blockages/collapsing

84
Q

What should be kept at minimum and why?

A

Reducing the number of bends and turns

85
Q

The evaporator coil is freezing up, name some air movement problems that would cause this to happen?

A

Ductwork too small
restrictions
no return air
plugged filter
plugged coils
fan not working

86
Q

What should be in each room?

A

One return duct

87
Q

How is a building divided when zoning is considered?

A

Zoning is a practice of dividing a building into distinct thermal zones, which have similar heating and cooling requirements.

interior zone
exterior zone

88
Q

What is the purpose of zoning?

A

Independent control of temperature and humidity

89
Q

What is the 24”rule?

A

24” of straight plenum before any fittings

90
Q

Plenum length is restricted to?

A

150’

91
Q

Electric duct heaters require ho such plenum length before they can be installed?

A

48”

92
Q

What type of fitting should be used?

A

Long and radius fittings instead of short and mitred

93
Q

When should ducting be insulated?

A

15F TD between inside of ductwork and outside is when insulation shall be considered?

94
Q

Two problems caused by leaky ductwork?

A

lack of airflow

increased costs of runtime

95
Q

What is an acceptable range of noise for the HVAC industry?

A

NC values for different types of buildings range from 30 to 40 decibels

96
Q

5 sources of noise in a ducted system?

A
  1. duct borne noise - flow of air
  2. radiate equipment noise - noise from equipment
  3. break in noise - equipment noise elsewhere
  4. break-out noise - transmits through wall
  5. terminal end noise - air flowing through register
97
Q

VAV drives should have what?

A

VFD

fast acting reliefs may be used but not nearly as effective

98
Q

What can be done to reduce equipment noise?

A

vibration insulators

insulate mech room

flexible fabric connections

99
Q

Procedures when using flexible ducting ?

A

Use only at terminal ends. Completely stretched and restricted to a 6ft length

100
Q

What can be done to reduce noise at diffusers?

A

acoustical flex between duct and diffuser

101
Q

What can be a cause of regenerated noise in a medium and high velocity system?

A

turbulence produces noise

102
Q

What does turbulence cause in a system?

A

Turbulence increases the noise levels and system operating costs.

103
Q

5 things to look at when testing systems?

A
  1. air flow balanced
  2. total supply as designed
  3. total return + total exhaust = total supply
  4. ducts, plenum, air handler tight
  5. static pressure correct
104
Q

Name 5 of 10 things to do when balancing

A
  1. inspect system
  2. open all dampers
  3. velocities at each outlet
  4. measure free grill area
  5. air flow volume at each outlet
105
Q

What device uses a pitot tube and what is it used for?

A

Inclined manometer to record pressure readings

Term 3 - Ductwork (4 decks)

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