REFRIGERATION & AIR

CONDITIONING:
DUCT DESIGN & AIR DISTRIBUTION
CIVIMEC

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INTRODUCTION
Air is processed in the Air Handling Unit (AHU) and
transmitted into the room which is to be airconditioned.

Air Distribution System consists of:

• Supply Air Duct
• Supply Air Outlet
• Return Air Duct
• Return Air Outlet
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CLASSIFICATION
Air moves through the ductwork in response to a pressure
difference created by the fan.

1. High Velocity Duct

• Velocities greater than 750 m/min are considered high
• Higher velocities cause greater noise and large pressure drops.
2. Low Velocity Duct
• Most air conditioning systems use low velocities due to noise
level considerations

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SHAPE OF DUCTS
1. Circular shape is most compact and requires less materials;
has least frictional pressure drop. It may be more difficult
to construct.
2. Square shape is also a compact design but not economical
to maintain throughout the length of ductwork.
3. Rectangular shape is most common for low velocity ducts
because it is easy to construct at worksite.

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ASPECT RATIO
Aspect Ratio is the ratio of larger to smaller dimension of
cross-section of duct. AR = W / D
Lower AR:
1. Less material due to lesser perimeter
2. Less material due to higher gauge of sheet ( Ga. 18 vs Ga. 24)
3. Lesser cost of installation
4. Lesser insulation cost
5. Lesser running cost due to lower
pressure drop
E.g. Duct at 4 sqft x 100ft:
a. 2’x2’, AR=1, 8x100 = 800 sqft
b. 1’x4’, AR=4, 10x100 = 1,000 sqft

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PRESSURE LOSSES
• Loss of pressure due to friction between moving particles of
fluid and the inner surface of duct; it occurs throughout the
duct length.
• Dynamic loss of pressure occurs due to change in cross
section of the duct; also due to change in direction of the
duct.

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Total Pressure, Static Pressure, Velocity Pressure

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VELOCITY REDUCTION METHOD
• Velocity Method is one of simplest ways of designing the
duct system for both supply and return air.
• Application of this method requires selection of suitable
velocities in different duct runs and requires experience.

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Velocity Reduction Method can be used when sizing
air ducts.
The method can be summarized:
1. Select suitable velocities for main and branch ducts from
the table below
2. Find the sizes of main and branch ducts from the air flow
rates and the velocities by using eq. 1 and the charts below
3. From velocities and duct dimensions - find the frictional
pressure loss in the main and branch ducts using the
friction chart below
4. Add minor dynamic loss
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Proper Velocities
A proper velocity depends on application and environment.
The table below indicate commonly used velocities:
Be aware that high velocity close to outlets and inlets may
generate unacceptable noise.
Type of COMFORT SYSTEMS INDUSTRIAL SYSTEMS HIGH SPEED SYSTEMS
Duct m/s ftm m/s ftm m/s ftm
1,575 – 1,670 –
Main ducts 4-7 780 – 1,380 8 - 12 10 - 18
2,360 3,540
Main
1,180 –
branch 3-5 590 - 985 5-8 985 – 1,575 6 - 12
2,360
ducts
Branch
1-3 200 - 590 3-5 590 - 985 5-8 985 – 1,575
ducts 10
Velocity Reduction Method
Velocities commonly used for different applications:
• upstream medium pressure VAV boxes: 2,000 to 2,500 fpm
(10 - 13 m/s)
• transport of fumes, mist or very light particulates: 2,400 fpm
(12 m/s)
• dust collection systems with small particulate: 3,500 fpm (18
m/s)
• dust collection systems with heavy particulate like metals:
5,000 fpm (25 m/s)
• Variable Air Velocity terminal unit, often called VAV box, is the zone-level flow control device. It is basically a
calibrated air damper with an automatic actuator. The VAV terminal unit is connected to either a local or a central
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control system.
Sizing Ducts
Sizes of ducts are then given by the continuity equation like:
A=q/v (1)
where
A = duct cross sectional area (m2)
q = air flow rate (m3/s)
v= air speed (m/s)

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Sizing Ducts
Alternatively in Imperial units
Ai = 144 qi / vi (1b)
where
A = duct cross sectional area (sq.in.)
q = air flow rate (cfm)
v= air speed (fpm)

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Sizing Ducts
1. The diagram is based on standard air 0.075 lb/ft3 in clean
round galvanized metal ducts.
2. 1 inch water = 248.8 N/m2 (Pa)= 0.0361 lb/in2 (psi) = 25.4
kg/m2 = 0.0739 in mercury
3. 1 ft3/min (cfm) = 1.7 m3/h = 0.47 l/s
4. 1 ft/min = 5.08x10-3 m/s
5. 1 inch = 25.4 mm = 2.54 cm = 0.0254 m = 0.08333 ft

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Air Ducts - Friction Loss Diagram
A major loss diagram for air
ducts - Imperial units ranging 10
– 100,000 cfm
• Example - Friction Loss in Air Duct
• The friction loss in a 20 inches diameter
duct with air flow 4,000 cfm can be
estimated to approximately 0.23 inches
water per 100 feet duct as shown in the
diagram above. The air velocity can be
estimated to approximately 1,850 feet
per minute.

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EQUAL FRICTION LOSS METHOD
1. This method is based on principle that friction rate per unit length
in the entire duct system would be maintained constant and duct
sizes would be calculated for different discharges by keeping friction
rate constant.
2. This method is generally suitable when ducts are not too long, and
it can be used for both supply and return ducts.
3. Similar to Velocity Method, the Equal Friction Method requires
partial closure of dampers in all but index run, which may generate
noise.

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STATIC PRESSURE RECOVERY METHOD
This method is commonly used for high velocity systems with long duct
runs, especially in large systems.
In this method, the static pressure is maintained same before each
terminal or branch.

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DUCT DESIGN

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OTHER DESIGN CONSIDERATIONS
1. Thermal mass 6. Infiltration
2. Natural light 7. Mixed air conditions
3. Solar shading 8. Energy in the air
4. Control strategies 9. Comfort level
5. Life safety issues 10. Amount of water in air

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AIR DISTRIBUTION SYSTEM
1. Induction of Air TYPES
2. Spread of Outlet 1. Vaned registers
3. Throw of Outlet 2. Slotted outlet
4. Drop of Outlet 3. Ceiling diffuser
5. Effect of vanes 4. Perforated panels

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AIR DISTRIBUTION SYSTEM
1. Supply Air LOCATIONS
2. Return Air 1. Wall
3. Ventilation Air 2. Ceiling
3. Floor
4. Relief Air
5. Exhaust Air

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Above Ceiling Air Distribution System

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Displacement Air Distribution System

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Displacement Principles
1. Conditioned air moves upward past persons’ breathing zones.
2. Invisible plumes continue with an upward movement toward the ceiling.
3. Persons will breathe lower lower contaminant levels compared with a
mixing design.
Advantages Disadvantages
1. Moderate supply air temperature 1. Higher initial cost
2. Low velocity 2. Less understood
3. Air delivered to comfort zone without mixing 3. Too quiet
4. Accommodate more outside air for same energy cost
5. Very quiet
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Underfloor Air Distribution System

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COMMERCIAL & INDUSTRIAL PLANT HVAC
SYSTEMS
PURPOSE
The purpose of the Heating, Ventilation, and Air Conditioning
(HVAC) System is to:
1. Provide heat and cooling climate control for plant, office /
control room, and electrical room areas of the steam and
chiller plants
2. Maintain Interior Air Quality (IAQ)
3. Maintain air pressure
4. Provide emergency exhaust
COMMERCIAL & INDUSTRIAL PLANT HVAC
SYSTEMS
Exhaust Air
a. Air expelled to remove contaminants from areas where
chemicals, hazardous materials, and toxic gases are present
b. Air exhausted from the building in case of emergency
5. Makeup Air
Air blown into the building by Makeup Air Units (MAUs) to
maintain air pressure balance when pressure is low
6. Relief Air
Air exhausted from the building to maintain air pressure
balance when air pressure is high
COMMERCIAL & INDUSTRIAL PLANT HVAC
SYSTEMS
HVAC AIR FUNCTIONS
1. Return Air - Air inside the building drawn back into the air
handlers
2. Outside Air - Fresh air drawn into the air handlers from
outside
3. Supply Air -Air blown into the building from the air handlers
WHY AIR BALANCE IS IMPORTANT?
1. Over Pressure and Under Pressure conditions negatively
affect building efficiency.
2. Over Pressure
a. When air pressure inside a building is higher than pressure
outside, the conditioned air inside the building is pushed
outside.
b. High air pressure can require excess energy to condition the
air in the building and compensate for energy lost.
WHY IS AIR BALANCE IMPORTANT?
3. Under Pressure
a. When building pressure is lower than pressure outside, the
air outside is drawn inside.
b. Outside air may come from undesirable sources (trash bins,
open doors, cracks, construction, and so forth).
c. Unfiltered or unconditioned air can affect the HVAC
system’s ability to maintain the desired environment.
PLANT HVAC COMPONENTS
1. Air Handler Units (AHUs)
2. Makeup Air Units (MAUs)
3. Fan Coil Units (FCUs)
4. Exhaust fans
5. Supply fans
6. Emergency exhaust fans
AHU COMPONENTS
1. Mixing Box Section contains:
a. Return Air Damper which connects to the Return Air Duct
b. Relief Air Damper also connects to the Return Air Duct
c. Outside Air Damper which connects to the duct to the building exterior
RETURN RELIEF AIR
AIR DAMPER
DAMPER

OUTSIDE
AIR
DAMPER
MIXING
BOX
AHU COMPONENTS
2. The Filter Section contains:
a. Access door for filter element replacement, with pressure gauge
b. W.C. Meter mounted in door
c. Filter is replaced when air pressure reaches maximum resistance
RETURN
W.C. METER AIR
RELIEF AIR

OUTSIDE
AIR

FILTER MIXING
BOX
AHU COMPONENTS
3. The Cooling Coil Section contains:
a. In and Out connections for chilled water, with temperature gauges
b. Cooling coil heat exchanger
RELIEF AIR

RETURN
AIR

OUTSIDE
AIR

COOLING FILTER MIXING

COIL BOX
AHU COMPONENTS
4. The Heating Coil Section contains:
a. In and Out connections for steam
b. Heating coil heat exchanger RETURN
AIR
RELIEF AIR

COOLING
COIL

OUTSIDE
AIR

HEATING FILTER MIXING

COIL BOX
AHU COMPONENTS
5. The Fan Section contains:
a. Fan and motor
b. Starter Box mounted on exterior or nearby
RETURN
AIR

HEATING
COIL OUTSIDE
AIR

COOLING FILTER MIXING

COIL BOX
MAU COMPONENTS
1. The Fan Section contains:
a. Fan and motor
b. Outside Air Damper which connects to the duct to the building exterior

OUTSIDE AIR
DAMPER
FAN
MAU COMPONENTS
2. The Coil Section contains:
a. In and Out connections for steam and chilled
water
b. Heating and cooling coil heat exchangers

OUTSIDE AIR
DAMPER
HEATING COOLING FAN
COIL COIL
MAU COMPONENTS
3. The Filter Section contains:
a. Filters
b. Supply Air Damper

SUPPLY AIR
DAMPER

FILTER

OUTSIDE AIR
DAMPER
HEATING COOLING FAN
COIL COIL
MAU COMPONENTS
4. The Starter box is located next to the MAU.
FCU COMPONENTS
1. Outside Air Damper
2. Electric Duct Heater

RETURN AIR
SUPPLY AIR

ELECTRIC
DUCT HEATER

OUTSIDE AIR
DAMPER
FCU COMPONENTS
1. Filter
2. Cooling Coil
3. Fan and Motor

FAN

COOLING COIL

FILTER
CHILLER PLANT AHU SYSTEM
1. Temperature Transmitters on the Return Air Duct, Supply Air Duct,
and Mixing Box of AHU-21 send readings to Local Controller Digital
Control System (DCS).

LOCAL CONTROLLER
DCS

RETURN AIR DUCT

AHU-21

SUPPLY AIR DUCT

MIXING BOX
CHILLER PLANT AHU SYSTEM
2. The local DCS controls heating and cooling by modulating the Cooling
Coil chilled water control valve and the steam valve supplying the
Heating Coil.
LOCAL CONTROLLER
DCS

COOLING HEATING
COIL COIL

MIXING BOX

CHILLED WATER CONTROL VALVE

STEAM VALVE
CHILLER PLANT AHU SYSTEM
1. The Outside Air Damper is set at 2,000 CFM.
2. The Return Air Damper is set at 30,000 CFM.

SUPPLY AIR
DAMPER
D-21E

AHU-21

MIXING BOX OUTSIDE AIR

DAMPER
D-21F
CHILLER PLANT AHU SYSTEM
1. 1,000 CFM exhausted continuously from Chemical Treatment area
by Exhaust Fan EF-23.
2. 350 CFM exhausted continuously from the Break Room, Men’s and
Women’s Restrooms, Shower, and Janitor’s Closet by Exhaust Fan
EF-24.
BREAK ROOM

JANITOR’S CLOSET

SHOWER

WOMEN’S RESTROOM EXHAUST FAN

EXHAUST TO OUTSIDE
EF-24
MEN’S RESTROOM

LOUVER L-28
CHILLER PLANT AHU SYSTEM
Supply Fans SF-21 and SF-22 run continuously to recirculate air within
the plant.
AHU-21

SUPPLY SUPPLY
FAN SF-21 FAN SF-22
EF-24
CHILLER PLANT AHU SYSTEM
Economizer Operation - Based on outside air enthalpy and outside air
temperature, when less than 60°F AHU-21 operates in economizer cycle
modulating the outside and return air dampers to maintain 60°F supply
air temperature.
LOCAL CONTROLLER
DCS

RETURN AIR DAMPER

OUTSIDE AIR DAMPER

CHILLER PLANT AHU SYSTEM
Economizer Operation - The chilled water valve to AHU-21 is closed during
economizer cycle.
In the event of damper failure or mixed air temperature below 35°F, a
freezestat shuts down AHU and sends an alarm.
LOCAL CONTROLLER
DCS

FREEZESTAT

CHILLED WATER
VALVE
CHILLER PLANT AHU SYSTEM
Economizer Operation - Relief Air Damper RD-21 is controlled by
differential pressure to maintain a 0.05 inch W.C. positive pressure in the
building by modulating the damper.
AHU-21

PRESSURE SENSOR
RELIEF DAMPER RD- A7
21 AT 0.05” W.C.
CHILLER PLANT AHU SYSTEM
Emergency Exhaust - During emergency when refrigerant accumulates
above 100 PPM due to leak or rupture, air recirculation dampers to
Exhaust Fans EF-21 and EF-22 close and exhaust dampers open forcing the
refrigerant from the building.
EMERGENCY
EXHAUST
DAMPER
L-26

EMERGENCY
EXHAUST
DAMPER
L-25
SOUTH
EMERGENCY
EXHAUST
EF-22
NORTH
EMERGENCY
EXHAUST
EF-21
CHILLER PLANT AHU SYSTEM
Emergency Exhaust
a. Supply Fans SF-21 and SF-22 shut down.
b. The outside air damper to AHU-21 opens fully and the return air
damper closes.
c. Additional outside air (30,000 CFM) brought into the building from
AHU-21 is exhausted through Positive Relief Damper RD-1.
d. When the refrigerant detectors no longer sense a low refrigeration
concentration of 100 PPM, exhaust air dampers to EF-21 and 22
close and the air recirculation dampers open. SF-21 and 22 then
energize and AHU-21 returns to a modulating damper cycle.
CHILLER PLANT AHU SYSTEM
Emergency Exhaust
If an emergency occurs when the supply air temperature is less
than 60°F, a temperature sensor opens steam valve TCV-AHU-
21B to maintain a supply air temperature of 60°F.
CHILLER PLANT AHU SYSTEM
AHU-22 supplies 10,000 CFM of cooling to the Electric Room.
The local DCS modulates the cooling coil chilled water valve in response to
temperature sensors in the supply and return air ducts and mixing box.
LOCAL CONTROLLER
DCS

COOLING
COIL

COOLING COIL CHILLED

WATER VALVE TCV-AHU-
22A
CHILLER PLANT AHU SYSTEM
The Outside Air Damper is set at 9,940 CFM.
60 CFM is exhausted continuously through the Gravity Relief Damper in
the mezzanine floor.

OUTSIDE AIR
DAMPER
L-23

AHU-22

GRAVITY
RELIEF
DAMPER
RD-22
CHILLER PLANT AHU SYSTEM
Economizer Operation
Based on outside air enthalpy and outside air temperature, when less
than 60°F AHU-22 operates in economizer cycle modulating the outside
and return air dampers to maintain 60°F supply air temperature.
LOCAL CONTROLLER
DCS

OUTSIDE AIR DAMPER

RETURN AIR DAMPER

CHILLER PLANT AHU SYSTEM
Economizer Operation – Relief Air Damper RD-21 is controlled by
differential pressure to maintain a 0.05 inch W.C. positive pressure in the
building by modulating the damper.

PRESSURE SENSOR A7
RELIEF DAMPER AT 0.05” W.C.
RD-21

AHU-22
CHILLER PLANT AHU SYSTEM
Economizer Operation
1. The chilled water valve to AHU-22 is closed during economizer cycle.
2. In the event of damper failure or mixed air temperature below 35°F, a freezestat
shuts down AHU-22 and sends an alarm.

LOCAL CONTROLLER
DCS

FREEZESTAT

CHILLED WATER VALVE

CHILLER PLANT AHU SYSTEM
FCU-21 supplies 1,285 CFM of cooling to the Control Room, break room, office, and
vestibule.
Cooling is controlled by modulating the cooling coil chilled water valve in response to a
temperature sensor in the Control Room.

LOCAL CONTROLLER
DCS

COOLING COIL CHILLED

WATER VALVE
CONTROL ROOM FCU-21A
COOLING
COIL
CHILLER PLANT AHU SYSTEM
Outside air is set at 175 CFM during cooling with 1,110 CFM of return air.
Heating is provided by a 6 KW Electric Duct Heater located in the supply air duct.

ELECTRIC DUCT
HEATER DH-21

FCU-21
CHILLER PLANT AHU SYSTEM
OPERATIONS MONITORING
Operation of the Plant HVAC Systems is monitored through the Control Room on the top floor of Chilled
Water Plant #2.
CHILLER PLANT AHU SYSTEM
PLANT ROUNDS
1. During an Operator’s routine shift, Rounds are accomplished
to record key information and visually assess HVAC Water
System operation.
2. Each Plant has a Rounds Sheet to be filled out by the
Operator.
3. Each plant has different requirements for plant rounds.
4. The information is recorded, not only for the current status,
but also to record system trends.
CHILLER PLANT AHU SYSTEM
EXAMPLE ROUNDS SHEET (front and back)
CHILLER PLANT AHU SYSTEM
HVAC DAILY ROUNDS: Chiller Plants #1 and #2
1. Outside temperature
2. Outside humidity
3. Outside enthalpy
4. AHU (On/Off)
5. Supply Fans (On/Off)
6. Exhaust Fans (On/Off)
CHILLER PLANT AHU SYSTEM
HVAC DAILY ROUNDS: West and East Campus Steam Plants
1. Outside temperature
2. AHU
a. Online/Offline
b. Space temperature
c. Supply air temperature
3. Supply Fan
a. Online/Offline
b. Space temperature
c. Supply air temperature
CHILLER PLANT AHU SYSTEM
HVAC DAILY ROUNDS: West and East Campus Steam Plants
1. Exhaust Fan (not all exhaust fans are checked for all
conditions)
a. Online/Offline
b. Space temperature
c. Supply air temperature
d. Firm alarm OK
e. Gas alarm OK
f. E-Stop alarm OK
CHILLER PLANT AHU SYSTEM
TYPICAL PROBLEMS ASSOCIATED WITH PLANT HVAC
SYSTEMS
1. Stuck dampers
Most likely cause is corrosion or debris
2. Blocked coils
Most likely cause is corrosion, faulty filters
3. Malfunctioning control components
Components damaged or out of calibration
4. Clogged condensate drains
Most likely cause is debris or damage to pipes
Additional References
1. www.EngineeringToolBox.com
2. info@cedengineering.com, HVAC How to Size and Design
Ducts

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