AIR DISTRIBUTION
Abdullah Nuhait, PhD
King Saud University
Air Distribution –cont.
• Questions:
• What is Air Distribution in HVAC?
• Why Does One Need to Study it?
Air Distribution –cont.
Air Distribution in HVAC:
• Distribution of Conditioned Air in Buildings and Rooms in
Order to Hold Temperatures, Humidities and Air
Velocities within Occupied Space at Acceptable
Conditions
Air Distribution –cont.: Air conditioning components
Air Distribution –cont.
With Some Knowledge of Air Distribution in HVAC, One:
• Can select optimum air outlets
• Can design optimum duct work
ROOM AIR DISTRIBUTION
• Distribution and Movement of Air within Conditioned
Space
• Selection and Location of Optimum Air Outlets Delivering
Proper Amount of Air:
• To Provide Comfort within Occupied Zone
• To Provide Suitable Indoor Quality within Occupied Zone
• To Meet Required Total Pressure
• To Produce acceptable Noise Level within Occupied Zone
Room Air Distribution –Cont.
• Requirements Necessary for Good Air Distribution:
• Temperature: to be Hold within Tolerable Limits
• Air Velocity: Table Illustrates Occupant Reaction to Various
Air Velocities in Occupied Space
Room Air Distribution –Cont.: Occupied Zone Air Velocities
Air Velocity
(FPM)
Reaction
Recommended Application
0-16
Complaints About Stagnant Air
None
25
Complaints About Stagnant Air
All Commercial Application
25-50
Probably Favorable but 50 FPM is Approaching
Maximum Tolerable Velocity for Seated People
All Commercial Application
65
Probably Favorable but 50 FPM is Approaching
Maximum Tolerable Velocity for Seated People
75
Upper Limit For People Moving About SlowlyFavorable
Retail and Department Store
Some Factory Air Conditioning Installations-Favorable
Factory Air Conditioning
Higher Velocities for Spot Cooling
75-300
Room Air Distribution –Cont.: Air Direction
• Air Direction: Sketches Give Guide to Most Desirable
Air Direction for Seated People
Room Air Distribution –Cont.
•
Air outlets can be classified into five groups:
• Group A: air outlets are mounted in or near ceiling that discharge air
horizontally
• Group B: air outlets are mounted in or near floor that discharge air
vertically in non-spreading jet
• Group C: air outlets are mounted in or near floor that discharge air
vertically in spreading jet
• Group D: air outlets are mounted in or near floor that discharge air
horizontally
• Group E: air outlets are mounted in or near ceiling that project air
vertically downward
Room Air Distribution –Cont.
Group A:
• High sidewall type register
• Used in mild climates
• Used on second and succeeding floors of multistory floors
• Not recommended for cold climate
• Diffuser
• Ceiling diffuser very popular in commercial applications
• Linear or T-bar diffusers favored in VAV applications due to
their better flow characteristics at reduced flow
Room Air Distribution –Cont. Group A
Room Air Distribution –Cont.
Group B:
•
Perimeter-type outlets with
Non-Spreading:
• Satisfactory for Cooling
• Less Desirable for Heating
Room Air Distribution –Cont.
Group C:
• Perimeter-type outlets with Spreading:
• Considered as superior for heating applications
• Diffusers with wide spread are best for heating because
buoyancy tends to increase flow
• Diffusers with wide spread are not good for cooling because
buoyancy tends to decrease flow
Room Air Distribution –Cont. Group C
Room Air Distribution –Cont.
Group D:
•
Diffuser for Special Applications
Room Air Distribution –Cont.
Group E:
•
Covers Downward Projected Air Jets for Special Application
Room Air Distribution –Cont.
Air outlets can be located on:
• Walls
• Floors
• Ceilings
Room Air Distribution –Cont.
Terminologies:
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Primary Air
Induced Air
Entrained Air
Terminal Velocity
Throw
Radius of Diffusion
Drop
Temperature Differential
Diffuser
• Linear
• Square
• Round
• T-Bar
• Perforated
Grille
Register
Damper
Spreading Jet
Non-Spreading Jet
Room Air Distribution –Cont
Throw and Drop for Air
Jet and Room Air Velocities and Temperature for Vo = 1000 ft/min and ∆t = - 20 F
Sound in HVAC
Sound becomes noise when:
• Too load
• Unexpected
• Uncontrolled
• Happens at wrong time
• Contains pure tones
• Contains unwanted information
• Unpleasant
Sound in HVAC
•
Audible frequency range for humans extends from 20 Hz to 20000 Hz
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Sound power and sound pressure
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Sound measured in decibel (dB):
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Frequency range called octave used in sound
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frequency bandwidth having upper band limit twice frequency of its lower band limit
All air outlets generate noise
Noise can be annoying to occupants
Noise level can be related to velocity of air through outlet:
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10 Log10 ( W/10-12 ) dB relative to 1 pW
10 Log10 ( P/2X10-5 ) dB relative to 1 µPa
Lower air velocity produces low level of noise
Higher air velocity makes air outlet noisy
Noise criterion (NC) curves widely used to describe noise level of air outlets
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Level below NC of 30 considered quiet
Level above NC of 50 considered noisy
Octave and 1/3 Octave Bands Series
NC Curves
Acceptable HVAC Noise Levels in Unoccupied Rooms
Linear Diffuser
Installation of Linear Diffuser
Installation of Linear Diffuser –Cont.
Zero-Bar Diffuser
Round Diffuser
Round Diffuser –Cont.
Perforated Diffuser
Grille
Square Diffuser
Slot-Bar Diffuser
Variable-Volume System (VAV)
•
VAV air distribution systems use of:
• Linear or T-bar diffusers
• Thermostat-controlled metering device
(called VAV terminal box)
Steps for Selecting Air Outlet
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Determine air flow requirement and room size
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Select type of diffuser to be used
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Determine room characteristic length
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Find throw
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Using performance data catalog, select appropriate diffuser
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Make sure any other specifications are met (noise, pressure drop …
etc.)
Table: Characteristic Room Lengths for Several Diffusers
Diffuser Type
Characteristic Length L
High sidewall grille (wall)
Distance to wall perpendicular to jet
Circular ceiling diffuser (ceiling)
Distance to closest wall or intersecting air jet
Sill grille (floor)
Length of room in direction of jet flow
Perforated diffuser (ceiling)
Distance to wall or mid-plane between
outlets
Performance Data for Round Diffuser
Performance Data for Square Diffuser
Example
• Room part of single-story office Building
• Building located in Riyadh
• Dimensions of room shown in sketch
• Ceiling height =10 ft
• Air quantity = 250 cfm
• Select Ceiling Diffuser
Example
Solution
• Noise level from above table, for office, NC < 35
• Flow rate, Q = 250 cfm
• Room almost square
• From above table, Characteristic length, L = 14/2 = 7 ft
• Throw = L = 7 ft
• Using Q = 250 cfm, throw = 7 ft and NC < 35
• From above performance table for round diffuser, size 10” will be
right size
• Q ok
between 220 cfm and 275 cfm
• Throw = 7.5 ft ok
• NC < 20 ok
• Pressure drop around 0.035 IWG ok
Fans and Building Air Distribution
• Second part of air distribution is distributing air in
buildings through duct work
• Will cover followings:
• Fans and fan performance
• Methods of design of duct
• Examples showing how to design duct work
• Shown, in next slide, components of air conditioning
system
Air Conditioning Components
Fans Used In HVAC
One essential component of HVAC - FANS
• Fan used to move air through ducts and air outlets
• Two type of fans used in HVAC:
• Centrifugal fan (Blower)
» Forward-tip fan
» Backward-tip fan
• Axial fan
» Vane-axial fan
» Tube-axial fan
Exploded View of Centrifugal Fan
Axial Fans
Method of Obtaining Fan Performance Curves
Typical performance Curves:
Forward-tip, Backward-tip, and Vane-axial Fans
Fans laws
Relationships between fan capacity, pressure, speed, and power:
• First three fan laws (most useful)
» Capacity proportional to fan speed (rpm)
» Pressure proportional to square of fan speed
» Power proportional to cube of fan speed
• Other three fan laws
» Pressure and power proportional to density of air at constant
speed and capacity
» Speed, capacity, and power inversely proportional to square root
of density of air at constant pressure
» Capacity, Speed, and pressure inversely proportional to density
and power inversely proportional to of square of air at constant
mass flow of air
Performance of fans
Manufacturers present their fan performance data in
form of:
• Graphs of pressure, efficiency, and power as
functions of flow rate
• Example: Centrifugal fan operating at point 1,
estimate capacity, pressure, and power at speed
1050 rpm, initial bhp = 2 hp
» Q2/Q1= rpm2/rpm1 Q2=5000
(1050/900)=5830CFM
» P2/P1= (rpm2/rpm1)2 P2=1.5(1050/900)2 =2.04
IWG
» W2/W1= (rpm2/rpm1)3 W2=2 (1050/900)3 = 3.2
hp
• Tables showing pressure, flow rate, rpm, and bhp
» Cannot use fan laws
Performance Curves for Fan
Pressure-Capacity Table
Selection of Fans
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System and fan characteristics
combined on one plot
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Intersecting of system and fan
characteristics is point of operation
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Range of Optimum matching of system
and fan shown
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Slope of system and fan characteristics
must be of opposite sign for stable
operation
Fan Installation
Performance of fan can be reduced due to:
• System effect factors
• Fan outlet connection
• Inlet conditions
• Enclosure restrictions
Fan and System Characteristics Showing Deficient Operation
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Point B is specific operation point
Test may show point A as actual
operation point
System Effect
Fan outlet Conditions
Outlet-Duct Elbow Positions
Inlet-Duct Elbow Configuration
Fans and Variable-Air-Volume Systems (VAV)
Inlet Vanes of Centrifugal Fan for
VAV
Air Flow in Ducts
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Pressure changes in duct
Three constant area horizontal sections
Two fittings
• Smooth converging transition
• Abrupt diverging transition
Duct Design
General considerations
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Low-velocity duct system
• Pressure loss per 100 ft of duct range between 0.08 to 0.15
• Pressure loss of 0.1 per 100 ft of duct is ok
• Pressure loss of 0.05 per 100 ft of duct used in most projects in KSA
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High-velocity duct system
• Pressure loss per 100 ft of duct range between 0.4 to 0.7
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Chart prepared to help designers to design duct cross section
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For flowing air in galvanized steel ducts
Forty (40) joints per 100 ft
Based on standard air and fully developed flow (constant area horizontal duct)
Chart gives round cross section
Table gives equivalent rectangular cross section
Air-Duct Calculators (Duct-lator) constructed by manufacturers
Pressure Loss Due to Friction
Circular Equivalents of Rectangular Ducts
Simple Duct Systems with Outdoor Air Intake and Relief
Shown Pressure Gradient Diagrams
Simple Duct Systems with Outdoor Air Intake and Relief – Cont.
Total Pressure Profile for Typical Unitary System
Shown Pressure Gradient Diagram
Air Flow in Fittings
Losses in fitting called dynamic (minor) losses
• Computed using
∆P = Co ( v2 )
• Tables give coefficients Co for different fittings
• Equivalent-length method used for fitting losses in lowvelocity duct (table gives equivalent length)
Total Pressure Loss Coefficient (Pleated Elbow r/D=1.5)
Total Pressure Loss Coefficient (mitered elbow with vanes)
Total Pressure Loss Coefficient (mitered elbow)
Total Pressure Loss Coefficient (transition, round)
Total Pressure Loss Coefficient (transition, rectangular)
Total Pressure Loss Coefficient (conical converging bell-mouth)
Total Pressure Loss Coefficient (smooth converging bell-mouth)
Total Pressure Loss Coefficient (converging tee)
Total Pressure Loss Coefficient (diverging wye)
Total Pressure Loss Coefficient (diverging tee)
Total Pressure Loss Coefficient (diverging tee)
Equivalent Lengths of Some Fittings in Feet with Meters in Parentheses
Design of Low-Velocity Duct Systems
Several methods can be used for design of low-velocity duct work:
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Equal-friction method
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Balanced-capacity method
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Constant-velocity method
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Reduced-velocity method
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Static-regain method
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T-method (optimization procedure)
Will cover only equal-friction method in detail and briefly cover balanced-capacity method
Equal-friction method
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Principle of equal-friction method to make pressure loss per foot of
duct length same for entire system
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Produce good balanced design for symmetrical duct layout
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Most duct systems have variety of duct runs ranging from long to
short
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Dampers may be used for short runs (may cause considerable
noise) in order to balance system
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Equal-friction method reduces air velocity in direction of flow
Equal-friction method –Cont.
a
20 ft
1
80 ft
300 CFM
2
25 ft
15 ft
5
3
60 ft
300 CFM
6
4 60 ft
7 30 ft
300 CFM
300 CFM
Equal-friction method –Cont.
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One way of starting design of duct work
• To select maximum air velocity in main after fan outlet (based on some
criterion)
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Using this velocity with flow rate, one can establish duct size of that
section and pressure loss per 100 ft
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Using this pressure loss per 100 ft for all sections, one continue to
find their diameters
Balanced-capacity method
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Principle of Balanced-capacity method, one makes loss in total pressure equal for all duct runs from fan to
outlets
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Each run may have different equivalent length
• Pressure loss per 100 ft may be different for each run
• This may result in high air velocity (noisy duct)
• Limit air velocity and use damper for balancing
a
20 ft
1
80 ft
300 CFM
2
15 ft
25 ft
5
3
60 ft
300 CFM
6
4 60 ft
7 30 ft
300 CFM
300 CFM
Balanced-capacity method –Cont.
• Longest run form fan to outlets must be determine
• Pressure drop (loss) per 100 ft will be same for sections
of longest run (same as equal-friction method)
• Establish pressure loss for branch by equating its
pressure loss to pressure loss of branch of longest run
• Find pressure loss per 100 ft by divide pressure
loss over equivalent length of that section
Constant- and Reduced-Velocity method
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From name of constant-velocity method, velocity selected and kept
fixed for all duct runs
• Used for exhaust (kitchen exhaust, grease, industrial ventilation)
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In velocity-reduction method, velocities of air set from fan to outlet
• reduces air velocity in direction of flow
Static-Regain method
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Static-regain method reduces air velocity in direction of flow in such
a way that increase (regain) in static pressure in transition just
balances pressure loss in following section
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Used in high-velocity systems
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Method require iterations
Examples
• Several example will be solved using mainly method of
equal friction
• Each example will be solved using computer software
• Ductlator will be used for designing some sections
• Examples done using single line duct work
Example # 1
300 CFM
55 ft
400 CFM
2
45 ft
a
30 ft
25 ft
1
3
25 ft
5
4
600 CFM
85 ft
6
7
60 ft
500 CFM
Example # 2
80 ft
2
a
20 ft
1
300 CFM
60 ft
300 CFM
6
25 ft
3
15 ft
5
4
7
30 ft
60 ft
300 CFM
300 CFM
Example # 3
90° ELBOW
400 CFM
90° ELBOW
P = 0.04 IWG
diff
10 ft
300 CFM
SHARP INLET
P = 0.04 IWG
10 ft
10 ft
10 ft
20 ft
5 ft
20 ft
PLENUM
P = 0.04 IWG
diff
200 CFM
90° ELBOW
diff
Example # 4
a
20 ft
1
80 ft
300 CFM
2
25 ft
15 ft
5
3
60 ft
300 CFM
6
4 60 ft
7 30 ft
300 CFM
300 CFM
Example # 5
80 ft
300 CFM
2
a
20 ft
1
25 ft
3
15 ft
5
4
60 ft
300 CFM
6
7
30 ft
60 ft
300 CFM
300 CFM
Example # 6 Fan produce 0.7 IWG and 0.35 IWG lost pressure in coil, filter and
furnace, divide remaining pressure 65% for supply duct and 35% for return duct
Duct layout
• Actual duct work of some projects shown using double
line duct with sizes shown
• Different diffuser types shown
• Air conditioning equipment shown
Duct Work with Square Diffusers
Duct Work with Linear Diffusers
Duct Work with Round Diffusers (shown concealed equipment)
Duct Work with Linear Diffusers (shown concealed equipment)
Roof-Top Packaged Unit With Duct Work (25 tons, plan)
Roof-Top Packaged Unit With Duct Work (25 tons, Side view)