AIR DISTRIBUTION
Abdullah Nuhait, PhDKing 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
Recommended Application Reaction Air Velocity (FPM)
None Complaints About Stagnant Air 0-16
All Commercial ApplicationComplaints About Stagnant Air 25
All Commercial ApplicationProbably Favorable but 50 FPM is Approaching Maximum Tolerable Velocity for Seated People
25-50
Probably Favorable but 50 FPM is Approaching Maximum Tolerable Velocity for Seated People
65
Retail and Department Store Upper Limit For People Moving About Slowly-Favorable
75
Factory Air Conditioning Higher Velocities for Spot Cooling
Some Factory Air Conditioning Installations-Favorable 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:
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
Sound power and sound pressure
Sound measured in decibel (dB): 10 Log10 ( W/10-12 ) dB relative to 1 pW 10 Log10 ( P/2X10-5 ) dB relative to 1 Pa
Frequency range called octave used in sound 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:
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 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
Determine air flow requirement and room size
Select type of diffuser to be used
Determine room characteristic length
Find throw
Using performance data catalog, select appropriate diffuser
Make sure any other specifications are met (noise, pressure drop etc.)
Table: Characteristic Room Lengths for Several Diffusers
Characteristic Length LDiffuser Type
Distance to wall perpendicular to jetHigh sidewall grille (wall)
Distance to closest wall or intersecting air jetCircular ceiling diffuser (ceiling)
Length of room in direction of jet flowSill grille (floor)
Distance to wall or mid-plane between outlets
Perforated diffuser (ceiling)
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 rootof density of air at constant pressure
Capacity, Speed, and pressure inversely proportional to density and power inversely proportional to of square of air at constantmass 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
System and fan characteristics combined on one plot
Intersecting of system and fan characteristics is point of operation
Range of Optimum matching of system and fan shown
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
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
Pressure changes in duct Three constant area horizontal sections Two fittings
Smooth converging transition Abrupt diverging transition
Duct Design
General considerations
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
High-velocity duct system Pressure loss per 100 ft of duct range between 0.4 to 0.7
Chart prepared to help designers to design duct cross section 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 low-velocity 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:
Equal-friction method
Balanced-capacity method
Constant-velocity method
Reduced-velocity method
Static-regain method
T-method (optimization procedure)
Will cover only equal-friction method in detail and briefly cover balanced-capacity method
Equal-friction method
Principle of equal-friction method to make pressure loss per foot of duct length same for entire system
Produce good balanced design for symmetrical duct layout
Most duct systems have variety of duct runs ranging from long toshort
Dampers may be used for short runs (may cause considerable noise) in order to balance system
Equal-friction method reduces air velocity in direction of flow
Equal-friction method Cont.
300 CFM
25 ft20 ft
300 CFM80 ft
60 ft
60 ft 300 CFM
15 ft
300 CFM
30 ft1
a3
4
57
62
Equal-friction method Cont.
One way of starting design of duct work
To select maximum air velocity in main after fan outlet (based on some criterion)
Using this velocity with flow rate, one can establish duct size of that section and pressure loss per 100 ft
Using this pressure loss per 100 ft for all sections, one continue to
find their diameters
Balanced-capacity method
Principle of Balanced-capacity method, one makes loss in total pressure equal for all duct runs from fan tooutlets
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
300 CFM
25 ft20 ft
300 CFM80 ft
60 ft
60 ft 300 CFM
15 ft
300 CFM
30 ft1
a3
4
57
62
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
From name of constant-velocity method, velocity selected and kept fixed for all duct runs
Used for exhaust (kitchen exhaust, grease, industrial ventilation)
In velocity-reduction method, velocities of air set from fan to outlet reduces air velocity in direction of flow
Static-Regain method
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
Used in high-velocity systems
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
600 CFM
25 ft30 ft
400 CFM400 CFM55 ft
85 ft
60 ft 500 CFM
25 ft
300 CFM
45 ft
1
2
3 5 6
47
a
Example # 2
25 ft20 ft
300 CFM80 ft
60 ft
60 ft300 CFM
15 ft
300 CFM
30 ft1
a3
4
5 7
62
300 CFM
Example # 3
90 ELBOW
90 ELBOW
90 ELBOW
10 ft
20 ft20
ft
10 ft
10 ft
5 ft
diffP = 0.04 IWG
400 CFM
200 CFM
300 CFM
PLENUM
SHARP INLETP = 0.04 IWGdiff
P = 0.04 IWGdiff
200 CFM
90 ELBOW
P = 0.04 IWGdiff
5 ft20 ft
PLENUM
90 ELBOW
SHARP INLET
P = 0.04 IWGdiff
400 CFM
10 ft
10 ft
90 ELBOW
300 CFM
P = 0.04 IWGdiff
Example # 4
300 CFM
25 ft20 ft
300 CFM80 ft
60 ft
60 ft 300 CFM
15 ft
300 CFM
30 ft1
a3
4
57
62
Example # 5
300 CFM
25 ft20 ft
300 CFM80 ft
60 ft
60 ft300 CFM
15 ft
300 CFM
30 ft1
a3
4
5 7
62
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)
AIR DISTRIBUTIONAir Distribution cont.Air Distribution cont.Air Distribution cont.: Air conditioning componentsAir Distribution cont.ROOM AIR DISTRIBUTIONRoom Air Distribution Cont.Room Air Distribution Cont.: Occupied Zone Air VelocitiesRoom Air Distribution Cont.: Air DirectionRoom Air Distribution Cont.Room Air Distribution Cont.Room Air Distribution Cont. Group ARoom Air Distribution Cont.Room Air Distribution Cont.Room Air Distribution Cont. Group CRoom Air Distribution Cont.Room Air Distribution Cont.Room Air Distribution Cont.Room Air Distribution Cont.Room Air Distribution Cont Jet and Room Air Velocities and Temperature for Vo = 1000 ft/min and t = - 20 FSound in HVACSound in HVACOctave and 1/3 Octave Bands SeriesNC CurvesAcceptable HVAC Noise Levels in Unoccupied RoomsLinear DiffuserInstallation of Linear DiffuserInstallation of Linear Diffuser Cont.Zero-Bar DiffuserRound DiffuserRound Diffuser Cont.Perforated DiffuserGrilleSquare DiffuserSlot-Bar DiffuserVariable-Volume System (VAV)Steps for Selecting Air OutletTable: Characteristic Room Lengths for Several DiffusersPerformance Data for Round DiffuserPerformance Data for Square DiffuserExampleExampleSolutionFans and Building Air DistributionAir Conditioning ComponentsFans Used In HVACExploded View of Centrifugal FanAxial FansMethod of Obtaining Fan Performance CurvesTypical performance Curves:Forward-tip, Backward-tip, and Vane-axial FansFans lawsPerformance of fansPerformance Curves for FanPressure-Capacity TableSelection of FansFan InstallationFan and System Characteristics Showing Deficient OperationSystem EffectFan outlet ConditionsOutlet-Duct Elbow PositionsInlet-Duct Elbow ConfigurationFans and Variable-Air-Volume Systems (VAV)Air Flow in DuctsDuct DesignPressure Loss Due to FrictionCircular Equivalents of Rectangular DuctsSimple Duct Systems with Outdoor Air Intake and ReliefShown Pressure Gradient DiagramsSimple Duct Systems with Outdoor Air Intake and Relief Cont.Total Pressure Profile for Typical Unitary System Shown Pressure Gradient DiagramAir Flow in FittingsTotal 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 ParenthesesDesign of Low-Velocity Duct SystemsEqual-friction methodEqual-friction method Cont.Equal-friction method Cont.Balanced-capacity methodBalanced-capacity method Cont.Constant- and Reduced-Velocity methodStatic-Regain methodExamplesExample # 1Example # 2Example # 3Example # 4Example # 5Example # 6 Fan produce 0.7 IWG and 0.35 IWG lost pressure in coil, filter and furnace, divide remaining pressure 65% for supDuct layoutDuct Work with Square DiffusersDuct Work with Linear DiffusersDuct 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)