Ducts and Diffusers Design

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 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


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


• 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


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


Group B:• Perimeter-type outlets with

Non-Spreading:• Satisfactory for Cooling• Less Desirable for Heating


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


Group D:

• Diffuser for Special Applications


Group E:

• Covers Downward Projected Air Jets for Special Application


Air outlets can be located on:

• Walls• Floors• Ceilings


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 µPa

• 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)

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)

Ducts and Diffusers Design

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