Ducts and Diffusers Design

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.

With Some Knowledge of Air Distribution in HVAC, One:

• Can select optimum air outlets

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

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 Application Complaints About Stagnant Air

25

All Commercial Application Probably 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

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 B:

• Perimeter-type outlets with Non-Spreading:

• Satisfactory for Cooling

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 D:

Room Air Distribution –Cont.

Group E:

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

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

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 L Diffuser Type

Distance to wall perpendicular to jet High sidewall grille (wall)

Distance to closest wall or intersecting air jet Circular ceiling diffuser (ceiling)

Length of room in direction of jet flow Sill grille (floor)

Distance to wall or mid-plane between outlets

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

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

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

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

Typical performance Curves:

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 _P 2=1.5(1050/900)2 =2.04 IWG » W₂/W₁= (rpm₂/rpm₁)3 _W 2=2 (1050/900)3 = 3.2 hp

• Tables showing pressure, flow rate, rpm, and bhp

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

Fan and System Characteristics Showing Deficient Operation

• Point B is specific operation point

• Test may show point A as actual

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

Simple Duct Systems with Outdoor Air Intake and Relief

Shown Pressure Gradient Diagrams

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 = C

_o

( v

2

₎

• Tables give coefficients C

_o

for different fittings

• Equivalent-length method used for fitting losses in

low-velocity duct (table gives equivalent length)

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)

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 to short

• Dampers may be used for short runs (may cause considerable noise) in order to balance system

Equal-friction method –Cont.

300 CFM 25 ft 20 ft 300 CFM 80 ft 60 ft 60 ft 300 CFM 15 ft 300 CFM 30 ft 1 a 3 4 5 7 6 2

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 to outlets

• 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 ft 20 ft 300 CFM 80 ft 60 ft 60 ft 300 CFM 15 ft 300 CFM 30 ft 1 a 3 4 5 7 6 2

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

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

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

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

Example # 1

600 CFM 25 ft 30 ft 400 CFM 400 CFM 55 ft 85 ft 60 ft 500 CFM 25 ft 300 CFM 45 ft 1 2 3 5 6 4 7 a

Example # 2

25 ft 20 ft 300 CFM 80 ft 60 ft 60 ft 300 CFM 15 ft 300 CFM 30 ft 1 a 3 4 5 ₇ 6 2 300 CFM

Example # 3

90° ELBOW 90° ELBOW 90° ELBOW 10 ft 20 ft 20 ft 10 ft 10 ft 5 ft diff P = 0.04 IWG 400 CFM 200 CFM 300 CFM PLENUM SHARP INLET P = 0.04 IWG diff P = 0.04 IWG diff 200 CFM 90° ELBOW P = 0.04 IWG diff 5 ft 20 ft PLENUM 90° ELBOW SHARP INLET P = 0.04 IWG diff 400 CFM 10 ft 10 ft 90° ELBOW 300 CFM P = 0.04 IWG diff

Example # 4

300 CFM 25 ft 20 ft 300 CFM 80 ft 60 ft 60 ft 300 CFM 15 ft 300 CFM 30 ft 1 a 3 4 5 7 6 2

Example # 5

300 CFM 25 ft 20 ft 300 CFM 80 ft 60 ft 60 ft 300 CFM 15 ft 300 CFM 30 ft 1 a 3 4 5 ₇ 6 2

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