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.:

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: • • • • • • • • •

• • • • •

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): • •

•

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: • •

•

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 • •

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

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 •

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 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:

•

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 to short

•

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.

a

20 ft 1

80 ft 300 CFM 2 25 ft 15 ft 5 3

4 60 ft

300 CFM

60 ft

300 CFM

6 7 30 ft

300 CFM

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

a

20 ft 1

80 ft 300 CFM 2 15 ft 25 ft 5 3

4 60 ft

300 CFM

60 ft

300 CFM

6 7 30 ft

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

•

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

300 CFM 55 ft

400 CFM

2

45 ft 30 ft a

1

25 ft

25 ft

3

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

4 60 ft

300 CFM

60 ft

300 CFM

6 7 30 ft

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)