Objectives
• Duct Design AND Diffuser Selection

Relationship Between Static and Total Pressure

P t
 
P s



V
1
2 
V
2
2

2

Duct Design
• Total and static pressure drops are proportional to square of velocity
  f
LV
2
D 2 g
 g    f
LV
2
D 2
 
P s
• Plot of pressure drop vs. volumetric flow rate (or velocity) is called system characteristic

System Characteristic

Electrical Resistance Analogy

Frictional Losses

Non-circular Ducts
• Parallel concept to wetted perimeter

Dynamic losses
• Losses associated with
• Changes in velocity
• Obstructions
• Bends
• Fittings and transitions
• Two methods
• Equivalent length and loss coefficients

» ΔP t
= C o
P v,0
Loss Coefficients C
1




V
1
V
0
C
0


 2

Example 18.7
• Determine total pressure drop from 0 to 4

Conversion Between Methods
  f
L eq
V
2
D 2 g

C
0
V
2
2 g
L eq

C
0
D f

Air Distribution System Design
• Describe room distribution basics
• Select diffusers
• Supply and return duct sizing

Forced driven air flow
Diffusers
»Linear diffusers
»Grill (side wall)
» diffusers
»Vertical
»Horizontal one side

»swirl diffusers
Diffusers types
»Valve diffuser
»ceiling diffuser
»wall or ceiling
»floor

»Perforated ceiling diffuser
Diffusers
»Jet nozzle diffuser »Round conical ceiling diffuser »Square conical ceiling diffuser
»Wall diffuser unit »Swirl diffuser »Floor diffuser »Auditorium diffuser
»Linear slot diffuser
»DV diffuser »External louvre
»Smoke damper
» http://www.titus-hvac.com/techzone/
» http://www.halton.com/halton/cms.nsf/www/diffusers

Low mixing Diffusers
Displacement ventilation

Diffuser Selection Procedure
V
 
 tot
 Δh

 sen
 Δt
»V = maximum volumetric flow rate (m 3 /s, ft 3 /min)
»Q tot
= total design load (W, BTU/hr)
»Q sen
= sensible design load (W, BTU/hr
»ρ = air density (kg/m 3 , lbm/ft 3 )
)
»Δt = temperature difference between supply and return air (°C, °F)
»Δh = enthalpy difference between supply and return air (J/kg,
BTU/lbm)
• Select and locate diffusers, divide airflow amongst diffusers

Find Characteristic Length (L)

Indicator of Air Distribution
Quality
• ADPI = air distribution performance index
• Fraction of locations that meet criteria:
• -3 °F < EDT < 2 °F or -1.5 °C < EDT < 1 °C
• Where, EDT = effective draft temperature
• Function of
V and
Δt
(Eqn 18.1)
• EDT=(t local
-t average
)-M(V local
-V average
) , M=7 °C/(m/s)
ADPI considers ONLY thermal comfort (not IAQ)

Ideal and Reasonable Throws

Select Register
• Pick throw, volumetric flow from register catalog
• Check noise, pressure drop

Summary of Diffuser Design
Procedure
1) Find Q sensible total for the space
2) Select type and number of diffusers
3) Find V for each diffuser
4) Find characteristic length
5) Select the diffuser from the manufacturer data

Reading asignement
• Chapter 18
• 18.1-18.4 (including 18.4)