Objectives

Document Sample

```					                   Objectives
- Analyze Rotary Heat Exchangers
- Use Psychometric chart

- Start Air Distribution Systems
- Diffuser selection
Sensible and Enthalpy Wheels

Example of Dual Wheel System
Psychrometrics of sensible and enthalpy wheel
Example NMS Building HVAC system:
Enthalpy wheel                                               Fans
Sensible wheel       Exhaust air
D

A                   B                  C                                       1
Return air                                                                       2                     Fresh air
form building

Hot and                                                                                            Filters
Preheater
fresh air to   7             6             5       4                        3
building

Heating coil
Cool and              Cooling coil

fresh air to
building
Air Distribution System Design
• Describe room
distribution basics
• Select diffusers
• Supply and return
duct sizing
Designing Room Airflow
• Very complex problem
• Pumped flow, buoyant flow (or mixed flow)
• What non-dimensional parameters govern each
regime?
• Archimedes number = Ar = gβLΔT/v2

L = characteristic length (m, ft)
g = acceleration due to gravity (m/s2, ft/min2)
T = absolute temperature (K, °R)
β = 1/T (1/K, 1/ °R)
v = kinematic viscosity (m2/s, ft2/min)
Computational Fluid Dynamics
CFD in Air Distribution Design

Contaminant concentration in a kitchen
Buoyancy driven flow:
Example of airflow in a stairway

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

Horizontal one side
Diffusers types
Valve diffuser
swirl diffusers                           ceiling diffuser

wall or ceiling

floor
Low mixing Diffusers
Displacement ventilation
18.7
Diffuser Selection Procedure

V = maximum volumetric flow rate (m 3/s, ft3/min)
Qtot = total design load (W, BTU/hr)
Qsen = sensible design load (W, BTU/hr)
ρ = air density (kg/m3, lbm/ft3)
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=(tlocal-taverage)-M(Vlocal-Vaverage) ,   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
Example 18.3
• Qtot = 38.4 kBTU/hr
• Δh = 9.5 BTU/lbma
Note omission in text