Terminology is important !
Draught
draught is
unwanted cooling of the body caused by air movement,
which depends on the air speed, air temperature, turbulence intensity, activity
and clothing of the building occupants.
The predicted percentage of people dissatisfied due to annoyance by draught
(DR) is limited to:10% for Class A and
20% for Class B and C environments,
All of which prescribe a mean air velocity within 0.13 to 0.22m/s
for a local air temperature of 24oC
Don’t nod off just yet notes • “Sister strategies” – focused air delivery & mixing air delivery. • Cooling & energy performance is intrinsically linked to air diffusion because ultimately air diffusion influences suction pressures.
• Air rarely moves predictably
• “Air diffusion first” can often reveal opportunities for saving energy
Natspec is OK, right?
Specification elements for Indoor Air Quality & Comfort
•
Potential Diffusion features:
• Air Diffuser Performance Index
(ADPI)
• Temperature
• CO2 concentration
•
• Furniture off gassing
• VOCs
• Sick Building Syndrome
Why is this
important ?
VAV System – Mixed flow ADPI
– Zone control critical
Displacement System
– Mixed flow for low ceilings
– Individual occupant control
– Pollutants rise to extract
• Chilled Ceiling
– 100% outdoor air @ 1.5/L/s per m2
– Zone control
Fundamentals are affected
• AS1668.2 (Fresh air) does not standardize how well the air is diffused.
• ISO7730 (PMV) does not specify velocities.
• The use of PMV in ASHRAE 55 is limited to velocity< 0.2m/s.
• Hot Air rises.
• ACE is strongly driven by air patterns, OA, TD • The higher the ADPI, the higher the % of occupants satisfied w/ thermal conditions. • Design is for heating & cooling extremes & everywhere inbetween
The “New Normal”
“The pace of change in the workplace is such that the mindset and disposition of the worker is the Number 1 CEO concern…” THE FUTURE OF WORK /ASPEN INSTITUTE
New systems with user
interaction
Comfort
...affects our performance
Comfort
...differs between gender
Air movement
...is fundamental
This is normal
Boss sits here!
Comfort & Well‐being
Complaints incl:
Choice
Productivity
“Do you think having control of air movement
will enhance or hinder productivity?”
Control
Air Diffusion Performance Index
ADPI is the % of measurements within an occupied volume where the
(Eff Draft Temperature) is between -1.5 to +1C of setpoint with air
velocity <0.35m/s.
Strategies
• Focused air delivery • Mixing air delivery
Focussed Delivery – workstation air
Focussed Delivery – Workstation air
3500
Focussed Delivery‐ UFAD / Displacement
Focussed Delivery‐ Displacement
Mixing Delivery ‐ Flexibility
Mixing Delivery – Wide Temperatures
Types of Variable Air Volume Systems
• VAV System
–
–
–
–
–
RSH < 200 W/m2
Supply air > 12°C
S/R air < 10 K
Standard Induction diffusers
< 500 L/s per diffuser
Low Temp VAV System
–
–
–
–
–
RSH < 200 W/m2
Supply air > 8°C
S/R air < 16 K
High induction air diffusers
< 500 L/s per diffuser
Variable Airflow & Coil Control
Cool KW = 55 Sens & 47KW Latent
COP = 3.1
Comp Input = 32.9 KW
Airflow = 5050L/s
Fan Energy 7.7KW absorbed
Cool KW = 79.5 Sens & 19.9KW Latent
COP = 4.2
Comp Input = 23.8 KW
Airflow = 5050L/s
Fan Energy 7.7KW absorbed
What are we playing for?
Combining entrainment and swirl
Is combining focus and mixing
Sinusoidal air movement is real
Below brief, above brief and compliant
3 minutes later
Airflow variation = arm wrestling between AHUs
Pressure sensors
Common Atrium/ Open space
Equilibrium Try “Air Diffusion First”
If I had my time again...
Scenario
Focussed or Mixed
Advantages
Large atrium or mall
Mixed
Little duct required
Focussed UFAD / Task Air
Mixed
LT VAV
High turndown capability
Flexibility of diffuser placement
Commercial office – high churn expectations 3.5 flr to flr
Both
Retail tenancies
Mixed
Try “Focussed” strategies first – they are intrinsically lower energy and duty.
Where it is impractical (large retail or warehouse spaces), try mixing
strategies.
Where VAV turndown is important, try mixing and possibly motorised
diffusers.
Summary
• Air Diffusion is essential to thermal comfort and air quality delivering air where people are.
• Style of air diffusion is related to coil and airflow decisions & thus energy
• Air rarely moves predictably.
• Try looking “Air diffusion first” for a change.
• Air diffusion needs to be deliberate in design.
• Our clients are searching for higher levels of comfort & IAQ without energy penalty.