Transcript Radiant Ceiling Cooling & Heating and Displacement Ventilation

Ventilation Effectiveness
&
Indoor Air Quality
Douglas Spratt
February 2001
The Indoor Environment
Indoor
Environment
HVAC
Air
The Purpose of HVAC Systems
H
V
A
C
Thermal Control
(ASHRAE - 55)
Air Quality
(Contaminant Control)
(ASHRAE - 62)
HVAC Systems:
 Maintain the space within the thermal comfort
 Remove/dilute air contaminants
Ventilation- Removes Emissions
Source
Reduce
Emissions
Remove
Ventilation Approaches

Natural

Displacement

Mechanical
Natural Ventilation

Advantages -
Low 1st Cost
Low Energy
Low Maintenance
Natural
Greater occupant
acceptance
Natural Ventilation

Forces
-
Buoyancy
-
Wind
Symptoms
rC
on
ce
ra
tio
n
s
es
a
ss
se
ss
n
nt
pl
in
e
Na
u
D
izz
in
e
e
n
ch
tio
Sealed Building
Po
o
Sl
ee
ita
da
Irr
He
a
Ey
e
Percentage of Respondents
Effect on Health Symptoms
Natural Ventilation
90
80
70
60
50
40
30
20
10
0
Impact on Work Environment
Sealed Building
Natural Ventilation
100
Percent of Respondents
90
80
70
60
50
40
30
20
10
0
Too Little
Air
Movement
Too Hot
Too Cold
Too Dry
Too Stuffy Unpleasant Too Noisy
Odour
Environmental Condition
Natural Ventilation

Limitations
– Limited to Low Occupant Densities
– Limited to Mild Climates
– Limited Distribution
Natural Ventilation

Considerations
– Security
– Outdoor air quality
– Wind gusts
– Noise
– Bugs
Displacement Ventilation
Displacement Ventilation

Continuously purges air pollutants

Typical features:
- De-couples ventilation
- Uses natural buoyancy forces
- Low noise
- No drafts
- Fresh air breathing zone
Air Distribution Effectiveness
Room Air Distribution
ERM
Ceiling supply of cool air
1.0
Ceiling supply of warm air and ceiling return
0.8
Ceiling supply of warm air and ceiling return if the
supply air stream reaches floor level
Floor supply of cool air and ceiling return if the
supply velocity is high enough to induce mixing of
room air (most underfloor systems)
Floor supply of cool air and ceiling return if low
velocity displacement ventilation achieves nearlaminar flow and thermal stratification
Floor supply of warm air and floor return
1.0
Source: ASHRAE 62N
1.0
1.2
1.0
Mechanical Ventilation
Practical Help
 Watch for short circuiting
 Min. O/A settings for VAV Systems
 Ensure continuous fan operation
Mechanical Ventilation

Methods
– Exhaust Fan
– Exhaust With Make-Up Air
– Mixed Supply Air


Fixed Dampers
Modulating Dampers
The Economizer Section
EXHAUST
RETURN
RECIRCULATED
M-2
OUTDOOR
MIXED
M-1
SUPPLY
Example
20 ton Roof-top unit
9,000 cfm supply
1,000 cfm toilet exhaust
9,000 ft2 floor space
Office space with 150 ft2/person
62-89 O/A = 20 cfm/person x 9,000 ft2
150 ft2/person
= 1,200 cfm
Ignoring Infiltration
200 cfm
1,000 cfm
R/A 8,000 cfm
7,800 cfm
1,200 cfm
S/A 9,000 cfm
T/E
% O/A = 1,200 cfm = 13%
9,000 cfm
Ventilation Summary
Source
Emissions
Removal
Source
Extraction
Natural
Dilution
Mechanical