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However, the more time a small volume of air spends in a room, the more it
will be contaminated by pollutants.
Since there is a large number of air particles all taking different paths, we
define a probability density f(
r
) that the age of particles arriving at a given
location is between and þ d and, a probability F(
r
) that this age is
larger than . These two functions are, by definition, related by:
fð
r
Þ¼
dFð
r
Þ
d
and Fð
r
Þ¼1
ð
0
fðt
r
Þdt ð3:1Þ
The local mean age of air at a point r is defined by the average age of all the
air particles arriving at that point:
r
¼
ð
1
0
tf
r
ðtÞdt ¼
ð
1
0
F
r
ðtÞdt ð3:2Þ
The room mean age of air hi is defined by the average of the ages of all the
air particles in the room.
Nominal time constant
The nominal time constant,
n
, of a ventilated zone, is the ratio of its volume,
V, to the supplied fresh volume airflow rate, q, (including infiltration), or
the ratio of the mass of air contained in the space, M, to the mass airflow
rate, Q:
n
¼
V
q
¼
M
Q
ð3:3Þ
Its inverse is the specific airflow rate or air change rate, n.
If the room or ventilated zone has a defined air exhaust, Sandberg (198 4)
has shown that the nominal time constant is equal to the mean age of air at
this exhaust:
n
¼
e
ð3:4Þ
Air exchange efficiency
This efficiency expresses how the fresh air is distributed in the room. The
time,
a
, required on average to replace the air present in the space is twice
the room mean age of air (Sandberg and Sjo
¨
berg, 1983):
a
¼ 2hið3:5Þ
At a given flow rate and zone volume, the shortest time required to replace
the air within the space is given by the nominal time constant. Therefore, the
air exchange efficiency,
a
, is calculated by:
a
¼
n
2hi
ð3:6Þ
The air exchange efficiency is equal to one for piston-type ventilation, where
the exhaust is reached at a time correspon ding exactly to the nominal time
40 Ventilation and Airflow in Buildings
- Claude-Alain Roulet 4
- Contents 6
- List of Figures and Tables 8
- List of Figures and Tables ix 10
- List of Figures and Tables xi 12
- Preamble 13
- Introduction 14
- Outdoor airflow rate 15
- Recirculation rates 16
- Exfiltration 17
- Ventilation efficiency 18
- Airtightness 20
- Energy efficiency 20
- Common techniques 21
- Airflow Rates in Buildings 22
- Tracer decay, no injection 24
- Constant injection rate 24
- Constant concentration 25
- Pulse injection 25
- Airflow Rates in Buildings 5 26
- Airflow Rates in Buildings 7 28
- Global system of equations 29
- Airflow Rates in Buildings 9 30
- Properties of the flow matrix 31
- Airflow Rates in Buildings 11 32
- Note: y Under condition 34
- Source: Sherman, 1990 34
- Airflow Rates in Buildings 13 34
- Airflow Rates 36
- Source: Roulet et al., 2000a 37
- Velocity traverse 38
- Test procedure 39
- Tracer gas dilution 39
- Source: Awbi, 2007 40
- Inflatable bag 41
- Flowmeter 41
- Compensated flowmeter 41
- Tracer gas injection ports 41
- Source: Roulet et al., 1999 42
- Node by node method 44
- Least square solution 48
- Eliminating some equations 48
- Simplest way 49
- Less than four tracer gases 53
- Planning tool 54
- Simple measurement using CO 55
- Age of Air and 60
- Ventilation Efficiency 60
- Nominal time constant 61
- Air exchange efficiency 61
- Source: Roulet, 2004 62
- Measurement method 63
- Concentration [ppm] 65
- Source: Roulet et al., 1998 69
- The model matrix M 73
- Condition of the model matrix 74
- Factorial designs 75
- 3-D designs 76
- Measurement methods 80
- Reductive sealing 82
- Density corrections 84
- Two measurement points 85
- Airtightness 65 86
- Airtightness of buildings 88
- External fan 89
- Internal fan 90
- Leakage visualization 90
- The stack effect method 91
- Airtightness 71 92
- Neutral height method 93
- Airtightness 73 94
- Pressurization method 95
- Flow rate difference method 96
- Conditioned space 97
- SealSeal 97
- Measurements and Measures 98
- Related to Energy Efficiency 98
- Energy in air handling units 100
- Heat exchangers 104
- Types of heat exchangers 105
- Rotating heat exchangers 106
- Glycol heat exchanger 107
- Heat pump 107
- Heat exchange efficiency 107
- Ventilated 110
- Global heat recovery efficiency 111
- Source: Roulet et al., 2001 114
- Examples of application 115
- Energy for ventilation 118
- Measurement of airflow rate 120
- Measurement of electric power 121
- Simulation results 126
- Recirculation 126
- Humidification 127
- Heat recovery 127
- Ductwork 128
- Contaminants in 129
- Air Handling Units 129
- Source: Bjo 130
- Humidifiers 131
- Source: Roulet et al., 2000 133
- Measurement protocols 134
- Selecting the panel 135
- Source: Bluyssen, 1990 136
- Training procedure 137
- Experimental day 138
- Principle of the method 138
- Injection technique 139
- Hot air blower (200 °C) 140
- Stainless steel 140
- Ø 6 mm copper tube 140
- Example of application 141
- Evidence for adsorption 144
- HVAC systems 146
- Common Methods 153
- Properties of tracer gases 155
- Source: Dietz et al., 1983 157
- Mixing tracer gases 158
- Tracer density problem 159
- Sampling methods 160
- Grab sampling 160
- Passive sampling 160
- Networks, pumps and pipes 161
- Tracer gas analysers 163
- Objective of the analysis 163
- Photo-acoustic detector 165
- Mass spectrometry 165
- Gas chromatography 167
- Chemical indicator tubes 167
- Identification methods 168
- Confidence in the coefficients 170
- Regression of the second kind 171
- Orthogonal regression 171
- Bayesian identification 172
- Error analysis 174
- Definitions 175
- A few statistics 175
- Covariance 177
- Statistical distributions 177
- What is the problem? 180
- Most simple error analysis 180
- Estimate of the variance 181
- Linear equations systems 181
- Upper bound of the errors 183
- References 187
- Unit Conversion Tables 192
- Pressure 194
- Volume flow rate 194
- Mass flow rate 194
- Glossary 195
- Glossary 175 196
- Glossary 177 198
- Glossary 179 200
- Glossary 181 202
- Glossary 183 204
- Glossary 185 206
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