Civil Engineering Reference
In-Depth Information
where
q
res
is the pulmonary ventilation rate (kg/s),
W
res
and
W
a
in
are respectively
the humidity ratio of the exhaled and ambient air (kg/kg).
In addition, under normal circumstances, the pulmonary ventilation rate,
q
res
is
estimated as a function of the metabolic rate
M
,asshowninEq.
4.40
(ASHRAE
2009
) where
A
D
is the DuBois area in (m
2
). Moreover, for typical indoor environ-
ment the humidity ratio of the exhaled air can be estimated using the relationship
proposed byMcCutchan and Taylor (
1951
) andwhich can be observed in Eq.
4.41
.
10
−
6
q
res
=
1
.
43
×
×
MA
D
(4.40)
W
res
=
0
.
0277
+
0
.
000065
×
T
a
in
+
0
.
2
×
W
a
in
.
(4.41)
4.2.3.3 CO
2
Concentration Model
There is a fairly new concept in the building construction area designated as
sick
building syndrome
, which can be considered as a synonym of a poor indoor air
quality, i.e. it can originate the possible appearance of health problems and the lack
of comfort, in air quality terms for users (Awbi
2003
). Therefore, indoor air quality
must be defined as a function of the human necessities. Basically, the occupants
of a certain environment demand to perceive fresh air instead of vitiated, loaded
or irritated air, and to know that the risk for health which could be derived from
breathing that air is depreciable (Hernández
1994a
).
In general, contaminant substances as odour, carbon dioxide concentration,
tobacco smoke, etc., are responsible for poor indoor air quality inside a building (Awbi
2003
). However, as carbon dioxide, CO
2
, is the main waste from human respiration,
it is used to estimate the outdoor air volume intake in order to dilute overall indoor-air
pollution. In addition, to control CO
2
concentration inside a certain building the most
used technique is ventilation (CO
2
Demand-Controlled-Ventilation), which can be
defined as the intentional introduction/extraction air in a certain space, in this case,
into a typical office room, in a natural or a mechanical way (Hernández
1994b
).
To reach this objective, it is necessary to have detailed knowledge of the dynami-
cal behaviour of CO
2
concentration. Therefore, in this section a CO
2
concentration
model based on a mass balance is proposed, see Fig.
4.15
.
As shown in Eq.
4.42
, the indoor CO
2
concentration dynamic model can be
estimated as a function of a natural mass balance between CO
2
gains and losses
occasioned by people (
MCO
2
p
), forced ventilation (
MCO
2
fv
), infiltrations (
MCO
2
inf
)
and natural ventilation (
MCO
2
nvnt
). Therefore, it changes as a function of the number
of people,
N
p
, and their CO
2
generation rate (
G
CO
2
) in (mg/s), and the existing air
flow in the modelled room in (m
3
/s). More specifically, it has been taken into account
the CO
2
exchange through the window (
q
nvnt
), the HVAC system (
q
fv
) and due to
infiltrations (
q
inf
). In the previous equation, CO
2
out
and CO
2
dc
are referred to outdoor
CO
2
concentration and double ceiling CO
2
concentration (mg/m
3
), respectively.
