Civil Engineering Reference
In-Depth Information
Table 4.10
Typical metabolic heat generation for office activities
Activity
W/m
2
met
Reading, seated
55
1.0
Writing
60
1.0
Typing
65
1.1
Filing, seated
70
1.2
Filing, standing
80
1.4
Walking about
100
1.7
Lifting/packing
120
2.1
Source:
(ASHRAE
2009
)
behaviour. In the case of a typical office room, the main humidity gain sources
are the occupants of the room and the activities that they usually do, infiltrations,
forced ventilation, etc. On the other hand, the principal humidity loss sources are the
exfiltrations and themechanically extracted air. Furthermore, in the proposedmodel it
has been supposed that the humidity ratio in the double ceiling (
W
a
dc
) is different from
the humidity ratio inside the modelled room (
W
a
in
). Hence, the humidity dynamic
model can be estimated as a natural mass balance between humidity gains and losses
occasioned by people (
Mw
p
), forced ventilation (
Mw
fv
), infiltrations (
Mw
inf
) and
natural ventilation (
Mw
nvnt
), as shown in Eq.
4.38
.
d
W
a
in
d
t
V
a
in
ρ
a
in
=
Mw
p
+
Mw
fv
+
Mw
inf
+
Mw
nvnt
=
N
p
G
W
+
q
fv
ρ
a
imp
(
W
a
imp
−
W
a
in
)
+
q
inf
ρ
a
out
(
W
a
out
−
W
a
in
)
+
q
nvnt
ρ
a
out
(
W
a
out
−
W
a
in
)
(4.38)
ρ
a
in
are the volume (
m
3
) and the density of the air inside the room
(kg/m
3
), respectively.
W
a
imp
and
where
V
a
in
and
ρ
a
imp
are the absolute humidity (kg/kg) and the
density of the impulse fancoil air (kg/m
3
), respectively, whereas
W
a
out
and
ρ
a
out
are
the humidity ratio (kg/kg) and the density of the outdoor air (kg/m
3
), respectively.
G
W
represents the absolute humidity rate generated per person
N
p
(kg/s), which
is directly related to the activity performed by people. Moreover,
q
fv
,
q
nvnt
are the
air exchange rate by means of the HVAC system and through the window (m
3
/s),
respectively. The air exchange rate through the window can be estimated according
to Eq.
4.31
.Atlast,
q
inf
is the air exchange rate due to infiltrations (m
3
/s), i.e. when
the room is completely closed and the HVAC system is disconnected, see Eq.
4.32
.
1.
Human Humidity Generation Rate
(
G
W
)
One of the main sources of humidity in a typical office room is the human being.
The human humidity generation rate, which appears in Eq.
4.38
, is a variable that
is directly related to the physical activity performed by the human at each time
instant. In addition, the relationship between the physical activity and the humid-
ity production has been studied by different authors (Christian
2009
;Harriman
