Civil Engineering Reference
In-Depth Information
Q
fv
=
A
ret
v
a
ret
ρ
a
ret
ζ(
C
p
imp
T
a
imp
−
C
p
ret
T
a
ret
)
(4.29)
where:
A
ret
isthereturnarea(m
2
),
v
a
ret
is the return air velocity (m/s),
ρ
a
ret
is
the return air density (kg/m
3
),
C
p
imp
and
C
p
ret
are the impulse and return specific
heat capacities (J/kgK), respectively,
T
a
imp
and
T
a
ret
are the impulse and return air
temperatures (
K
), respectively. Finally,
ζ
is a correction factor from the calibration
curve of the hot-wired anemometer (
−
) which is located inside the HVAC system,
and is used to measure
v
a
ret
.
4.
Heat Gain caused by Natural Ventilation
(
Q
nvnt
)
Natural ventilation is one of the most commonly used mechanisms to control
indoor air temperature reducing, at the same time, energy consumption, since by
means of the use of this technique it is possible to reduce the use of the HVAC
system. On the other hand, natural ventilation can be defined as the outdoor air
flow through the intentioned openings of the room (ASHRAE
2009
). The term
intentioned opening makes reference mainly to the windows and door of the
modelled room. Hence, heat gain caused by natural ventilation can be calculated
as shown in Eq.
4.30
.
Q
nvnt
=
q
nvnt
ρ
a
in
C
p
a
(
T
a
out
−
T
a
in
)
(4.30)
where
q
nvnt
represents the total ventilation rate through intentioned openings
(m
3
/s), whose calculation is described in the following section,
ρ
a
in
is the indoor
air density (kg/m
3
),
C
p
a
is the specific heat capacity of the indoor air (J/kgK).
•
Ventilation Rate through Intentioned Openings
.
In general, airflow through an intentioned opening is caused by a pressure
difference. At the same time, this pressure difference can be occasioned by
two main forces: (a) the buoyancy forces and (b) the wind forces (Boulard and
Baille
1995
) which depend on intentioned openings sizes. Therefore, to model
this parameter a combination of both forces has been chosen, see Eq.
4.31
.
wth
w
io
Cd
3
T
a
in
−
T
a
out
T
a
out
g
q
nvnt
=
2
g
2
v
out
3
2
−
Cv
v
out
(
T
a
in
−
T
a
out
)
3
(4.31)
×
hth
w
io
+
Cv
×
×
T
a
out
where
wth
w
io
and
hth
w
io
represent the width and the height of the intentioned
opening (m), respectively,
Cd
is the
discharge coefficient
(
−
),
g
is the universal
gravity constant (m/s
2
),
Cv
is the
wind coefficient
(
−
) and, finally,
v
out
is the
outdoor air velocity (m/s).
5.
Heat Gain occasioned by Infiltrations
(
Q
inf
)
The term infiltrations can be defined as the air flow fromoutdoor to indoor through
non-intentioned openings and by means of the typical use of in/out doors. Infiltra-
tions depend on pressure and temperature differences, themagnitude and direction
