Architecture 418 Spring 2012 Designing with Natural Forces Peter ...

Architecture 418 Spring 2012
Designing with Natural Forces
Peter Simmonds
require
that
all
residences
have
operable
windows,
even
though
in
some
cases,
these
windows
are
not
useful
to
the
occupant.
In
general,
natural
ventilation
works
best
when
the
outside
air
temperatures
are
just
below
what
would
generally
be
considered
comfortable
indoor
conditions.
However,
the
outdoor
temperature
that
will
lead
to
acceptable
indoor
temperatures
depends
greatly
on
the
internal
heat
loads
(from
human
occupants,
equipment,
lights,
solar
gain,
etc.)
and
the
flow
rate
that
can
be
achieved.
The
American
Society
of
Heating,
Ventilating
and
Air
Conditioning
Engineers
(ASHRAE)
Standard
55
(2010)
has
a
discussion
on
thermal
comfort
that
includes
a
chart
highlighting
the
acceptable
indoor
temperatures
in
naturally
ventilated
buildings.
The
key
is
that
occupants
must
be
given
some
measure
of
control
over
their
environment.
Design
of
natural
ventilation
in
a
complex
building,
or
where
natural
ventilation
is
the
only
form
of
ventilation,
can
involve
intuitive
experience
as
well
as
hard
science.
In
each
case,
the
form
of
the
building
will
be
important
and
various
features
of
the
building’s
architecture
can
enhance
natural
ventilation
or
work
against
it.
Designing
natural
ventilation
is
the
art
of
balancing
driving
forces
and
pressure
losses
to
achieve
a
desired
minimum
air
flow
rate.
The
required
minimum
flow
rate
need
not
be
constant.
In
general
there
are
three
different
criteria
for
identifying
the
minimum
flow
rate:
(1)
The
human
biological
requirement
established
in
codes
and
by
organizations
(e.g.
ASHRAE,
NBCC,
CIBSE)
and
sometimes
simply
referred
to
as
20
cfm/person
(10
L/s/person);
(2)
The
flow
required
to
maintain
human
or
equipment
temperature
limits.
ASHRAE
55
suggests
that
acceptable
temperatures
range
from
17º
to
31º
C
depending
on
the
outdoor
temperature;
(3)
The
flow
to
maintain
contaminants
(e.g.
carbon
dioxide
concentrations)
at
a
maximum
allowable
limit.
Various
tools
are
available
to
assess
natural
ventilation
and
the
use
of
one
tool
over
another
depends
on
the
driving
forces
and
critical
nature
of
the
flow.
Stack effect
Stack
effect
is
a
phenomenon
present
in
all
vertical
shafts
that
are
at
different
temperatures
from
outdoors.
This
includes
chimneys
and
buildings.
A
building’s
shafts
include
the
vertical
HVAC
risers,
elevator
and
stairwell
shafts.
The
temperature
difference
sets
up
a
scenario
where
there
is
a
density
difference
indoors
to
out.
This
pressure
difference
provides
a
driving
force
for
air
movement.
If
there
are
openings
in
the
building
façade
(even
if
they
are
very
small
cracks)
then
there
will
be
air
flow
in
at
the
bottom
and
out
at
the
top
for
a
heating‐climate
scenario.
Stack
effect
flows
in
shorter
buildings
and
chimneys
(e.g.
5
story’s)
are
a
few
Pascal’s
(PSI),
compared
to
wind
pressures
which
can
be
measured
in
10s
of
Pascal’s
(PSI).
Hence
it
is
possible
for
a
slight
wind
to
overwhelm
the
natural
stack
effect.
A
robust
design
of
natural
ventilation
in
a
building
that
uses
stack
effect
as
a
driving
force
will
be
configured
so
that
at
worst
the
wind
effects
will
be
benign
and
if
possible
they
will
assist.
It
is
important
that
all
wind
conditions
and
directions
be
considered
because
the
“prevailing”
wind
is
not
always
one
that
exists
more
than
50%
of
the
time.
You will be graded on attendance and participation in the following three areas:
Classroom Discussion /Team presentation 10%
Design Charrettes 20%
Midterm Papers (2x) 40%
Final exam & Quizzes 30%
Total 100%
Required reading:
Mechanical and Electrical Systems, by Grondzik, Kwok, Stein and Reynolds,
CIBSE, Natural Ventilation Design Guide.
Course Syllabus Page 2 of 3