Ecolibrium
THE OFFICIAL JOURNAL OF AIRAH
MARCH 2017 · VOLUME 16.2
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FE ATURE
STRA
THE AU
ST
LIAN IN
FRIGER
OF RE
ITUTE
ATION
NDI
, AIR CO
TIONIN
HE ATIN
G AND
G
AIR CON
DITIONING,
COOLING
AND COM
FORT IN
HOT HUM
ID TROPICA
L CLIMATE
S
DA20
Building d
e
and system sign
selection
3.1 Secti
3
DA20
Climate zone
AIR CON
DITIONING,
COOLING
s base
Western Aust
have been
exclu
ralia. Zone
s
and temperate ded are known as
desert, grass which
zones (see
land
the whole
of New Sout Figure 2.1) which inclu
h Wales, Sout
ACT and Tasm
de
h Australia,
The tropical ania and large parts
Victoria,
of
regions of
Australia whic Western Australia.
zones and
warm hum
h include hot
id
portion of
humid
the total area zones add up to a signi
ficant
of the cont
inent.
A climate analy
sis
and zone map
and heat pum
ping for air
p devices carrie
conditioners
New Zeala
d out for
nd
for air cond governments proposed the Australian and
itioning; Hot-h
a
Mixed (Coo
umid (pred three zone map
ling
heating) refer and heating) and Coldominantly cooling)
to Figure 2.3.
(predomin
antly
Australia
and New Zeal
and
cold and hot-h
regions with umid
mixed
demand betw
een
Figure 2.3:
Three zone
heating/co
2.2.3 Dew-p
oint temper
atures
in tropical
regions
One definition
for the
10
HOT HUM
ID TROPICA
0
2
A
D
L CLIMATE
Note: The
data conto
urs are not
conditions,
as high as
summer desig
being only
average 3pm
month and
n
data
year.
and time perio Figures showing other for a particular
months
ds are availa
ble from the
BoM.
Vapour
Pressure,
22
24
hPa
26
28
30
32
34
Dew36
Point
Temp, °C 19.0 20.4 21.7
23.0 24.1
25.2 26.2
27.2
36 hPa
34 hPa
32 hPa
30 hPa
28 hPa
26 hPa
24 hPa
22 hPa
20 hPa
18 hPa
16 hPa
14 hPa
12 hPa
10 hPa
8 hPa
6 hPa
4 hPa
2 hPa
3pm Vapo
ur pressure
(hPa)
January 2013
oling regio
on introdu
ction
This section
discusses the
and HVAC&R
considerations
designers
that build
must
and systems
for applicatio give to designing building
Information
n in tropical
ing
and advice
climates.
is provided
on
Passive desig
n strategies;
• Low ener
gy design
strategies;
• Effects
and design
implications
of tropical
climate chara
cteristics;
• Building
moisture migr
ation desig
• Design
n strategies;
strategies
for minimisin
the infiltration
g and cont
of outdoor
rolling
air.
This section
also provides
comfort syste
guidance on
m selection
appropriate
evaporative
including;
Ventilative
air
cooli
spot cooling cooling, refrigerativ
e air condition ng,
and hybrid
applications.
ing,
•
Ceiling fans
are a low cost,
option and
low energy
should be
cooling
cons
They can be
highly effec idered where possible.
tive during
the drier mon
Quality ceilin
ths.
g insulation
heat flows
reduces dow
.
nward
• Low-e
glazing redu
ces inter
Tropical locat
ions can expe nal radiant heat.
rience high
• All exter
diffuse radia
nal and inter
tion.
nal
should be
lightly colou solar exposed surfaces
red.
• Eaves on
the northern
and
reduce wall
and window southern walls
heat gains
.
• Externally
shad
oriented walls ed windows on the
east and west
reduce heat
load.
• Shade
trees to the
east and west
reduce heat
• Low mass
load.
cons
time but may truction will cool quick
er at night
bridging shou heat quicker during
the
is used then ld be eliminated. If high day. Thermal
internal insul
Passive desig
thermal mass
particularly
n
if the build ation may be considere
of buildings can improve the inter
ing is to be
Solar expo
d,
nal condition
that are not
air
sed
cond
significantly
air condition
s
itioned.
with summ high thermal masses
reduce the
ed
er shading.
should be
cooling loads and can
that are to
provided
be air cond
of buildings
itioned.
• Cooling
breezes
Well insulated
elements such can be encouraged
roofs, optim
by design
as wing walls
eaves both
al solar orien
to
face
north and
.
Avoid
tation, overh
breezes.
orienting build
south
on both east
ings
and west, shad, external window shad anging
• Reflective
ventilated
es
e trees, and
spac
insulation
should be
allowing free es large openable wind for naturally
insulation
used
materials to
air
decrease the with other
desirable. Light movement and cross ows with louvres
absorption
-ventilation
• Sealing
capacity and weight construction
of heat.
are
the building
reduces therm
therefore cools
to ensure a
vapour barri
al
continuous
quickly after
er encompa
sunset.
The follow
especially
ing are all impo
if the build sses the building,
ing is to be
strategies
air condition
for buildings rtant passive building
If the build
ed.
in tropical
design
of these strate
ing is to be
climates.
air
sealed and
gies reduces
comfort in
insulated and conditioned it should
cooling loads The application
the building:
be well
high perfo
provided with
and improves
rman
well positione
• Solar orien
incorporating ce glazing. Outdoor
d
air ventilatio
tation with
heat
recov
occupanc y
n
long
ery should
(i.e. the long
spaces.
be provided systems
er walls shou axis aligned east/west
in high
and south)
ld
be
orien
redu
ted to the
Infiltration
north
and windows. ces the thermal load
can
on
such as shop be hard to minimise
and solar hot This also provides optim external walls
in building
ping centr
type
es,
water orien
(particularly
al photovolt
tations.
foyers) and shops, restaurants, hote s
aic
of doors. Posit
the
ls
ive pressurisa like, with frequent open
(see 3.5.4)
tion is essen
ing
or air curta
tial and
ins (see 3.5.5
) can be utilis air locks
ed.
S
•
•
3.2 Passi
ve de
strategies sign
ICAL
HUMID TROP
AIR
G
CONDITIONIN
l HVAC
system zone
■ Hot-humid
s
(air conditionin
■ Cold (heati
g)
■ Mixed (heati
ng required)
ng and coolin
•••
FORT IN
consideration
humid tropi when designing com
cal
fort systems
summer dew climates. The use of
for hot
suita
-point cont
the likeliness
ours are usefu ble maps showing
of condensa
l
tion and air when assessing
moisture cont
Figure 2.4
ent.
show
pressure whic s the average 3pm
Janu
using the tableh relates directly to dew ary vapour
-point temp
the BoM clima . These maps reflec
t the informati erature
te zone and
above but
on in
climate class
quan
ification figur
and also show tifies the moisture
es
cont
inland from s the drop in moisture ent in the air
the
and east coas northern coastline content moving
and down
ts of Australia.
the west
d on temp
■ Hot humi
erature and
d summer
humidity
■ Warm summ
■ Hot dry
er, cold winte
summ
■ Hot dry
r
■ Warm humi er, mid winter
summer, cold
d summer
winter ■
Mild/warm
summer, cold
winter
Figure 2.2:
Six Australian
Climate Zone
Northern Territ
s.
ory and
Residentia
AND COM
•••
33
g)
ns map.
“Hot Humid
“areas wher
Tropics” could
e the summ
er outdoor
temperatu
re exceeds
design dew be;
the
-point
temperatu
re”. Dew-poin indoor design dry-b
ulb
t temperatu
res are a critic
al
Figure 2.4:
Average 3pm
January vapo
2.2.4 Eva
porative
in tropical cooling
regions
ur pressures.
A
A
MANU
T IO N
PPLI C A
L
Evaporative
air coolers
are not suita
cooling in
hot
ble for com
fort
Zone 1. Evap humid tropical clima
tes, see Figur
orative air
coolers are
coastal area
suitable in e 2.5
and most of
the
see Zones
the inland
2 and 3.
areas of Aust southern
ralia,
Topical and tropical
AIRAH has revised and republished application manual DA20 Humid Tropical Air Conditioning – 136 pages of detailed technical information and data on the challenges and solutions for air conditioning, cooling and comfort in a hot tropical environment. The manual provides the HVAC industry with hundreds of years of combined experience of the AIRAH review group members, all experts in their fields designing and installing air conditioning and cooling systems in the tropical north. Vince Aherne, M.AIRAH, steps us through the revised manual, highlighting some of the trickier aspects of this surprisingly complex topic.
WHY TROPICAL
AIR CONDITIONING?
Air conditioning is everywhere.
Air conditioned residences and workplaces
are the norm, particularly in metropolitan
Australia. The ever-increasing penetration
of air conditioning into all levels of
Australian society is well established.
Globally, air conditioning use is predicted
to continue to increase. This is particularly
true of the tropical north of Australia,
where almost all commercial and
institutional buildings and the majority
of residential buildings are air conditioned.
26
E CO L I B R I U M • M A R C H 2 0 17
In a hot and humid climate, it is natural
for people to associate air conditioning
with comfort, but the reality is not that
straightforward.
also provides the solutions that allow
designers to overcome the specific
comfort and environmental challenges
presented by hot humid climates.
There have been plenty of instances
where air conditioning in the tropics
has failed to provide adequate comfort.
This is often due to issues with humidity,
condensation, moisture damage, m ould,
draughts, pest infestation, storm damage,
and coastal salt.
A BUILDING NEEDS
GOOD BONES FOR
GOOD COMFORT
OUTCOMES –
START AT THE START
Air conditioning has to be designed
and applied correctly to provide comfort
in the tropics. DA20 highlights many
of the known causes of failures, but
Yes, you can air condition an uninsulated
glass box so that the internal temperature
meets specified criteria, but the result
will most likely not be comfortable.
FE ATURE
To make sure that
the comfort solution
is both effective
and affordable,
designers have to
start with some
building basics
Comfort depends on more than just
the air temperature of the room and
the harsh 21st century reality is that
cooling thermally inefficient buildings
is expensive and emissions-intensive.
To make sure that the comfort solution
is both effective and affordable, designers
have to start with some building basics:
insulate the fabric, shade glazing and
thermal mass, seal the structure for air
and moisture, and control ventilation well.
In addition to the technical design
of air conditioning systems, it is also
important for the building services
engineer to understand the architectural
decisions and the building operation
protocols that will influence the
building’s energy productivity,
indoor air quality, and comfort levels.
To reduce a building’s energy
consumption, reduce the loads
on the cooling system:
• Apply insulation and shade to
control solar heat loads and glare,
and to reduce artificial lighting;
• Design and control outdoor air intake
and exhausts to control building
pressurisation and infiltration;
• Manage internal heat loads
from lights, equipment, processes,
and people.
HVAC designers also need to
have a broad understanding of how
a building and its materials will interact
with an HVAC system and its controls,
throughout the full range of its indoor
operational settings and outdoor climatic
conditions.
The DA 20 manual steps users through
these issues and considerations before
getting into the hardcore technical
details of air conditioning design.
THE SCIENCE OF
THERMAL COMFORT
AND OCCUPANT
SATISFACTION
A broad overview of thermal comfort
is discussed based on ASHRAE 55,
the thermal comfort bible. What are
the things that make people comfortable
and uncomfortable? How can these things
be influenced in a tropical environment
to provide a high level of comfort while
still minimising energy use?
Typical HVAC design practice
assumes that at least 80 per cent of
occupants will be thermally comfortable.
Moving beyond that 80 per cent satisfied
level requires a bit of thinking – to build
a better-than-minimum building fabric/
shell and service it with a better-thanminimum HVAC system.
DA20 outlines the key variables in human
comfort (including the air temperature,
the operative temperature, the air
movement velocity, and the air humidity).
It also explains the methods of extending
the comfort envelope by manipulating
these variables, as well as cultural aspects
and the theory of adaptive comfort.
The manual also discusses the analysis
of comfort variables to predict the
Predicted Percentage of Dissatisfied
(PPD) people, and their manipulation
to achieve a targeted PPD result.
So what does all this mean for tropical
air conditioning design? If comfort
bands can be widened and internal
air temperatures increased without
creating dissatisfaction, there is a huge
capacity for energy saving. It costs a lot
less to maintain an indoor temperature
at 26°C than it does at 22°C.
If the air movement and humidity
can be controlled to within the thermal
comfort envelope, systems can keep
occupants satisfied at significantly
reduced energy costs and emissions,
or alternatively satisfy more occupants
(lower PPD) for the same costs.
Wider internal comfort bands also
increases the potential window
of application of alternative
non-refrigerative cooling systems.
IT’S ALL ABOUT
THE MOISTURE
In a tropical climate, by which we
mean a hot and humid climate, a major
challenge for the air conditioning system
is to control and remove the moisture
from the air. This includes the moisture
entering the building with the introduced
outdoor ventilation air, the moisture
in any infiltration air, and the moisture
generated internally in the building.
It is not just the temperature of the
internal air that must be controlled,
it is the enthalpy, which means the
sensible heat due to temperature and
the latent heat due to moisture within
the air.
Latent heat is removed when the
moisture in the air condenses, called
dehumidification. Temperature doesn’t
change but relative humidity drops.
In humid air, the quantity of latent
heat needed to remove the moisture
can be significantly greater than the
quantity of sensible heat needed to drop
the temperature. This sensible:latent
load characteristic is an issue that affects
the design of tropical systems and
the selection of plant and equipment.
CONTROLLING ENTHALPY
The DA20 manual warns against
off-the-shelf packaged air conditioning
equipment that may have coils
with poor performance in respect
to latent-heat removal. Coils have
a sensible:latent heat capacity ratio,
which varies with design aspects
such as coil and fin materials, fin
configuration, depth of rows, etc.
Conventional systems typically have
cooling coils with sensible heat ratios
(SHRs) in the range of 0.75 to 0.85
(i.e., 75–85 per cent of their cooling
capacity is sensible). Some tropical air
conditioning design scenarios could
require a coil cooling SHR of 0.50 or
50:50. This type of load characteristic
should direct the designer to use preconditioning or custom-designed coils.
Using a coil with inadequate capacity
for latent-heat removal means the air
is cooled, but not dehumidified enough.
Internal conditions with a high relative
humidity are generated, producing a
cold, clammy and often uncomfortable
environment.
One option to deal with highly humid
outdoor air in a tropical design is to use
a dedicated outdoor-air system (DOAS).
In this approach, all outdoor air is
centrally cooled and dehumidified
to a specified or neutral condition
and distributed to each zone.
M A R C H 2 0 17 • E CO L I B R I U M
27
FE ATURE
Over-cooled rooms
lead to grumpy,
dissatisfied
occupants
Individual zone air conditioners
do not then have to deal with these
high latent loads. The DOAS can be
specifically designed to achieve the
required latent-heat removal and can
incorporate desiccants, phase change
and heat/enthalpy recovery to make
the process less energy intensive.
This leaves standard plant able
to deal with local zone loads.
Construction management matters too.
Building materials should be protected
from water during storage and construction
where possible and practicable (this can
be tricky during the big wet).
The building and its materials must be
dried out after construction and prior
to commissioning. If the HVAC system
is used to assist materials drying, it should
be ensured that the moisture removed
does not build up in HVAC system
components, such as insulation or filters.
CONTROLLING
AIR TEMPERATURE
To avoid mould growth on internal
surfaces, keep the indoor-air dew point
below 13°C (12.8°C) at all times.
Monitor and control the dew point,
not the relative humidity. Once the
system controls know the temperature
and relative humidity, it is a simple
matter to determine dew point.
Manipulating the target-room
air temperature is the business of
comfort and air conditioning systems.
If supply-air temperature is allowed
to increase to save on cooling energy,
then the dew point must be kept low
and air velocity increased to achieve
an equivalent level of comfort.
If reheat is used, then raising the
supply air temperature in this way
may actually consume more energy.
Over-cooled rooms lead to grumpy,
dissatisfied occupants.
28
E CO L I B R I U M • M A R C H 2 0 17
Economy cycles and free cooling
(with unconditioned outdoor air)
should only ever be used in the tropics
with extreme caution. Economy cycle
controls must never introduce humid
air into the building and never allow
the building to be depressurised, which
produces a similar result.
Control the cycle on dew point, and
never enable it when the outdoor-air
dew point is higher than the target
indoor-air dew point (which is most
of the time in many tropical regions).
INTERSTITIAL
CONDENSATION,
DEW POINT AND
THE VAPOUR BARRIER
One of the common failures in a tropical
air conditioned building is interstitial
condensation forming inside walls and
roofs, and the resulting mould growth
and rot/damage to the structure.
air barriers help to air-seal the building,
and water barriers resist the penetration
of liquid water.
In a nutshell, vapour barriers must
be used in air conditioned buildings
in the tropics, while vapour retarders are
used for non-air conditioned buildings.
Corners of the building fabric, window
and door assemblies, and penetrations
of the barrier by building services, deserve
particular detailing and attention.
Construction management matters
and it is important that any trade that
may subsequently penetrate the installed
vapour barrier and/or air barrier
(i.e., plumbers, electrician, HVAC)
are competent in the methods to seal
the penetration to maintain the integrity
of the barrier. This should be checked
as part of the pre-commissioning work.
VENTILATION IS
IMPORTANT TOO
The DA20 manual explains why this
occurs and how to prevent it, particularly
by highlighting the correct selection
and installation of the vapour barrier.
One of the main loads on the system
is of course the outdoor air that must be
introduced into the building for occupant
ventilation.
The manual illustrates the dew-point
analysis of a composite wall construction
where the likely position of interstitial
condensation is identified, in relation to
the position of the vapour barrier relative
to the insulation.
Of course, ventilation is required,
and AS 1668.2 stipulates, a base rate
of 10 L/s. per person is needed. If this
air is to successfully provide for adequate
IAQ, it is important that it is supplied
both clean and dry. Drier air expands
the potential comfort range.
In an air conditioned building in
the tropics, the vapour barrier always
goes on the external (outdoor) side
of the thermal insulation. This may not
be the case for other comfort systems
and other climate zones.
The selection, installation and
construction management of the vapour
barrier is an essential part of moisture
management within the design and
construction process. The type of barrier
to use, where to put it, and in which
situation, is an issue that the construction
industry regularly grapples with. DA20
clears up many of the misconceptions.
Vapour barriers, vapour retarders,
air barriers, and water barriers are all
different functions that may or may not
be achieved by a single barrier membrane
or a composite construction containing
multiple barriers.
Vapour barriers (Class 1 and 2) resist
vapour penetration, vapour retarders
(Class 3 and 4) allow moisture to pass,
Air drying prevents mould, and
air cleaning reduces nutrient sources.
But air is expensive to clean, dry, cool
and push around a building, particularly
in the tropics, so it should be used
in a controlled and measured way.
The moisture content of the outdoor
air is high in tropical regions. In Darwin
for example, a kilogram of outdoor air
in summer may contain 20 to 21 grams
of moisture. A quick calculation shows
that, for an equivalent indoor air
condition, the outdoor air in Darwin
may need to have 80 times the amount
of moisture removed than outdoor air
in Melbourne.
The way that outdoor air must be
managed in tropical climates is different
to the way it is treated in other climate
zones. Delivering untreated outdoor
air directly to air conditioned areas
is a recipe for disaster, a recipe
for “internal rain”.
FE ATURE
It costs a lot less
to maintain an
indoor temperature
at 26°C than
it does at 22°C
The balance between outdoor air intake
and exhaust air or spill air discharge
largely determines the resulting pressure
profile within a building. In the tropics,
HVAC must act to slightly pressurise
the building to eliminate infiltration.
The quantity of outdoor air intake
should be greater, in volume, than the
total building exhaust discharge plus
any air that exfiltrates through the
building gaps, openings, and doorways
at the operating pressure.
Building exhausts include kitchens,
bathrooms, laundries and toilets,
garbage rooms, battery rooms, gas meter
rooms, and also any non-comfort local
exhausts, for equipment or processes
(think laboratories, healthcare and
workshops). Exhaust systems also provide
the potential for heat recovery (good)
or enthalpy recovery (better), particularly
where a DOAS is used. DA20 outlines
the various recovery options and their
strengths and weaknesses.
Remember, if demand-controlled
ventilation is employed, to vary the
outdoor air intake during periods
of low occupancy, then building
exhaust/spill air will also need to
be varied to maintain the required
pressurisation.
ventilation over-pressurisation of
the building is the key to managing
infiltration and air-based moisture
migration across the building fabric.
Seal all air ducts as well, including
exhaust air ducts. Poorly sealed exhaust
ducts can create local areas of negative
pressure which can act to increase
infiltration from outdoors. Avoid using
masonry or concrete shafts as air ducts,
they are notoriously difficult to seal
and keep clean.
Cleaning filters is also important
because filters that are clogged with
dirt will change the HVAC/building
pressure profile and potentially harbour
moisture and microbes, delivering
them into the airstream.
AND DON’T
FORGET THE STORMS
Storms, storm surges, tropical
cyclones, heavy rainfall, strong winds,
and extreme heat are all relatively
common in the hot humid tropics.
HVAC system designers need to
account for these events in their design
and construction. Outdoor air intakes
and exhaust discharges, the method
of supporting pipes and ducts, plant
and equipment supports all need to
be considered in terms of wind forces,
rain penetration, hail impact, and floods
or storm surges.
Carefully consider the implications,
and required mitigations, of installing
plant and equipment either below the
flood line (e.g., sump pump) or on the
roof (e.g., strapping and hail guards).
AIR TIGHTNESS MATTERS
System components and assemblies
that dry easily or are moisture resistant
are preferred, particularly for equipment
located outdoors.
Of course, if outdoor air is entering
the building through a leaky façade,
air conditioning or indeed any comfort
system will always struggle to cope.
NOT ONLY AIR
CONDITIONING
Correctly sealing those small
construction gaps and the correct
detail, installation, penetration, and
sealing of an air barrier is the best
way to achieve a well-sealed building.
The manual explains the purpose and
application of air-pressurisation testing
to prove build seal quality.
DA20 also constantly reinforces
the message that maintaining a slight
DA20 is not just about air conditioning.
Alternative comfort cooling systems
such as ventilative cooling and
evaporative cooling are also discussed
– their obvious advantages and
disadvantages, but also how they
can be applied in a hybrid or mixeduse approach to comfort and adaptive
comfort. For example, combining
ventilative cooling (e.g. ceiling fans)
with air conditioning is discussed.
The manual does not preclude
natural ventilation options, for
some or all of the time, but notes that
ventilation windows and doors need
to be sealable when closed. Draughts
and infiltration still need to be controlled,
even in a naturally ventilated building.
Air conditioning needs to be
accompanied with mechanical
ventilation. Never rely on the opening
of windows and doors for ventilation
in an air conditioned building, and
never introduce unconditioned humid
air into air conditioned spaces.
BUT WAIT, THERE’S MORE
The DA20 manual provides detailed
guidance on all of this and a range
of other topics including:
• Exploring the advantages and
disadvantages of chilled water versus
direct-expansion designs and central
versus distributed-design approaches.
• Outlining some of the control
strategies that are better suited
to tropical applications and some
of the common pitfalls in standard
approaches.
• Identifying the kinds of bugs and
pests that invade your HVAC system,
and where you might find them.
• Explaining what air pressurisation
testing is – why it’s important.
Why do it in tropical climates?
And, how to do it.
• Providing information on monitoring
and metering, and how to manage
the unoccupied building.
• Discussing the benefits of
regular HVAC tune-ups and system
maintenance for microbial control
(filters, cooling towers, cooling coils,
trays and drains) which can be more
problematic in the tropics.
The manual also includes a full-page
reproduction of ASHRAE climate data
(air temperature, air moisture, local
precipitation, and incident sunshine)
for each of the 39 Australian and
regional tropical locations represented
in the ASHRAE climate data set. Would you
like to know more?
For more information about DA20,
go to www.airah.org.au/DAmanuals
M A R C H 2 0 17 • E CO L I B R I U M
29