To the Building Environmental Engineer it is generally not the overall size of a building that creates the challenge it is the internal height and the lack of suitable locations for indoor climate control systems. Large span structures are synonymous with high open spaces. The Engineer seeks to control not only thermal conditions but also Indoor Air Quality (IAQ) both to achieve comfortable conditions within the occupied space and to maintain a healthy environment free from pollutants (of which there are many). Ideally the Engineer would seek to condition the occupied space rather than the whole volume and hence benefit from both reduced plant capacity and reduced energy consumption and C02 emissions. This is not always possible. The temperature within a large space can be controlled using air systems or radiant systems. Indoor Air Quality (IAQ) can only be controlled using fresh air (usually outdoor air). Many systems tend to combine the temperature regulation function with the IAQ function.
Trang 1CONTROLLING THE INDOOR CLIMATE
IN WIDE SPAN ENCLOSURES
4 CASE STUDIES
Nick Cullen Hoare Lea & Partners - Consulting Engineers
S Y N O P S I S
This paper presents four case studies of different large
span structures, describing the characteristics of, and the
systems used to control, the indoor climate
The first two studies consider the difficulties inherent in
designing systems that 'fight' against the basic laws of
physics The first of the two, the British Aerospace
Aircraft Assembly Hall is based on work undertaken in
the 1980's and highlights the significance of buoyancy
forces and the difficulty in mixing airstreams of different
temperatures The second case study, the ExCel
exhibition centre in London's Docklands, highlights the
need for compromise in the design of Engineering
systems
The second two studies review projects in which the
designs made use of the natural forces of gravity and
buoyancy in order to maintain thermal and Indoor Air
Quality (IAQ) conditions The first, the Millennium
Stadium Cardiff, features a fully retractable roof and
relies upon Natural Cooling and Ventilation enhanced
with the operation of the smoke extract fans as necessary
The final Study details the work undertaken at the House
of Representatives, Brasilia the Capital of Brasil It
discusses the significance of control and alternative
strategies
I N T R O D U C T I O N
To the Building Environmental Engineer it is generally
not the overall size of a building that creates the
challenge it is the internal height and the lack of suitable
locations for indoor climate control systems Large span
structures are synonymous with high open spaces
The Engineer seeks to control not only thermal
conditions but also Indoor Air Quality (IAQ) both to
achieve comfortable conditions within the occupied
space and to maintain a healthy environment free from
pollutants (of which there are many) Ideally the
Engineer would seek to condition the occupied space
rather than the whole volume and hence benefit from
both reduced plant capacity and reduced energy
consumption and C 02 emissions This is not always
possible
The temperature within a large space can be controlled using air systems or radiant systems Indoor Air Quality (IAQ) can only be controlled using 'fresh air' (usually outdoor air) Many systems tend to combine the temperature regulation function with the IAQ function
The problem faced by Engineers is that hot air rises, or more accurately, cold air falls and forces warmer air to high level leading to temperature stratification within the space This fundamental law of physics can work to the Engineers advantage A case in point being Displacement Ventilation Systems (natural or mechanical), which rely upon buoyancy and gravity forces to drive them However displacement air systems require the supply air
to be introduced at low level and at regular -albeit fairly large -intervals This is rarely compatible with the needs
of large span structures and indeed is often in conflict to the use of such structures
The consequences of stratification are twofold Firstly, the increased temperature differential at roof level results
in a greater heat loss increasing energy consumption and thereby C 02 emissions Secondly, thermal conditions within the occupied zones may at times be unsatisfactory, depending of course, on the location of the occupants
High spaces are generally conditioned using mixing systems with the supply air introduced at high level, the objective being, to minimise stratification by producing a fully mixed environment The designer has to ensure that when heating the supply air can deliver heat to low level and when cooling the air arrives at low level without causing discomfort due to cold drafts In the process of creating a mixed condition, pollutants, produced within the space, are diluted by 'fresh' air
The alternative, that of displacement ventilation, seeks to condition and removes pollutants only from occupied zone
Trang 2C A S E S T U D Y N O l
B R I T I S H A E R O S P A C E A I R C R A F T
A S S E M B L Y H A L L , B R I S T O L
" T H E B R A B A Z O N H A N G E R "
BACKGROUND
The aircraft assembly hall was constructed in the 1940's
for the specific purpose of constructing the Brabazon
aircraft, the largest aircraft in the world at the time The
building's clear height (23m) was determined by the
height of the Brabazon tailfin and its clear internal span,
by its wingspan Its overall internal height reaches 35m
At the time the building was completed, it was one of the
largest clearspan structures of its type in the world Its
floor area was approximately 30,000m2 and enclosed a
volume of 1,000,000m3 (Figures 1&2)
Height to Eaves 26 m Height to Apex 35 m Floor Area 30,000 m 2
Total Volume 1 mi 1.1 ton
Fig I The Brabazon Hanger - Exterior View
Fig 2 T h e Brabazon Hanger - Interior View
EXISTING HEATING SYSTEM
The original (1940's) heating system comprised steam unit heaters at catwalk level blowing air vertically down into the space At the perimeter of each bay were located
a row of "swan neck" steam heaters which drew cool air from low level, heated it, and discharged the warm air down towards the hangar floor from a height of about 10m (Figure 3) By 1980 the steam pipework was beyond its useful life and had significant leakage problems The pipework was poorly insulated, mainly with asbestos and as a consequence, apart from the health issues of asbestos the operating efficiency of the system was extremely poor Furthermore, under test it was found that the unit heaters at catwalk level gave insufficient velocity to the hot air to overcome its inherent buoyancy The heated air lost any momentum after the first few metres and rose back up to high level Thus, only the perimeter "swan neck" heaters provided any useful heat to the hangar floor, the remaining capacity being used to heat the roof space Temperatures
at roof level rose regularly towards 40°C in the vain attempt to hold a comfortable temperature within the occupied zone (Figure 4)
He,! Lot* through Roof
Down draught heaters dine!
airdowtw >nry3m Swan Neck Darcharge
C R O S S S E C T I O N - C E N T R E S P A N )
Fig 3 Existing Heating Sytem
Improved thermal performance Reduced heat Ion leading to increase tn temperature
CROSS SECTION -CENTRE SPAN) ORIGINAL ROOT 4 HEATING SYSTEM
CROSS SECTION -CENTRE SPAN) NEW ROOF 4 HEATING SYSTEM
The building has always been difficult to heat effectively
In the early 1980's a complete re-cladding of the building
was undertaken to upgrade the performance of the
building envelope to comply with the Building
Regulations standards of the day Sadly, the cost of
upgrading the doors was prohibitive, a feature which we
will return to later
Fig 4 Temperature Profiles
NEW HEATING SYSTEM
Immediately following the recladding contract, Hoare Lea & Partners were commissioned to design a new direct gas fired heating system to replace the original steam fired system The concept was to replace the
Trang 3existing steam heaters at catwalk level with direct gas
fired unit heaters, blowing vertically downwards from a
height of 23m (Figure 5) The existing perimeter heaters
were to be modified, and instead of blowing warm air
down to low level, they were to draw cool air from low
level and to discharge the air vertically upwards, mixing
the cool air with warm air at high level, inducing
destratifying circulation currents within the space
Fig 5 Proposed New Heating System
The concept had been developed in conjunction with Bristol
University who carried out performance monitoring on the
existing system and then on a trial mock up, modifying one
of the perimeter "swan neck" heaters Initial results were
promising, showing a much reduced temperature gradient in
the space
The team identified the proposals as carrying significant,
technical risk, there being no precedent for use of reverse
destratification system, least of all, on a building of this size
In order to offset this risk, the team applied to the EEC for a
Thermie Grant which was subsequently awarded, in
recognition of the innovative nature of the project
The client embarked on a significant construction contract,
comprising the removal of the existing heating system,
including the steam pipework installation, asbestos
insulation and heaters In its place was installed a new gas
pipework, new power distribution system, fan powered unit
heaters complete with discharge jet nozzles The complete
installation was undertaken, at a height of 23m, whilst
maintaining production on the factory floor This required
significant protection measures to be provided to allow the
building occupants to continue working safely Key design
considerations involved reducing C 02, and moisture levels
in the space to acceptable levels by introducing fresh air
through perimeter units The design of the heaters, and
"swan neck" discharge nozzles was also critical to give good
air mixing and air distribution
The designers struggled to balance the design parameters of
heat input, air velocity, noise and power consumption and
cost and eventually arrived at a "best fit" solution
P E R F O R M A N C E After completion of the installation, the performance of the heating system was monitored to assess whether the predicted performance was achieved in practice The results were dramatic
The delivery of air at 45°C to the hangar floor from a height of 23m required a substantial discharge air velocity At part load conditions, when the discharge temperature was lower, the high discharge velocity was not dissipated, so that a very high air movement occurred
at low level It was decided to accept a restricted turndown ratio on the units, typically to a minimum of 80% of full heat output, the fans being controlled
"on/off below this level
The building fabric, and particularly the old hangar doors, were found to allow a considerable amount of cold air to infiltrate into the building As a consequence of the density of this cold infiltration, it tended to collect at low level creating a cold "lake" of air at about 10°C in the first 2m above the hangar floor, the very zone that was required to be heated
Under full load output from the heaters, operating in response to temperature sensors located in the cool occupied zones, the buoyant warm air was found to have lost most of its momentum by the time it arrived at the bottom 2m zone The discharge air suddenly moving in 10°C set, not 20°C ambient air, effectively "bounced" at this 2m level, providing very little heating effect in the occupied zone As a consequence, the whole volume of the hangar was being heated to a temperature of 20-25°C, in order to maintain I0°C in the occupied zone (Figure 6)!
^t^+HMt Low through Roof
, ""1
a Entrained Air item high lev e mixes ' 20*C t§ With raster dBchtrge Air
C R O S S S E C T I O N - C E N T R E SPAN)
Fig 6 Actual Performance
M A I N ACCESS D O O R
Paradoxically, the solution to this problem was to reduce the maximum heat output of the gas heaters, lessening the buoyancy of the supply air, which enabled proper penetration by the supply air into the occupied zone, and good mixing in that space
Trang 4The modified "swan neck" destratification units were
found to have minimal effect in destratifying the space,
the temperature profiles and airflow patterns being
determined primarily by the velocity and discharge
temperature of air from the direct fired gas heaters
Of course with hindsight the solution should have
included:
(i) an increase the thermal performance of the
doors
(ii) a reduction in the infiltration leakages of the
building
Had it been practical within the constraints of an
operational production facility, the provision of a warm
floor by embedded piping or by overlaid radiant heaters,
may have overcome many of the problems
C A S E S T U D Y N O 2
E X C E L L O N D O N , R O Y A L V I C T O R I A
D O C K
INTRODUCTION
Across the river from the Millennium Dome on the North side of the Thames a New "State of the Art" exhibition centre is about to open Phase 1 of the project will provide 93,500m2 of accommodation including 64,000
m2 of exhibition space split between two halls Each hall
is designed with a minimum clear height of 10m The entire exhibition space is located above a car park A boulevard running the length of the building separates the two column free halls The whole building can operate as a single exhibition space or be sub-divided down into individual halls each of 4000m2 (Figure 7)
Fig 7 Excel Exhibition Centre - London Docklands
VALUE MANAGEMENT
The indoor climate control system was divided according
to the minimum module size A single air-handling unit serves each module and is located at high level within the structural depth of the roof Supply air ductwork from the air-handling unit is distributed at high level (Figure 8) Out door air is drawn in via a 'beehive' air intake the amount being determined either, by Indoor Air Quality (IAQ) as measured by C Oz sensors, or according to the free cooling opportunities As extract air is drawn it passes directly from the space and discharges to out doors
Intake Air Exhaust Air
Supply Air via Long throw Diffisers
EzUbitlonHal]
I Clm
r
Boulevard Supply Air via Long throw Diffisers
EzUbitlonHal]
I Clm
r
90m
Fig 8 Excel Exhibition Centre - London Docklands - Diagramatic
Trang 5The exhibition space required both cooling and heating
The supply air system therefore had to operate to deliver
warm buoyant air to low level during heating, and cool
non-buoyant (heavier) air during cooling The obvious
answer was to vary the trajectory of the supply air
according to the supply air temperature by using
adjustable geometry diffusers This however proved to be
too costly and would probably prove to be unreliable and
an alternative approach was required
The alternative proposal envisaged a fixed airflow
trajectory with long throw nozzles fixed directly into
ductwork and arranged in groups With volume flow rate
and design supply air temperatures, fixed, two variables
remained under the designers control, discharge velocity
and trajectory (Figure 9) Using Computation Fluid
Dynamics combinations of the different parameters were
tested in both heating and cooling modes
Computational Fluid Dynamics, not available at the time
the design of Brabazon Hanger Design was employed to
assess options and performance of the design (Figure 9)
RESULTS
The results from the analysis showed that the cold slab
(due to the unheated car park below) would create a
'lake' of cold air at low level which could be reduced in
depth by increasing the momentum of the supply air, but
could not be completely overcome Once again the
conclusion pointed to the need for a warmed floor which
was beyond the budget (Figure 10)
The CFD modelling images brought instance 'Deja vu' to
the (by now Partner) engineer who years earlier had
experienced the Brabazon hanger or refurbishment and
its outcome
It was recognized that the primary circumstance likely to
occur was that of cooling and so parameters were
selected to satisfy the associated thermal comfort
conditions
Engineering designers learn very early that compromise
will be called for, that compromise often involves
designing to satisfy the primary circumstances When
warmth from exhibits and people will require a cool air
supply from the building systems That lessens the
outstanding probability that when a few people rent a
small amount of the space in colder weather they may
find a bracing experience requiring a pullover Satisfying
the majority that is now called value judgement and is an
essential part of an engineer's experience
Figure L5 (a) Temperature distribution at height of 1 5m
Fig 9 Results - Computational Fluid Dynamics
2 3 B
Winter model, no occupancy Winter model- Low level occupancy
Fig 1 0 Results - Computational Fluid Dynamics
Trang 6Fig 11 Millennium Stadium Cardiff - Exterior View
C A S E S T U D Y N O 3
T H E M I L L E N N I U M S T A D I U M C A R D I F F
I N T R O D U C T I O N The £120million Millennium Stadium Cardiff has a capacity of 72,500 people and is the first UK arena to have a fully retractable roof It provides a multi-use all weather venue with completely un-restricted views The grass pitch is completely removable allowing the arena to
be put to use as a concert venue The stadium takes the form of a bowl complete with retractable roof This form Fig 12 Millennium Stadium Cardiff - Interior View clearly limits the Natural ventilation and cooling
mechanisms that act around stadia with open corners
The retractable roof (Figure 11 & 12), when closed, created a number of problems that the designers needed
to resolve Firstly the space needed to be ventilated to remove unwanted heated and metabolic pollutants Ventilation was also an important factor in maintaining a healthy grass pitch Secondly it had to be safe, allowing spectators to escape in the event of a fire
Trang 7The arena was conceived as being Naturally Cooled and
Ventilated using the vomitory passage ways and a
high-level louvre system as air paths Numerous different
scenarios were considered using Computational Fluid
Dynamics (CFD) The Criteria set for the Stadium was
for all occupied areas to remain below 28°C at design
summer conditions (26°C) The effect of different sized
openings, their number and location were investigated
The initial analysis assumed a worst-case scenario of
stack driven ventilation only without wind assistance
The analysis showed the need for two sets of parallel
louvres running at high level , one at the junction
between the retractable roof and the fixed roof and the
around the back of the upper tier seating Temperatures at
high level varied only slightly between the various
options (Figure 13) The arrangement operated primarily
using Natural buoyancy effects and, when available,
wind pressure to drive air through the arena The smoke
extract fans are made available to guarantee a minimum
volume of fresh air movement through the arena
CFD modelling showed that the combination of vomitary
and high level openings produced acceptable conditions
with the roof closed even without the beneficial effects of
wind or with the fans running
& P A R T N E R S
26 0
Cardiff Millennium Stadium
Fig 13 Results - Ventilation and Cooling CFD Results
FIRE The fire engineering for public arenas is vitally important The objective was to determine whether, in the event of a fire, there would be sufficient time for the audience to escape This time for full evacuation from the arena was calculated as 12 minutes taking into account detection, investigation, action and evacuation times In addition a smoke temperature limit of 200°C and a visibility distance of 25m to a reflective sign were adopted, as design criteria
Being primarily a sports stadium the potential fire load was minimal It was considered that a pop concert with a stage located at one end of the pitch was the worst case scenario The effect of the operation of the mechanical extract system was investigated using Warrington's Fire Research CFX CFD software
The results highlighted two important factors Firstly that the depth of the smoke was worst at the end of the stadium closest to the fire (Figure 14) The time available for escape in these areas did not meet the design criteria and people could not be located in these areas
Secondly the operation of the fans provided an additional
2 minutes escape time extending the period to 14 minutes for the topmost seats The extract temperature of the smoke was estimated as being between 39°C and 43°C, well within the operational capability of the fans (Figure 15)
Time: + 1 2 minutes
0 0 0 0 0
Fig 15 Computational Fluid Dynamics - fire/smoke - fans operational
Fig 14 Computational Fluid Dynamics - fire/smoke - no fans
Trang 8C A S E S T U D Y N O 4
H O U S E O F R E P R E S E N T A T I V E S ,
B R A S I L I A N C O N G R E S S B U I L D I N G S ,
B R A S I L I A , B R A S I L
In late 1997 Hoare Lea & Partners Research and
Development group were asked to offer advice on the
problem of acute 'Sick Building Syndrome' in the House
of Representatives at the Brasilian Congress The
particular Building, is that pictured and constructed in
the 1960's to designs by the renowned Architect Oscar
Niemeyer (Figure 16)
Fig 16 House of Representatives, Congress Building, Brasilia
-Exterior View
The House of Representatives is one of two chambers
(plenaria) in the Congress building complex and it measures
some 30 m in diameter and 15m high The plenaria has
capacity for up to 550 people made up both of Representatives
and a smaller number of journalists A raked gallery for
'spectators' overlooks the chamber, encompassing 3/4 of the
high level perimeter, but this is isolated from the chamber by a
glass screen (Figure 17 & 18)
Fig 18 House of Representatives, Congress Building, Brasilia -Interior View towards Podium
The building had been reported as 'sick', indeed a Government Minister had passed away it was said,
"because of the amount of his time he had spent in the building" An initial visit and inspection of the air supply system indicated that the system was clearly at the end of it's serviceable life It also had some inherent design problems most notably the absence of any system of air extraction other than by tortuous route out of the chamber via the main entrance doors which had to be left open (Figure 19)
Fig 19 House of Representatives, Congress Building, Brasilia -Schematic representation of existing ventilation and cooling system
Hoare Lea and Partners were asked to put forward a scheme which after much consideration was based on Displacement Ventilation principles Unlike the first two case studies displacement ventilation is a system that relies upon natural forces to function Cool fresh air is introduced at low level and is drawn towards any heat source where is warms and is 'displaced' to high level taking with it unwanted heat and pollutants The polluted air can be extracted and thrown away having first passed through heat exchangers
Trang 9Two alternative schemes were studied and each was
modelled using Computational Fluid Dynamics The
favoured scheme envisaged the installation of a
compartmented raised floor through which air would be
delivered to air terminals integrated into the seat The
floor would double as a conduit for power and data
cabling (Figure 20)
Schematic of Proposed New
Displacement Ventilation for
dulled AHeattng Water from Existing Central Plant
-Fig 20 Schematic of proposed new displacement ventilation
The alternative method was to introduce the air around the
perimeter of the space a scheme that would have required
only a small raised platform
The size of the space highlights another inherent problem of
large spaces not so far mentioned, that of locating control
sensors This problem exists irrespective of the parameter
being measured
Ideally the sensor should be located at regular intervals
within the occupied zone, but without a surface on which to
mount the sensor an alternative strategy is required The
walls around the chamber offered possible locations but were
rejected due to their variable surface temperature and
unrepresentative location
The main concern was the IAQ within the space and the
main pollution sources both of heat, chemical and biological
contamination were the occupants themselves The quantity
of air could therefore be varied according to the number of
occupants within the space Whilst C 02 sensors are regarded
as a good measure of IAQ when people are the main
pollutant source, they were considered to be too much of an
on-going maintenance item requiring regular re-calibration
Two alternative strategies were conceived The first was the
inclusion of a variable volume damper within the
construction of the seat itself This would enable the
associated diffuser to deliver fresh air only when the seat was
occupied A background supply would be guaranteed
through other diffusers The alternative was simply to count,
electronically the number of people within the space and
then deliver an appropriate volume of fresh air This would
rely upon the characteristic of displacement ventilation for
the air to be drawn to the heat sources within the room Both
these options would have resulted in energy and C 02
consumption reductions
Ka.-il.aii loi'iKc&t - under Lest tolut>
• • i ->' - of Temper&l'j'e tl)
Fig 21 C F D Results - Velocity Vectors - Temperature Supply
Riri/'iian Conor Case 2 S
Vekioty Victors Gotwrt C> Vckc-oty Magn-tuOo (nvs)
RMHAJNS 4.2 pa to i :
Thu Apr te-W8 ;
Fig 22 C F D Results - Velocity Vectors - Perimeter Supply
Rrf.7t»u*nCuiiti(e** C&ce 2»
Vei»:*v Victors Ccwaoa Dv V**fflv Magirtudo (m/s) Cioss section at tortus • 20m
eiuonfUNS 4.2 0(1 he) Thu Ap» 16 -aSK) Fluefil inc
Fig 23 CFD Results - Velocity Vectors - Perimeter Supply
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Trang 10RESULTS
The results confirmed the design supply air volume was sufficient to maintain thermal conditions within acceptable limits in both cases (Figure 21) It did however identify that the alternative perimeter supply solution generated a 'dough-nut' vortex which had the effect of driving high level polluted air to low level back down into the occupied zone This was due to three factors Firstly the massing of heat sources created a coalescence of individual plumes which rose to high level Secondly the thermally cool surfaces of the glass divide between the gallery and plenaria generated a down flow of air Thirdly the rising plumes of air drew air from the perimeter supply points The combination of these three characteristics generated the vortex (Figure 22 & 23) In contrast the favoured option with the supply air introduced on a seat by seat basis showed a less vigorous air movement with a general, albeit un-steady drift of air flow to high level (Figure 24)
The project proposals await approval and finance from the government which, unlike our own, of whatever party, is very concerned not to spend money on it's own accommodation whilst there are calls for money from its populace
CONCLUSION
Wide span structures enclosing large volume high spaces present the Building Engineer with significant challenges The Building Environmental Engineer seeks
to control the conditions within the occupied space with the minimum of 'environmental impact' Numerous different scenarios often need to be considered The function of the space along with cost restrictions often force the Professional Engineer to design systems that fight the basic laws of physics and to seek compromises
in performance The advent of CFD has given the Engineer an invaluable tool enabling the prediction of the performance and comparison of different engineering systems Despite the rapid growth in computer power we are still limited to making only global assessments of large spaces