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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.
117 CONTROLLING THE INDOOR CLIMATE IN WIDE SPAN ENCLOSURES 4 CASE STUDIES Nick Cullen Hoare Lea & Partners - Consulting Engineers SYNOPSIS 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. INTRODUCTION 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 C0 2 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 C0 2 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. 118 CASE STUDY NO.l BRITISH AEROSPACE AIRCRAFT ASSEMBLY HALL, BRISTOL "THE BRABAZON HANGER" 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 30,000m 2 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 The 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 CROSS SECTION -CENTRE SPAN) 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 119 existing 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. CROSS SECTION -CENTRE SPAN) MODIFIED HEATERS 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 C0 2 , 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. PERFORMANCE 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 el mixes ' 20*C t§ With raster dBchtrge Air CROSS SECTION-CENTRE SPAN) Fig 6 Actual Performance MAIN ACCESS DOOR 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. 120 The 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. CASE STUDY NO.2 EXCEL LONDON, ROYAL VICTORIA DOCK 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 93,500m 2 of accommodation including 64,000 m 2 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 4000m 2 (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 CO z 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 ^s-— .,.,••„- )IINI?jahu| ••••• "7—\ 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 121 The 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 23B Winter model, no occupancy Winter model- Low level occupancy Fig 10 Results - Computational Fluid Dynamics 122 Fig 11 Millennium Stadium Cardiff - Exterior View CASE STUDY NO.3 THE MILLENNIUM STADIUM CARDIFF INTRODUCTION 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. 123 The 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. & PARTNERS 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: +12 minutes _ 0.0010 0.0000 Fig 15 Computational Fluid Dynamics - fire/smoke - fans operational Fig 14 Computational Fluid Dynamics - fire/smoke - no fans 124 CASE STUDY NO.4 HOUSE OF REPRESENTATIVES, BRASILIAN CONGRESS BUILDINGS, BRASILIA, BRASIL 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. Fig 17 Plan and Section through House of Representatives Two 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 C0 2 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 C0 2 consumption reductions. Ka il.aii loi'iKc&t - under Lest tolut> . •. • i . ->' - of Temper&l'j'e tl) Fig 21 CFD Results - Velocity Vectors - Temperature Supply Riri/'iian Conor Case 2S Vekioty Victors Gotwrt C> Vckc-oty Magn-tuOo (nvs) RMHAJNS 4.2 pa to i : Thu Apr te-W8 ; Fig 22 CFD 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 B'tU'haii Cu tyieu Cass 1 ••'-•>:•< V6CKHSCokxM r. .e. • " Cress free non at racJus - 25nt E H«1H. : NS4.2(3J M»2.W. unstMQY Fn Apt 17 1998 Hurt IK, Fig 24 CFD Results - Velocity Vectors - Under Seat Supply 126 RESULTS 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.