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VENTILATION The world is ventilated by natural movement of air. Inequalities of heat distribution drive the climatic air movements. The convection currents in the atmosphere are generally powerful enough to dilute the pollutants which we generate. However, pollution due to industrialisation is not always removed by the wind. The great fog in London in 1952, which killed in excess of 4000 people who had vulnerable lungs, and the photochemical smog in Los Angeles are examples where the natural air movements in an anticyclone are not strong enough to ventilate a city. When considering the ventilation of individual parts of a town, there is the concept of canyon streets where pollution is dispensed slowly and the concentration of Carbon Monoxide is a problem.
105 THE ENVIRONMENTAL CONSEQUENCES OF A BUILDING WITH A WIDE SPAN Max Fordham Max Fordham & Partners Fig 1 Model for a competition in Pottsdam designed by Straub & Vogler This paper examines the environmental consequences of a building with a wide span. A bridge is wide span but it does not make the kind of environmental impact which concerns me. I take it that much of this symposium is concerned with structures like the Dome :- Sports arenas (Sydney 2000 Stadium), cricket schools, greenhouses (the Great Glasshouse at the National Botanic Garden of Wales), cities in Alaska, garden centres (Manheim), the Albert Hall, the Crystal Palace, the Pantheon, Gothic Cathedral. Enclosure of Manhattan (Buckminster Fuller) A wide span building is a building where the enclosing envelope is the top surface and has a wide span. This means that the top surface is light in weight and is designed to carry only the minimum load. The wide spanning surface can hardly carry the load of another storey of accommodation and so we are considering single storey buildings. I am not a structural engineer but I suspect there are reasons for wide span structures to be tall at least in places. Even though the enclosure of a wide span structure may be light, gravitational forces will be developed. These forces are vertical and a vertical component of forces generated in the structure will have 106 at least to equal the weight. Members which are inclined to the horizontal can generate vertical forces even without bending moments, so most wide span structures seem to be carried with arches and catenaries. I dare say the structural papers will expand on this theme and give some credit to Frei Otto who developed ideas for forming and finding shapes which could support wide span enclosures with bending moments only needing to be developed for perturbing forces. The wind is a big perturbation. So we get single storey, wide, high spaces with lightweight cladding. At a smaller scale there are cyclones which are turbulent and chaotic and provide the variability in our weather. The scale here is about 1000km. Cumulo nimbus clouds are limited by the height of the atmosphere where the plumes of buoyant gas fan out when they reach the tropopause. The point about this introduction is that the air in very large spaces is mixed by turbulent convection currents. The other point to notice is that at the tropopause there is a temperature inversion (the temperature increases with height), and the atmosphere is stratified. VENTILATION The world is ventilated by natural movement of air. Inequalities of heat distribution drive the climatic air movements. The convection currents in the atmosphere are generally powerful enough to dilute the pollutants which we generate. However, pollution due to industrialisation is not always removed by the wind. The great fog in London in 1952, which killed in excess of 4000 people who had vulnerable lungs, and the photochemical smog in Los Angeles are examples where the natural air movements in an anticyclone are not strong enough to ventilate a city. When considering the ventilation of individual parts of a town, there is the concept of canyon streets where pollution is dispensed slowly and the concentration of Carbon Monoxide is a problem. The thermal equilibrium of any part of the world is affected by ventilation. We do not particularly think of the hottest parts of the world as being places with inadequate ventilation, but the hottest parts of the world are generally places subject to anticyclonic weather with low air movement, where the temperature builds up. A wide span structure clearly modifies the ventilation of the space it encloses. Ventilation is needed to disperse pollution and to control the thermal conditions in a space. The ventilation of a fire is a particular case where these two requirements are combined. I believe we have to understand some basic principals about fluid flow in enclosures so that we accept the possibility that ventilation can look after itself. Imagine looking at a plan of Paddington station and wondering how to provide adequate ventilation in this deep structure with combustion engines inside. In fact, the enclosure is adequately ventilated by the openings in the top and at one end. We live in a very large enclosure. It is about 12km high and 7000km radius. Convection currents drive strong air movements. The major convection cells (the trade winds) have a scale of about 10,000km, even though the atmosphere is only 12km high. In a large span structure, convection cells are likely to develop, with a scale characterised by the size of the space itself. When a strong temperature inversion develops at the top of the space, a stratified layer is likely to develop. Hot gas<ts flowing out through vent 1> V" twfyl" V \ A;r «ntrc.nad by rising % y strccm ot goscs ^LAl Formation of a layer of hot gases Fig 2 Design of roof-venting systems for single storey buildings, Fire Research Technical Paper No 10; 1964, Fig 2 page 4 Picture on Fire Research. Warm air discharge Fig 3 Heat source forming a plume in a dome Fig 4 Multiple heat sources and plumes in a dome 107 If we need to ventilate a space so as to remove heat from 'h' the lower occupied section of a tall space 'H\ then the amount of air rising in the plumes to height 'h' must be extracted from the upper reservoir and replacement air allowed to enter at the base of the space. During sunny weather the temperature of the skin will build up. The temperature build up depends on the wind speed, the reflection coefficient to short wave radiation - light, and the emission coefficient to long wave radiation. It is likely to be 10°C to 20°C above ambient. The entering air must not cause uncomfortable air movements in the occupied zone and it must not disrupt the stability of the hot air reservoir. Most of the envelope of a wide span structure is likely to be subject to a low pressure zone while the elevated pressure stagnation zone will be developed at low level on the windward face. It may be possible to rely on input air flowing only from the windward with discharge at the top. Often with strong winds, too much air will come from the windward, and it will flow out to leeward. If the resulting air currents are tolerable then the wind driven ventilation is a good solution. If the air currents are too strong then the air inputs on the windward side have to be throttled off so that the air enters on the leeward faces driven by the thermo syphon effect of the reservoir of hot gas. This pattern of air movement requires input around the perimeter and air outputs at the top. I was tempted to illustrate how this thinking might be applied to the Millennium Dome. It is important that the warm air is stratified and stable above the occupied zone of the building. Where a hot zone of air is lying above a cooler zone, there is a region with a strong vertical temperature gradient. If this region has strong air movements with the air speed changing with height, then the stratification and the turbulence will act in opposition. The Richardson number (R S Scorer 1) is defined as :- dd_ where, 6 = z = U = dU lz temperature height horizontal velocity acceleration of gravity If Ri > 1 then the turbulence will die down and the horizontal air flow will be restrained below the stratified layer. Fig 5 Hypothetical flow for the Millennium Dome The internal area is, say, 80m high. Strong air movement can be tolerated round the perimeter, say 0.5m/s at the centre. The heat from the exhibition buildings will be given out as driven ventilation plumes at a temperature of, say, 25°C. I suggest these discharges should be ducted above the reservoir. It is tempting to apply the ideas of stratification and displacement ventilation to the heat loads in the Dome, but the following calculation shows how quickly a plume cools down and how much input air is needed. Say there are 50,000 people in the centre of the Dome (25,000 people in the exhibition buildings, and 25,000 people clustered into 13m diameter arrays of 400 people each, producing 53kW per array, then with the addition to solar gain reaching the floor at, say, 100W/m 2 , the temperature rise in the plume is much less than 1°C. Qf heatsource r 0 = 5a 0 radius of heatsource Fire Research Paper No 7 HMSO 1983 1968 reprlr acceleration due to gravity 9 8" m-'sec' temperature above ambient : C temperature "K heatsource kW density of air at source temp 1 2 kg/rrf' specific heat of a>s kJ/kg height from point source to plane defined by "y Fig 6 Fire Research Paper 7 108 The air flow into the reservoir would be over 4000m 3 /s. In fact the ventilation requirement for 50,000 people is nearer 1000m 3 /s and the plume is likely to recirculate inside the building. lOOOmYs of ventilation picking up a heat gain of, say, 100W/m 2 plus 50,000 people is 12,000kW and raises the temperature of the air by 10°C. If this stratifies in the top 30m of the Dome, the stack effect is about lOPa, giving a velocity of about 3m/s through the vents. So 300m 2 of roof vents, which need to be controlled to prevent too much ventilation in cold and windy weather, should be supplemented with, say, 400m 2 of vent round the perimeter. Of course, keeping the rain out has to be addressed. This crude analysis should be a precursor to more modern techniques such as salt water modelling and CFD (computational fluid dynamics). Salt water modelling gives an accurate representation of convective turbulence, but it is not able to model the momentum of incoming air, nor the thermal capacity of the bounding surfaces. National Theatre warm stage - cool auditorium smoke flows towards fly-tower cool stage - warm auditorium smoke flows towards auditorium Fig 7 Air flows in the National Theatre Then at the Royal Exchange Theatre Manchester which is a very large enclosure with a theatre inside it We worked with Professor Manfredi Nicoletti on an entry for the Cardiff Bay Opera House Competition which was to be a large glass enclosure. The modelling of a thermal plume has to be carefully considered. A person is modelled as a 100W heat input. The model should equate to 100W emitted from, say, a 450mm diameter source with an initial plume of, say, 20 1/s at a 5°C temperature rise. It should not be a 100W gls lamp, say, 100mm diameter with a plume of, say, 5 1/s at * 20°C temperature rise. The difference here is represented by different flow rates of salt solution at different densities. Computational fluid dynamics cannot deal properly with turbulence. Assumptions about the amount of turbulence have to be built into the finite difference equations as dummy transfer constants. The constants have been developed for heat transfer in jet turbines, nuclear power station boilers, meteorological forecasting, and other rich applications. The constants for buildings certainly need examining as far as I am concerned before I am happy to have them used in very large and very small spaces with very different Reynolds numbers. Fig 8 Competition entry for the Cardiff Bay Opera House - Interior. Designed by Manfredi Nicoletti These ideas about ventilation and fluid flow are based on experience. When commissioning the National Theatre, the convection currents seemed to be preventing the ventilation operating as required by the fire officer. We turned up the thermostat controlling the stage fan convectors and the convection currents reversed. Fig 9 Competition entry for the Cardiff Bay Opera House Thermal Balance 109 Fig 10 CFD for the Cardiff Bay Opera House - Velocity Vectors LIGHTING One of the immediate consequences of the single storey property of wide span structures is that the spaces can be roof lit The requirements for light to enable people to see are not to be defined too simplisticaUy. The eye is like a camera with a variable aperture pupil to control the amount of light entering and with a light sensitive membrane retina to generate signals which are focused and transmitted to the brain. The signals are processed by the brain and the nerves in the retina to generate sensations which we interpret as "seeing". The retina can integrate the photons it receives in a variety of ways. I CflRDff 86V OPERA MOUSF 'eMPERATI Fig 11 CFD for the Cardiff Bay Opera House - Temperature Profile The air flow and temperature were modelled using CFD as shown on the figure above. The air flow model immediately suggests that the air entry slot should be above head level and slanted upwards. There was another issue about the temperature plot. We initially modelled the solar gain onto every node of the lowest part of the floor level. The temperature plots showed very high temperatures on the floor which we couldn't understand. However, I vaguely remembered a lecture during which it was explained that mirages in the desert sometimes occurred when the morning sun heated the desert surface and the invert layer of air to a high temperature causing a very strong temperature gradient close to the ground and making the mirage. The hot air did not rise as a convection current because there was no trigger to start the convection at any point. The relaxations calculation process of CFD would be similar if all nodes were at the same temperature with the same heat gain. The heat gain would be equated to the temperature rise and the relaxation process might be stopped before getting a proper answer. We changed the input, putting in a double batch of solar input to half the nodes and immediately the answers looked sensible like the figure above. I am sure I will raise as many questions as I answer in this brief summary of seeing but I am presenting you with a working designer's basic knowledge. In the absence of light the pupil is wide open and too few photons are received to see anything. The sensitivity of the eye/brain system is increased and it takes about half an hour for full sensitivity to be developed. The part of the retina most sensitive to low light levels is not in the same location as the area most sensitive to detail, so at night air force pilots are trained to look away slightly from a dim object. At low light levels, the retina integrates different coloured photons and integrates them over a period of time before transmitting a signal to the brain. Thus the image seen is monochrome and in these conditions the retina cannot catch fast moving objects. The sensitivity varies with age, but a fully adapted night eye can "see" a light coloured object under an overcast night sky at 0.001 lux. SECURITY (;N'«IM;t.;K]Ni; Ronge of 'Vidicon' EOffiWa ojtenuW by doling leni iris Operating theatre Wcli iH chart Drawing ofiice Offices, shops Stairs, corridor Weil EH s» Upp»f limit of VFiion tolerance Approximate rong« of 'Newvici camera, using auto ilia .«n* 10 ? • 10"*' i Clear nsght 10"*- i OttKCOtt slight Ay 10 •'• (0-4t*| f Range of 'V.dicorV and extended b, infra-red * loop* |2r,lx| | 102ml«| 1 iO Oorolsjj Fig 12 Figure 4.8 from the CIBSE Applications Manual AM4: 1991 Relative light level chart showing operational ranges of several types of camera. 110 "See" must mean differentiate between two levels of brightness of objects of a certain size (not too small) so the reflective properties of the field of view are important. When I say "see" in quotes I mean the whole eye/brain process. As the general light level increases, the eye/brain adapts to the changed signals. With more photons arriving the cells can differentiate between different colours and can send/fire off signals to the brain at shorter intervals and the brain can "see" colour and fast moving images. There is a limit to the rate at which a cell can send signals to the brain. After receiving light and sending a signal it has to reset itself. So, as the light received by a cell increases, the frequency of sending signals increases until it is saturated and can simply report maximum brightness. Indeed, if too much light is received the cell can overheat and die. I was reminded of the ancient Egyptians who worshipped the sun god Ra. A person accused of offending the god was tied down on their back in the open and their eyelids cut off. At the end of the day, if they could still see they were innocent but usually the retina was burnt out by the bright sun and this was taken as a proof of guilt. Bright sun at 100,000 lux (1000W/m 2 ) is definitely too much. If there is an unevenly lit field of view but all the cells report saturation, the field of view is not being "seen" very well. The eye needs to adapt so that the bright part of the field of view just fails to saturate any cells. Of course the dimmest part of the field must provide enough light for good, fast colour vision. Thus, there is a maximum ratio of brightest to dimmest light for good vision. The eye adapts to the average illumination. If a field of view is uniformly lit then the eye simply receives a uniform vision and there is very Utile differentiation between planes and objects. I experienced this once when I went very early to an exhibition at the Satehi gallery. The gallery was painted white all over and lit with fluorescent uplighters. Light was diffusely reflected off the ceiling in all directions. There were very few exhibits and very few people so everything was white and I felt very disorientated. In order to "see" the field of view must not be uniformly bright. Hold a pencil vertically up on a sheet of paper and look at the shadows. I hope there are some - usually several. A shadow represents a place where one of the brightest light sources cannot shine on the paper behind the shadow. The light in the shadow comes from all other sources. The contrast between the brightness of the paper generally and the brightness in the shadow represents the contrast between the general diffuse light and the directional light from the source. Set this up in a more disciplined way and we can define the vector/scalar ratio of an illumination field. Imagine looking at a hemisphere on a table. In diffuse light where every surface is uniformly illuminated the hemisphere looks like a disc. In purely vector light, the shaded part looks black (like a half moon). A mixture of diffuse and directed light is needed in order to perceive the shape of the hemisphere. Horizontal light from right Generally diffuse light 30% diffuse Fig 13 From Table 10 - Relationship of vector/scalar ratio to assessment of directional qualities of the lighting. IES Code for Interior Lighting 1977 The overcast sky is very diffuse and the design minimum figure is taken at about 5000 lux. Under an overcast sky the field of view reflects back a diverse field of light because of different reflection coefficients and different colours. The lighting/seeing is pretty adequate despite being viewed in very diffuse light. Inside a building the outside light is usually introduced from a transparent window. The light then has a vector component, and shapes and shadows can be seen even on an object with a uniform surface. Compared to outside the light level is reduced but the "seeing" is improved because the light has a stronger vector component. As an illustration of the need for diffuse light, think back to the difficulty of seeing anything in the region of a matt black motor car engine by the light of a single torch, even when it doesn't wobble. The reason for this discussion is to try to get you away from the view that the amount of light needed in a space can be defined by a single, simple figure of the amount of light. The contrast between inside and outside is important. The shading over a motorway underpass assumes that the speed limit is being kept and the light level can be halved every 3 seconds. In the Mediterranean one walks from the bright sunlight outside at 100,000 lux into a room with the shutters closed. It takes some time before you can see anything. I guess the shuttered room allows about 0.1% of the diffuse sky light (10,000 lux) into the room so that the light level is about 10 lux. So on entering the room the light level drops from bright sunlight at 100,000 lux to 10 lux or 1:10,000, ie 213.3 = 10,000. It takes 3 seconds for the eye to adapt to a halving of the light level and therefore takes 40 seconds (3 x 13.3 + 40) to adapt to a light level of 10 lux. In the UK, well designed rooms with fixed windows keep most direct sun out and then provide a daylight factor of about 1% or 2%. I believe this should be increased for new buildings so as to reduce the amount of fossil fuel and electricity used for lighting. However, this aim of mine carries an increased risk of buildings getting too hot in summer. The indoor cricket schools at Lord's and Edgbaston try to exclude most direct sun and to provide a daylight factor of 5% to 6% and so give 1000 to 1200 lux on an overcast day. These buildings do not fit my idea of wide span enclosures but the cricket school at Lord's was won in a competition where we expected the opposition would offer air supported or other lightweight solutions. We put forward a case for natural lighting. A diffuse skin with a transparency of 20 to 25% provides a light level of 1000 to 1250 lux as required. However, in strong sun the internal light level rises to 20,000 to 26,000 lux and the direct solar gain rises to 200 to 260W/m 2 as well as long wave radiation from the translucent skin. Another disadvantage of the overall diffuse skin is that it gives a very diffuse light inside. The lighting inside a tent or marquee is very diffuse and gives poor figuring to three dimensional shapes. In saying this I am offering opinions which could inform the development of the design of lightweight, wide span enclosures. I realise that diffuse skins are provided for indoor tennis centres, millennium domes and so on. As structural engineers gain confidence in making lightweight wide span structures using glass as the membrane, then I think the design, for example, of ventilating roof lights will be able to be developed. The thinking about the type of lighting needs also to be developed. A solution with 75% to 80% opaque area with 20% to 25% horizontal, transparent area supplies the same level of light as a diffuse skin. It is then possible to insulate the opaque area. 100,000 Lux in direction of sun Light Cloud Blue Sky Overcast Sky 20,000 Lux 10,000 Lux £,000 Lux No to option one - dark building fitted with fluorescent lighting. Poor ambience, and lighting consumes 450KWh/m 2 a year. No to option two - fabric/translucent roof In sunlight 24,000 Lux internally can quickly be reduced to 2400 by a small cloud. \ l;>>7 JJJJlJJJJJhiflHB 1 1200 -4800 Lux I Fig 15 Interior of the Indoor Cricket School at Lord's Ground. Photograph by Dennis Gilbert. The transparent areas possibly need blinds or shades to protect the space from direct sunlight. Or the transparent areas can be diffusing with high transparency. Yes to option three -north roof light. Only diffuse light admitted. Light levels in excess of 1200 Lux except after sunset in winter. Fig 14 Sawtooth roof arrangement at the Indoor Cricket School at Lord's Ground designed by David Morley Architects 112 JHK Fig 16 Interior of Bespak Stage 1 showing diffusing roof lights. Designed by the Cambridge Design Group. Fig 17 The Menil Collection, Houston. Designed by Renzo Piano. Photograph by Hickey Robertson. The lighting solution for the Menil Gallery is a special case where the external condition was generally strong direct sun (100,000 lux) and the light level inside had to be kept low for conservation purposes (50 to 100 lux), so 0.1% of the light was required and multiple reflections provided a really clever solution. So a wide span, single storey building can easily provide adequate light. A light transparency of 2 to 25% is feasible and provides adequate light. A lighting strategy can be developed to provide shading for direct sunlight and improve the overall thermal efficiency of the skin at the expense of a less homogenous solution. HEATING The air movement in a large, tall space is likely to be violent. The subject of ventilation and air movement has been addressed. The roof of the structure is likely to subtend an angle of 2p steradians from a person so that its radiant temperature will be an important factor. One of the main issues with wide span buildings is that if the skin is to be light and transparent then a single or possibly double skin of fabric is unlikely to meet the Building Regulations for energy conservation. A justification might run :- 1. A conventional building deemed to satisfy the Building Regulations We need a light level of 1000 lux and a building with an opaque roof will need electric light at 30W/m 2 , using fossil fuel at a rate of 90-120W/m 2 . An opaque roof is allowed to lose 3W/m 2 of heat generated by fossil fuel, ie a U-value of 0.3 x 10°C mean. Total energy requirement 93-123W/m 2 . 2. Lightweight wide span skin During a 24 hour mean day with a mean inside to outside temperature difference of 10°C, we have :- Opaque Conventional Roof Transparent Roof single skin double skin U-value Temperature Difference Heat Loss Heat Loss in 24 hours Electric Light (12 hours) (at 30W/m 2> Watt hours per day 0.3 6 3 10°C 10°C 10°C 3W 60W 30W 72Whr 1440Whr 720Whr lOoOWhr 1152 1440 720 This argument can be developed for different conditions. The light saved in the summer will improve the argument but for a lower light level, say 500 lux, the electrical energy saved is less impressive. Of course if the space is unheated the insulation value is unimportant. For a competition entry for the Cardiff Bay Opera House (shown previously in Figures 8, 9, 10, and 11), we had postulated a roof of 50% double glazing and 50% insulated panels. The enclosed space was a foyer so the electrical energy saved by lighting to 100 lux or so was not significant. 113 However spaces which formed the brief were huddled together rather like a Greek village or the National Theatre. Fig 18 Sketch by Max Fordham for Cardiff Bay Opera House competition with Manfredi Nicoletti. The surface area of this convoluted shape implied a heat loss through walls and windows with ventilation which we evaluated and compared to the heat loss of the simply shaped envelope. The envelope had a lower heat loss than that deemed to satisfy the building so the Building Regulations were satisfied. The internal buildings could have simple, un-insulated walls which notionally helped to pay for the whole scheme. Fig 19 Buckminster Fuller Dome over mid town Manhattan This argument follows Buckminster Fuller's for the dome over mid town Manhattan where the extended heat transfer of the buildings is replaced by the smooth, reduced envelope of the dome. In both cases there is a problem enabling heat and pollution to escape from the inner layer of buildings and this is basically the ventilation problem addressed in the next section. Heating large high spaces depends on several levels of consideration :- Heat Loss Firstly we have to decide on the heat loss. The U-value of the cladding is important. Next, the amount of winter ventilation; how airtight will the enclosure be and what stack effect is likely. How much temperature gradient will there be to increase the heat loss at the top. Ventilation air will tend to come in at the bottom and on the windward side. The incoming cold, fresh air needs to be heated before it can lead to discomfort. A 4 or 5m/s wind speed coming through the windward cracks or open doors must be heated. Most 50m high buildings have lobbied entrances A 50m high stack with a 20°C temperature difference will produce air movement through openings of about 8m/s. The temperature gradient in the space depends on the types of heat source. It is difficult for any part of the space to get hotter than any individual object inside. Direct fuel fired warm air heating is designed to be cheap by recirculating air into a space at around 70°C and using heaters of 300 to 600kW capacity. The air flow is of the order 6 to 12kg/s and the air has to be supplied at a very high velocity to ensure that it mixes into the room before losing momentum and drifting up to the ceiling. At the necessary velocity noise generation is the problem. The parameters of air flow, heat load, noise generation, and temperature gradient have to be considered. In working out warm air heating we have relied on a hypothesis advanced by Holmes and Caygill [2] and repeated by P J Jackman [3], that :- if thermal forces are not to dominate the pattern of air circulation. This relationship was originally postulated for a specific set of conditions but we have used it successfully in much more extreme situations. Where a heating system provides W kg/s of air at q°C specific heat c kJ/kg = 1 at velocity V then the momentum M = WV and the heat load q = Wcq The relationship, where H = height, becomes :- 114 wv f 0.07 WOH or V > qH 0.07 We have used the relation at Churchill College, St Mary's Church Barnes, and the CZWG office in Bowling Green Lane. At St Mary's Church Barnes we deliver 4mVs of air at lOm/s from a nozzle at 70°C into a 10m high space. This does not generate a noise. Fig 20 St Mary's Church, Barnes At Churchill College, the space is 10.5m x 18.5m x 22m and the air supply to a dining room is at 12m/s. I have started with crude warm air heating because I believe it is suitable for large open space of indeterminate use. Of course, radiant heat has its advocates for tall spaces. I don't want to give a detailed case as to why I am not in favour. Where competing design solutions coexist in a market then the reasons favouring one rather than the other are probably marginal. Of course, if a group of open air dining spaces were under a wide span canopy, radiant heaters to each space would be a good solution. The behaviour of the air in a space with heat sources inside it is largely defined by the behaviour of the plumes. A plume is a rising current of air which is warmer than the surroundings. The behaviour of plumes is described in the book "Environmental Aerodynamics" by Scorer and it has become a very important topic for fire engineers. The plume is a particular case of jet flow. It is a hot jet. Jets are also described by Scorer and are very important to HVAC engineers in considering how air flows in space and how grilles need to be sized. The best visualisation of a jet which I know and which I expect most of you can visualise is a stream running under a humped back bridge. I idealise the flow in the following figure. W i, i i i i * « V » i i r 1 I 1 a n * ^— Streamline flow ^— Flow starts to converge Flow remains radial Velocity on circle/sphere ^— Stagnant: weeds thrive ^— Rapids form at centre of stream Level drops T— Eddies feed flow into side of jet Jet expands Velocity drops Momentum is conserved ^— Quantity of flow in jet increases then reduces as flow is bled off to serve the eddies ^— Status quo reinstated Fig 21 Idealised flow at a hump backed bridge at a quiet stream. CONDENSATION Moisture movement in buildings is not perfectly understood. We should remember that the moisture content of air and water vapour has an upper limit. The upper limit is a function of temperature. Air and water • separate out into cloud/mist , . Water content Upper limit of water content Temperature Fig 22 [...]... special feature of wide span structures are that the spaces tend to be tall, rooflit, and poorly insulated The size is not a reason for abandoning natural ventilation because air movement is more likely to be turbulent in a large space than a small one I would like to see light admitted as a series of discrete, transparent areas rather than an overall diffusing surfaces Then the opaque areas can be... 0.6% of the roof area I think I came to the understanding while at the conference that wide span structures really do have to be designed to minimise the weight On the other hand, some resistance to wind uplift is a benefit 2 For a target insulation resistance of R = (say) 3°C/m W the mass per unit area = p R/k where p is the density and k is thermal conductivity So p/k is the significant property The. .. top of the membrane but that will be drained away as though it were rain OPTION 2 The insulation layer below the membrane With this option, the membrane is at or near the outside temperature Any moisture which flows through the insulating layer has to be removed by a ventilation path to outside 2 Standard UK practice (Building Regulations ) gives as guidance that vnetilation openings should be about...115 The air in a building may be represented by point 'A' at a certain moisture content and temperature If the air is cooled down until it is saturated - at the limit line - moisture will deposit as condensation or dew The temperature is then a measure of the moisture content It is called the dew point The dew point of the air inside a building will be the same as the dew point outside... added Generally in well ventilated slightly wanned spaces condensation rarely occurs even on single membranes For condensation to occur, the dew point of the air has to be above the temperature of any surface On a clear night, heat is radiated to the sky and a lightweight surface quickly cools down to below the air temperature and often to below the dew point so that dew forms Of course, dew will also... to be 100mm thick and of mass 3kg/m ; OPTION 1 In cold climates the insulating layer can be resistant to rain penetration and placed on top of the waterproof membrane The membrane is then kept warm and condensation is prevented Gravel, paving slabs or other means of holding insulation in place Roof membi Fig 23 Fig 24 116 In warm moist climates with air conditioned enclosures condensation is likely... recorded in the literature In buildings which are enclosed, moisture is likely to be produced A person produces 0.022g/s of water and if ventilated at 10 1/s raises the dew point by 3-4°C The surface temperature of a membrane has to be raised by this temperature above outside to prevent condensation A person raises the temperature of 10 1/s of air by about 8°C and the temperature of a single skin is raised... improved thermal resistance The main complication arises because of handling interstitial condensation problems 4 The volume of air associated with each 1 sq m of roof is roughly the height of the space 1 To reduce the dew point of 50m of air by 1°C and so prevent further condensation inside, we need to condense 25g of moisture per sq metre lOg/m* looks like this An insulating layer is typically going... condensation outside than inside As condensation takes place, moisture is taken out of the air The slope of the saturation curve varies with temperature but at 10°C ± 10°C the moisture content changes by about 5g/kg for a 10°C change in dew point (E B H Stevens and M Fordham ) Condensation on the underside of lightweight sheeted roofs - warehouses, dutch barns, and so on - is not a problem which has been... form on the underside of a single membrane under this condition As moisture condenses on the surface, latent heat is released so that the surface temperature tends to stabilise a little below the dew point The boundary layer on the outside has a higher conductivity than the boundary layer on the inside so more dew will form outside than inside The conductivity ratio is about 3 so there will be about