List of fi gures
2.1.2 Air movement
Key recommendations in brief:
* Begin to design for ventilation outside the build- ing, with adequate siting and supporting land- scaping;
* Utilise cross-ventilation as the most effective method of moving air through the building;
* Cross-ventilation is most efficient in narrow, open-plan buildings orientated at around 45to- wards the oncoming winds and breezes;
* Avoid creating obstructions to airflows both in- side and outside the building.
Figure 2.24 Ground tube cooling.
Relative air velocity, a measure of air movement over the skin, influences heat loss by convection and, if the skin is wet, further cooling results from evapo- ration. Its physiological effects, and hence its impor- tance, is bigger in warmer climates where clothing is normally being reduced and the body is more ex- posed to air movement, and where increased air mo- tion can greatly increase the tolerance for higher temperature and humidity levels. Still, the air temper- ature, to a great extent, affects the body’s perception of air velocity. When it is close to the temperature of the skin, an air current of 0.5 m/s may be impercep- tible but individuals may vary quite markedly in their perceptions and reactions to air movements.
There is no doubt that this climate can be seen as uncomfortable when most of the time is spent indoors and rather pleasant when we can retire to the shade outdoors. They are mainly due to one reason: reduc- tion of air speed within a building – usually limited to well below 1.0 m/s. Within buildings, air speeds are generally less than 0.2 m/s and air movement above 1.5 m/s may cause inconvenience in most domestic situations. The heat loss corrections (chill effect) for various wind speeds given in literature are adequate for conditions typical in moderate climates. In the tropics, where clothing is normally much reduced, one can use the following formula combining effects derived from several different calculation methods for air speed up to 2 m/s (Szokolay, 2000):
dT¼6vev2e ð2:5Þ where veis the ‘effective’ air speed (slightly less than the actual air velocity ve= v0.2 m/s). The resulting correction to the perceived air temperature may take a value of up to 5.6 K.
When relative humidity levels are high then the air movement becomes the most important mecha- nism for removing the heat that the body generates.
Fanger (1970), for example, demonstrated that – con- servatively defined – comfort can be maintained at 27.7C and 100 per cent RH or at 32.2C and 50 per cent RH as long as air movement of approx. 1.5 m/s is provided. Others suggest still higher temperature/
humidity values. Practically all beneficial effects dis- appear at air temperature above around 34C (approx- imate skin temperature), when body heat gain results regardless of air movement speed (Figure 2.25).
It should be noted that certain parts of the body (back of the neck, feet) are more sensitive to air move- ment and, if the stream is directed onto them, the cooling effect is increased. The cooling effect can also be improved by elimination of directional changes of the air stream. The drying effect of higher air veloci-
ties, which in cooler climates would be associated with discomfort, in the hot and humid conditions of the tropics is beneficial and generally improves comfort perception. It is worth remembering, howev- er, that when air temperature is higher than that of skin, the benefit of the enhanced evaporative cooling is countered by the detrimental effect of increased convective heat gain.
Air movement can influence the thermal perfor- mance of buildings in various ways. One such influ- ence, evident in the tropics, is the variation in heat transfer rate with different air speeds over surfaces of building materials (Figure 2.26).
There are two methods of estimating airflows through large openings in buildings resulting from winds or breezes. One is to use wind tunnel studies of scaled models to determine velocity coefficients at critical points inside the model. These coefficients relate speed of an indoor air movement to a reference wind speed outdoors. When related to long-term records (speeds and directions) of air currents near the building site, the method is quite accurate. An- other method of estimating velocities of air move- ment through the building is to use wind pressure differences and discharge coefficients. Wind pressure can be estimated from pressure coefficients and local wind speed data using techniques such as the one described in Australian Standard 1170 Part 2 (the cur- rent wind loading code). Discharge coefficients for typical openings and other formulas to estimate air- flow rates due to pressure differences caused by wind and/or stack effect can be found in many sources.
The wind velocity readings at meteorological sta- tions apply generally to heights of 10 m. They need to Figure 2.25 Estimated minimum air speed required to restore thermal comfort for a range of air temperatures and relative humidity values.
be reduced to a level appropriate for a given building.
This can be done using a formula, for example one taken from the British Standard Code of Practice CP3:
vh=v10 ¼0:2337ð1:00þ2:81log½hþ4:75Þ ð2:6Þ where h is height above ground; vhis wind velocity at height h; v10is wind velocity at a height of 10 m
The formula has been subsequently revised and in open flat country, such as on the coast, the follow- ing simplified version can be applied:
vh=v10¼c ha ð2:7Þ where, in the given type of terrain typical for the coast (flat and open), c = 0.68 and a = 0.17 (Santamouris, 1993).
Unfortunately, the number of stations supplying wind velocity data suitable for the present considera- tions is very small. Most stations that make such observations provide wind data only for 9am and 3pm, and some do it only for 9am. This situation poses a considerable problem of assessment of night-time wind velocities, which are the most im- portant from the point of view of tourist accommo- dation design.
Air movement can be referred to as wind, breeze or draught. Wind is the air movement caused by mac-
ro-scale changes in atmospheric pressure due to the heating effect of the sun. Air movements caused by localised phenomena are usually called ‘breezes’. Un- wanted air currents present inside a heated room (in cooler climates) would normally be described as draughts. In this publication, ‘draught’ will be the term given to any directionally consistent flow of air indoors.
There are a number of ways air movement could be taken into comfort considerations. It can influ- ence the physiological and mental state of people, but it can influence the thermal and acoustic per- formance of a building as well. In warm and humid climates, it is the former that is more apparent. Air movement is probably the most important comfort- bringing component of the climate in the wet tro- pics. Some experts believe that in hot and humid climates, air movement is most often the only nat- ural method of reducing heat stress. In these condi- tions it comes second only to adjustment of clothing as the most popular means of improving thermal comfort. In warm weather, the air can be utilised as a cooling medium when hot indoor air is replaced by cooler air from the outside. In addition, the ‘chill factor’ of the movement, in both cooling of the body and cooling the building structure, can be taken into account.
Figure 2.26 Surface conductance as a function of wind speed.
There are three ways to induce and promote air movement. This can be done either through differing temperature (the ‘stack effect’), by differing air pres- sure (‘cross-ventilation effect’) between two or more points, or by redirecting the existing airflows. Venti- lation and accompanying cooling effects can be ac- complished by creating conditions for the induction of air movement, or through deflecting winds and breezes in the desired direction. There are a number of factors that determine whether sufficient air ex- change can be achieved by natural means or if me- chanical ventilation has to be provided. In still-air conditions, ventilation is due to buoyancy. The ven- tilation rate is determined by the area and height of openings, and the temperature difference between indoors and outdoors. In wind assisted airflow, the rate is determined by the speed and direction of the wind at the building face, and by the number, area and type of openings providing passage for the air through the building (Figure 2.27).
When available, air movement due to ventilation can be a significant factor of improvement to the indoor environment. The airflow can increase the rate of evaporation of perspiration from the skin and hence induce the physiological cooling effect.
However, it can be argued that achievement of the desired velocities maintained across the full ‘living zone’ (up to 2 m above the floor level) would require several hundreds of air changes per hour. This seems to cast doubt over the feasibility of cooling using this
method. Moreover, when outside air temperatures are high, ventilation and the resulting admission of hot air add to undesirable heat load. In tropical cli- mates, while physiological cooling occurs at any tem- perature below skin temperature, ventilation is advantageous when it enables the warm indoor air to be replaced with preferably cooler air. Although the air moving from the outside will remove heat only if its temperature is lower than the air indoors, sensible velocity produces physiological cooling even when its temperature is slightly higher. On the coast, for instance, such cooler air can come with afternoon breezes off the sea.
Several factors need to be considered if ventila- tion is to be fully utilised:
* direction of prevailing winds and breezes;
* effect of the surrounding area on speed, strength, direction and temperature of airflows, for instance obstructions;
* design and location of openings; and
* internal layout of the building and the resulting air paths through it.
2.1.2.1 Driving force: wind pressure
The movement of air across a site is from high pres- sure zones to low pressure zones. When wind strikes a building, a high pressure zone results on the exposed side and a low pressure zone on the opposite – or sheltered – side. Usually, both the speed and direction Figure 2.27 Effectiveness of stack/single-sided ventilation and cross-ventilation expressed as the recorded indoors air speed.
of local winds are variable. However, a building can often be positioned in relation to neighbouring build- ings, planted vegetation, and other obstacles, so that the wind is moving in a known constant direction at a reasonably steady rate. Orientation broadside to pre- vailing breezes provides the greatest effect. Neverthe- less, it is the size and position of the openings that will determine the availability, speed and direction of air currents within the building. The resulting air speed indoors is greatest when the wind strikes the wall with openings at an angle of around 45 degrees, and when the apertures by which air leaves the building are bigger than (adequately sized) inlets. The best distribution of fresh air throughout the building is achieved when openings are diagonally opposite each other and airflow is not hindered by partitions and furniture (Figures 2.28 and 2.29).
Further improvement to air movement through the building can be accomplished by application of
‘Venturi-’ or ‘ridge-effect’ ventilation. This effect is created by air moving over a pitched or vaulted roof.
Since it has a longer distance to travel than the sur- rounding air, it must travel faster. It cannot gain mo- mentum, which results in a pressure drop (this phenomenon is known in fluid mechanics as the Ber- noulli principle). Consequently, air pressure near the ridge is always negative, irrespective of wind direc- tion. If exhaust points are provided in the ridge, the suction caused by the Venturi effect would ensure
considerable rates of air extraction from relevant spaces (Figure 2.30).
Obstructions to breezes by incorrect placement and structure of walls should be avoided. Any parti- tions located in the internal flow path impede air circulation. In order to promote unrestricted air movement, partitions should be adjustable and located so as to offer least resistance to airflow. The best is an ‘open-plan’ strategy, which seems quite Figure 2.28 Cross-ventilation is facilitated by areas of positive and negative pressure around buildings.
Figure 2.29 Recommended orientation for best ventila- tion results.
appropriate for many functional components of a resort (Figure 2.31).
In general, wind speed increases with height above ground. This increase, or wind velocity gradi- ent, is greater in an open area than among trees or buildings, which cause turbulence in airflows.
2.1.2.2 Driving force: buoyancy
Middle East wind towers, which can be considered vernacular examples of solar chimneys, use the en- hanced temperature gradient or stack effect as the
principal mechanism to move air through the adjoined building. Their thermal action is invoked by deliberate exposure of their massive structures to solar radiation. This results in heating the air they contain, inducing air movement from the building through the towers. The mechanism of interacting with the environment for indoor climate control demonstrated in wind towers in the Middle East is very complex and the temperature gradient effect is only one aspect of this mechanism. Anyway, convec- tive movements of air resulting from stack effects are rarely sufficient to induce appreciable airflows. For
Figure 2.31 Wind gradient in various terrains.
Figure 2.30 Irrespective of roof pitch, the ridgeline experiences negative pressure (suction) also known as the ‘ridge’ or
‘Venturi’ effect and this can be utilised to induce air extraction (compare with Figure 3.17).
this, pressure differential or hybrid pressure and tem- perature gradient systems must be utilised (Figure 2.32).
The stack effect is being employed also in the Trombe-Michel wall, a passive solar heating and cool- ing device useful in buildings occupied during the day for substantial periods of time (Figure 2.33).
Kenyan researchers disproved an earlier belief that a high-ceilinged room (above 2.4 m) was neces- sary for comfort in hot regions. They argued that increases in the height of a room resulted in an in- crease of the external wall area to be shaded. It
appears that subsequent heat gains would substan- tially increase and ultimately cause an even greater discomfort. As it follows, improvement of indoor comfort would then require roof insulation or vent- ing hot air from under the ceiling. Nevertheless, we have to stress that provision of space for air convec- tion is quite important as it might be that the Kenyan findings are applicable only to the daytime scenario.
It will be a different case in the absence of daytime heat gains.
Cathedral and other high ceilings allow hot air stratification above the occupancy height. Moreover, Figure 2.32 Solar chimney principle.
Figure 2.33 Trombe-Michel wall’s cooling action.
high ceilings promote useful rates of air exchange.
Some promising results have been reported from experiments in rooms with the thermal shade partial- ly drawn while leaving the window open at the top so as to get both sun control and sun-boosted venting.
Incorporation of supplementary means of airflow generation, such as ceiling fans, also assists, provided that they are energy efficient and move the air up rather than push it down.
2.1.2.3 Insect control and airflows
Although insects and pests do not in themselves con- stitute a climatic factor, preventing their presence impacts on bioclimatic design. Fly-screens, for in- stance, can severely restrict internal airflows, espe- cially when winds or breezes are light. Reduction in total airflow caused by a typical wire screen is about 25 per cent at wind speeds in excess of 2.75 m/s and about 60 per cent at 0.7 m/s. Cotton screens can re- duce the velocity of incoming air by 70 per cent, while smooth nylon screens reduce it by 35 per cent.
They reduce the rate of structural cooling at night and comfort ventilation at all times. This undesirable ef- fect may be reduced if fly-screens extend over a larger area than the opening alone, and if they are placed at some distance from them. Screening an entire veran- da/balcony (i.e. screen area larger than opening area) is better than screens installed in the respective door/
window frames (Figure 2.34).