Last updated on May 15th, 2024 at 08:02 am
Knowledge of the climate zone to which a town or settlement belongs and possession of published regional climatic data does not eliminate the need for careful investigation of site climate conditions.
It does, however, usually provide enough information for the designer to make a preliminary assessment of the climate and may be sufficient to form the basis of sketch designs.
Every city, town or village may have its own climate, slightly different from the climate described for the region (the macroclimate).
Information published by the nearest meteorological observatory describes the macroclimate such information may be a useful guide for the climate of the site but is seldom sufficient in accuracy as conditions can vary considerably within a short distance from the point of observation.
Factors that may cause local deviation from this are:
a. Topography, i.e. slope, orientation, exposure, elevation, hills or valleys at or near the site.
b. Ground surface, whether natural or man-made, its reflectance, permeability, and soil temperature, as these affect vegetation and this, in turn, affects the climate (woods, shrubs, grass paving, water, etc.)
c. Three-dimensional objects, such as trees, or tree belts, fences, walls, and buildings as may influence air movement, may cast a shadow, and may sub-divide the area into smaller units with distinguishable climate features.
d. A logical method will be to follow the sequence of climate elements, and see how each of these may be affected by the above-mentioned factors.
Explain the factor which causes the deviation of the urban climate from the regional macroclimate.
Man-made environments can create microclimates of their own, deviating from the macroclimate of the region to a degree depending on the extent of man’s intervention.
Such intervention with the natural environment is greatest in large towns or cities, thus it is justifiable to speak of an urban climate.
The factors causing deviations of the urban climate from the regional macroclimate are the following
a. changed surface qualities (pavements and buildings)-increased absorbance of solar radiation, reduced evaporation.
b. buildings – casting a shadow and acting as barriers to winds, but also channelizing winds, possibly with a localized increase in the ventilation or by storing absorbed heat in their mass and slowly releasing it at night.
c. energy seepage-through walls and ventilation of heated buildings; the output of refrigeration pants and air conditioning (removing heat from the controlled space to the outside air) heat output of internal combustion engines and electrical appliances, heat loss from industry, especially furnaces and large factories.
d. atmospheric pollution-waste products of boilers and domestic and industrial chimneys; exhaust from motor- cars, furnes, and vapours, which both tend to reduce direct solar radiation but increase the diffuse radiation and provide a barrier to out-going radiation.
The presence of solid particles in the urban atmosphere may assist in the formation of fog and induce rainfall under favorable conditions.
The extent of deviation may be quite substantial.
Air temperature in a city can be 8 deg C higher than in the surrounding countryside and a difference of 11 deg C has been reported.
The relative humidity is reduced by 5 to 10% due to the quick run-off of rainwater from paved areas, the absence of vegetation and a higher temperature.
Wind velocity can be reduced to less than half of that in the adjoining open country, but the funneling effect along a closely built-up street or through gaps between tall slab blocks can more than double the velocity.
Strong turbulences and eddies can also be set up at the leeward corners of obstruction.
What do you mean by ventilation? Explain.
Humans consume oxygen, taken from the air by breathing and exhale carbon dioxide. An average person, depending on his activity, inhales about 0.5 to 5 m³/ h. In a closed environment, the oxygen content is reduced and the carbon dioxide content is increased by man’s presence.
Biologically the limit of existence is 5.0 % CO, content (by volume) but a 0.15% content already gives a markedly used air effect. Body smells, fumes, and vapours produced by a variety of processes, such as smoking; all add to the deterioration of an enclosed volume of air.
A supply of fresh air at a rate substantially higher than the volume of actually inhaled air will be necessary. In many situations, an adequate air supply can be ensured simple by keeping the windows and doors open.
Climate zones and world climate types
If, however, there is a large difference between externally given and internally created (comfort) conditions, the air exchange rate must be regulated, particularly if the interior is heated or cooled by the expenditure of energy.
Often a certain degree of control can be achieved by the occupants opening and closing the windows at will, but in many cases, there are no windows or a more precise and centralized form of control is necessary-therefore mechanical ventilation should be installed.
This should supply air at the rate of 12 to 28 m³/h per person, depending on the volume of space and the activities carried out.
What are the various types of mechanical systems available for ventilation? Discuss in detail.
In mechanical ventilation, the air is moved by motor-driven fans, which can be.
a. propeller type or axial flow fans
b. impeller type, centrifugal or tangential flow fans
These can be local, e.g. built into a window or a wall, or maybe central in which case ducts will be necessary to deliver and distribute the air to where it is required. The installation can take the following forms:
1. an exhaust system-removing the used air and letting fresh air find its way in through grilles and opening (room under reduced pressure)
2. a plenum system-supplying air into the space and forcing out used air through grilles, etc.(slight overpressure in the room)
3. a balanced system-both supplying and removing air. The most dependable, but most expensive, system is used when combined with warm air heating, as it permits partial recirculation. With a plenum or balanced system the air will normally be filtered at the point of intake, by one of the following means:
a. dry filters, fibrous or porous materials (paper, cloth or glass fibers) usually disposable
b. wet filters, metal turning or some loose material with a large specific surface. where all surfaces are coated with oil, normally by dipping. These can be cleaned and reused.
c. washing, by a curtain of water flowing down the face of a metal or porcelain grille, or a spray through which the air is drawn
d. electrostatic filters, in which the suspended dust particles are ionized by a high static electrical charge and stick to the face of electrode plates.