In the same way as the earth has a climate the inside of buildings also have a climate, with measurable values for air pressure, humidity of radiated heat. Efficient control of these factors leads to optimum room comfort and contributes to man’s overall health and ability to perform whatever task his is engaged in. Thermal comfort is experienced when the thermal processes within the body are in balance ( i.e when the body manages it’s thermal regulation with the minimum of effort and the heat dissipated from the body corresponding with the equilibrium loss of heat to the surrounding area)
RECOMENDATION FOR INTERNAL CLIMATE:
An air temperature of 20- 24°c is comfortable both in summer and in winter. The surrounding surface areas should not differ by more than 2-3°c from the air temperature. A change in the air temperature can be compensated for by changing the surface temperature ( e.g with decreasing air temperature, increase the surface temperature) If there is too much difference between the air and surface temperature, excessive movement of air takes place. The main critical surfaces are those of the windows.
For comfort, heat condition to the floor via the feet be avoided ( ie the floor temperature should be 17°c or more). The surface temperature of the ceiling depends upon the height of the room. The temperature sensed by humans is somewhere near the average between room air temperature and that of surrounding surfaces.
It is important to control air movement and humidity as far as possible. The movement can be sensed as draughts and this has the effect of local cooling of the body. A relative air humidity of 40-50% is comforted with a lower humidity (e.g 30% dust particles are liable to fly around.
To maintain the quality of the air,controlled ventilation is ideal. The Co² content of the air must be replaced by oxygen. A Co² content of 0.10% by volume should not exceeded, and therefore in living rooms and bedrooms provide for two to three air changes per hour. The fresh air requirement of humans comes to about 32.0m3/h so the air change in living rooms should be 0.4 – 0.8 times the room volume per person/hour.
Temperature Regulation and heat Loss From the Body.
The Humans body can raise or lower the rate at which it losses heat using several mechanism: increasing blood circulation in the skin, increasing the blood circulation speed, vascular dilation and secreting sweat. When cold, the body uses muscular shivering to generate additional heat.Heat is lost from the body in three main ways: Conduction, Convection and Radiation.
- Conduction is the process of heat transfer from one surface to another surface when they are in contact (e.g feet in contact with the floor) . the rate of heat transfer depends on the surface area in contact,the temperature differential and the thermal conductivies of the materials involved.copper for example, has a high thermal conductivies while that of air is low, making it a porous insulating materials.
- Convection is the process of body heat being lost as the skin warms the surrounding air. This process is governed by the velocity of the circulating air in the room and the temperature differential between the clothed and unclothed areas of the body. Air circulation is also driven by convection: air warms itself by contact with hot objects ( e.g radiators), rise’s, cools off on the ceiling and sink again. As it circulates the air carries dust and floating particles with it. The quicker the heating medium flows (e.g water in a radiator) the quicker is the development of circulation.
- Radiation: All objects including the human body emit heat radiation in accordance to temperature difference between the body surface and that of the ambient area. It is proportional to the power of 4 of the body’s absolute temperature and therefore 16 times as high if the temperature doubles. The wavelength of the radiation also changes with temperature: the higher the surface temperature, the shorter the wavelength. Above 500°c, heat becomes vissible as light. The radiation below this limit is called infra-red/ heat radiation. It radiates in all directions, penetrates the air without heating it, and is absorbed by ( or reflected off) other solid bodies. In absorption the radiation, these solid bodies ( including human bodies) are warmed. This radiant heat absorption by the body ( e.g from tiles, stoves) is the most pleasant sensation for humans for physiological reasons and also the most healthy.Other heat exchange mechanisms used by the human body are evaporation of moisture from the sweat grands and breathing. The body surface and vapour pressure differential between the skin and surrounding areas are key factor here.
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