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Hygienic fundamentals
- Heat balance of people
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Hygienic fundamentals
A personrsquo;s body temperature is approximately 37 °C. However, the average skin temperature is 33 °C. People generate heat by chemical ”combustion” (oxidation) of foods. This is, in principle, energy from the sun needed to grow food in the form of plants that is again being released.
At a 33 °C skin surface temperature, the body temperature of people in the Euro- pean area is higher than the temperature of the environment almost throughout the year. People thus release heat continuously approximately as follows:
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- 35% through heat conduction and convection
- 35% through thermal radiation
- 24% through water vapor (sweating, breathing)
- 6% to heat ingested foods, drinks and breathed-in air (Fig. 2-40)
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Fig. 2-40 Percentage heat release of people
The percentages given above are averages. In summer or during intensive activity, the heat is released more through evaporation; in winter, more by convection and radiation.
However, in whichever form the heat is transferred, the body always strives to maintain its normal temperature, since it is only at this temperature that life func- tions can be carried out normally. In winter, therefore, the body reduces its heat transfer by contracting the skin: Heated blood can no longer reach the outer capil- laries. On the other hand, in summer or in heated rooms, these capillaries expand,
so that more heat can be transferred through evaporation. There are limits to this natural temperature control, however. Continuous contraction of the blood vessels can lead to frostbite, while continuous expansion can lead to an extreme drop in blood pressure (heat stroke). People supplement this automatic temperature con- trol mechanism by wearing suitable clothes, by adjusting food intake and by heat- ing or cooling the rooms they inhabit.
The total amount of heat given off by a body is not only a function of the tempera- ture of its surroundings, but even more of its momentary physical activity. (Fig.
2-41).
Fig. 2-41 Heat release in Watts related to activity for a grown person
These heat quantities become interesting when designing heating, ventilation or air conditioning systems mainly for rooms frequently occupied by large numbers of persons (e.g. department stores, offices, schools, movie theatres or restaurants).
Interior heat gains
Because of the good insulation of buildings and the thick building walls, the heat increase caused by interior heat sources, such as lamps, computers, copying ma- chines, etc., is often so large in peak periods that department stores must even be cooled in winter. The resulting heat is referred to as interior heat gains.
This example shows that a comfortable heating and ventilation system must not only be configured for the normal case, but that even in the design phase the maximum and minimum personnel loading conditions must be taken into account. In winter, the interior heat gain can be recovered as a heat contribution, thus reduc- ing energy consumption. In summer, on the other hand, the heat gain has to be ”cooled away” using considerable energy.
In a medium-sized movie theatre, 300 people produce about 30 kW which, for a three-hour showing, amounts to a thermal output of approximately 100 kWh.
People do not become conscious of the ongoing heat release from their bodies as long as their bodies have not trouble maintaining a heat balance with the surround- ings. Only when this limit is exceeded and persons begin to shiver or perspire, that is, when they feel uncomfortable, only then do they notice that they have ”a tem- perature” and also notice that, due to this temperature, they have an ongoing heat exchange with their surroundings.
The goal of HVAC technology is thus to treat the rooms which people occupy in such a manner that their bodies can effortlessly maintain a temperature balance with the room environment. This task is certainly not an easy one because the feel- ing of comfort for each person is as different as each personrsquo;s personality.
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- Comfortable room temperature
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Seen from a thermal point of view, people are bodies having a surface temperature of approximately 33 °C. If people are inside a room, be it a living room, working room or recreation room of some sort or another, these people are – with this 33 °C surface temperature – in a constant heat exchange process with the ceiling, the floor, the windows, the radiators, yes, even with the furniture and lamps, in short, with their environment Fig. 2-42). If the temperature of the environment is too low, the people give up too much heat. They shiver and feel cold and uncomfortable (left half of figure). If the temperature of the surroundings is too high, then bodily heat cannot be given up fast enough. People begin to sweat and feel uncomfortable in this situation also (right half of figure).
Fig. 2-42 Heat exchange between a person and the surroundings
So what is the proper temperature, the really comfortable room temperature, a temperature where we neither shiver nor perspire? And what other criteria also play an important role?
The comfortable room temperature initially depends on how active the people are, because we know, the larger the bodily effort, the larger the heat production. And the body must be able to give off this heat if people are to feel comfortable. Addi- tional criteria for determining a comfortable room temperature are:
- Personal temperament
- Heating habits
- Clothing habits
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