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THERMAL COMFORT


MASTERCLASS


Professor Doug King


This month’s article takes a few steps back to re-examine the fundamentals of ‘operative temperature’ to see how we can move forward with low-energy heating and cooling systems


A


s warm-blooded animals, humans produce their body heat internally. But this means that their internal organs


need to be regulated within the fairly tight temperature range of 36.5C to 37.5C. If our core temperature drops below 35C, we suffer from hypothermia; and if it rises above 37.8C, we are said to be suffering from hyperthermia, sometimes called heat stroke. Mild hypothermia is characterised by


shivering and a loss of coordination. Severe hypothermia, where the core temperature falls below 28C, causes irrational behaviour and leads to death if unchecked. Nevertheless, people have recovered from profound hypothermia, with temperatures as low as 20C. Hyperthermia is characterised by hot, dry


skin, but the loss of mental faculty happens at a much smaller deviation from the norm than for hypothermia. By the time the core temperature rises to 40C, the condition is life threatening. Clearly humans are much more susceptible to overheating than to the cold. So we need mechanisms to lose heat to the environment in order to remain cool, but not lose too much heat or we become over-cooled. The human metabolism converts calories


from food to energy in order for the body to function. Those calories that we do not use to do work, such as moving an object or moving ourselves, are converted into heat. We can lose heat to the environment through convection or conduction to the surrounding air, through radiant exchange with surrounding surfaces, and through evaporation of moisture from our skin and respiratory tract. The body has a number of mechanisms that automatically regulate


52 CIBSE Journal June 2011


our rate of heat-loss by these various means to maintain the correct core temperature. If we get too cool we can increase our


rate of heat generation or increase our level of insulation. Shivering is involuntary muscular activity designed to increase heat production. Vasoconstriction restricts blood flow near the skin to reduce heat- loss, and goose bumps appear when hair follicles contract in order to make the hairs stand up, trapping an insulating layer of air against the skin. We can also increase our clothing insulation, which is a cultural, rather than a biological, adaptation to the cold. If we become too warm, vasodilatation increases the flow of warm blood to the body surface for cooling. Panting and sweating are both means of increasing heat-loss through evaporation of moisture, either from our respiratory tract or from sweat glands beneath the skin. The presence of liquid sweat on the surface of the skin is actually an indicator that this cooling mechanism is already overloaded. Thus, in order to do full justice to our


thermal adaptability, it is necessary to have an index for comfort that takes into account the rate of metabolic heat generation, clothing insulation, air movement over the body and the processes of heat transfer by radiation, conduction, convection and evaporation. The standard method for assessing thermal comfort with all of these parameters is the predicted mean vote (PMV). One purpose of this Masterclass series


is to look for simpler methods of analysis that can lead to insights about the design of building services. Thus, as most heating or cooling systems affect the sensible temperature rather than the humidity or air


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