This page contains a Flash digital edition of a book.


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


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

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72