2.9 Heat Balance
21
2.9 Heat Balance
The core body temperature of any person is maintained constantly at about 37 °C,
although the actual value varies slightly from person to person. In cold climates, the
body temperature should be above that of the external environment, so there is an
internal source of heat in order to maintain the temperature difference; in hot climates,
the reverse phenomenon is required. The body must maintain thermal balance for
comfort, i.e. the metabolic heat generated in addition to the heat received from the
external environment must be matched to the heat loss from the body. If there is no
balance between the heat gain and the heat loss, the body temperature will potentially
either rise or fall, leading to a serious threat to life. Heat is generated in the human
body by the process of metabolism. It is an established fact that, of the total energy
extracted from food, only 15–30% is converted into useful work, and the remaining
70–85% is wasted as heat. Higher levels of physical activity will generate additional
amounts of heat energy that must be dissipated; otherwise, the body temperature will
rise. A lower level of or no physical activity leads to a fall in body temperature if
the available heat is not conserved by insulation. If a person is in heat balance (i.e.
comfortable) at rest, then a bit of hard work will produce excess heat and perspiration,
which need to be dissipated. Similarly, if the person is in heat balance while doing
strenuous exercise, then ceasing the work will lead to heat loss and feeling cold [65].
The heat exchange with the environment plays a key role in the thermal state of the
human body [66]. Thermal comfort is defined as the condition of mind that expresses
satisfaction with the thermal environment. Dissatisfaction is caused by a warm or cool
environment. Discomfort for the body, in general, is expressed by the predicted mean
vote (PMV) and predicted percentage of dissatisfaction (PPD) indices. However,
thermal dissatisfaction may also be caused by unwanted heating or cooling of one
particular part of the body (local discomfort). The PMV and PPD indices express
warm and cool discomfort for the body as a whole [67]. Due to individual differences,
it is impossible to specify a thermal environment that will satisfy everybody. A
percentage of occupants can always be expected to be dissatisfied. Nevertheless, it
may be possible to specify environments predicted to be experienced as acceptable
by a certain percentage of occupants. In the new standards, comfort requirements
are specified and predicted to be acceptable for at least 80% of occupants [68]. Heat
loss through respiration is only around 10% of the total heat loss from the body.
Humidity and temperature of the inhaled air have, therefore, only a small impact on
the thermal sensation for the human body as a whole [69].
2.10 Modes of Heat Transfer
Heat transfer or heat exchange occurs when an object or fluid is at a different temperature than its surroundings or another object. The heat transfer occurs until the body
and the surrounding reach thermal equilibrium. Heat transfer always occurs from the
21
2.9 Heat Balance
The core body temperature of any person is maintained constantly at about 37 °C,
although the actual value varies slightly from person to person. In cold climates, the
body temperature should be above that of the external environment, so there is an
internal source of heat in order to maintain the temperature difference; in hot climates,
the reverse phenomenon is required. The body must maintain thermal balance for
comfort, i.e. the metabolic heat generated in addition to the heat received from the
external environment must be matched to the heat loss from the body. If there is no
balance between the heat gain and the heat loss, the body temperature will potentially
either rise or fall, leading to a serious threat to life. Heat is generated in the human
body by the process of metabolism. It is an established fact that, of the total energy
extracted from food, only 15–30% is converted into useful work, and the remaining
70–85% is wasted as heat. Higher levels of physical activity will generate additional
amounts of heat energy that must be dissipated; otherwise, the body temperature will
rise. A lower level of or no physical activity leads to a fall in body temperature if
the available heat is not conserved by insulation. If a person is in heat balance (i.e.
comfortable) at rest, then a bit of hard work will produce excess heat and perspiration,
which need to be dissipated. Similarly, if the person is in heat balance while doing
strenuous exercise, then ceasing the work will lead to heat loss and feeling cold [65].
The heat exchange with the environment plays a key role in the thermal state of the
human body [66]. Thermal comfort is defined as the condition of mind that expresses
satisfaction with the thermal environment. Dissatisfaction is caused by a warm or cool
environment. Discomfort for the body, in general, is expressed by the predicted mean
vote (PMV) and predicted percentage of dissatisfaction (PPD) indices. However,
thermal dissatisfaction may also be caused by unwanted heating or cooling of one
particular part of the body (local discomfort). The PMV and PPD indices express
warm and cool discomfort for the body as a whole [67]. Due to individual differences,
it is impossible to specify a thermal environment that will satisfy everybody. A
percentage of occupants can always be expected to be dissatisfied. Nevertheless, it
may be possible to specify environments predicted to be experienced as acceptable
by a certain percentage of occupants. In the new standards, comfort requirements
are specified and predicted to be acceptable for at least 80% of occupants [68]. Heat
loss through respiration is only around 10% of the total heat loss from the body.
Humidity and temperature of the inhaled air have, therefore, only a small impact on
the thermal sensation for the human body as a whole [69].
2.10 Modes of Heat Transfer
Heat transfer or heat exchange occurs when an object or fluid is at a different temperature than its surroundings or another object. The heat transfer occurs until the body
and the surrounding reach thermal equilibrium. Heat transfer always occurs from the