BODY HEAT BALANCE
Ordinarily the body remains at a fairly constant temperature of 98.6°F. It is very important that this body temperature be main-tained and, since there is a continuous heat gain from internal body processes, there must also be a continuous loss to maintain body heat in balance. Excess heat must be absorbed by the surrounding air or lost by radiation. As the temperature and humidity of the environment vary, the human body automatically regulates the amount of heat it gives off. However, the body’s ability to adjust to varying environmental conditions is limited. Furthermore, although the body may adjust to a certain (limited) range of atmospheric conditions, it does so with a distinct feeling of discomfort. The following discussion will help you understand how atmospheric conditions affect the body’s ability to maintain a heat balance.
Body Heat Gains
The human body gains heat
(1) by radiation, (2) by convection, (3) by conduction, and (4) as a by-product of thephysiological processes that take place within the body (for example, the conversion of food into energy). Heat gain fromradiation comes from our surroundings. However, heat always travels from areas of higher temperature to areas of lower temperature. Therefore, the human body receives heat from those surroundings that have a temperature higher than body surface temperature. The greatest source of heat radiation is the sun. Some sources of indoor heat radiation are heating devices, operating machinery, and hot steam piping. Heat gain from convection comes only from currents of heated air. Such currents of air may come from a galley stove or an operating diesel engine.
Heat gain from conduction comes from objects with which the body comes in con-tact. Most body heat comes from within the body itself. Heat is produced continuously inside the body by the oxidation of food, by other chemical processes, and by friction and tension within muscle tissues.
Body Heat Losses
There are two types of body heat losses: loss of sensible heat and loss of latent heat. Sensible heat is given off by
(1)radiation, (2) convection, and (3) conduction. Latent heat is given off in the
Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation. Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC (heating, ventilation, and air conditioning) design engineers.
Thermal neutrality is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. The main factors that influence thermal comfort are those that determine heat gain and loss, namely metabolic rate,clothing insulation, air temperature, mean radiant temperature, air speed and relative humidity. Psychological parameters such as individual expectations also affect thermal comfort.
The amount of thermal insulation worn by a person has a substantial impact on thermal comfort, because it influences the heat loss and consequently the thermal balance. Layers of insulating clothing prevent heat loss and can either help keep a person warm or lead to overheating. Generally, the thicker the garment is, the greater insulating ability it has. Depending on the type of material the clothing is made out of, air movement and relative humidity can decrease the insulating ability of the material
The air temperature is the average temperature of the air surrounding the occupant, with respect to location and time. . Air temperature is measured with a dry-bulb thermometer and for this reason it is also known as dry-bulb temperature.
3.Mean radiant temperature
The radiant temperature is related to the amount of radiant heat transferred from a surface, and it depends on the material’s ability to absorb or emit heat, or its emissivity. The mean radiant temperature, depends on the temperatures and emissivities of the surrounding surfaces as well as the view factor, or the amount of the surface that is “seen” by the object. So the mean radiant temperature experienced by a person in a room with the sunlight streaming in varies based on how much of her body is in the sun.
Operative temperature attempts to combine the effects of air and mean radiant temperatures into one metric. It is often approximated as the average of the air dry-bulb temperature and of the mean radiant temperature at the given place in a room. In buildings with low thermal mass, the operative temperature is sometimes considered to be simply the air temperature.
Air speed is defined as the rate of air movement at a point, without regard to direction. According to ANSI/ASHRAE Standard 55, it is the average speed of the air to which the body is exposed, with respect to location and time. The temporal average is the same as the air temperature, while the spatial average is based on the assumption that the body is exposed to a uniform air speed, according to the SET thermo-physiological model. However, some spaces might provide strongly nonuniform air velocity fields and consequent skin heat losses that cannot be considered uniform. Therefore, the designer shall decide the proper averaging, especially including air speeds incident on unclothed body parts, that have greater cooling effect and potential for local discomfort.
Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor that the air could hold at the specific temperature and pressure. While the human body has sensors within the skin that are fairly efficient at feeling heat and cold, relative humidity (RH) is detected indirectly. Sweating is an effective heat loss mechanism that relies on evaporation from the skin. However at high RH, the air has close to the maximum water vapor that it can hold, so evaporation, and therefore heat loss, is decreased.
Most sun charts plot azimuth versus altitude throughout the days of the winter solstice and summer solstice, as well as a number of intervening days. Since the apparent movement of the Sun as viewed from Earth is nearly symmetrical about the solstice, plotting dates for one half of the year gives a good approximation for the rest of the year. Thus, to simplify the diagram, some sun charts show days for different months as the same, e.g. March 21 equals September 21. The accompanying sun chart for Berlin accounts for deviations in symmetry between the two halves of the year through the use of the analemma, represented by each figure eight on the chart.
A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting greenhouse or livestock building environmental problems and in determining solutions. Understanding psychrometric charts helps visualization of environmental control concepts such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation.
A psychrometric chart contains a lot of information packed into an odd-shaped graph. If we dissect the components piece by piece, the usefulness of the chart will be clearer. Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio (moisture content) scale on the vertical axis, and an upper curved boundary which represents saturated air or 100 percent moisture holding capacity.
An understanding of the shape and use of the psychrometric chart will help in diagnosing air temperature and humidity problems. Note that cooler air (located along lower, left region of chart) will not hold as much moisture as warm air. A rule of thumb, inside typical greenhouses or animal buildings during winter conditions, is that a 10oF rise in air temperature can decrease relative humidity 20 percent.