Air and human comfort
Space explorations to the moon, other planets, and beyond have made people appreciate more than ever what a favoured planet the earth is. No other planet or satellite in our solar system possesses the unique mixture of gases that we call the earth’s atmosphere.
The ocean of air at the bottom of which we live and breathe and work is, in reality, a very thin envelope surrounding the earth, held in place close to the earth’s surface by the force of gravity, in other words, by its own weight. The total ‘weight of air pressing down on the earth’s surface per unit surface area is called atmospheric Pressure. The normal (sea-level) value of atmospheric pressure is
1 atm = 14.7 lb/in2
= 29.92 in. Hg
= 760 mmHg
= 101.3 kilopascals (kPa)
= 407 in. water
At an altitude above sea level of about 2 miles, many people begin to find the air a bit too “thin” for comfortable breathing. A comparison of this distance with the diameter of the earth (3960 mi), makes one realize how very shallow our ocean of air is.
Pure air is by no means an inexhaustible resource, as the residents of urban areas now fully realize. Industrial pollution and contaminants from automobile exhausts literally poison the air in scores of big cities all over the world. Besides air pollution, a number of other factors and conditions determine whether or not air is conducive to health and comfort. Three of the most important of these are air temperature, relative humidity, and air motion.
Air Temperature and Human Health
Extremes of either hot or cold weather are not only uncomfortable but can be unhealthy. In extremely cold weather, the heart must work harder to circulate the larger flow of blood required to maintain body heat. Cold air chills the respiratory tract and irritates the mucous membranes of the sinuses and bronchial tubes. The real cause of the common cold is not yet known, but prolonged exposure to extremely cold air frequently is a factor in the onset of colds and in inducing secondary infections such as tonsillitis, influenza, bronchitis, and pneumonia.
In hot weather, being comfortable involves dissipating excess heat to prevent the body temperature from rising. This is accomplished by a diversion of blood from internal organs to the body surface so that heat losses by conduction, radiation, and evaporation can occur. This net loss of blood from the vital organs may produce conditions of illness such as indigestion, headache, dizziness, and a general feeling of debility. Proper nutrition during hot weather is difficult to attain because of loss of appetite in general and a predilection for cold drinks and foods lacking in protein content. If the surrounding air temperature is so hot (and humid) that heat losses from the body cannot equal the body’s heat production, then body temperature rises above 37°C (98.6°F)and “fever” sets in. This indicates that the temperature control mechanism of the body can no longer cope with the situation and the danger of heat prostration is present.
The human body, through its processes of metabolism, produces a great deal of heat. The amount of heat produced depends, of course, on the degree of activity, but even moderate activity produces as much body heat as would be given off by a 200w electric lamp. The heat produced is dissipated by an increased blood circulation through the arteries and capillaries of the body surface. This puts a greater than normal load on the heart, and for this reason prolonged hot weather is dangerous for heart patients.
Profuse perspiration, though essential to body cooling, carries away certain salts and minerals which the body needs. These are not ordinarily replaced as fast as they are dissipated, and the net result of this depletion is muscular fatigue and sometimes cramps. The loss of certain mineral salts, notably salts of potassium, may upset the balance of electrolytes in human cells and interfere with proper muscle action, including the heartbeat. As a further complicating factor, hot, humid weather interferes with normal sleep habits, with the result that body organs and tissues do not have a chance to repair themselves before the beginning of another exhausting day.
Body-Temperature Regulation
The average body temperature, taken under the tongue, is 98.6°F (37°C). This is not to say that all persons exhibit this body temperature or that any individual maintains exactly this temperature day and night, summer and winter. For people in good health, however, 37°C (98.6°F) is the average body temperature, and it will not vary more than about half a degree regardless of the surrounding air temperature as long as the body’s temperature control mechanism is functioning properly. The human body is continually gaining (and producing) heat and losing heat to its surroundings to maintain temperature equilibrium. The balance between heat gains and heat losses is maintained in the following manner.
Processes involved in body heat equilibrium. Body heat gains (internal and external) must equal body heat losses if body temperature is to remain constant.
Body heat gains come from two sources:
- Heat produced within the body itself as a result of metabolic processes
- Heat gained by the body from external sources, by radiation from the sun or other hot objects or surfaces, and by conduction and convection from the surrounding ambient air.
Heat is lost from the body by:
Conduction: When the body surface is at a higher temperature than the surrounding air, heat is conducted from the skin to the air. Clothing slows down the rate of conduction, and the nature of the clothing (i.e., wool or cotton, padded or thin) influences the rate of conduction loss.
Convection: Heat conducted from the body to the layer of air immediately adjacent to the skin must be removed or no further conduction will take place. The warm air next to the skin, being less dense, tends to rise, and convection currents are thus set up. If air is circulating past the body, heat transfer by convection is speed up, thus allowing the rate of conduction to increase also. This explains why we feel cooler in a breeze or sitting by a fan, even though there has been no actual drop in temperature. Proper air motion is an important factor in human comfort. Neither “dead air” nor a strong draft is considered comfortable.
Convection heat loss also occurs as a result of respiration.
Radiation: Any object at a higher temperature than its surroundings radiates heat to objects at a lower temperature. Thus, the human body will radiate heat to walls, ceilings, floors, windows, and to the out-of-doors if these surfaces are at a lower temperature than the body surface. Conversely, the body gains heat by radiation from the sun or from any surface warmer than the skin surface. Body skin temperature ranges between 30 and 34°C (86 and 94°F) with an average of about 32°C (90°F) for a healthy person engaged in light activity.
Evaporation: Body fluids find their way to the surface through the sweat glands. Actually the skin is always slightly moist, but as air tempera~ tune rises, the body’s temperature-control mechanism stimulates the sweat glands to greater activity and the rate of perspiration is increased. Evaporation of perspiration from the skin represents heat loss and assists in cooling the body. In fact, if the surrounding air and all nearby objects are at a temperature greater than that of the skin, heat loss by conduction and radiation does not occur, and the only methods by which the body can maintain its temperature equilibrium are to lose heat by the evaporation of perspiration or through respiration. On extremely hot days the body will gain heat by both conduction and radiation, and these gains as well as body metabolic heat must be lost through evaporation from the skin.
Evaporation of perspiration is speedy if the relative humidity of the air is low, and consequently the body can maintain a satisfactory heat loss under very high dry-bulb temperature conditions 43°C (110°F) and more if the relative humidity remains below about 20%. On the other hand, a much lower dry-bulb temperature say 32°C (say 90°F) is uncomfortable if the relative humidity is over 85%.
Conditioned air allows the body’s temperature control to function normally without strain on the vital organs. Under controlled conditions of temperature and humidity the body easily conserves the necessary heat in winter and dissipates excess heat in the summer to maintain temperature equilibrium.
Ventilation Requirements
Not only does air affect people, but people affect the surrounding air. The human body gives off perspiration and dissipates heat, as we have noted. Table 6-l gives data on the amounts of sensible and latent heat given off by people at rest under a variety of ambient air conditions.
It will be noted that as the dry-bulb temperature of the ambient air rises, the body sensible-heat emission decreases while latent-heat emission increases.
When the ambient-air temperature is higher than body temperature, the entire body heat loss is latent heat.
Carbon dioxide is exhaled as a product of the respiratory process, and oxygen in the air is used up. Outdoors the effect of contamination by people is not often noticed, but in buildings where large numbers of people congregate, contaminants and the heat produced must be continuously removed or an unhealthy air condition will result. Smoking and cooking in confined spaces add to the problem, of course, and these factors must be taken into account in determining the amount of fresh air to be supplied in order to maintain a comfortable and pleasant air condition.
The process of supplying fresh air to buildings in the proper amount to offset the heat and contaminants produced by people is known as ventilation. In addition to bringing in uncontaminated air and carrying away contaminants and body heat, ventilation air (that is air from outdoor) feel, in contrast to the “stale or “dead” feel of air in crowded rooms. The “freshness” of air seems to be related to the ion content at the time. Molecules in air which has been tossed by winds and breezes scrub against one another with the result that electrons are scrubbed off some atoms and collect in excess on others. Positively and negatively charged atoms, called ions, are thus created. and the sense of freshness in air seems to depend on inn content. Outdoor air usually has the ion content associated with freshness, while indoor air in crowded spaces does not. Merely circulating the indoor air with fans or blowers does not bring the ion content up to satisfactory levels. Even though other factors (temperature, contaminants, humidity) may not require it, the need for fresh air indicates that proper ventilation, with some outdoor air, should always be provided.
In many countries the amount of ventilation air is stipulated by law such as in Australia; the Building Code of Australia.
The Concept of Effective Temperature
People react in widely differing ways to atmospheric conditions around them. A comfortable condition for one person may be judged as “chilly ” by another or as too warm by a third individual. Some people like the moist climate of the sea coast, others prefer the dryness of a desert or mountain climate.
Air that is dry in summer helps the body dissipate heat by evaporating perspiration rapidly, whereas dry air in the winter produces discomfort by lowering the skin temperature to uncomfortable levels even though the surrounding dry bulb temperature is relatively high. Dry bulb temperature is not a reliable indication of how warm or cold a room will feel to human beings. Both relative humidity and air movement are factors which must be considered.
The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has conducted research over many years, involving hundreds of persons in different localities, in attempts to relate the factors of temperature, humidity, and air movement to human comfort. From the results of these tests a concept of effective temperature (ET) ad operative temperature (OT) has been developed. Effective temperature is a measure of comfort which involves the combined effects of dry bulb temperature, relative humidity and air movement as judged by the subjects in the research studies. The operative temperature take into consideration of the surrounding radiant heat effect. It is emphasized that these are subjective judgments statements by people as to how they personally feel under different conditions of air and radiant temperatures, humidity, and air movement.
It was found that for a given air velocity, there were a number of different combinations of dry bulb temperature and relative humidity which would give the same feeling of comfort to a high percentage of the subjects. All these combinations were said to have the same effective temperature. Charts were then constructed by drawing lines through the points at which the majority of people equally clothed and equally active reported the same feeling of comfort. These lines are called effective temperature lines.
The subjects tested varied their responses some what from summer to winter, indicating that although a given air condition is comfortable at one season of the year, it may not be so rated a few months later. Air conditioning systems designed to produce exactly the same temperature, humidity, and air motion the year round may therefore not produce optimum comfort the year round. For example, 98% of all subjects tested agreed that a summer condition of 24°Cdb (76°F) and 50 percent RH was ideal. In winter, however, with the relative humidity maintained as before at 50 percent, and the same air velocity, the dry bulb temperature had to be reduced to about 22°C (72°F) before 97 percent of the subjects agreed that the condition was one of optimum comfort
Human Comfort
It is important to note that human comfort is based on these factors:
- air temperature
- clothed normally for indoor living
- engaged in no more than light activity-reading, office work, occasional walking about the room
- air velocity of 0.08 to 0.125m/s (15 to 25 fpm)
- radiation effects
Factors of Compromise in Design
Engineering has been defined as the application of science and the arts to the needs and desires of people. Such a definition implies that engineering must be a very practical thing, that scientific findings will be made to dovetail with human needs and wishes, and that, on occasion, rigorously engineered design can be compromised to suit a special situation dictated by the exigencies of the moment. Frequently, after determining what is desirable, the engineer next has to determine what is possible and then by compromise bring desirability and possibility as close together as possible.
So it is in designing, selling, and installing comfort air conditioning systems. Consideration for human comfort, when properly applied, indicate the design which will produce optimum comfort. But many prospective purchasers of summer cooling want some relief from heat and humidity but are not financially able (or willing) to invest the amount required for a system which will produce optimum comfort conditions. Witness, in this connection, the tremendous mass market air conditioners which has developed in the past 20 years. From the dollar viewpoint, the application engineer must sometime depart somewhat from the optimum in designing a summer cooling system. It is worth noting, however, that although many people will compromise on less than optimum summer cooling, nobody wants less than optimum winter heating.
It is the professional obligation of the engineer to explain the exact nature of the compromise to the prospective customer, so that he or she understands that in purchasing an air conditioning system, just as in purchasing a motor car, the performance is closely related to the price.
In addition to economic factors, certain other considerations may indicate a departure from optimums. Duration of occupancy is one of these. When occupancy is to be of short duration (for example, banks, department stores, supermarkets, drugstores) the comfort of the entering patron may be a controlling factor in setting the design rather than the comfort of the employees. When a person enters a very cool space after long exposure to high outside temperatures, the first sensation is one of pronounced chill. Perspiration still flows to the skin and evaporates, since sweat glands do not close immediately. The evaporation adds further to the sensation of coldness, and actual shivers may run up and down the spine. Such sudden temperature changes may bring on summer colds in some people, and in any event the body temperature is temporarily disturbed.
For spaces in which short occupancy is the controlling factor, comfort-zone optimums should be modified somewhat. Inside temperature should not be much more than 15°C (20°F) below outside temperature for short-occupancy spaces, no matter how hot it is outside. Relative humidity may vary between 40 and 65 percent, but for the benefit of employees in the space, the lower figure would be better.
Air motion is another factor which enters into the practicalities of design. The effect of air motion on comfort also critical. Note that, as air velocity rises, the effect as judged by people is the same as if the dry bulb temperature had been reduced. At 26°C (78.8°F) and 50 percent RH, for example, an air velocity of 0.1m/s (20 fpm) gives an effective temperature of 23.5°C (74.3°F), whereas at 1m/s (200 fpm) the effective temperature is 22°C (71.6°F).
Regardless of this reduction in effective temperature, air velocities much in excess of 0.5m/s (100 fpm) must be avoided. Refrigerated air from a register is quite cold, usually about 14°C (58°F), and air at this temperature creates a feeling of discomfort if it blows directly on a person’s skin. In the case of evaporative cooling, with an air-off the register temperature of 19 to 24°C (66 to 75°F) air velocities may be much higher without causing a feeling of chill.
Most people are far more sensitive to drafts of warm air than to those of cold air. Warm air blowing from registers may be at 43°C (110°F) or more and a draft of air this hot is very unpleasant. For winter air conditioning, air motion should not exceed 0.2m/s (40 fpm) across the skin, or discomfort will result.
Air velocities in ducts and out of registers and grilles may be high, but when entrained into the room air circulation, it should be planned so that the velocities above recommended are not exceeded below the 1.8m (6ft) level.
Humidity is a third factor of design which can be compromised to some extent without serious consequences. A relative humidity of 50 percent is considered ideal, but this ideal is sometimes very difficult to attain for both winter and summer systems. Since the majority of people are reasonably comfortable within the relative humidity range from 30 to 70 percent, most comfort systems are designed to operate within these limits. Persons exhibiting more than normal sensitivity to either dry or moist air would, of course, constitute a special problem. For summer cooling, experience indicates that it is best to stay in the range below 50 percent RH if possible, for winter heating, 20 to 40 percent is satisfactory. However, it costs money to dehumidify in summer and to humidify in winter, and cost factors may again necessitate a departure from comfort optimums. In actual practice, and where health or allergic conditions are not controlling factors, relative humidities may be allowed to go as high as 65 percent for summer cooling and as low as 20 percent for winter heating without actual discomfort, providing the associated dry-bulb temperatures bring the condition to or within the seasonal comfort zone.
Allergic reactions to contaminants in the air afflict many people, some to the point of serious illness. Pollens, dust, molds, spores, and other small particles contain allergens which may induce serious respiratory difficulties, including sinus upsets and asthma attacks. All well designed air conditioning systems, will include filtration for both outside air and return air. In actuality, all contaminants are not removed, but up to 95 percent of pollens and other troublesome particles can be removed if the filtration system is serviced properly and if it features electrostatic precipitation or far UV irradiation.
In general noxious gases are removed unless they are soluble in the condensed moisture on the cooling coil (summer cycle), or unless chemically active materials (for example, activated carbon are used with the filter system.
Comfort Versus Energy Conservation
Our energy security and challenges threaten to become more severe with each passing year. The cost of fuels and electricity, along with the possibility of not having enough energy at any cost, is a real problem for the air conditioning industry. In the early 2000s the typical family’s heating bill amounted to less than one-fourth of the family food bill. As of 2010, especially in the colder regions of the country, some families are spending as much for fuel as they do for food. Electric energy, too, has skyrocketed in cost during the 10 years.
One of the most effective ways of reducing overall energy consumption for both winter and summer air conditioning is to minimise heat transmission into and out of the conditioned spaces. Another is to make sure that all heating and cooling equipment is operating efficiently, at or near design levels. Many controlled studies have shown, for example, that more than half of the heat produced in the typical home is wasted-either lost up the flue or lost as a result of heat transmission through walls, doors, and windows.
It is safe to say that older homes in Australia are well insulated, one has to inspect only a few home to realise the deplorable state of maintenance and efficiency that characterises it. The same comments can be made about most summer cooling systems. Residential units are only a part of the problem. Factories, office buildings, commercial structures, schools, and institutions also have to be heated and cooled. We could conserved energy by meeting more rigid insulation standard to older homes as well.
Another approach to energy conservation in air conditioning involves a gradual change in our ideas of what constitutes “comfort. We have become accustomed to central heating and winter indoor temperatures in the range 22 – 25°C (72-77°F). Most people do not like to wear sweaters or any kind of ‘warm’ clothing while indoor in winter. These habits will probably have to change.
Redesigning control systems so that only selected spaces will be kept comfortably warm may be necessary, and wearing warmer clothes indoors will be a noticeable trend.
Careful attention to relative humidity control (at, say, 40%) could allow a 19°Cdb (66°F) temperature to approach the winter comfort zone for about 50 percent of normally clothed persons. It is again emphasised that the increased comfort resulting from a higher humidity does not come free; energy (fuel) must be used to evaporate water into the air stream.
For summer cooling too, conservation of energy must receive top priority. Air conditioning engineers have long objected to the ridiculously cold temperatures that many building managers maintain during the summer cooling season. Many banks, restaurants, supermarkets, theaters and public buildings are so cold that people walking in from warm outside air are “chilled to the bone.” The persons in control of these “refrigerated air” systems would seem to define air conditioning as a means “to keep my building colder in the summer than it is in the winter!”
Most of the benefits of summer (comfort) air conditioning can be attained with dry-bulb temperatures in the conditioned spaces no colder than 26°C (78°F) if the relative humidity is kept at or below 50 percent.
Re-adjustment to these energy dictated realities will have to be made quite rapidly. Most people’s concept of “comfort” may have to be rather drastically redefined. And renewed attention will have to be given to the efficient operation of equipment and to insulation, weather stripping, and caulking to minimize transmission gains and losses.