Heating, Cooling and HVAC

A good ventilation system is designed to keep the temperature and humidity at comfortable levels, to create enough air movement of acceptable velocity and direction to produce a feeling of freshness and comfort, and to maintain the purity of the air reducing the concentrations of dust, bacteria, odours and carbon dioxide to desirable levels for health and hygiene. The main factors then to be considered for satisfactory comfort conditions are:

• Temperature
• Humidity
• Air Movement
• Purity

The first three factors are to large extent interdependent, as at normal room temperature say 20ºC, an increase in humidity increases one’s sense of warmth, whereas at an outside winter temperature of say 2ºC a similar increase in humidity makes one feel cooler, and an increase in air movement about the body in either case would give one a sense of a reduced air temperature.

If the temperature drops below 18ºC or if it is draughty, people do not feel comfortable at home, office workers are not fully productive, and the frequency of influenza increases significantly. Likewise, if it is stuffy or smoky, or if the temperature and humidity are high, we feel sluggish. In fact, to function properly we require an environment whose physical, chemical and biological properties are narrowly defined. Since nature does not provide us with such a climate, we must create an artificial indoor habitat which better fits our needs.

The indoor environment is determined by the outdoor climate, building design, building management, and the actions of a building’s occupants.

A sedentary or slowly walking person has a metabolic efficiency of about 5%. A trained weight lifter can deliver about 20%. Thus, the metabolic efficiency of a human being in converting chemical energy into mechanical energy is about the same as that of a car engine. Both can only function if a steady negative temperature gradient is maintained toward their environment so that waste energy can be discarded. The human energy balance has been thoroughly studied during the last 200 years and it is now known that a sedentary adult dissipates heat at a fairly constant rate of about 50-80W. This corresponds to the heat emitted by a normal incandescent light bulb.

Clothing

The heat transfer is strongly influenced by the insulation value of clothing. A nude person with a fully exposed skin surface is most comfortable at an air temperature of 31ºC. Heavily clothed people prefer an air temperature between 16 and 24ºC.

Medical doctors have recognised for several hundred years that vanity can make people insensitive to their thermal needs. Thus, most business people invariably wear a three-piece suit, closed collar and tie. Moreover, the suit is often made of synthetic fibre cloth, which is impermeable to moisture but an excellent conductor of heat. Thus, the wearer freezes in winter in the same clothing that acts as a sweat suit in summer. Ideally, fabrics should provide for moisture transmission, moisture absorption and heat insulation. It is little short of a wonder that workers in an office can peacefully coexist at any one chosen temperature. Natural vegetable fibres, such as cotton or linen are ideal for hot climate clothing. Man-made fibres worn close to the skin are uncomfortable in these conditions.

Heat Dissipation

If a person performs physical work, the body produces waste heat in an amount proportionate to the physical load.

If the body temperature of a working person is to remain the same as that of a sedentary person, heat transfer to the environment must be increased. This may be achieved by regulating sweating, or by modifying the properties of the environment. Sweating is highly efficient because the heat of vapourisation of water is 540 cal/g.

Thermal Comfort

As mentioned above, conditions are very rarely ideal for heat transfer from the body, and the perception of comfort changes constantly as a person’s needs change. Obviously, a hard-working athlete’s perception of the comfort of a heated arena will differ from that of his sedentary audience. Hot dry air is comfortable because it enhances evaporation of perspiration; hot moist air is uncomfortable because the moisture gradient is insufficient for the air to absorb perspiration as it forms and accumulates on the skin. Air movement can offset the unpleasant effect of partly saturated moist air because of the increased volume of air that touches the skin.

The human body is capable of surviving very large air temperature gradients for considerable time. Some saunas are kept at 90°C and the earth provides temperature extremes reaching from -62°C in the Yukon to 56°C in Death Valley, California.

In contrast, the human body is extremely sensitive toward internal heat accumulation and therefore requires continuous heat dissipation.

The human body copes with this constant challenge by a variety of straight-forward and instinctive actions and reactions. These involve a large number of complex physical, biological and chemical mechanisms that defy simple scientific characterisation. In fact, these interactions are so complex that apparently simple daily observations - the sensation of sudden cold that is experienced on stepping out of a cold shower into warm air, for example - provoke lengthy (and inconclusive) discussions among experts.

Typical time budgets for family members disclose that on the average, everybody spends at least 20 or more hours each day indoors. Add to this travelling time (train, bus, car) and it may be that many people spend less than 30 minutes per 24 hours outside. The main exceptions being people who play (and watch) outdoor sports and dog-walkers. It is incongruous that smokers may now have more fresh air, as many buildings ban smoking indoors.

The overwhelming majority of the current housing stock was built before 1970 when energy was freely available. At that time, it made sense to save on insulation and capital investment because heating and cooling buildings cost relatively little.

During the last 10 years the cost of energy has increased by almost a factor of 10. The UK Government, private industry and the commercial sector have all developed plans for better designed buildings and for retrofitting the current building stock with better insulation and other energy conservation devices.

For example, it is easier to reduce heating costs by cutting the influx of fresh air than by installing insulation. As a result, many people currently live and work in tightly sealed structures in which 100% of the air is recirculated. This has led to complains of eye irritation, headaches, dizziness, fever, nausea, sleepiness and poor concentration from every segment of the population and every type of building, as well as from the passengers and crew aboard aeroplanes because airlines can achieve a 1% fuel savings by reducing ventilation.

The Temperature of an occupied space is increased as heat is passed to it from:

• The structure, i.e., walls, windows and roof, which are heated by the sun or warmer outside air
• Electrical or other heat-producing equipment
• The body heat given off by the occupants

For bodily comfort the heat produced by the body, which may normally vary between 120 and 440 watts depending on the amount of activity, must be dissipated to preserve the inner body temperature at about 36.9°C, and this is done naturally by heat loss from the skin. If the air surrounding the body is cool, heat loss is rapid, but if very warm air surround the body it may gain heat from the air, and this additional heat must be dispersed as well as that generated by the body. The processes used by the body for regulating its temperature are:

• Radiation
• Convection
• Evaporation

Source:Vent Axia Ventilation Hand Book