Heating, Cooling and HVAC

Air is a gas mixture composed mainly of oxygen and nitrogen with small percentages of carbon dioxide and water vapour (moisture) in varying amounts. Dust particles and bacteria area also present, and may have to be dealt with at some point in the ventilation system, depending on the nature of the job, as also would odours given off by people, animals or vegetable matter. The air in an occupied space will gradually become less pure due to the bodily functions of the occupants and the sort of work they are doing.

Carbon dioxide and water vapour from the lungs, organic impurities from the body, smoking, fumes, gases and dust from industrial processes all tend to increase the temperature, humidity, dust and odours and to reduce the percentage of oxygen in the air to make it less comfortable to live with. Volatile organic compounds (VOC’s) are given off from materials used in building construction and furnishings. Formaldehyde from carpets, upholstery and certain wall cavity insulation. Solvents from paints can include Toluene and Xylene, both considered carcinogenic. In the interests of health, hygiene and efficiency, some dilution of the impurities, and removal of undesirable heat and moisture must be carried out by ventilation.

Composition of Ambient Air at Sea Level and 70°F/21°C

Constituents……………………. Volume

N₂………………………………… 78.084 -0.004

O₂ ………………………………..20.946- 0.002

CO_{2……………………………………….}0.033 - 0.001

A…………………………………0.934 - 0.001

H₂O……………………………………0 - 2.2

For most humans the daily oxygen demand is about 300-800 litres.

This corresponds to about 0.750 - 1.5kg of oxygen, or 1500 - 4000 litres of air.

Not all oxygen is extracted from the air inhaled with each breath. Only about 4% is absorbed; exhaled air contains 16% residual oxygen. This surplus is necessary to maintain the oxygen gradient to which the human lung has adapted. Human beings are capable of living in air containing 12-60% oxygen. An atmosphere that is more than 60% oxygen is toxic; air with less than 12% oxygen will not maintain human life.

In the lung oxygen is exchanged against carbon dioxide (CO₂). If inhaled air already contains CO₂ the breathing ratio has to be increased to purge CO₂ at the rate at which it is produced. A safe level fir indoor living is considered to be 0.1%. Excessive carbon Dioxide triggers increased breathing and eventually narcosis. High CO₂ levels can accumulate in deep wells and in wine cellars because CO₂ is heavier than air.

Exhaled air is almost fully saturated with moisture at the body temperature of 37º regardless of the breathing rate. The water exchange between air and the human body differs from that of oxygen and CO₂ in two fundamental ways. First, water is not only exchanged in the lung. But is also released via the skin; second, the water release does not serve a chemical purpose, but supplements conductive, convective, and radiative heat transfer as necessary to ensure disposal of metabolic waste heat. If inhaled air is already moist, water transfer in the lung is simply reduced. For room air at 21ºC and 50% humidity, the water transfer amounts to about 0.4kg/day per person. In a room with 25% humidity, 0.7kg of water will be exhaled. This is only a fraction of the water that is released by perspiration through the skin. Typical vaporisation losses are 1.5 to 4.5kg/day.

This water adds to the moisture already present in air. If several people share a small room, the water flux constitutes a significant burden on the capacity of the indoor air. For example, three sedentary people watching TV in a living room will generate about 130g of water per hour. During an exciting show, however, the release can easily triple. Since a 5 x 5 x 2.5m living room at 20º and 50% humidity can absorb only 34g of water, air in tightly sealed room will reach 100% humidity in about 15 minutes and water will condense on the coolest surface, usually the floor, windows, or walls. In order to maintain a stable room humidity of, say, 70%, a ventilation rate of 40-85m³/h must be maintained. This corresponds to about one air change per hour.

Almost all water enters the body as liquid. Whether water is excreted as liquid or as vapour is determined by physical needs. As far as water flux through the skin and lung is concerned, the best comfort condition is achieved if the air humidity is about 40-60%, regardless of temperature. This level provides for continuous removal of metabolic products via respiration and perspiration.

Air also has Weight

1m³ of air weights approximately 1.2kg. Therefore, energy has to be used to move it through the atmosphere, to push it along ducting, to turn it around corners, and to squeeze it through grilles and filters. The energy required to do this work increases as the cube of the air velocity, whilst the resistance of the particular ventilation system increases as the square of the air velocity. For instance, if ducting in a system is reduced from 400 x 400mm to 300 x 300mm with the same volume of air. The resistance of the ducting to airflow would be increased more than 3 times. This subject will be expanded later when duct resistance is discussed.

From:The Vent Axia-Ventilation Hand Book