provide at least 4 cfm of fresh air per person to provide enough oxygen and to remove carbon dioxide. If six people occupy a 1,000-square- foot space with a 10-foot ceiling, there is 10,000 6, or 1, 667 cubic feet per hour for each person, or 1,667 60 = 27.7 cfm (.78m/min). This meets or exceeds ventilating code requirements.
One must remember that the air can be handled either to produce positive pressure (higher than atmospheric pressure) in a building or negative pressure (below atmospheric pressure). A positive pressure eliminates infiltration of air from the outside or from other spaces. Positive pressure is produced by using special air intakes to the blowers. A positive pressure assures that all air entering a building can be filtered and cleaned before reaching the occupied space. Hospitals use positive air pressure and require a 100- percent fresh air intake.
Negative pressure increases the infiltration at windows and doors. This air is untreated and may be dirty. If the amount of impurities in the inside air-such as odor, smoke, and bacteria - is great enough to require air cleaning, the remedy may be either more ventilation (using fresh air) or improved air cleaning.
Ventilation is usually based on air changes per hour for the conditioned space. If the space is 10,000 cubic feet, for example, three changes per hour would mean 3,000 cubic feet per hour or 50 cmf. Three changes every hour is the minimum for a residence during the heating season. As high as 12 changes an hour (in the above case, 200 cfm) are recommended for cooling.
It is a good practice to keep the air blowers running all the time to provide good ventilation to all parts of the building. Variable speed blowers are sometimes used. They provide more air movement when the heating or cooling systems are off.
An adequate air supply is the best way to control comfort. Body comfort is controlled by evaporation, convection, radiation, and respiration. You must, therefore, control the temperature of the walls, floors, or ceilings to make sure they are not too warm or too cold (radiation). You must also supply enough air to promote good respiration, evaporation, and convection. If the specified conditions are not known, it is best to design for 2 cubic feet per minute per square foot and/or 12 changes of air per hour. It is also very important to remember that people occupying a closed space give off considerable heat. A sleeping person gives off about 200 Btu/hr; a person doing heavy work gives off up to 2,400 Btu/hr.
Another way to determine ventilation requirements is to design for 4 cfm to 6 cfm of fresh air per person and for about 25 cfm to 40 cfm of recirculated air per person. This means the system should handle a total of 29 cfm to 46 cfm per person. (1 cfm = 0.0283 cu m/min.)
Natural ventilation, or gravity ventilation, uses the natural forces of wind, stack effect, and breathing of structures caused by the interior- exterior temperature difference to induce air circulation and removal. Generally, air enters through openings at or near the floor level in a building and escapes through openings high in the walls or ventilators on the roof.
Natural ventilation is used only where the necessary quantity of ventilation can be induced by natural forces. Applications that require a continuous supply of outdoor air for human comfort, or the safe use of space (or process), should not be designed for natural ventilation. In such cases, natural ventilation is not reliable because of wide variations in the natural forces, such as wind velocity and direction and the inside- outside temperature difference.Continue Reading