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Technical information Air-Conditioning Basic Ventilation Requirements

The Main Ventilation Requirements

As noted earlier, the air-gas mixture. As a rule, the air contains approximately 21% oxygen. Human systems that requires the specific content of oxygen contained in the air:
  • To sustain life.
  • To be comfortable.
If the number is tightly, any person in this room they consume oxygen. Amounts of carbon dioxide, water vapour and various impurities will also increase. This may cause drowsiness or even death. Human living space, must be the air with good oxygen content. This air should be kept at an acceptable temperature. It is very important that fresh air to be taken to provide oxygen.

In the past, this fresh air entered into space by an infiltration (leakage). Infiltration, as a rule, occurs through the door and window openings and cracks in the structure. However, the modern building is the reduction of air leakage. Air conditioning apparatus, must provide fresh air. Modern devices have controlled fresh air.

This fresh air conditioned and mixed with recirculated air before it reaches the room.

Some air-conditioned air to leave the house through the doors, Windows and other construction joints. Some of them also goes through the same hole (exfiltration). Any exhaust fan removes conditioned air. It is best to bring in replacement of fresh air through an air system. When this is done: air can be cleaned and cooled or heated; positive pressure can be maintained in the building to help keep the air dirt, dust and pollen (negative pressure, lowers the efficiency of the exhaust fans and fuel, furnace); and a certain amount of air (makeup), attracted for medical purposes (oxygen content).

Some construction zones must be slightly less positive pressure than the rest of the building. Below the positive pressure (from 10% to 15% less than the remainder of the building), reduces the spread of odors. Such areas may include kitchen, bathrooms, and where certain industrial operations produce pairs.

For the calculation of fresh air requirements and air changes per hour, the following factors: number of passengers, use of space, dry bulb temperature, relative humidity, quantity of fresh air is admitted by seepage and effectiveness of the device. One basic rule when the system is in cooling mode, is at least 15 cfm air per person. This will ensure a sufficient amount of oxygen and remove carbon dioxide. Six people, occupying 10 000 ft3 space is 90 fresh air cfm (6 x 15 cfm = 90 cfm). It takes 10,000/90 = 111 minutes (1.85 hours)to completely replace the air into the space. It's pretty slow.

Remember, however, that the purpose is not to replace the air quickly. It is very expensive to use heat as fast as the need to replace the air immediately. Air can be processed either positive or negative pressure in the building. (Positive pressure above atmospheric pressure. Negative pressure below atmospheric pressure.) Positive pressure will exclude the penetration of air from the outside or from other places. This is done using special air intakes for blowers. Positive pressure assures that all air entering the building can be filtered. It is purified to the achievement of the occupied space. Negative pressure increases the penetration of the Windows and doors. The air cannot be messy.

Houses, use fuel-burning furnaces are in need of air for combustion. Combustion air, going out through the chimney, can create a slightly negative pressure inside the house. The amount of impurities in the air may be large enough to require air cleaning. (Smell, smoke, and bacteria can be a part of such impurities.) The tool can be either ventilation, using the fresh air, or improvement of air cleaning. Ventilation is usually based on the air changes per hour for the conditioned space. 1,000 ft3 space, for example, three changes per hour will mean 3000 ft3/h or 50 cfm. Three changes every hour minimum for the heating season. As high as 12 changes per hour (in this case-200 ft. / min) are recommended for cooling in a public gathering. Fig. 23-6 shows a typical change of air for heating and cooling seasons...

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