Determination of air exchange
When mixed ventilation air in the room is usually dictated by local suction performance. Total extract, completed by local, usually no more than two-three times the exchange (according to the rules - at least one exchange). In exceptional cases - when a very large cubic space and unproductive local exhaust - exhaust general exchange prevails over local. Such ventilation is approaching a purely general exchange.
If together with gases, vapors and aerosols can be significant heat, sometimes the volume of air, the need for heat removal, greater than necessary to deal with these hazards. Then air is dictated by the heat, and when mixed general exchange ventilation hood can exceed the local. Usually, when gases, vapors and aerosols, the bulk of the air falls na local pumps.
The amount of air L, which must be removed from the shelters of various types, as well as umbrellas and screens, is determined by a formula
L = 3 600A; [m3 / h], (27)
where F-rated area of open holes, cracks and leaks in the shelter m2 \ v - minimum speed of air in the open doorway in m / sec.
For umbrellas and curtains estimated area F is the area of the receiving section, most often horizontal. To shelter-type enclosures, cabinets, etc., area F is the sum of squares of all the open holes, cracks and gaps. In general, the main distinction in the shelter, or a worker, an aperture, the auxiliary openings (inspection, sampling), the gaps and, finally, the occasional leaks, depending on the implementation of constructive and shelter material.
Under the minimum design speed and the lowest mean for the shelter value of conditional mean velocity in the openings of the shelter or an umbrella. Velocity v is some average value that is different from the average rate of, say, in air ducts, as in any shelter can not create patterns of distribution of velocities over the cross-sectional area of the same opening, not to mention several different openings. The velocity of that was found by experience, build on existing in reality, uneven distribution of velocities in the doorway or in the receiving section of an umbrella. If the definition of the area F - question the thoroughness and clear picture of the cover design, the correct determination of the velocity v is much more complicated.
With this design the shelter and the specific value of F magnitude speed determines the amount of air that must be removed from the shelter or an umbrella, to ensure that no harm knockout.
Air flow through the open doorway shelter - a process transient. Speed at certain points in the cross section vary over time and depend mainly on which takes place in the shelter of the process (movement of the arms, machine work) and from the movement of air outside the shelter (presentation of the material, human motion).