Ventilation Devices

Few confined spaces have adequate natural or mechanically induced air movement, and in most spaces, it is necessary to remove harmful gases or vapors by ventilation with a blower, fan, and capturing hoods. The most common method of ventilation uses a large flexible hose attached at one end to a blower with the other end lowered into the space. The blower will push fresh air into the space to purge the area of hazardous substances. The blower allows the fresh air to enter the space at the lowest point possible. Because the atmosphere in a confined space can change quickly, ventilation should be continuous. The OSHA requires the establishment of numerous performance and design standards for ventilation systems. There are OSHA standards for specific substances, for operations, and for specific hoods. However, the recommended minimum design capture velocity could be from 500 to 1000 ft/min if contaminants are released with high velocity into turbulent air (Danielson 1973; McDermott 1976).

EXAMPLE 5.13

A cleaning bath for electronic parts emits 0.5 g/s of CFC‐12 into a small work room of dimensions 3 m × 2.45 m high. Calculate the concentration in the room under average and poor ventilation conditions if the air velocity in the room is 0.3 m/s and compare the results to the OSHA PEL for CFC‐12 = 4950 mg/m³.

SOLUTION

When the air speed is 0.3 m/s, the volume of air flowing through the room will be

and the concentration of CFC‐12 in the air will be

A comparison with the PEL of 4950 mg/m³ indicates that even under poor ventilation conditions, the OSHA PEL will not be exceeded and respiratory protection will not be needed to safeguard the health of a person working in the room.

EXAMPLE 5.14

Calculate the concentration of Freon in the cube, 1 m on a side, surrounding the top of the cleaning bath in Example 5.13 if the air exchange rate with the remainder of the room is 1 m³/s. Repeat the calculation for an air exchange rate of 0.5 m³/s.

SOLUTION

From Example 5.13, the rate of release of Freon from the cleaning unit is 0.5 g/s and the ventilation rate in the room is 2.21 m³/s. Thus, in the area closest to the cleaning bath, the concentration of Freon can be calculated as follows:

or

As would be expected, the local concentration of the chemical increases when the ventilation is less. Localized ventilation is an effective method of dispensing airborne chemicals and reducing exposures of workers to chemicals.