**2. Local ventilation**

Effective and properly designed ventilation offers a solution to the problem of protecting workers. Ventilation can also serve to control moisture, odor, and other undesirable environ-

The health hazard potential of an airborne substance is characterized by the threshold limit value (TLV). TLV refers to the airborne concentration of a substance. It represents the conditions that under which it is believed that nearly all workers may be exposed day after day

The ventilation systems used in industrial plants are of two kinds. The "supply" system is used to supply air to a work space. The "exhaust" system is used to remove the contaminants which are generated by an operation to maintain a healthful work environment. When a dilution ventilation system is used to control or isolate contaminants in a special area of the overall plant, this may be desirable. Often, this condition occurs simply because of installing local exhaust systems and not considering the corresponding replacement air

A well-designed supply system will consist of an air inlet section, heating and/or cooling equipment, filters, a fan, and register/grilles and ducts for distributing the air within the work space. Exhaust ventilation systems are classified into three groups: (1) the "local" exhaust system. The general exhaust system can be used for controlling heat and/or removing nontoxic contaminants generated in a space by flushing out a given space with large quantities of air and (2) the "general" exhaust system. The air may be tempered and recycled when used for heat control. And when used for controlling contaminant (the dilution system), enough outdoor air must be mixed with the contaminant, so that the average concentration is reduced to a safe level. Then, the contaminated air is typically discharged to the atmosphere. A supply system is usually used in conjunction with a general exhaust system to replace the air exhausted. Local exhaust ventilation systems operate on the principle of capturing toxic contaminants at or near its source. It is the preferred method of control because it is more effective and compared to high flow rate general or dilution exhaust requirements, the smaller exhaust flow rate results in lower heating or cooling load costs. Dilution ventilation systems are normally used for controlling toxic contaminants when local exhaust is impractical or is not economic, as the large quantities of tempered replacement air required to offset the exhausted air can lead to high operating costs. The present emphasis on industrial air pollution control highlights the need for efficient air cleaning devices on industrial ventilation systems, and the smaller flow rates of the local exhaust systems result in lower costs for air cleaning devices and better efficiency. Local exhaust systems consist of four basic elements: The hood(s), the duct system, the air cleaning device, and the fan. The purpose of the hood is collecting the contaminant generated in an air stream directed toward the hood. Then, a duct system must transport the contaminated air to the air cleaning device (collectors) or to the fan. In the air cleaner, before the contaminant is exhausted to environment, it is removed from the air stream. While producing the intended flow rate, the fan must overcome all the losses due to friction of hood entry, ducts, and fittings in the system. Most of the time, the duct on the fan outlet discharges the air to the atmosphere in such a way that it will not be re-entrained by the replacement

mental materials.

systems.

systems.

without adverse health effects.

146 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

Designing local exhaust systems aims to capture and remove process emissions prior to their escape into the workplace environment. The local exhaust hood is the point of entry into the exhaust system. Regardless of their physical configuration, it is defined herein to include all suction openings. The hood primarily is to create an air flow field which effectively captures the contaminant and transports it into the hood. In addition, local ventilation system contains four parts that its characteristics are calculated as following:
