Magazine article Occupational Hazards

Confined Space Ventilation

Magazine article Occupational Hazards

Confined Space Ventilation

Article excerpt

What's the best way to ventilate a confined space? Should you blow air in or exhaust it out? How many air changes are sufficient? How do you arrange ventilation equipment for maximum efficiency? Unfortunately, there are no simple answers to these and other questions related to confined space ventilation. Furthermore, since each space is unique, there is no single set of rules that applies to every situation.

As a result, identical ventilation equipment may be arranged one way when ventilating a street manhole, and a different way when ventilating a chemical process vessel. Some spaces may be ventilated with a single blower. Others will require multiple blowers arranged so that some force air into the space, while others exhaust it out.

Recognizing that there is virtually an infinite variety of spaces and correspondingly infinite ways to ventilate them, this article will not focus on specific ventilation techniques. Instead, it will explain some basic ventilation principles and concepts that may be applied to all spaces. These concepts and principles, when married with experience and sound professional judgment, will determine the best way to ventilate a specific space.

Purging vs Ventilation

Although the terms purging and ventilating are often used interchangeably, they are not the same.

Purging is the process by which a space is initially cleared of contaminants by displacing the hazardous atmosphere with air, steam or an inert gas.

Ventilation is the process of continuously moving fresh, uncontaminated air through a space. Continuous air movement accomplishes three things. First, it dilutes and displaces any air contaminants that may be present in the space. Second, it assures that an adequate supply of oxygen is maintained during the entry. Third, it exhausts contaminants formed by processes such as welding, oxy-fuel gas cutting or abrasive blasting.

General Ventilation and Local Exhaust

General ventilation is the process of introducing clean outside air into a space in such a way that it mixes with the inside atmosphere to dilute contaminants and restore oxygen. It is useful for providing comfort-cooling and for removing unpleasant odors; however, it is ineffective for controlling highly toxic contaminants, welding fumes and heavy dusts generated by grinding, chipping and abrasive blasting.

Local exhaust provides a positive means of removing contaminants at their source. Local exhaust systems are well suited for removing contaminants generated by point sources like arc welding and oxy-fuel gas cutting. They can also be used to remove vapors formed by the local application of solvents such as those encountered during dye penetrant testing or touch-up painting.

Ventilation Equipment

Commercially available ventilation systems may be generally divided into three categories: self-contained ventilators, motor-driven blowers, and units driven by compressed air.

Self-contained ventilators use either a small LP-gas or gasoline engine to directly drive the fan blades. These units are highly portable and are available in a number of sizes. Typical air delivery rates range between about 600 to 4,000 cubic feet per minute (CFM). Some units may also be fitted with an electric generator that can be used to operate auxiliary equipment such as lights and power tools.

Motor-driven blowers depend on an external source of electrical power provided either by fixed commercial wiring or by a portable generator. Smaller units which are as portable as self-contained ventilators can deliver about 1,500 to 3,000 CFM. Larger units, which because of their weight and size cannot be moved as easily, can deliver more than 20,000 CFM. As an extra option, some manufacturers will install explosion-proof motors to allow use in "classified" locations.

Compressed air driven units may be subdivided into three types: air eductors, turbines and reactor fans. …

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