Ducts fume control

When primary fume capture is performed by the enclosure, furnace off-gas combustion efficiency is lower than experienced by furnace direct evacuation control. The off-gas, rich in carbon monoxide (CO), rises from furnace roof openings and partially burns and cools with enclosure air. Significant levels of CO have resulted in the enclosures and exhaust ducting from this type of combination. These levels are not explosive but present a potential hazard to personnel working in the enclosure or in downstream fume cleaning equipment. 

The purpose of the control plant is to maintain a working environment that is acceptable in terms of any statutory regulations and the custom and practice within an industry. The effectiveness of a control system is measured by the amount of dust or fumes it controls. Efficiency, on the other hand, is measured by the amount of power it takes to do the work. It is the job of the dust-control engineer to produce the most effective plant in the most efficient way, and the techniques of control will vary from one industry to another. All control plants will have either four or five elements, as shown in Figure 46.1, i.e. hoods, ducting, fan, collector and disposal. 

The laboratory shall be equipped with a fume hood. The fume hood should meet any specific safety requirements mandated by the nature of the research program. A discussion of hood design parameters will be found in a later section, but for high hazard use the interior of the hood and the exhaust duct should be chosen for maximum resistance to the reagents used the blower should either be explosion-proof or, as a minimum, have non-sparking fan blades the hood should be equipped with a velocity sensor and alarm should the face velocity fall below a safe limit the interior hghts should be explosion-proof, and all electrical outlets and controls should be external to the unit. It may be desirable to equip the unit with an internal automatic fire suppression system. 

Laboratory fume hoods and the associated exhaust ducts should be constructed of nonflammable materials. They should be equipped with either vertical or horizontal sashes that can be closed The glass within the sash should be either laminated safety glass that is at least 7/32 inch thick or other equally safe matraial that will not shatter if there is an explosion within the hood. The utility control valves, electrical receptacles, and other fixtures should he located outside the hood to minimize the need to reach within the hood proper. Other specifications regarding the construction materials, plumbing requirements, and interior design will vary, depending on the intended use of the hood. (See Chapter 6, sections 6.C.1.1 and 6.C.I.2.)... 

Although hoods are most commonly used to control concentrations of toxic vapors, they can also serve to dilute and exhaust flammable vapors. Although theoretically possible, it is extremely unlikely (even under most worst-case scenarios) that the concentration of flammable vapors will reach the lower explosive limit (LEL) in the exhaust duct. However, somewhere between the source and the exhaust outlet of the hood, the concentration will pass through the upper explosive limit (UEL) and the LEL before being fully diluted at the outlet. Both the hood designer and the user should recognize this hazard and eliminate possible sources of ignition within the hood and its ductwork if there is a potential for explosion. The use of duct sprinklers or other suppression methods in laboratory fume ductwork is not necessary, or desirable, in the vast majority of situations. 

During the preparation of medicines, steam, vapour, aerosols, dust and fumes can be released, which may pose a health risk for the operator. It is not always possible to change the process releasing these hazardous substances. As a consequence it can be necessary to protect operators in preparation or quality control areas from exposure to the product or the active substance. This can be done by active ventilation and exhaust and by filtration in order to protect the environment (see also Sect. 26.4.1). The appropriate equipment may be fume cupboards, moveable exhaust ducts, powder exhaust units, (bio)safety cabinets and isolators. Fumes, gas mixtures and volatiles might be absorbed by special filters, but in pharmacy practice only the technique of exhausting and screen filtration is usually used. 

Most of the chemical fume hoods considered here consist of a cabinet or enclosure set at waist level (above a table or storage cabinet) that is connected to a blower located above the hood or external to the hood through a duct system. The cabinet has an open side (or sides) to 2illow a user to perform work within. A movable transparent sash separates the user from the work. Most chemical fume hoods have a sill that functions as an airfoil at the work surface below the sash. The connection to the blower might be by use of a v-belt, or it may be direct drive. This allows provision of a smooth flow of air with minimal turbulence. In some installations, axially mounted blowers are used, especially if multiple hoods are ducted into a common blower. Baffles located in the rear of the cabinet provide control of the air flow patterns, and can usually be adjusted to provide the best air flow around the experiment or procedure being performed. Many chemical fume hoods are equipped with air flow indicators, low flow monitors and alarms, and differenti2d pressure sensors to allow the user to operate safely. The major types of chemical fume hoods include the standard/conventional, W2dk-in, bypass, variable air volume, auxiliary air, or ductless types. Additional types include snorkels and canopies that are portable. Each type must be understood to be operated most efficiently within specifications (see the section below on safe operation). 

---