Document mm3XdmZyb50GJyOgdmjRLkkRd

834 CHAPTER 46 1948-Guide performed mechanically in wholly enclosed booths, the volatile vapors may reach Injurious or explosive concentrations. At all times the con centrations of these vapors, and particularly those containing benzol, should be kept well below 100 parts per million in the breathing zone of the worker. Spray booth-vapors are dangerous to the health of the worker and care should be taken to minimize exposure to them. It.is recommended in the design of spray booths that the exhaust duct be located at the end of the booth opposite the opening; In front of this duct should be placed baffle plates which will cause a uniform air velocity distribution across the frontal area. The air volume should be sufficient to maintain a velocity of not less than 100 fpm over the open area of the booth (150 fpm is preferable where benzol or lead is present in the paint) and the vapors should be discharged through a suitable stack to permit dilution. It is good practice to pass the fumes or vapors through baffle type washers or scrubbers designed for efficient spray removal. Hoods for Chemical Laboratories Hoods used in chemical laboratories are generally provided with sliding windows which permit positive control of the fumes and vapors evolved by the apparatus. Their design should offer easy access for the installation of chemical equipment and should- be well lighted. Air velocities should exceed 50 fpm when the window is fully open. Kitchen Hoods , The length and width of kitchen hoods'should be.such .as to extend beyond the extreme projection of the ranges,, broilers, etc., over which they are installed.. The minimum projection or overlap should be 12 in. Where space conditions permit, range hoods should be about 2 ft high so as to provide a reservoir to confine momentary bursts of smoke and steam until .the exhaust system can evacuate the hood. Range hoods should be located as low as possible to increase, their effectiveness. A steel plate placed horizontally within a kitchen hood and located sufficiently high above the level of the bottom of the hood to permit use of a row of lights within the hood along the edge has been found effective in improving the operation. The area between the edges of the plate arid the edges of the hood should be selected according to the' exhaust volume rind velocity, required. . ' In general the amount of air to be exhausted from restaurant range hoods is at the rate of 100 fpm per square, foot of face area. Thus, a hood 4.5 ft wide by 30 ft long has a face area of 135 sq ft, which multiplied by 100 fpm velocity results in a total air quantity to be exhausted of 13,500 cfm.' In some cases, where the applicrition Is principally frying and where it is not practical to iristall a hood 2 ft high, it is recommended that the face velocity be increased from 100 to 150 fpm, dependirig on peak load con ditions ' in the kitchen. Exhaust connections to range hoods should always be iriride at the top and back of hoods, and should be spaced pref erably not more than 6 ft apart arid be rectangular in shape with the long side parallel to the back of the hood. Exhaust openings into range hoods should be designed to maintain a velocity of 1500 to 1800 fpm. An approved fire damper with fusible link should be (and is required by code in many states) installed in the main exhaust duct or branch adjacent to'the range hood. Should there be more than orie hood con nected to a coriimon duct, then the branch duct to each hood should be provided with a fire damper. Access doors should be provided at the fire Exhausting and Conveying Systems 835 damper for purpose of inspection, cleariing, or for renewal of fusible link. All exhaust piping to range hoods, commonly called grease ducts, should be provided with tight fitting cleanout doors of adequate size to permit easy removal of grease. Some engineers use filters to advantage in hoods which are subject to grease conditions. Hoods over steam tables should be of similar construction to range hoods. It is good practice to design such hoods with a face velocity of 60 to 70 fpm. Hoods over dishwashing machines are usually relatively small and generally 1500 to 2000 cfm per hood is allowed,'which is equivalent to a velocity of approximately 100 fpm per square: foot of-face area. Range hoods in diet kitchens are constructed, the same as-restaurant range hoods but with less exhaust air per square foot of face .area, depending upon the nature of the food cooked. Hoods are not often used in private residences unless they are'quite large and the consideration of expense is not important. For such residences the hoods should be designed on the same basis as diet kitchens. Most all residence kitchens can be effectively and economically venti lated by the installation of a built-in kitchen ventilator, which should be located in an outside, wall and in close proximity, to the kitchen range. It has been found that the capacity of the built-in kitchen ventilator should be at least 350 cfm regardless of the size of kitchen. This can be justified on the basis that the smaller the kitcheri the more concentrated the heat will be thus requiring a more rapid rate of air change. Standrird. size built-in kitchen ventilators are generally available in three sizes, namely 350, 500 and .800 cfm. The proper size to use. will depend on design conditions and available wall space. DUCT SYSTEM DESIGN In designing a duct system it is necessary to recognize a few funda mental principles (see also Chapter 41). Knowing the quantity of air required, the size of the duct may be computed from Equation 5:' where A = cross-section area of duct, square feet. .. Q = air quantity to be handled by the duct, cubic feet per minute. . V = velocity of air, feet per minute. . Air Velocities in Ducts ' Where it is necessary to transport the particulate material collected in \arv exhaust system, minimum carrying velocities must be maintained in the ducts preceding the collector. It has been found that good results are obtained when design air velocities in horizontal runs are not less than 2000 fpm or not greater than 5000 fpm. When the dust being carried is organic and other than wood flour, or similar material, a velocity of 2500 fpm is adequate. Approximate required conveying velocities are given in Table'6. For duct systems wherein the air has .no:.dust or solid load, a lower velocity is desirable, which may range from 1200 to 2000 fpm. In view of the fact that the horsepower required by a system depends directly on the resistance and the resistance is a . function of the velocity, eco nomical design requires velocities of this magnitude.