Document R0ja0DoL820ZBVje6oGY1ME

702 Chapter 40 1945 Guide 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. i 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 and the edges of the hood should be selected according to the exhaust volume and 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 application is principally frying and where it is not practical to install a hood 2 ft high, it is recommended that the face velocity be increased from 100 to 150 fpm, depending on peak load con ditions in the kitchen. Exhaust connections to range hoods should always be made at the top and back of hoods, and should be spaced'pref erably not more than 6 ft apart and 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 one hood con nected to a common duct, then the branch duct to each hood should be provided with a fire damper.* Access doors should be provided at the fire damper for purpose of inspection, cleaning, 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 boods. 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 Industrial Exhaust Systems 703 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 kitchen the more concentrated the heat will be thus requiring a more.rapid rate of air change. Standard 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 31). 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 an 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 systein depends directly on the resistance and the resistance is a function of the velocity, eco nomical design requires velocities' of this magnitude. ' The equal friction method is generally used for designing a duct system as this insures equal resistance to air flow in all branches throughout the system (see Chapter 31). Long main ducts do not generally provide the most economical layout. Where it is necessary to ventilate a large'number of machines, or machines which are widely separated, it is desirable to Table 6. Approximate Conveying Velocities Material. Conveyed Large particles, heavy loads, moist materials;......................................... Design Velocity FPM 2,000 3,000 3.500 3,500-4,500 4.500 and over