Document 17mxGK4rN49zqOK1Ez76L1zd

American Society of Heating and Ventilating Engineers Guide, 1935"^ Chapter 21--Industrial Exhaust Systems Solution. Substitute in Equation 1 and solve for Y, thus Y 100 - Y 0.1 X 64 5X5 from which Y = 20.4 per cent of the velocity at the opening of the hood. 400 X 144 Velocity at opening = 64 = 900 fpm Hence, the velocity at the point in question is 900 X 0.204 = 184 fpm Air Flow from Static Readings The volume of air flow into any hood may be determined from the : following equation: where Q = 4005 fa VhT gj Q = volume of air flow, cubic feet per minute. a -- area of connecting duct, square feet. At = static suction at throat of hood, inches of water. / = orifice or restriction coefficient, which varies from 0.6 to 0.9 depending on the shape of the hood. An average value of/is 0.71, although for a well-shaped opening'a value of 0.8 may be used. If it is assumed that the entrance loss of a hood is proportional to the velocity head, / can be determined by the relation: where Av = the velocity head. Ae = the entrance loss. V/ - Av Av + Ae (3) For duct ends and abrupt openings^ = Av and for flared openings he = 0.5AV. The term static suction is not a good measure of the effectiveness of a hood unless the area of the opening and the location of the operation with respect to the hood are known. This is clearly indicated by Equation 1 which shows that the velocity function at any point along the axis varies directly as the area of the opening and inversely as the square of the distance. However, this formula coupled with Equation 2 should serve to indicate the velocity conditions to be expected when operations are conducted external to the hood opening. Large Open Hoods Large hoods, such as are used for electroplating and pickling tanks, should be subdivided so the area of the connecting duct is not less than one-fifteenth of the open area of the hood. Frequently, it will be found necessary to branch the main duct in order to obtain a uniform distnbution of flow. Canopy hoods should extend 6 in. laterally from the tank for every 12-in. elevation. In most cases, hoods of this type take advan tage of the natural tendency of the vapors to rise, and air velocities may be kept low. Cross drafts from open doors or windows disturb the rise of 350 f ^ * rs and therefore provision must: be made for them. The air ^-/tjfc^iirequired also depend upon the character of the vapors given off, fumes, for example, requiring an air velocity of approximately. |'iV,cyad5cl:on tj,e surface of the tank and acid and steam vapors requiring low as 25 to 50'fpm. The total volume of air flow necessary these velocities may be approximately determined from the <:/' Q = 1.4PDV inker? -'total volume of air handled by hood, cfm. : j, = perimeter of the tank, feet. p = distance between tank and hood opening, feet. V = air velocity desired along edges and surface of tank, fpm. (4) Spray Booths .vjn the design of an efficient spray booth, it is essential to maintain an eVen distribution of air flow through the opening and about the object fg: being sprayed. While in many instances spraying operations can be performed mechanically in wholly enclosed booths, the volatile vapors may reach injurious or explosive concentrations/ At all times the con- jf; Centrations of these vapors, and particularly those containing benzol, should be kept below 100 parts per million. Spray booth vapors are I dangerous to the health of the worker and care should be taken to mini- ? mize exposure to them. ?, ; It is recommended in the design of spray booths that the exhaust duct \ be located in a horizontal position slightly above the object sprayed. t Stagnant regions within the booth should be carefully avoided or should k be provided with a vertical exhaust. The air volume should be sufficient r to maintain a velocity of 150 to 200 fpm over the open area of the booth t and the vapors should be discharged through a suitable stack to permit ^ dilution6. i Hoods for Chemical Laboratories i '. 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 I installation of chemical equipment arid should be well lighted. Air * velocities should exceed 50 fpm when the window is opened to its mixi- | miim height. / i DESICN OF DUCT SYSTEMS !; The duct system should be large enough to transport the fumes or ? material without causing serious obstruction to the air flow. It is good l practice to proportion the ducts to obtain,, the desired velocities' and | suction pressures at the hoods, although in many cases only an approxir mation to an ideal design is possible. Many exhaust hoods, and par- F _ a discussion of spray booths, see Special Bulletin No. 16, Spray Painting in Pennsylvania, Depart- ; *Qt of Ubor and Industry, 1926, Harrisburg, Pa. ., 351 A