Local Exhaust Hoods Local Exhaust Hoods Introduction v

Local Exhaust Hoods

Local Exhaust Hoods Introduction: v Designed to capture and remove harmful emissions from various processes prior to their escape into the workplace. v Hood is the place where the process emission enters the exhaust system. v Main function of the hood is to capture the contaminants and transport them into the hood. v An air field is created in the hood for the above function. v Fig. 3 -1, page 3 -3, ACGIH manual shows nomenclature associated with local exhaust hoods. Local Exhaust Hoods 2

Contaminant Characteristics v Inertial effects: movement with respect to air depends on their inertia. v Effective specific gravity: specific gravity affects the density which in turn effects the motion of particles with the air. v Wake effects: A turbulent wake is created due to air flow around an object. Local Exhaust Hoods 3

Hood Types Enclosing hoods: v Hoods which completely or partially enclose the contaminant generation point these are preferred wherever the process configuration and operation will permit. Exterior hoods: v Hoods located adjacent to the source. Examples are slots along the edge of the tank or a rectangular opening on a welding table. Criteria for hood selection: - Physical characteristics of the equipment. - Contaminant generation mechanism. - Equipment surface. Fig 3 -3, page 3 -5, ACGIH manual shows different types of hoods. Local Exhaust Hoods 4

Factors Affecting Hood Design v v v Capture velocity Hood flow rate determination Effect of flanges and baffles Air distribution Rectangular and round hoods Worker position effect Local Exhaust Hoods 5

Capture Velocity v It is the minimum hood induced air velocity necessary to capture and convey the contaminants into the hood v It is the result of hood air flow rate and hood configuration Factors affecting selection of values of capture velocity v Lower end of rangev Room air currents minimal or favorable to capture v Contaminants of low toxicity or of nuisance value only v Intermittent, low production v Large hood – large air mass in motion v Upper end of rangev Distributing room air currents v Contaminants of high toxicity v High production, heavy use v Small hood - local control only Local Exhaust Hoods 6

Hood Flow Rate Determination v For an enclosure capture velocity at the enclosed opening is the exhaust flow rate divided by opening area v The capture velocity at a given point in front of the exterior hood will be established by the hood air flow through the geometric surface which contains the point v For a theoretical unbounded point suction source Q = v * a = v * 4 * π * x 2 = 12. 57 * v * x 2 Where Q = air flow into suction point, cfm V = velocity at distance X, fpm A = 4 * Π * X 2 = area of sphere, ft 2 X = radius of sphere, ft Local Exhaust Hoods 7

Hood Flow Rate Determination v For an unbounded line source Q = v *2 * π * x * l = 6. 28 * v * x * l Where L = length of line source, ft v In general the equation used is Q = v * (10 * x 2 + a) Where Q = air flow, cfm V = center line velocity at X distance from hood, fpm X = distance outward along axis of flow in ft A = area of hood opening, ft 2 D = diameter of round hoods or side of essentially square hoods, ft Local Exhaust Hoods 8

Effect Of Flanges And Baffles Flange: v It is a surface at and parallel to the hood face which provides a barrier to unwanted air flow from behind the hood. Baffle: v It is a surface which provides a barrier to unwanted air flow from the front or sides of the hood. . Functions of flanges and baffles: v Reducing the flow area which in turn reduces the flow rate required to achieve a given capture velocity. v Flow rate is approximately reduced by 25% in practice. v For most applications the flange width should be equal to the square root of the hood area (√ A ). Local Exhaust Hoods 9

Air Distribution v Slots are generally used for uniform air distribution. v Hoods with an opening width - to - length ratio of 0. 2 or less are slot hoods. v They provide uniform exhaust air flow and adequate capture velocity over a finite length of contaminant generation. v Slot velocity does not contribute toward capture velocity. v Slot length and exhaust volume effect the capture velocity. Local Exhaust Hoods 10

Rectangular And Round Hoods v Air distribution for rectangular and round hoods is achieved by air flow within the hood rather than by pressure drop as for the slot hood. v The area of the hood changes with the shape of the hood. v The effect of slot is different in both cases. v Different kind of distribution techniques can be used. Local Exhaust Hoods 11

Worker Position Effect v Workers position considerably effects the exposure. v Local exhaust ventilation is designed to be near the point of contaminant generation. v The worker should be such oriented that contaminants flow with air flow. v The contaminants should not enter the breathing zone. Local Exhaust Hoods 12

Hood Losses v Entry losses occur due to formation of venacontracta at the entrance of duct. v The hood entry loss represents the energy necessary to overcome the loss as the air enters the duct. v The losses increase with increase in flow area. v Hoods with two or more points of loss are compound hoods. The basic equations used are (for simple hood) SPh = hed + VPd Where SPh = hood static pressure, “wg hed = entry loss transition (Fh * VPd ) VPd = duct velocity pressure Local Exhaust Hoods 13

Hood Losses For compound hoods: SPh = (FS) (VPS) + (FD) (VPD) + VPD This is when duct velocity is greater than slot velocity. Where: SPh = hood static pressure, “wg FS = entry loss factor for slot VPS = slot velocity pressure, “wg FD = entry loss factor for duct VPD = duct velocity pressure, “wg Local Exhaust Hoods 14

Minimum Duct Velocity v Depends on type of material being transported. v Used to calculate duct velocity pressure and hood losses. Factors affecting minimum duct velocity: v Plugging or closing of branch. v Damage to ducts (e. G. Denting). v Leakage of ducts. v Corrosion or erosion of fan wheel. v Slipping of fan drive belt. v Velocities should be able to pick up dust particles which may have settled due to improperation. Table 3 -2, page 3 -19, ACGIH manual shows values of typical duct velocities. Local Exhaust Hoods 15

Special Hood Requirements Ventilation of high toxicity and radioactive processes: v Extraordinarily effective control methods are to be used. v Knowledge of hazards and adequate maintenance required that includes monitoring. v Enclosing type of hood preferred. v Replacement air should be introduced at low velocity and in a direction so that it does not produce disruptive cross drafts at the hood opening. Laboratory operations: v Glove boxes should be used. v For low activity radioactive laboratory work, a laboratory fume hood may be acceptable. Local Exhaust Hoods 16

Push - Pull Ventilation v It is a kind of variation to exterior hoods. v A jet of air is pushed across contaminant source into the flow v v field of hood. Contaminant control is primarily achieved by the jet. Exhaust receives the jet and removes it. Advantage is that jet can travel greater distance in a controlled manner. The system is harmful if not properly designed, installed or operated. Local Exhaust Hoods 17

Hot Process v Designed differently than normal hoods. v Thermal draft created due to convection and conduction. v Draft causes upward air current with high velocities. Equation used to find flow rate for rectangular and circular high canopy hoods Dc = 0. 5 * xc 0. 88 Where: DC = column diameter at hood face. XC = y +z = the distance from the hypothetical point source to the hood face, ft Y = distance from the process surface to the hood face, ft Z = distance from the process surface to the hypothetical point source, ft Z = (2 * DS)1. 138 Where: DS = diameter of hot source, ft Local Exhaust Hoods 18

Hot Process Qt = Vf * Ac + Vr * (Af - Ac) Where: Qt = total volume entering hood, cfm Vf = velocity of hot air column at the hood face, fpm Ac = area of the hot air column at the hood face, ft 2 Vr = the required velocity through the remaining hood area, fpm Af = total area of hood face, ft 2 For low canopy hoods: Qt = 4. 7 * ( Df)233 * (Δt)0. 42 Where: Qt = total hood air flow, cfm Df = diameter of hood, ft Δt = difference between temperature of the hot source, and the ambient, F. Local Exhaust Hoods 19