Local Exhaust Hoods

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

Local Exhaust Hoods3 Contaminant Characteristics  Inertial effects: movement with respect to air depends on their inertia.  Effective specific gravity: specific gravity affects the density which in turn effects the motion of particles with the air.  Wake effects: A turbulent wake is created due to air flow around an object.

Local Exhaust Hoods4 Hood Types Enclosing hoods:  Hoods which completely or partially enclose the contaminant generation point these are preferred wherever the process configuration and operation will permit. Exterior hoods:  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 Hoods5 Factors Affecting Hood Design  Capture velocity  Hood flow rate determination  Effect of flanges and baffles  Air distribution  Rectangular and round hoods  Worker position effect

Local Exhaust Hoods6 Capture Velocity  It is the minimum hood induced air velocity necessary to capture and convey the contaminants into the hood  It is the result of hood air flow rate and hood configuration Factors affecting selection of values of capture velocity  Lower end of range-  Room air currents minimal or favorable to capture  Contaminants of low toxicity or of nuisance value only  Intermittent, low production  Large hood – large air mass in motion  Upper end of range-  Distributing room air currents  Contaminants of high toxicity  High production, heavy use  Small hood - local control only

Local Exhaust Hoods7 Hood Flow Rate Determination  For an enclosure capture velocity at the enclosed opening is the exhaust flow rate divided by opening area  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  For a theoretical unbounded point suction source Q = v * a = v * 4 * π * x 2 = * 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 Hoods8 Hood Flow Rate Determination  For an unbounded line source Q = v *2 * π * x * l = 6.28 * v * x * l Where L = length of line source, ft  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 Hoods9 Effect Of Flanges And Baffles Flange:  It is a surface at and parallel to the hood face which provides a barrier to unwanted air flow from behind the hood. Baffle:  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:  Reducing the flow area which in turn reduces the flow rate required to achieve a given capture velocity.  Flow rate is approximately reduced by 25% in practice.  For most applications the flange width should be equal to the square root of the hood area (√ A ).

Local Exhaust Hoods10 Air Distribution  Slots are generally used for uniform air distribution.  Hoods with an opening width - to - length ratio of 0.2 or less are slot hoods.  They provide uniform exhaust air flow and adequate capture velocity over a finite length of contaminant generation.  Slot velocity does not contribute toward capture velocity.  Slot length and exhaust volume effect the capture velocity.

Local Exhaust Hoods11 Rectangular And Round Hoods  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.  The area of the hood changes with the shape of the hood.  The effect of slot is different in both cases.  Different kind of distribution techniques can be used.

Local Exhaust Hoods12 Worker Position Effect  Workers position considerably effects the exposure.  Local exhaust ventilation is designed to be near the point of contaminant generation.  The worker should be such oriented that contaminants flow with air flow.  The contaminants should not enter the breathing zone.

Local Exhaust Hoods13 Hood Losses  Entry losses occur due to formation of venacontracta at the entrance of duct.  The hood entry loss represents the energy necessary to overcome the loss as the air enters the duct.  The losses increase with increase in flow area.  Hoods with two or more points of loss are compound hoods. The basic equations used are (for simple hood) SP h = h ed + VP d Where SP h = hood static pressure, “wg h ed = entry loss transition (F h * VP d ) VP d = duct velocity pressure

Local Exhaust Hoods14 Hood Losses For compound hoods: SP h = (F S ) (VP S ) + (F D ) (VP D ) + VP D This is when duct velocity is greater than slot velocity. Where: SP h = hood static pressure, “wg F S = entry loss factor for slot VP S = slot velocity pressure, “wg F D = entry loss factor for duct VP D = duct velocity pressure, “wg

Local Exhaust Hoods15 Minimum Duct Velocity  Depends on type of material being transported.  Used to calculate duct velocity pressure and hood losses. Factors affecting minimum duct velocity:  Plugging or closing of branch.  Damage to ducts (e.G. Denting).  Leakage of ducts.  Corrosion or erosion of fan wheel.  Slipping of fan drive belt.  Velocities should be able to pick up dust particles which may have settled due to improper operation. Table 3-2, page 3-19, ACGIH manual shows values of typical duct velocities.

Local Exhaust Hoods16 Special Hood Requirements Ventilation of high toxicity and radioactive processes:  Extraordinarily effective control methods are to be used.  Knowledge of hazards and adequate maintenance required that includes monitoring.  Enclosing type of hood preferred.  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:  Glove boxes should be used.  For low activity radioactive laboratory work, a laboratory fume hood may be acceptable.

Local Exhaust Hoods17 Push - Pull Ventilation  It is a kind of variation to exterior hoods.  A jet of air is pushed across contaminant source into the flow 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 Hoods18 Hot Process  Designed differently than normal hoods.  Thermal draft created due to convection and conduction.  Draft causes upward air current with high velocities. Equation used to find flow rate for rectangular and circular high canopy hoods D c = 0.5 * x c 0.88 Where: D C = column diameter at hood face. X C = 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 * D S ) Where: D S = diameter of hot source, ft

Local Exhaust Hoods19 Hot Process Q t = V f * A c + V r * (A f - A c ) Where: Q t = total volume entering hood, cfm V f = velocity of hot air column at the hood face, fpm A c = area of the hot air column at the hood face, ft 2 V r = the required velocity through the remaining hood area, fpm A f = total area of hood face, ft 2 For low canopy hoods: Q t = 4.7 * ( D f ) 233 * (Δt) 0.42 Where: Q t = total hood air flow, cfm D f = diameter of hood, ft Δt = difference between temperature of the hot source, and the ambient, F.