Industrial Ventilation General Principles of Industrial Ventilation

What Is Industrial Ventilation? Environmental engineer’s view: The design and application of equipment for providing the necessary conditions for maintaining the efficiency, health and safety of the workers Industrial hygienist’s view: The control of emissions and the control of exposures Mechanical engineer’s view: The control of the environment with air flow. This can be achieved by replacement of contaminated air with clean air General Principles

Industrial Ventilation Objectives To introduce the basic terms To discuss heat control To design ventilation systems General Principles

Why Industrial Ventilation? To maintain an adequate oxygen supply in the work area. To control hazardous concentrations of toxic materials in the air. To remove any undesirable odors from a given area. To control temperature and humidity. To remove undesirable contaminants at their source before they enter the work place air. General Principles

Application Of Industrial Ventilation Systems Optimization of energy costs. Reduction of occupational health disease claims. Control of contaminants to acceptable levels. Control of heat and humidity for comfort. Prevention of fires and explosions. General Principles

Solutions To Industrial Ventilation Problems Process modifications Local exhaust ventilation Substitution Isolation Administrative control Personal protection devices Natural ventilation General Principles

Ventilation Design Parameters Manufacturing process Exhaust air system & local extraction Climatic requirements in building design (tightness, plant aerodynamics, etc) Cleanliness requirements Ambient air conditions Heat emissions Terrain around the plant Contaminant emissions Regulations General Principles

Source Characterization Location Relative contribution of each source to the exposure Characterization of each contributor Characterization of ambient air Worker interaction with emission source Work practices General Principles

Types Of Industrial Ventilation Systems Supply systems Purpose: To create a comfortable environment in the plant i.E. The HVAC system To replace air exhausted from the plant i.E. The replacement system General Principles

Supply Systems Components Air inlet section Filters Heating and/or cooling equipment Fan Ducts Register/grills for distributing the air within the work space General Principles

Exhaust Systems Purpose An exhaust ventilation system removes the air and airborne contaminants from the work place air The exhaust system may exhaust the entire work area, or it may be placed at the source to remove the contaminant at its source itself General Principles

Exhaust Systems Types of exhaust systems: General exhaust system Local exhaust system General Principles

General Exhaust Systems Used for heat control in an area by introducing large quantities of air in the area. The air may be tempered and recycled. Used for removal of contaminants generated in an area by mixing enough outdoor air with the contaminant so that the average concentration is reduced to a safe level. General Principles

Local Exhaust Systems(LES) The objective of a local exhaust system is to remove the contaminant as it is generated at the source itself. Advantages: More effective as compared to a general exhaust system. The smaller exhaust flow rate results in low heating costs compared to the high flow rate required for a general exhaust system. The smaller flow rates lead to lower costs for air cleaning equipment. General Principles

Local Exhaust Systems(LES) Components: Hood The duct system including the exhaust stack and/or re-circulation duct Air cleaning device Fan, which serves as an air moving device General Principles

What is the difference between Exhaust and Supply systems? An Exhaust ventilation system removes the air and air borne contaminants from the work place, whereas, the Supply system adds air to work room to dilute contaminants in the work place so as to lower the contaminant concentrations. General Principles

Pressure In A Ventilation System Air movement in the ventilation system is a result of differences in pressure. In a supply system, the pressure created by the system is in addition to the atmospheric pressure in the work place. In an exhaust system, the objective is to lower the pressure in the system below the atmospheric pressure. General Principles

Types Of Pressures In A Ventilation Systems Three types of pressures are of importance in ventilation work. They are: Static pressure Velocity pressure Total pressure General Principles

Why is air considered incompressible in Industrial Ventilation design problems? The differences in pressure that exist within the ventilation system itself are small when compared to the atmospheric pressure in the room. Because of the small differences in pressure, air can be assumed to be incompressible. Since 1 lb/in2 = 27 inches of water, 1 inch = 0.036 lbs pressure or 0.24% of standard atmospheric pressure. Thus the potential error introduced due to this assumption is also negligible. General Principles

Velocity Pressure It is defined as that pressure required to accelerate air from rest to some velocity (V) and is proportional to the kinetic energy of the air stream. VP acts in the direction of flow and is measured in the direction of flow. VP represents kinetic energy within a system. VP is always positive. General Principles

Static Pressure It is defined as the pressure in the duct that tends to burst or collapse the duct and is expressed in inches of water gauge (“wg). SP acts equally in all directions SP can be negative or positive General Principles

Static pressure can be positive or negative.Explain. Positive static pressure results in the tendency of the air to expand. Negative static pressure results in the tendency of the air to contract. For example, take a common soda straw, and put it in your mouth. Close one end with your finger and blow very hard. You have created a positive static pressure. However, as soon as you remove your finger from the end of the straw, the air begins to move outward away from the straw. The static pressure has been transformed into velocity pressure, which is positive. General Principles

Velocity Pressure VELOCITY PRESSURE (VP) VP = (V/4005)2 or V = 4005√VP Where VP = velocity pressure, inches of water gauge (“wg) V = flow velocity, fpm General Principles

Total Pressure TP = SP + VP It can be defined as the algebraic sum of the static as well as the velocity pressures SP represents the potential energy of a system and VP the kinetic energy of the system, the sum of which gives the total energy of the system TP is measured in the direction of flow and can be positive or negative General Principles

How do you measure the Pressures in a ventilation system? The manometer, which is a simple graduated U-shaped tube open, at both ends, an inclined manometer or a Pitot tube can be used to measure Static pressure. The impact tube can be used to measure Total pressure. The measurement of Static and Total pressures using manometer and impact tube, will also indirectly result in measurement of the Velocity pressure of the system. General Principles

Basic Definitions Pressure It is defined as the force per unit area. Standard atmospheric pressure at sea level is 29.92 inches of mercury or 760 mm of mercury or 14.7 lb/sq.inch. General Principles

Basic Definitions Air density It can be defined as the mass per unit volume of air, (lbm/ft3 ). at standard atmosphere (p=14.7 psfa), room temperature (70 F) and zero water content. The value of ρ=0.075 lbm/ft3 General Principles

Basic Definitions Perfect Gas Equation: P = ρRT Where P = absolute pressure in pounds per square foot absolute (psfa). ρ = gas density in lbm/ft3. R = gas constant for air. T = absolute temperature in degree Rankin. For any dry air situation ρT = (ρT)std ρ = ρstd(Tstd/T) = 0.075 (460+70)/T = 0.075 (530/T) General Principles

Basic Definitions Volumetric Flow Rate The volume or quantity of air that flows through a given location per unit time Q = V * A or V = Q /A A = Q/V Where Q = volume of flow rate in cfm V = average velocity in fpm A = cross-sectional area in sq.ft General Principles

Example The cross-sectional area of a duct is 2.75 sq.ft.The velocity of air flowing in the duct is 3600 fpm. What is the volume? From the given problem A = 2.75 sq. ft. V = 3600 fpm We know that Q = V * A Hence, Q = 3600 * 2.75 = 9900 cfm General Principles

Basic Definitions Reynolds number R = ρDV/μ Where ρ = density in lbm/ft3 D = diameter in ft V = velocity in fpm μ = air viscosity, lbm/s-ft General Principles

Darcy Weisbach Friction Coefficient Equation hf = f (L/d)VP Where hf = friction losses in a duct, “wg f = friction coefficient (dimensionless) L = duct length, ft d = duct diameter, ft VP = velocity pressure,”wg General Principles

Duct Losses Types of losses in ducts Friction losses Dynamic or turbulence losses General Principles

Duct Losses Friction losses Factors effecting friction losses: Duct velocity Duct diameter Air density Air viscosity Duct surface roughness General Principles

Duct Losses Dynamic losses or turbulent losses Caused by elbows, openings, bends etc. In the flow way. The turbulence losses at the entry depends on the shape of the openings Coefficient of entry (Ce) For a perfect hood with no turbulence losses Ce = 1.0 I.E V = 4005ce√VP = 4005 √VP General Principles

Duct Losses Turbulence losses are given by the following expression Hl= FN*VP Where FN = decimal fraction General Principles

Terminal Or Settling Velocity V = 0.0052(S.G)D2 Where D = particle diameter in microns S.G = specific gravity V = settling velocity in fpm General Principles