Block 4 SGWB Student 2021 Final-pages-2.pdf

Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 3: Protective Clothing Concepts PENETRATION Penetration is the flow of a chemical through zippers, weak seams, pinholes, cuts, or imperfections in the protective clothing on a nonmolecular level. Even the best...

Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 3: Protective Clothing Concepts PENETRATION Penetration is the flow of a chemical through zippers, weak seams, pinholes, cuts, or imperfections in the protective clothing on a nonmolecular level. Even the best protective barriers are rendered ineffective if punctured or torn. 16 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation BLOCK IV – UNIT 4: INTRODUCTION TO VENTILATION Objective 4a: State the basic industrial ventilation system principles. The Air Force invests a lot of resources toward the health and well-being of its workers. In the BE career field, we do our part by evaluating and recommending controls for potential occupational health hazards. You’ve already seen a considerable amount of information about chemical hazards. We will now discuss one of the principal methods of controlling them –ventilation. The costs associated with the installation, operation, and evaluation of the performance of ventilation systems can be significant. Comparatively few people have an adequate grasp of the parameters that must be considered for an efficient ventilation system. Many times in the past, this has resulted in waste, insufficient protection, and a false sense of security for workers. KEY CONCEPTS AND DEFINITIONS CONSERVATION OF MASS Conservation of mass is a principle that states that mass cannot be created or destroyed. For a system operating at a steady state, this means that the mass entering must equal the mass exiting the system. For a ventilation system, this means that the amount of air coming in equals the amount of air going out. This can be stated by the equation (Q1 = Q2) where each Q is the volumetric flow rate at a point in the system. One reason this concept is so valuable is that a BE technician may be able to take a measurement at a convenient location in the system and apply that flow rate to another part of the system. CONSERVATION OF ENERGY Conservation of energy is a principle that states that energy cannot be created or destroyed; it can only change form. The energy of a ventilation system can be observed in the movement of air. A fan increases the energy in the syastem, accelerating the air. There are also energy losses due to bends in the ducts, friction, and system inefficiencies. DUCT AREA The duct area is the area of a cross-section of duct. For a round duct, the cross section is a circle. For a rectangular duct, the cross-section is a rectangle. Duct area can be found using the following equations: • • For a round duct: 𝐴𝐴 = 𝜋𝜋𝑟𝑟 2 For a rectangular duct: 𝐴𝐴 = 𝐿𝐿 ∗ 𝑊𝑊 VOLUMETRIC FLOW RATE (Q) Volumetric flow rate, Q, has units of volume per time, such as cubic feet per minute (cfm). This is a measurement of air flowing through a point in the system. The volumetric flow rate can be determined using the following formula: Q = Av where “A” is the area of the duct and “v” is the 17 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation average velocity across the duct's cross-section. Area has units of distance squared (i.e. ft2) and velocity has units of distance per time (i.e. ft/min). When these are multiplied, we see that Q is expressed as volume per time (i.e. ft3/min or cfm). STATIC PRESSURE (SP) Static pressure can be thought of as the potential energy of the ventilation system. Static pressure is exerted in all directions and is the result of a volume of air occupying the space in the duct. It is the pressure that tends to either collapse (negative) or expand (positive) the ductwork with the greatest amount of pressure near the fan. Static pressure is negative upstream of the fan (where air is “pulled” toward the fan) and positive downstream of the fan (where air is “pushed” away from the fan). VELOCITY PRESSURE (VP) Velocity pressure can be thought of as the kinetic energy in a ventilation system. Velocity pressure is the pressure required to accelerate air (kinetic energy) to its current velocity. NOTE: The impact of moving air is always measured in the direction of flow, which will always make VP positive. TOTAL PRESSURE (TP) Total pressure is the sum of VP and SP (TP = SP + VP). It carries the sign of SP (either positive or negative) and measures the energy content of the air stream. CAPTURE VELOCITY Capture velocity is the air velocity required to capture and transport a contaminant into a ventilation hood. The volumetric flow rate (Q) of a ventilation system must be high enough to achieve capture velocity where the contaminant is generated. FACE VELOCITY Face velocity is the measurement of airflow across the opening of a local exhaust capture hood. Face velocity is measured in feet per minute (fpm). It is one method of determining the performance of a hood and one of the ways used to determine if the ventilaiton system meets design criteria and baseline parameters. TRANSPORT VELOCITY Transport velocity is the amount of airflow required inside a duct to keep a contaminant entrained throughout the system. It is expressed in feet per minute (fpm). 18 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation COMPONENTS OF A VENTILATION SYSTEM INLET Where the air is drawn into the hood, which captures, contains, or controls the emission source. DUCT The duct carries the contaminant to the air cleaner and then to the outside area. AIR CLEANER The air cleaner scrubs, separates, removes, or filters the contaminant from the air. FAN The fan generates static pressure and moves the air. OUTLET The outlet disperses the cleaned air into the outside environment. Figure 3: Components of a Ventilation System 19 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation DUCT AREA AND VOLUMETRIC FLOW RATE CALCULATIONS DUCT AREA The calculation of duct area will depend on the shape of the duct. We generally want duct areas to be expressed in square feet (ft2), since we are measuring velocities in feet per minute (fpm) and volumetric flow in cubic feet per minute (cfm). Rectangular Duct: The cross-sectional area of a rectangle is length times width. A=l∗w Example 1: Calculate the cross-sectional area of a rectangular duct measuring 8 by 14.5 inches. A = 8 in ∗ 14.5 in = 116 in2 To convert from square inches to square feet, we use dimensional analysis. We apply the inch to foot conversion ratio twice since we need to cancel inches squared. A= A= 116 in2 1 ft 2 116 in2 1 ft 2 116 in2 1 ft 1 ft � �� �= � � = � � 1 12 in 12 in 1 1 144 in2 12 in 116 2 ft = 0.806 ft 2 144 Example 2: Calculate the cross-sectional area of a rectangular duct measuring 19 by 27 inches. Circular Duct: The cross-sectional area of a circular duct is pi times the radius squared. A = πr 2 Example 3: Calculate the cross-sectional area of a circular duct with a diameter of 10 inches. First, we find the radius, which is half the diameter. d 10 in = = 5 in 2 2 Plug radius into circular area equation and convert to square feet (ft2). r= 20 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation A = πr 2 = π(5 in)2 = π(52 𝑖𝑖𝑖𝑖2 ) = 25𝜋𝜋 𝑖𝑖𝑖𝑖2 A= 25π in2 1 𝑓𝑓𝑓𝑓 2 25𝜋𝜋 2 � �= 𝑓𝑓𝑓𝑓 = 0.545 𝑓𝑓𝑓𝑓 2 2 144 1 144 𝑖𝑖𝑖𝑖 Example 4: Calculate the cross-sectional area of a circular duct with a diameter of 13 inches. VOLUMETRIC FLOW RATE The volumetric flow rate is the volume or quantity of air that crosses an area per unit of time. The volumetric flow rate is calculated by multiplying the velocity of the air by the cross-sectional area the air is passing through. The relationships between the volumetric flow rate (Q), cross-sectional area (A), and velocity (v) can be visualized by the triangle shown in Figure 4. The triangle represents these three relationships: Q=A∗v A= Q v Q v= A Figure 4: Volumetric Flowrate Triangle ft Air velocity (v) is measured in feet per minute � or fpm�. Cross-sectional area (A) should be min 2 calculated in units of square feet (ft ). This way, when A and v are multiplied, we get the unit for volumetric flow rate (Q) as cubic feet per minute � ft3 min or cfm�. Example 1: Find the volumetric flow rate of a ventilation system with a cross-sectional area of 0.034 ft2 and air velocity of 1500 feet per minute (fpm). Since we are solving for the volumetric flow rate (Q), we use the first equation. Where, Q=A∗v A = 0.034 ft2 v = 1500 fpm Q = (0.034 ft 2 ) ∗ (1500 fpm) Q = 51 cfm 21 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation Example 2: Find the cross-sectional area of a duct with an air velocity of 4800 fpm and a volumetric flow rate of 2500 cfm. Since we are solving for the cross-sectional area (A), we use the second equation. Where, A= Q v A= 2500 cfm 4800 fpm Q = 2500 cfm v = 4800 fpm A = 0.52 ft2 CHARACTERISTICS OF STATIC PRESSURE, VELOCITY PRESSURE, AND TOTAL PRESSURE Most ventilation systems rely on one or more fans to move air throughout the system. A fan creates the pressure necessary to begin and maintain the flow of air. The fan must overcome losses in the system, such as the pressure drop from a filter or from friction over long lengths of duct. There are two components of the total pressure at each location in a system: static pressure (SP) and velocity pressure (VP). STATIC PRESSURE (SP) Static pressure (SP) is created when the fan is turned on and moves air from one side of the system to the other. Separated by the fan, the two sides are called upstream (suction side), and downstream (blowing side). SP is the potential energy of the ventilation system. It is exerted in all directions and is the pressure that tends to either collapse or expand (or burst) the ductwork. The sign of SP (positive or negative) changes depending on if measurements are taken upstream or downstream. If it is upstream from the fan, static pressure is negative. If it is downstream from the fan, static pressure is positive. Figure 5 shows how static pressure is measured. 22 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation Figure 5: Static Pressure VELOCITY PRESSURE (VP) Velocity pressure (VP) is the pressure exerted by air in motion. VP is the pressure required to accelerate air from zero to the current velocity. It is the kinetic energy of the system. It is always measured in the direction of airflow which will always make VP positive. Figure 6 shows how velocity pressure is measured. VP, like all of the pressure measurements we make in ventilation systems, is expressed in inches of water. This is abbreviated as “in wg” (inches water gauge); however, standards for air flow are based on feet per minute (fpm) or cubic feet per minute (cfm). When performing a ventilation survey, VP readings must be converted to velocity. Figure 6: Velocity Pressure 23 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation TOTAL PRESSURE (TP) Total pressure (TP) is the sum of the static and velocity pressures at a given point in the duct with due regard for sign. Determining TP as a point in the system is accomplished by the following equation: TP = SP + VP Note that the sign for TP will always be the same as the sign for SP. The signs for SP, VP, and TP are shown in Table 2. Table 2: Pressure Signs Upstream (Suction Side) Downstream (Blowing Side) SP + Figure 7Figure 6 shows how TP is measured. Figure 7: Total Pressure 24 VP + + TP + Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation Example 1 SP = 0.86 in wg VP = 0.45 in wg TP = ? TP = SP + VP TP = 0.86 in wg + 0.45 in wg TP = 1.31 in wg On what side of the fan are these readings taken? Example 2 SP = ? VP = 0.45 in wg TP = -0.65 in wg TP = SP + VP -0.65 in wg = SP + 0.45 in wg SP = -0.65 in wg - 0.45 in wg SP = -1.1 in wg On what side of the fan are these readings taken? TYPES OF PRESSURE LOSSES AND THEIR CAUSES There are pressure losses throughout a ventilation system, starting with the entry of the air into the hood or opening as negative static pressure causes air to be drawn into the system. Most system losses are due to friction encountered in the system and can be divided into the following categories: • • • Friction Losses Dynamic Losses Contractions 25 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation Friction Losses Friction losses are due to the interaction of air molecules with the sides of the duct. This produces a drag on the airflow. Four major factors contribute to friction loss: • • • • Duct Material: Rough surfaces cause more drag and friction than smooth surfaces. Air Velocity: Faster air causes more friction. Duct Diameter: Small duct with high velocity will have much more friction losses than a larger duct with the same velocity. Duct Length: Friction increases as the length of the duct increases. A rough metal surface will cause more friction loss than a smooth one; so ducting materials available on the market are generally smooth. Because of losses at the duct surface, the velocity pressure (VP) will be highest at the center of the duct. Dynamic Losses Dynamic losses are caused by changes to air movement and turbulence. Dynamic losses can be caused by a change in duct diameter (getting bigger or smaller), a change in direction (such as an elbow, or turn, in the duct), a branch (duct splitting into two), or a “Y” fitting (two ducts combining into one). The more sudden the changes in flow, the greater the turbulence and dynamic loss. To reduce these losses, diameter changes should be tapered over a length of duct, branches should split and merge at an angle, and hoods should be flanged or tapered to allow smoother entry of air. Contractions As air moves from a larger area to a smaller area, it will contract as it passes through the opening. This happens when moving from a large duct into a smaller duct, or from the ambient air into a hood opening. As air is pulled into the duct opening from the area outside the duct, the individual air streams converge to a higher velocity air stream that does not completely fill the duct. The point where the airstream diameter is the smallest is called the vena contracta (which means a contraction of the airstream diameter). Since the actual diameter of the airstream (the cross-sectional area through which the air is moving) is smaller at the vena contracta than at the other points in the duct, the velocity at the vena contracta is higher than the average duct velocity in the rest of the system. At the vena contracta, the air velocity has increased and can only come at the expense of SP, which decreases at the vena contracta. Thus, a small energy loss occurs in this conversion of SP to VP; however, as the air passes through the vena contracta, the air slows down as the airstream diameter enlarges to fill the duct. At this point, VP is converted back to SP. This is important when discussing vent design. If the duct is too small in diameter, then as the air passes through it and expands into the duct, there is a significant drop in VP. In other words, this drop in velocity will result in false readings at the face of the vent. This is also important when trying to overcome SP to move contamination from the work area. 26 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 4: Introduction to Ventilation To reduce contractions, hoods should be flanged to create a smoother transition of airflow into the duct. Figure 8 shows the effect of three different hood types: open duct, flanged duct, and tapered duct. Figure 8: Vena Contracta for Different Hood Types MAKE-UP AIR Adequate make-up air is often neglected when designing ventilation systems. This is a serious problem that is seen frequently with larger exhaust systems such as paint booths. Make-up air is critical to the proper functioning of large-volume systems. Natural draft make-up through windows, doors, and cracks can be used if enough air can get in, but is often unsatisfactory. The amount of air entering a room must be about the same as that leaving through all exhaust sources. Mechanical make-up is much better in providing this. It is a good idea to have a little more than needed (or a fan with a drive belt that can be adjusted for different flows) just in case more exhaust systems are needed in the future. Unheated air being drawn into a room causes uncomfortable drafts on workers that could be eliminated with proper make-up air. It is undesirable to have air coming in from different directions instead of a smooth temperature-controlled source of make-up air. This also helps to lower costs of heating by distributing the air over the room to eliminate hot or cold spots. Inadequate make-up air causes a negative pressure in the workroom and can limit the amount of air exhausted. It adds to the total system resistance that must be overcome by the exhaust fan. The amount of air coming into the room will ultimately equal the amount exhausted, but the system capacity will be reduced. This could result in insufficient contaminant control. The installation of a make-up system can get the system’s capacity up to the air volumes for which it was designed. 27 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation BLOCK IV – UNIT 5: DILUTION VENTILATION Objective 5a: State basic principles associated with dilution ventilation. Local exhaust ventilation (LEV) systems are generally superior to dilution ventilation systems because LEV systems remove the contaminant from the work environment, reduce overall airflow rates, and are more economical. There are, however, many industrial operations where LEV is impractical, not cost effective, or impossible to install. Such operations often use dilution ventilation. Comfort and dilution ventilation are used to ventilate general areas rather than a specific point of contaminant release, as with LEV. Because of this, some references group them together under either the heading general ventilation or the dilution ventilation. We need to make some distinction, however. Comfort ventilation is meant to do just what its name implies: keep workers comfortable, at least comparatively. Dilution ventilation, on the other hand, is used to control some type of airborne hazard in the workplace. DEFINITIONS DILUTION VENTILATION Dilution ventilation introduces uncontaminated air for the purpose of reducing airborne concentrations and controlling potential airborne health hazards, fire and explosive conditions, odors, and nuisance contaminants. With dilution ventilation, the contaminant is allowed to disperse to some extent and is then gradually removed. NATURAL VENTILATION Natural ventilation is air movement within a workplace caused by wind, temperature differences, or other factors where no fan or other mechanical air mover is used. Natural ventilation can be used when hot process equipment such as ovens and furnaces cause the air to warm, expand, and rise. If openings are provided in the roof, the hot, rising air exits the building. This is known as “The Chimney Effect.” MECHANICAL VENTILATION Mechanical ventilation is ventilation accomplished with the use of a fan. It is usually distinguished by providing either negative or positive pressure. Any vent system that uses a fan provides more reliable control than a system operating by natural ventilation. A combination of supply (make-up air) and exhaust is preferred for proper distribution and dilution. Supply is often used alone with satisfactory results, but exhaust alone cannot usually provide proper dilution. To demonstrate this, Figure 9 shows a supply outlet and an exhaust inlet both with a diameter of 1 foot and identical face velocities of 1000 fpm. Both of these setups will have the same volumetric flow rate (Q) and have the same size fans. The velocity of air from the blower will be about 100 fpm at 30 feet from the outlet face. The velocity of air to the 28 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation exhaust inlet will be about 100 fpm at only 1 foot from the inlet face and nonexistent at 30 feet. Supply provides a far more directional airflow for better mixing with the contaminant. Supply air should travel past the worker and then through the source of contaminant generation. You do not want a design where the chemical vapor(s) are blown into the worker’s breathing zone. Figure 9: Supply Vs. Exhaust Air Flow If the area adjacent to the room with the dilution system is normally occupied, then the dilution room should have a slight negative pressure (volumetric flow rate greater for the exhaust). If unoccupied, it is better that the dilution room be slightly positive (volumetric flow rate greater for the supply). In Figure 10, fewer lines from the fan mean less velocity. Negative pressure will keep contaminants from crossing into the adjacent area through the cracks in the door. Figure 10: Negative and Positive Pressure 29 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation FACTORS IN SELECTING DILUTION VENTILATION Dilution ventilation in the industrial environment should be considered only if the following are true: • • • • • • • • Other, more cost-effective controls are unavailable. Air contaminants have a relatively low toxicity. o Dilution does not completely remove the contaminants from the work area, so there will usually be some level of exposure. Note: Dilution ventilation should not be used to control chemicals with an OEL < 100 ppm. Air contaminants consist of gases, vapors, or small aerosols (diameter < 25 μm). Emissions occur uniformly in time (constant generation rate). o Uneven “spikes” of contaminant generation during the day can tax the system’s ability to control the substance. Because a dilution system gradually removes contaminants, once the concentration spikes, it can take a long time to reduce to a safe level. Emission sources are widely dispersed and are not close to the employee breathing zone. The source of supply air is not contaminated. The facility is located in a moderate climate. The Heating, Ventilation, and Air Conditioning (HVAC) system used to condition dilution air is capable of maintaining appropriate temperatures and humidity. ADVANTAGES AND DISADVANTAGES ADVANTAGES There are many reasons why dilution ventilation may be chosen over local exhaust ventilation (LEV). Some of the following are examples of dilution ventilation advantages: • • • • Usually lower equipment and installation costs Requires less maintenance Effective control for small amounts of low toxicity chemicals (OEL > 100 ppm) Systems typically not co-located with the source of contamination, which prevents interference with the worker’s ability to perform a specific operation DISADVANTAGES The following are disadvantages of using a dilution ventilation system: • • • • Does not completely remove contaminants Cannot be used for high toxicity chemicals (OEL < 100 ppm) Ineffective for large amounts of dusts, fumes, gases or vapors Ineffective for handling surges of gas or vapor emission or irregular emissions 30