Bioenvironmental Engineering Apprentice Block IV: Chemical Controls PDF

Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation DILUTION VENTILATION EQUATIONS ROOM VOLUME (V) Dilution ventilation requirements are given as a certain number of air changes per unit time. To determine how many times a room’s air is...

Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation DILUTION VENTILATION EQUATIONS ROOM VOLUME (V) Dilution ventilation requirements are given as a certain number of air changes per unit time. To determine how many times a room’s air is changed, we need to first calculate the room volume. Since air flow is in units of cubic feet per minute (CFM) we need room volume to be calculated in cubic feet (ft3). Room Volume (ft3) = Room Length (ft) x Room Width (ft) x Room Height (ft) Or V = l * w* h Example: Calculate room volume for an office with walls measuring 20 ft and 14 ft and a ceiling height of 12 ft. V = l * w* h V = 20 ft * 14 ft * 12 ft V = 3360 ft3 VOLUMETRIC FLOW RATE (Q) To determine the number of times this volume of air is changed in the room, we need to determine the volumetric flow rate from the dilution system. We have already covered this equation: Q=A∗v Example: A room has one round duct with a diameter of 15 inches. The air velocity at the duct opening was measured to be 250 feet per minute (fpm). What is the volumetric flow rate (Q) of this system? First calculate the area of the duct opening: r= d 15 in = = 7.5 in 2 2 A= 176.7 in2 1 𝑓𝑓𝑓𝑓 2 176.7 2 � �= 𝑓𝑓𝑓𝑓 = 1.23 𝑓𝑓𝑓𝑓 2 2 144 1 144 𝑖𝑖𝑖𝑖 A = πr 2 = π(7.5 in)2 = π(7.52 𝑖𝑖𝑖𝑖2 ) = 176.7 𝑖𝑖𝑖𝑖2 Now use the equation for volumetric flow rate: Q=A∗v Q = (1.23 𝑓𝑓𝑓𝑓 2 )(250 fpm) Q = 307.5 cfm 31 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation AIR CHANGES PER HOUR (AC/HR OR ACH) Air changes per hour (AC/hr or ACH) is how many times the air volume of the room is replaced in one hour of time. If we know the room volume (ft3) and the volumetric flow rate (cfm), we can determine our air changes per minute (AC/min) by the equation below. When a room has both a supply and exhaust flow rate, use the greater of the two. Note that AC is a ratio of the volume being moved and the volume of the room (𝑓𝑓𝑓𝑓 3 ⁄𝑓𝑓𝑓𝑓 3 ). 𝑄𝑄 (𝑐𝑐𝑐𝑐𝑐𝑐) 𝑉𝑉 (𝑓𝑓𝑓𝑓 3 ) Once AC/min is calculated, it can be converted to AC/hr by applying a conversion ratio for minutes to hours (1 hr = 60 min). 𝐴𝐴𝐴𝐴/𝑚𝑚𝑚𝑚𝑚𝑚 = 𝐴𝐴𝐴𝐴/ℎ𝑟𝑟 = � 𝐴𝐴𝐴𝐴 60 𝑚𝑚𝑚𝑚𝑚𝑚 �� � 1 ℎ𝑟𝑟 𝑚𝑚𝑚𝑚𝑚𝑚 Example: Calculate the air changes per hour (AC/hr) for a room that has a supply of 500 cfm, an exhaust of 400 cfm, and a room volume of 4000 ft3. First, calculate air changes per minute. Use the greater of the supply and exhaust flow rates (500 cfm > 400 cfm). 𝑄𝑄 (𝑐𝑐𝑐𝑐𝑐𝑐) 𝐴𝐴𝐴𝐴 = 𝑉𝑉 (𝑓𝑓𝑓𝑓 3 ) 𝑚𝑚𝑚𝑚𝑚𝑚 500 𝑐𝑐𝑐𝑐𝑐𝑐 𝐴𝐴𝐴𝐴 𝐴𝐴𝐴𝐴 = = 0.125 3 𝑚𝑚𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑚𝑚 4000 𝑓𝑓𝑓𝑓 Convert to AC/hr: 𝐴𝐴𝐴𝐴 𝐴𝐴𝐴𝐴 𝐴𝐴𝐴𝐴 0.125 𝐴𝐴𝐴𝐴 60 𝑚𝑚𝑚𝑚𝑚𝑚 �� � = 0.125 ∗ 60 =� = 7.5 ℎ𝑟𝑟 ℎ𝑟𝑟 ℎ𝑟𝑟 1 ℎ𝑟𝑟 𝑚𝑚𝑚𝑚𝑚𝑚 REQUIRED VOLUMETRIC FLOW RATE (Q) If we know the volume of a room (V) and the air changes per hour (AC/hr) required by a standard, we can calculate the required volumetric flow rate (Q). This allows us to determine what size fan should be installed in the system. Example: Given a room with walls measuring 20 ft by 25 ft, a height of 9 ft, and a required air change rate of 6 AC/hr, determine the air flow the fan must provide in CFM. First, calculate the room volume: V = l * w* h = 20 ft * 25 ft * 9 ft = 4500 ft3 32 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation Now change the air change rate to AC/hr 6 𝐴𝐴𝐴𝐴 𝐴𝐴𝐴𝐴 6 𝐴𝐴𝐴𝐴 1 ℎ𝑟𝑟 � �= = 0.1 60 𝑚𝑚𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑚𝑚 ℎ𝑟𝑟 60 𝑚𝑚𝑚𝑚𝑚𝑚 Use the equation for AC/min to calculate the required Q. 𝑄𝑄 (𝑐𝑐𝑐𝑐𝑐𝑐) 𝐴𝐴𝐴𝐴 = 𝑉𝑉 (𝑓𝑓𝑓𝑓 3 ) 𝑚𝑚𝑚𝑚𝑚𝑚 0.1 𝑄𝑄 𝐴𝐴𝐴𝐴 = 𝑚𝑚𝑚𝑚𝑚𝑚 4500 𝑓𝑓𝑓𝑓 3 Remember that AC represents 𝑓𝑓𝑓𝑓 3 ⁄𝑓𝑓𝑓𝑓 3 . 𝑄𝑄 = 0.1 𝐴𝐴𝐴𝐴 (4500 𝑓𝑓𝑓𝑓 3 ) = 450 𝑐𝑐𝑐𝑐𝑐𝑐 𝑚𝑚𝑚𝑚𝑚𝑚 SURVEY EQUIPMENT Various types of equipment may be used during a dilution ventilation survey depending on what type of system is being evaluated. SMOKE TUBES Some rooms have a requirement to be under either positive or negative pressure. It can sometimes be difficult to tell what the conditions are because the pressure difference may be very slight. To visualize the direction of air flow, you can use smoke tubes (Figure 11). If the room is under negative pressure, smoke will flow into the room through any opening, such as a space under the door. Figure 11: Smoke Tubes 33 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 5: Dilution Ventilation MAGNEHELIC GAUGE Some systems may have one or more magnehelic gauge permanently installed to check the system’s static pressure. These gauges should not be used to perform the survey, but should be checked during the survey and marked with an acceptable range (+/- 10% of reading) if the system is operating correctly. This allows the worker to verify the system is operating correclty. If the gauge is not on a calibration schedule, the acceptable range may need to be adjusted as it could drift over time. BALOMETER Figure 12: Magnehelic Gauge The standard piece of equipment used for determining air changes is the balometer, shown in Figure 13. A balometer uses a hood to envelop the supply or exhaust opening and funnel the airflow through the instrument. This allows the instrument to measure the air velocity over a fixed cross-sectional area, allowing the instrument to directly measure the volumetric air flow (Q). One advantage of the balometer is that it does not matter what shape the duct opening is, so you do not need to do a complex area calculation. Figure 13: Balometer 34 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 6: Local Exhaust Ventilation BLOCK IV – UNIT 6: LOCAL EXHAUST VENTILATION Objective 6a: Identify types and basic principles of local exhaust ventilation (LEV) systems. Protecting an employee from inhalation exposure is achieved by reducing the amount of contaminant in the worker’s breathing zone. Remember, the breathing zone is the area around the worker’s nose and mouth (within about nine inches). Dilution ventilation does not remove the contaminant from the breathing zone; it only maintains concentrations at a low level. Local exhaust ventilation (LEV) removes the contaminant before it can be released in the work area. LOCAL EXHAUST VENTILATION (LEV) SYSTEMS A local exhaust ventilation (LEV) system is designed to capture and remove emissions prior to their escape into the workplace environment. Typical LEV systems consist of hoods, exhausted enclosures, ductwork, air cleaners, fans, and stacks. LEV is a primary means of controlling employee exposure to gases, vapors, and particles in traditional workplaces. The exhaust systems are termed “local” in the sense that the hood providing suction is located close to the source of contamination, as shown in Figure 14. If properly designed, such an arrangement removes a contaminant directly from its source before it has an opportunity to disperse into the general workplace atmosphere, where it could be inhaled by the worker. Figure 14: Local Exhaust Ventilation ADVANTAGES AND DISADVANTAGES As with most controls, there are some undesirable and limiting aspects of applying local exhaust ventilation (LEV); however, in many cases the advantages of local exhaust systems outweigh the disadvantages. ADVANTAGES Air Volume With rare exception, such as paint booths and other large systems, there is much less air that must be moved with local exhaust than there is when dilution is used. This means make-up air (supply) may not always be necessary, and when it is necessary, less is required. Workers may be bothered less by drafts than they would be if the large volumes of dilution air were used. The reduced requirement for heating or cooling air is also a benefit and has a lot to do with the cost of operating the system. 35 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 6: Local Exhaust Ventilation Cost While initial costs for local exhaust systems is usually high when compared to a dilution system, local exhaust is usually cheaper in the long run due to lower operating costs. Dilution systems require a large amount of energy to condition the air being used to achieve high air change rates. Contaminant Dispersion Another benefit of LEV is that it limits the spread of the contaminant. This can help in protecting equipment from the wear and tear caused by contamination. For example, keeping wood dust out of the moving parts of saws in a carpenter shop can prolong the equipment’s life. Less effort would also be required in conducting housekeeping/cleaning of the area. Worker Exposure Ultimately, the purpose of an LEV system is to minimize worker exposure to the contaminant. Not only is the worker at the process protected, but controlling dispersion protects those not directly involve. DISADVANTAGES Worker Access For LEV to be effective, operations must be done close to the exhaust system to ensure that the contaminant is captured. The closeness of the hood to the process can restrict the worker’s access. It may be difficult to work comfortably and may limit visibility of the work being done. In addition to discomfort and annoyance, this could be a serious safety hazard that requires immediate correction. In some cases, workers may simply think it is too much trouble or annoying to get close enough to the hood. Noise A LEV system may produce hazardous noise levels due to the closeness to the fan, motor, airflow into the hood, and vibrations caused by the system. These can be addressed to an extent during the design of the system, but add to the initial costs. Cost Initial expense is often quite high. Because of this, units sometimes delay the purchase or installation of these system. Elaborate hoods are needed for some processes and expensive air cleaners for others. The cost of having a system designed in the first place can be high. False Security Although a worker’s exposure is meant to be completely controlled, this knowledge may lead to carelessness. A worker may not follow safe practices, may cause splashing or other direct contact with the substance, or may interfere with the proper operation of the system (such as causing disturbing drafts away from the hood). Use of the system may even be discontinued because “nothing ever happened before.” 36 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 6: Local Exhaust Ventilation TYPES OF HOODS The process of local exhaust ventilation (LEV) design begins with decisions made regarding the hood. The hood is the most important part of the entire LEV system. Proper selection and design of the hood can go a long way toward achieving the goal of capturing and removing the contaminant at its source before it has a chance to reach the worker’s breathing zone. In fact, this is precisely what hoods are designed to do. There are many different hoods with different shapes and geometries. The type of hood selected depends on the process and nature of the emissions the hood is expected to control. The typical convention is dividing hoods into two categories: enclosing and exterior hoods. ENCLOSING HOOD An enclosing hood completely or partially encloses the contaminant. Examples include large paint booths, glove boxes, bead blasting units, and laboratory hoods (as seen in Figure 15). An enclosing hood surrounds the contaminant source, thereby isolating the process from the worker and the workspace. When the contaminant is emitted, it is already either totally or at least partially inside the hood. The contaminant is contained inside the enclosure by an inward flow of air through the hood opening(s) and is thereby prevented from escaping into the work area. EXTERIOR HOOD Exterior hoods are located adjacent to an emission source but do not enclose the emission source. Figure 15: Laboratory Hood Exterior hoods come in many shapes, sizes, and varieties. These hoods are designed to provide a certain capture velocity, or air velocity at the point of contaminant generation. Examples of exterior hoods are slots along the edge of the tank or a rectangular opening on a welding table. A canopy is an example of an exterior hood. Figure 16 shows a band saw with a capture hood. The contaminant is directed into the hood by the flow of air. Figure 16: Band Saw with Capture Hood 37 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 6: Local Exhaust Ventilation DESIGN PARAMETERS AND REFERENCES There are many different references that can be used to determine the design criteria of a ventilation system. The first logical place to look for the design criteria of a ventilation system is the Manufacturer’s Guidance. The manufacturer of the ventilation system should publish the minimum parameters that the ventilation system must meet before the system is deemed “operational.” The key parameters to be concerned with are the minimum duct velocity of the system, the capture velocity, and the volumetric flow rate (Q) of vent system. The next reference to use is ACGIH’s Industrial Ventilation, a Manual of Recommended Practice for Design (Figure 17), also known as the Industrial Ventilation Manual. The Industrial Ventilation Manual will have key parameters that a ventilation system must meet before it is put into place. The manual has specific requirements based on the type of contaminant and process being performed. Another useful reference to find key parameters of a ventilation system is in DOEHRS, or your shop may have its own ventilation pre-survey form. Figure 17: Industrial Ventilation: A Manual of Recommended Practice for Design, 30th Edition The ventilation pre-survey form will be discussed in a later lesson, but it is important to note that if a ventilation system has already been evaluated, then there is a high probability that a BE technician has already researched the ventilation system key parameters and annotated those parameters and reference on a pre-survey form. See Figure 18 for an example of a ventilation pre-survey form. This form is primarily a guide for taking notes during a survey. All information from the form should be documented in DOEHRS. 38 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 6: Local Exhaust Ventilation Figure 18: Ventilation Pre-Survey Form 39 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 7: Ventilation Survey Requirements BLOCK IV – UNIT 7: VENTILATION SURVEY REQUIREMENTS Objective 7a: List ventilation survey requirements. Ventilation is a widely used and time-tested approach to exposure control. It is one of the most important engineering control techniques available for improving or maintaining the quality of the air in the occupational work environment. Broadly defined, ventilation is a method of controlling the environment with airflow. Your duties will involve evaluating ventilation systems to ensure they perform as intended. There are four types of ventilation system surveys BEs will need to perform: • Pre-Survey - The pre-survey is used to plan a full-scale ventilation survey (initial/baseline measurements and future routine system checks). • Initial Survey - The initial survey determines initial acceptance standards (after installation of system). After the system is installed, it should not be accepted and put into service until BE verifies it meets the design criteria. • Baseline Survey - The baseline survey is accomplished after the system has been put into service. The purpose of the survey is to ensure that the system is capable of controlling a hazard below the appropriate OEL and establish baseline parameters for future routine performance tests. • Routine Survey - A routine survey is accomplished periodically and measures the same parameters that were measured during the baseline survey. TYPES OF SURVEYS PRE-SURVEY The first survey to complete is the pre-survey. During this survey BEs document specific aspects of the system-- who, what, where, when, why, and how. They look at the process, the material being used in the process, how it is being used, how the system is operating, type of system, and key parameters. Designers should know the minimum duct design velocities or range for a given contaminant. This design criterion is what will be used by the contractor to build the system. During this survey we will define how the system should perform (control the contaminant below the OEL) by documenting the key parameters identified for the system (volumetric flow rate, capture velocity, and duct/transport velocity). Here are some example velocity requirements based on density of aerosol contaminants: • • • • Gases, vapors, smokes: 1000 fpm Fumes: 2000 fpm Grinding dust: 3500 fpm Heavier dusts, chunks: 4500 fpm INITIAL SURVEY The initial test of a ventilation system is usually the most involved, but if done carefully and properly, need only be done once. Ideally, BEs conduct an initial survey before a system is put into operation–-when it is new and the process it is to control has not yet started. This implies that calculations indicated that contaminant levels would be over the OEL or that good industrial hygiene practices dictate the use of a system. Other systems are installed on the basis of air 40 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 7: Ventilation Survey Requirements sampling results. BEs perform the initial survey to evaluate whether or not the key parameters for the system have been met to ± 10 percent of the design criteria. Measurements of the airflow are made in each of the branches or subsystems. To determine if initial survey results are within standards, reference the Industrial Ventilation Manual or the manufacturer’s design criteria. BASELINE SURVEY BEs conduct baseline ventilation surveys to evaluate whether or not a worker is being properly protected from an exposure to a chemical. Baseline surveys are conducted after the system has been put into operation. The ultimate test of a ventilation system’s performance is how well it controls a contaminant. To determine the effectiveness of the system, air sampling is performed at the time of the baseline survey. During the baseline survey, measurements are done to record the airflow at the time of the survey. This is preferably a static pressure (SP) check that reflects the volumetric flow rate of the system. During this survey, BE determines the point at which they will take the SP reading for a given branch, and that SP will represent the airflow volume in that branch. The purpose of air samples are to see if the system is controlling the contaminant below the OEL. The air sampling results let us determine if the measurements were taken while the system was controlling the hazard as required. If the results are below the OEfitL, the measurements are established as the baseline readings. In some cases, a system will not meet the key parameters established by the Industrial Ventilation Manual or manufacturer but will still control hazards below the OEL. If this is the case, the system is still operating adequately and the baseline can be established. ROUTINE SURVEY A routine survey ensures the system is operating within the parameters of the baseline survey and is accomplished periodically. These surveys may be done annually, every 6 months, every 3 months, or at a locally determined frequency. It is normally the practice of BE to accept a routine survey within +/-10 percent of the baseline. If the system is operating within this range, it indicates that exposures are still being controlled. During the routine survey, BE identifies any upgrades in the system or process (speak with the shop workers often during the survey). The shop workers will have knowledge of any changes. If there have been changes, more than likely BE will need to do another initial/baseline survey to identify key parameters and to ensure it is still controlling a hazard below the OEL. BE will also identify the need for maintenance. If the routine survey is not within the baseline criteria, then the system needs maintenance. Maintenance should check the filter condition, fan rotation, fan rpm, belt condition (such as too loose), or clogs in the system. The routine survey: • • • • Should be within 10 percent of baseline results Is only required for systems that are controlling a hazard Measures the same parameter(s) and locations as the baseline survey Compares parameters to baseline measurements 41 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 7: Ventilation Survey Requirements SURVEY EQUIPMENT The selection of ventilation survey equipment should depend on the range of values to be measured, the required accuracy, and the conditions of the measurement. The measurements taken during a routine survey should be the same type performed during the baseline survey. The two main types of measurement we perform are pitot traverse and face velocity. FACE VELOCITY METHOD This survey measures the velocity at the face of the hood. The volumetric flow rate of air can then be obtained by multiplying the air velocity by the area of the hood face. This method can be used with either the baseline or routine survey, but not the initial survey. Thermoanemometer Thermoanemometers are commonly used during a face velocity survey. Figure 19 shows an example of a thermoanemometer. This type of instrument employs the principle that the amount of heat removed by an airstream passing a heated object is related to the velocity of the airstream. The air to be measured flows past a heated wire in the probe, and the instrument senses the cooling effect of the air, which it translates into a velocity reading. A second unheated wire (or component with the same purpose) senses the air temperature to compensate for a wide range of temperature variations. Be careful to ensure that there are no combustible gases present when using thermal anemometers as the heated wire can cause them to ignite. Figure 19: Thermoanemometer PITOT TRAVERSE METHOD The Pitot traverse survey method is used to perform duct velocity or transport velocity surveys as well as static pressure checks. Because the airflow in the cross-section of a duct is not uniform, it is necessary to obtain an average by measuring velocity pressure at points in a number of equal areas across the cross-section. Use the Pitot tube to measure velocity pressure and static pressure within a duct for later calculation of velocity and volume. See Figure 20 for an example measurement diagram. 42 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 7: Ventilation Survey Requirements Figure 20: Pitot Traverse Measurement Points Manometer A manometer is used with a pitot tube (Figure 21) to measure static pressure (SP) and velocity pressure (VP) inside a duct. The pitot tube has a right-angle curve so that the tip can be pointed into the airflow inside the duct. There are holes on the side and tip of the tube to measure both types of pressure. The pitot tube may be attached to an inclined manometer or a digital manometer. An inclined manometer is a manual instrument that uses red gauge oil to show the pressures. Tubing must be attached to the pitot tube and incline manometer in the correct orientation to read the pressures (Figure 22). Figure 21: Pitot Tube A digital manometer uses no oils and detects the pressures directly. As with the inclined manometer, tubing connects the pitot tube to ports on the manometer. Always follow the manufacturer’s guidance on tube orientation. 43 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 7: Ventilation Survey Requirements Figure 22: Inclined Manometer 44 Bioenvironmental Engineering Apprentice Block IV: Chemical Controls B3ABY4B031-0A1B Unit 8: Pitot Traverse Surveys BLOCK IV – UNIT 8: PITOT TRAVERSE SURVEYS Objective 8a: Given references, survey equipment, and a ventilation system, perform a pitot traverse ventilation survey and static pressure check IAW PT-IV-8a. Nothing has been mentioned thus far about comparing the existing duct velocity to the key parameter duct velocity requirement. You already know that you can calculate this after you have measured at the face of an opening and found the existing Q. You can also measure the velocity pressures inside a duct and convert those pressures to velocities to get more accurate readings. This method is called a pitot traverse survey. SURVEY MATERIALS AND EQUIPMENT To perform a survey, we use a manometer (inclined or digital) with a pitot tube. The size of the pitot tube will depend on the size of the duct being surveyed. Additionally, we use the following materials: • Ventilation survey form – This is where we record the measurements • Calculator – A calculator is needed to determine the measurement depths and to convert from velocity pressure (VP) to duct velocity • Duct tape – We typically use duct tape to cover the hole where measruements are taken so that system performance is not impacted • Measuring Tape or Ruler – To determine sampling points, we must measure the duct and distances along the pitot tube. PITOT TRAVERSE SURVEY STEPS To properly conduct a pitot traverse survey, you must accomplihsh the following steps: 1. Determine key parameters using a Ventilation Pre-Survey Form, ACGIH Industrial Ventilation Manual, or manufacturer guidance. Note minimum duct velocity. 45