 This program presents a practical overview of industrial ventilation, covering local exhaust and dilution systems, operation and maintenance, troubleshooting, and OSHA ventilation standards. OSHA requires employers to provide a safe and healthful workplace for their employees. No employee should have to give up good health in order to have a job. But many jobs require employees to work with or around hazardous chemicals. This program shows how industrial ventilation can help protect employees from dangerous over-exposures to harmful chemicals. But first, a few definitions from the glossary found in the handout materials provided with this program. Let's start right at the beginning. Air is important because it can transport contaminants to the worker's breathing zone. But it can also be used to help control exposures to contaminants. Air is a complex mixture of gases and vapors like nitrogen, oxygen, water vapor, carbon dioxide, argon, and many others in trace quantities. Air seems light and airy, but it's actually quite heavy, weighing about 75 pounds per 1,000 cubic feet at sea level. If we move 1,000 cubic feet of air per minute through ductwork, we're moving about 75 pounds of air every minute. This requires choosing fans and motors large enough to handle the load. The speed or velocity called V of air is measured in feet per minute, or FPM. Air volume flow rate called Q is expressed in cubic feet per minute, or CFM. We cannot directly measure Q. It is equal to the product of the average velocity in the cross-sectional area through which the air flows. That is, Q equals V times A. Air is pressurized by gravity, the barometric pressure. At sea level, the barometric pressure is about 29.9 inches of mercury as measured with a barometer. The barometric pressure is also called the absolute static pressure. Air starts moving because there is a difference in pressure between two points. Air moves from an area of higher pressure to an area of lower pressure. The lower pressure in exhaust systems is generated by the fan and is called the static pressure. The static pressure created by the fan is less than barometric pressure on the suction or inlet side of the fan, and greater than barometric pressure on the discharge or outlet side. It acts equally in all directions and can be measured using a water manometer. Static pressure is expressed in inches of water. You might hear the fan generates two inches of negative static pressure, which means that the pressure inside the duct on the suction side of the fan is two inches of water less than the pressure outside the duct. The pressures encountered in ventilation applications are relatively small. The use of conventional units of pressure measurement, such as inches of mercury or pounds per square inch, would result in pressures expressed as very small numbers. It is more convenient to express pressure as the height of water column that the pressure in the duct will support. The PEL is OSHA's permissible exposure limit. The PEL is the highest concentration in air of a chemical substance to which employees may be exposed. The LEL, or lower explosive limit, is a much higher concentration, at which the chemical gas or vapor will burn or explode if an ignition source is present. Methyl alcohol, for example, has an LEL of 6.7% in air. This is equivalent to a concentration of 67,000 parts per million, more than 300 times higher than its PEL of 200 parts per million. IAQ is a popular term standing for indoor air quality. You've probably heard a lot about this in the recent past. When we talk about indoor air quality problems, we're talking about improper temperatures, high or low humidity, odors, oxygen depletion, carbon dioxide buildup, or high concentrations of other air contaminants in the workplace, such as tobacco smoke, biological agents like mold spores, or industrial chemicals. Normally, the building's heating and air conditioning systems are expected to control temperatures, humidities, odors, oxygen depletion, carbon dioxide buildup, mold spores, and tobacco smoke. Industrial ventilation systems are often used to control the buildup of hazardous concentrations of industrial chemicals in air. Where do such airborne chemical contaminants come from? Directly from process equipment, mobile equipment, employee activities, wind and poor housekeeping, and fugitive emissions from industrial process control equipment are some of the sources. As you can imagine, we must identify emission sources before we can effectively control them. Intensive study of the source is often required before controls are installed. The hazards of emission sources can be controlled by many methods. One good way is to substitute a less toxic material, for example, using hot soapy water in place of solvents for cleaning. Another is to provide a change in the process, for example, using paint dipping instead of paint spraying. Yet another approach is to enclose the source or provide a barrier between the source and the employee. Sometimes worker education or worker placement or other administrative control can reduce emissions and exposures. Approved respirators may be used as a control method in some situations, but usually the most reliable approach to reduce employee exposure is through the use of engineering and administrative controls. Ventilation control is one such engineering method. This approach has become known as industrial ventilation. There are two basic types of industrial ventilation, local exhaust and dilution ventilation. Local exhaust systems capture or contain contaminants from the worker's source before they escape into the work environment. Dilution ventilation systems, on the other hand, allow the contaminants to escape into the work environment. The contaminant concentrations are lowered to safe levels by dilution with fresh air before reaching the employee's breathing zone. Where employees are close to the emission source, local exhaust is usually superior to dilution if it is feasible. Both types of ventilation require tempered, heated or cooled make-up air to replace air exhausted through the ventilation system. Think about it. Air removed must always be replaced. It is best to do this through designed and installed make-up air systems. Sometimes the make-up air is provided by the building's heating and cooling system. The local exhaust system consists of five important components, hood, duct, air cleaner, fan, and stack. The hood is the most important part of the system. No local exhaust system will work properly unless enough of the contaminants are retained or captured by the hood so that the concentration of contaminants in the workroom air is below acceptable limits. Duct work or piping transports the contaminants to an air cleaner or to the outdoor environment. Many local exhaust systems must be equipped with air cleaning equipment in order to meet air pollution requirements. The fan has to develop enough static pressure to pull a desired amount of air and contaminants into the hoods and through the ducts to the point of discharge. Finally, a stack is used to disperse the exhaust air stream and whatever contaminants get past any air cleaner installed. Let's look at each of these components in more depth. There are three major types of hoods, those which enclose the emission source, those which capture emissions, and those which merely receive the emissions such as a canopy hood. The most effective hood usually encloses the emission source as much as possible. When using capture and receiving hoods, place the hood as close to the source as possible. Canopy hoods should only be used on hot processes. The canopy cannot be used when workers must lean over the tank or process when contaminants are rising since they will breathe the contaminated air. Let's look at how good industrial ventilation systems are designed and installed. At the beginning of the design process, important questions are answered. At the emission source, one question is, how fast must the air travel to capture or contain the emission source? The answer to this question tells us the hood capture velocity. Similarly, how much air must be moved through the hood to control the emissions? The answer to this question becomes the volume flow rate or Q in CFM required for effective control. Next, how fast must the air travel through the ductwork in order to prevent dust particles from settling out in the duct? This airspeed is called transport velocity. Is an air cleaner required? The answer to this question must be obtained from the local air pollution authority that will issue a permit. How large should the fan be? Fans are chosen to match the requirements of the exhaust ventilation system. The fan must move the correct volume of air against the resistance to air flow caused by friction and turbulence in the system. The designer will estimate the static pressure required. How high should the stack be and where should it be located? The stack must be high enough or far enough away from intakes to avoid re-entrainment of the contaminants into the building air handling system. These questions are usually answered through careful study of the industrial process, worker activities, the air movement in the workplace, and by searching design references for appropriate standards and design information. For example, OSHA has included some design criteria in its ventilation regulations. We'll discuss more about this later. Now, let's look at a few good practices to follow when designing, building, and operating a local exhaust system. Be sure trained personnel are used, such as ventilation engineers or industrial hygienists. Remember to enclose the source as much as possible or bring the hood as close as possible to the source. The hood must be compatible with the worker and work practices. If it interferes with the work, productivity will be lost or the worker may alter the hood, making it less effective as an emission control. This receiving hood over an induction furnace was cut by employees because they could not see inside to check the melt. Adding a flange to a hood will normally increase its effectiveness by reducing airflow from areas behind the hood where no contaminant exists and improving velocity distribution in front of the hood where contaminants are generated. Flanging can reduce airflow requirements by as much as 50%, resulting in lower capital and operating costs for the system. A duct should be of the proper diameter and constructed of appropriate material. For example, when ventilated air contains sulfuric acid, stainless steel duct is often used. Be sure to maintain an adequate transport velocity in the duct or the duct may fill up with dust. An alternative is to provide clean-out openings at intervals throughout the ductwork. This, of course, increases maintenance requirements. Avoid 90-degree T's in the ductwork. Turbulence resulting from changes in air velocity and direction causes pressure losses in the system of which must be overcome by the fan. Branches should enter at gradual expansions at angles of 45 degrees or less to maintain duct velocities while minimizing turbulence. Avoid two branches entering directly opposite each other to reduce turbulence loss. Avoid sharp elbows such as this mitered elbow. The magnitude of pressure losses caused by elbows depends on the radius of the elbow curvature compared to the duct diameter. Where space permits, elbows should have a centerline radius of two to two-and-a-half duct diameters. Replace ductwork when it wears out due to erosion and impact damage from airborne particles. Maintain all ductwork in good working order. To design, remember to check with your local air pollution control agency to obtain a permit. The agency can give you guidance for selecting an air cleaner. The major types of particulate air cleaners include fabric filter devices such as bag houses, electrostatic precipitators, and cyclone separators. Gases and vapors are often removed from the airstream by scrubbers. Remember to provide protection for those handling materials collected from the air cleaner. Most users install monitors on the air cleaner to warn when the air cleaner is not functioning properly. When choosing a fan, be sure to select one which will provide the necessary static pressure and volume flow rate. Remember though, the volume flow rate is always determined by emission control requirements at the hood. The fan is to serve the hood's need, not vice versa. Fans are divided into two main classifications. Centrifugal in which the airflow is at right angles to the axis of rotation of the rotor. And axial in which the airflow is parallel to the axis of rotation of the rotor. Centrifugal fans are mostly used in local exhaust systems. The two important types are the radial blade fan and the backward inclined blade fan. The radial blade fan is the workhorse of the dust control industry because it is self-cleaning, rugged, and simple. Unfortunately, it is inefficient and noisy. The backward inclined blade fan is quieter, more efficient, and less likely to overload the motor at high volume flow rates. Unfortunately, it tends to clog or abrade when the exhaust air is laden with particulate material. The axial fan is used to move large volumes of air against small static pressures. It is often used in dilution ventilation systems and is usually noisier than equivalent centrifugal fans. Stacks should be tall enough to ensure that contaminants are adequately dispersed into the atmosphere. Stack exit velocities should be about 3,000 feet per minute. One rule of thumb is that the top of the stack should be at least 10 feet above the adjacent roof line or air intake if either are within 50 feet of the stack. This will help avoid re-entrainment of exhausted contaminants into the building. Rain protection is often used. This illustration shows a slightly larger diameter duct mounted on the exhaust stack. Rain never falls exactly straight down, so when it hits the inside of the larger duct, it drips down outside the stack. To avoid re-entrainment, don't use the slanted rain cap if the stack is close to the air intake for the building. Let's talk a little about troubleshooting existing systems. A major complaint is a lack of sufficient airflow into the hood. Look for plugged or dented ducts, or crimped or damaged ducts, or slipping fan belts, or the fan turning in the wrong direction, or worn out fan blades, or misadjusted gates and dampers in the system, or a clogged air cleaner. When constant plugging of ducts is the problem, check to transport velocity. Is it adequate to prevent settling? If employees complain about the hood or refuse to use it, check to see if the hood interferes with work, or perhaps the hood just doesn't do the job of providing good emission control. All ventilation systems require maintenance. A good maintenance program consists of, first, a written program. Second, a good record-keeping system to store drawings, plans, specifications, modifications, operating instructions, monitoring and testing results, and so forth. Third, a good program provides for regular testing and inspection of the system. Checklists can help. One is provided in the handout materials included with this program. All good ventilation systems are built to recognized standards. Some standards are mandatory, such as those found in the OSHA regulations and fire and building codes. Others are consensus or suggested standards. The handout materials contain a list of appropriate codes and standards. We have already mentioned OSHA's permissible exposure limits. The PELs limit airborne concentrations of chemicals to which employees may be exposed and are found in 29 CFR1910 subpart Z of the OSHA regulations. OSHA's ventilation standard, 29 CFR1910.94, also provides construction and operating guidelines for ventilation of open surface tanks, spray finishing operations, grinding, polishing and buffing operations, and abrasive blasting. Other OSHA regulations provide ventilation requirements for welding, confined spaces, sawmills, and chemical storage facilities. If you would like more information about OSHA or about how to comply with OSHA ventilation regulations, contact your nearest OSHA area office. This program has presented a practical overview of industrial ventilation. The handout materials provided with this program summarize and expand on the presentation. Remember, ventilation is only one way of protecting workers from exposure to air contaminants. However, it is one of the most important engineering control techniques for improving or maintaining the quality of the air in the occupational work environment.