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The characteristics of the ventilation system that must be documented during an investigation include equipment operability, physical measurements of the system, and use practices. Before taking velocity or pressure measurements, note and record the operating status of the equipment.
For example, are filters loaded or clean? Are variable-flow devices like dampers, variable-frequency drives, or inlet vanes in use? Are make-up units operating? Are system blueprints available? Squeeze off a quick burst of smoke. Time the smoke plume's travel over a two-foot distance. Calculate the velocity in feet per minute. For example, if it takes two seconds for the smoke to travel two feet, the velocity is 60 fpm. Hood static pressures SPH should be measured about duct diameters downstream in a straight section of the hood take-off duct.
The measurement can be made with a pitot tube or by a static pressure tap into the duct sheet metal see Figure III Figure III The approximate relationship of capture velocity V c to duct velocity V d for a simple plain or narrow flanged hood is illustrated in Figure III At two duct diameters from the hood opening, capture velocity decreases by a factor of 10, to 30 fpm.
Figure III shows a rule of thumb that can be used with simple capture hoods. If the duct diameter D is 6 inches, then the maximum distance of the emission source from the hood should not exceed 9 in. Similarly, the minimum capture velocity should not be less than 50 fpm. Use of the six-and-three rule ensures better design by providing for a minimum loss at six diameters of straight duct at the fan inlet and a minimum loss at three diameters of straight duct at the fan outlet Figure II Stack height should be 10 ft higher than any roof line or air intake located within 50 ft of the stack Figure III For example, a stack placed 30 ft away from an air intake should be at least 10 ft higher than the center of the intake.
Drawings are usually drawn to scale. Check dimensions and lengths with a ruler or a scale to be sure that this is the case. Good practices to follow when reviewing plans and specifications are listed in Table III Table III A well-designed system and a continuing preventive maintenance program are key elements in the prevention and control of ventilation system problems. If you suspect microbial agents, check for stagnant water in the ventilation system.
The presence of mold or slime is a possible sign of trouble. Table III lists preventive measures for controlling microbial problems in ventilation systems.
If an organic or reactive chemical e. OSHA has published a proposed rule for IAQ including tobacco smoke in the workplace , and this rulemaking is likely to be completed in the near future.
Smoking policies should include provisions for dedicated smoking areas. Dedicated smoking areas should be configured so that migration of smoke into nonsmoking areas will not occur.
Such areas should:. Industrial Ventilation, a Manual of Recommended Practice. Handbooks and Standards. Burgess, W. Ventilation and Control of the Work Environment. New York: Wiley Interscience. Guidance for Indoor Air Quality Investigations.
Washington, D. Government Printing Office. Before an appropriate ventilation system can be selected, the employer should study emission sources, worker behavior, and air movement in the area. In some cases the employer may wish to seek the services of an experienced professional ventilation engineer to assist in the data gathering.
Table III shows factors to consider when selecting a ventilation system. Combinations of controls are often employed for HVAC purposes. General exhaust ventilation, also called dilution ventilation, is different from local exhaust ventilation because instead of capturing emissions at their source and removing them from the air, general exhaust ventilation allows the contaminant to be emitted into the workplace air and then dilutes the concentration of the contaminant to an acceptable level e.
Dilution systems are often used to control evaporated liquids. To determine the correct volume flow rate for dilution Q d , it is necessary to estimate the evaporation rate of the contaminant q d according to the following equation:. The number of air changes per hour is the number of times one volume of air is replaced in the space per hour. In practice, replacement depends on mixing efficiency.
When using dilution ventilation:. A typical local exhaust ventilation system is composed of five parts: fans, hoods, ducts, air cleaners, and stacks. Local exhaust ventilation is designed to capture an emitted contaminant at or near its source, before the contaminant has a chance to disperse into the workplace air.
Fan Selection. To choose the proper fan for a ventilation system, this information must be known:. Once this information is available, the type of fan best suited for the system can be chosen. Many different fans are available, although they all fall into one of two classes: axial flow fans and centrifugal fans. The hood captures, contains, or receives contaminants generated at an emission source.
The hood converts duct static pressure to velocity pressure and hood entry losses e. A hood's ability to convert static pressure to velocity pressure is given by the coefficient of entry C e , as follows:.
To minimize air-flow requirements, the operation should be enclosed as much as possible, either with a ventilated enclosure, side baffles, or curtains. This helps both to contain the material and to minimize the effect of room air. When using a capture or receiving hood, the hood should be located as close to the contaminant source as possible. Reducing the amount of contaminants generated or released from the process reduces ventilation requirements. The hood should be designed to achieve good air distribution into the hood openings so that all the air drawn into the hood helps to control contaminants.
Avoid designs that require that the velocities through some openings be very high in order to develop the minimum acceptable velocity through other openings or parts of the hood. The purpose of most ventilation systems is to prevent worker inhalation of contaminants.
For this reason, the hood should be located so that contaminants are not drawn through the worker's breathing zone. This is especially important where workers lean over an operation such as an open-surface tank or welding bench. Most hood design recommendations account for cross-drafts that interfere with hood operation.
Standard hood designs may not be adequate to contain highly toxic materials. The hood should be designed to cause minimum interference with the performance of work. Positioning access doors inside an enclosure that must be opened and closed often means that in practice the doors will be left open, and locating capture hoods too close to the process for the worker's convenience often means that the hood will be disassembled and removed.
Hoods should never increase the likelihood of mechanical injury by interfering with a worker's freedom to move around machinery. Air flows turbulently through ducts at between 2,, feet per minute fpm. Ducts can be made of galvanized metal, fiberglass, plastic, and concrete. Friction losses vary according to ductwork type, length of duct, velocity of air, duct area, density of air, and duct diameter.
Air Cleaners. The design of the air cleaner depends on the degree of cleaning required. Regular maintenance of air cleaners increases their efficiency and minimizes worker exposure.
Different types of air cleaners are made to remove particulates e. Stacks disperse exhaust air into the ambient environment. The amount of reentrainment depends on exhaust volume, wind speed and direction, temperature, location of intakes and exhausts, etc.
When installing stacks:. Exhaust ventilation systems require the replacement of exhausted air. Replacement air is often called make-up air.
Replacement air can be supplied naturally by atmospheric pressure through open doors, windows, wall louvers, and adjacent spaces acceptable , as well as through cracks in walls and windows, beneath doors, and through roof vents unacceptable. Make-up air can also be provided through dedicated replacement air systems.
Generally, exhaust systems are interlocked with a dedicated make-up air system. If make-up air is not provided, a slight negative pressure will be created in the room and air flow through the exhaust system will be reduced.
HVAC heating, ventilating, and air-conditioning is a common term that can also include cooling, humidifying or dehumidifying, or otherwise conditioning air for comfort and health. HVAC also is used for odor control and the maintenance of acceptable concentrations of carbon dioxide. Air-conditioning has come to include any process that modifies the air for a work or living space: heating or cooling, humidity control, and air cleaning. Historically, air-conditioning has been used in industry to improve or protect machinery, products, and processes.
The conditioning of air for humans has become normal and expected. Improved human productivity, lower absenteeism, better health, and reduced housekeeping and maintenance almost always make air-conditioning cost effective. Mechanical air-handling systems can range from simple to complex. All distribute air in a manner designed to meet ventilation, temperature, humidity, and air-quality requirements established by the user.
Individual units may be installed in the space they serve, or central units can serve multiple areas. HVAC engineers refer to the areas served by an air handling system as zones. The smaller the zone, the greater the likelihood that good control will be achieved; however, equipment and maintenance costs are directly related to the number of zones. Some systems are designed to provide individual control of rooms in a multiple-zone system.
Both the provision and distribution of make-up air are important to the proper functioning of the system. The correct amount of air should be supplied to the space.
Supply registers should be positioned to avoid disruption of emission and exposure controls and to aid dilution efforts. Considerations in designing an air-handling system include volume flow rate, temperature, humidity, and air quality. Equipment selected must be properly sized and may include:. Although not generally recommended, recirculation is an alternative to air exchanging. Where used, recirculation should incorporate air cleaners, a by-pass or auxiliary exhaust system, regular maintenance and inspection, and devices to monitor system performance.
Key points to consider in the use of recirculation are shown in Table III Health-Related Ventilation Standards. This list includes some, but not necessarily all, OSHA standards that address the control of employee exposure to recognized contaminants. This list includes some, but not necessarily all, OSHA standards that do not contain airflow requirements but are located in the health-related ventilation standards.
Fire and Explosion-Related Ventilation Standards. This list includes some, but not necessarily all, OSHA standards that are intended to prevent fire and explosions.
Special Conditions Standards. Copies of the consensus standards are published and available directly from the organization issuing the standard. A minimal fee is often required. Most of the following checks can be made by visual observation and do not require extensive measurements. Toggle navigation Emergency Preparedness and Response Home. Troubleshooting an Exhaust System--Some Helpful Hints For problems with accessibility in using figures and illustrations in this document, please contact the Office of Science and Technology Assessment at NOTE: This table is best viewed on tablets, notebooks, and desktop computer screens.
Common Ventilation Conditions and Causes Condition Possible cause s Worker complaints, improper use of system, nonuse of system, alteration of system by employees. The hood interferes with work The hood provides poor control of contaminants.
Excessive employee exposures although flow volumes and capture velocities are at design levels. Employee work practices need improvement. The ventilation system interferes with work or worker productivity and leads workers to bypass the system.
Employee training is not adequate. Design of system is poor. Constant plugging of duct. Plugged ducts occur when transport velocity is inadequate or when vapor condenses in the duct, wets particles, and causes a build-up of materials. These problems are caused by poor design, open access doors close to the fan, fan problems, or other problems.
Reduced capture velocities or excessive fugitive emissions. The cause of these conditions is usually reduced flow rate, unless the process itself has changed. Reduced flow rate occurs in the following situations: plugged or dented ducts slipping fan belts open access doors holes in ducts, elbows closed blast gate to branch, or opened blast gates to other branches, or corroded and stuck blast gates fan turning in reverse direction This can occur when lead wires are reversed and cause the motor and fan to turn backwards.
Emission source Where are all emission sources or potential emission sources located? Which emission sources actually contribute to exposure? What is the relative contribution of each source to exposure?
Characterization of each contributor: chemical composition temperature rate of emission direction of emission initial emission velocity pattern of emission continuous or intermittent time intervals of emission mass of emitted material Air behavior Air temperature Air movement direction, velocity Mixing potential Supply and return flow conditions, to include pressure differences between space and surrounding areas Sources of tempered and untempered make-up air Air changes per hour Influence of existing HVAC systems Effects of wind speed and direction Effects of weather and season Employee Worker interaction with emission source Worker exposure levels Worker location Worker education, training, cooperation.
Understand the scope of the project. What is to be included and why? Look for conciseness and precision. Mark ambiguous phrases, "legalese," and repetition. Do the specifications spell out exactly what is wanted?
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