Written October 17, 2019
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.
Local Exhaust Ventilation Systems
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.
To choose the proper fan for a ventilation system, your HVAC engineer will typically recommend the proper system as determined by the following :
- air volume to be moved
- fan static pressure
- type and concentration of contaminants in the air (because this affects the fan type and materials of construction) and the importance of noise as a limiting factor.
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. To minimize airflow 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 currents.
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 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.
Heavier-than-air vapors tend to settle to the workroom floor and therefore can be collected by a hood located there. A small amount of contaminant in the air (1000 PPM means 1000 parts of contaminant plus 999,000 parts of air) has a resulting density close to that of air, and random air currents will disperse the material throughout the room.
Air moves through ducts between 2000-6000 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.
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.g., precipitators, cyclones, etc.); and
- gases and vapors (e.g., scrubbers).
Stacks disperse exhaust air into the ambient environment. The amount of re-entrainment depends on exhaust volume, wind speed and direction, temperature, location of intakes and exhausts, etc. When installing stacks:
- provide ample stack height (a minimum of 10 ft above adjacent rooflines or air intakes)
- place stack downwind of air intakes
- provide a stack velocity of a minimum of 1.4 times the wind velocity
- place the stack as far from the intake as possible (50 ft is recommended)
- place the stack at least 10 ft high on most roofs to avoid re-circulation; and
- avoid rain caps if the air intake is within 50 ft of the stack.
General Exhaust (Dilution) Ventilation Systems
General exhaust ventilation, also called dilution ventilation, is different from local exhaust ventilation. 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.g., to the PEL —Permissible Exposure Limit — or below). Dilution systems are often used to control evaporated liquids.
Make-up Air Systems
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. Other reasons for designing and providing dedicated make-up air systems are that they: avoid high-velocity drafts through cracks in walls, under doors, and through windows avoid differential pressures on doors, exits, and windows and provide an opportunity to temper the replacement air. If make-up air is not provided, a slight negative pressure will be created in the room and airflow 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. Although the initial costs of air conditioning are high, annual costs may account for only about 1% to 5% of total annual operating expenses. 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:
- outdoor air plenums or ducts
- supply fans and supply air systems
- heating and cooling coils
- humidity control equipment
- supply ducts
- distribution ducts, boxes, plenums, and registers
- return air plenums
- exhaust air provisions
- return fans and
- controls and instrumentation.
Although not generally recommended, re-circulation is an alternative to air exchanging. Where used, re-circulation should incorporate air cleaners, a by-pass or auxiliary exhaust system, regular maintenance and inspection, and devices to monitor system performance.
- Protection of employees must be the primary design consideration.
- The system should remove as much of the contaminant as can economically be separated from exhaust air.
- The system should not be designed simply to achieve PEL levels of exposure.
- The system should never allow re-circulation to significantly increase existing exposures.
- Re-circulation should not be used if a carcinogen is present.
- The system should have fail-safe features, e.g., warning devices on critical parts, back-up systems.
- Cleaning and filtering devices that ensure continuous and reliable collection of the contaminant should be used.
- The system should provide a by-pass or auxiliary exhaust system for use during system failure.
- The system should include feedback devices that monitor system performance, E.g., static pressure taps, particulate counters, amperage monitors.
- The system should be designed not to re-circulate air during equipment malfunction.
- The employer should train employees in the use and operation of the system.
Clean Room and Static Control
Controlling dust is a necessity for all screen printers. Screen printing onto objects such as mirrors, membrane switch components, circuit boards, plastic items especially where a full coverage of ink is printed, airborne particulates are especially troublesome and might require a higher level clean room classification. In areas where static is especially troublesome, “static-dissipative” table top surfaces are useful as well as many other items that dissipate the static potential.
Training for cleanroom facilitating and equipping your plant, is available. At these training facilities you are provided with information on construction, equipment, furniture, supplies, apparel, laundry service & equipment service.
The following list will provide you with questions you will need to know before determining the appropriate classification of cleanroom suitable for your operation.
- What is a cleanroom?
- What are the basic types of cleanrooms?
- What is contamination?
- What are sources of contamination?
- What are the principles of contamination?
- Are there proper gowning techniques?
- What are the do’s/don’ts in cleanrooms?
Modular Cleanrooms: This configuration of a cleanroom can be positioned anywhere within the plant. Many modular units have long vertical clear plastic strips or curtains that allow full visibility and permit easy entrance/exiting of the enclosed area. Full-length acrylic sheeting is also available with rigid access doors.