General ventilation supplies and/or exhausts air to provide heat relief, dilute contaminants to an acceptable level, and replace exhaust air. Ventilation can be provided by natural or mechanical supply and/or exhaust systems. Industrial areas must comply with ASHRAE Standard 62.1-2013 and other standards as required (e.g., by NFPA). Outdoor air is unacceptable for ventilation if it is known to contain any contaminant at a concentration above that given in ASHRAE Standard 62.1. If air is thought to contain any contaminant not listed in the standard, consult relevant federal, state, provincial, or local jurisdictions for acceptable exposure levels. In addition to their role in controlling industrial contaminants, general ventilation rates must be sufficient to dilute the carbon dioxide produced by occupants to a level acceptable under ASHRAE Standard 62.2.

For complex industrial ventilation problems, experimental scale models and computational fluid dynamics (CFD) models are often used in addition to field testing.

When large volumes of air are exhausted to provide acceptable comfort and safety for personnel and acceptable conditions for process operations, this air must be replaced, either through intentional design strategy or through paths of least resistance. A safe and effective ventilation design should be strategic about the mechanism, locations, and physical parameters by which makeup air enters the occupied space. Makeup air, consistently provided by good air distribution, allows more effective cooling in the summer and more efficient and effective heating in the winter. When makeup air design is not incorporated into the ventilation design scheme, it may lead to inefficient operation of local exhaust systems and/or combustion equipment and cross-drafts that affect occupant comfort and environmental control settings. Relying on windows or other air inlets that cannot function in year-round weather conditions is discouraged. Some factors to consider in designs for makeup air include

For more information on potential adverse conditions caused by specific negative pressure levels in buildings, see ACGIH (2013) and Chapter 28 in the 2016 ASHRAE Handbook—HVAC Systems and Equipment.

Sufficient air must be supplied to replace air exhausted by process ventilation and local exhausts, dilute contaminants (gases, vapors, or airborne particles) not captured by local exhausts, prevent the entry of contaminants or hazardous (ignitable) substances from any surrounding atmosphere during ingress or egress, and provide the required thermal environment. The amount of supplied air should be the largest of the amounts needed for temperature control, dilution, and replacement.

Air supply to industrial spaces can be by natural or mechanical ventilation systems. Although natural ventilation systems driven by gravity forces and/or wind effect are still widely used in industrial spaces (especially in hot premises in cold and moderate climates), they are inefficient in large buildings, may cause drafts, and may not sufficiently solve air contamination problems, because the prerequisite environmental conditions may not be available during all required periods of need or there is no practical filtration method available. Thus, most ventilation systems in industrial spaces are either mechanical (fan-driven) or a combination of mechanical supply with natural exhaust, using louvers or doors for air pressure relief (or for air replacement in exhaust systems).

The most common methods of air supply to industrial spaces are mixing, displacement, and localized.

Mixing Air Distribution. In mixing systems, air is normally supplied at velocities much greater than those acceptable in the occupied zone. Supply air temperature can be above, below, or equal to the air temperature in the occupied zone, depending on the heating/cooling load. The supply air diffuser jet mixes with room air by entrainment, which reduces air velocities and equalizes the air temperature. The occupied zone is ventilated either directly by the air jet or by reverse flow created by the jet. Properly selected and designed mixing air distribution creates relatively uniform air velocity, temperature, humidity, and air quality conditions in the occupied zone and over the room height. Note that supply systems should introduce air into the workspace in such a way as to not interfere with contaminant control systems such as ventilation hoods. If possible, ventilation hoods (e.g., fume hoods) should have quiescent air conditions (~100 fpm) at their face.

Displacement Ventilation Systems. Conditioned air that is slightly cooler than the desired room air temperature in the occupied zone is supplied from air outlets at low air velocities (~100 fpm or less). Because of buoyancy, the cooler air spreads along the floor and floods the room’s lower zone. Air close to the heat source is heated and rises upward as a convective air stream; in the upper zone, this stream spreads along the ceiling. The height of the lower zone depends on the air volume and temperature supplied to the occupied zone and on the amount of convective heat discharged by the sources.

Typically, outlets are located at or near the floor, and supply air is introduced directly into the occupied zone. In some applications (e.g., in computer rooms or hot industrial buildings), air may be supplied to the occupied zone through a raised floor. Exhaust or air returns are located at or close to the ceiling or roof.

Displacement ventilation is common in European countries. It is an option when contaminants are released in combination with surplus heat, and contaminated air is warmer (more buoyant) than the surrounding air. It is not a good choice when air turbulence can interfere with convective conveyance of heat and contaminants. Further information on displacement air distribution systems can be found in Chen and Glicksman (2003) and Goodfellow and Tahti (2001).

Localized Ventilation. Air is supplied locally for occupied regions or a few permanent work areas (Figure 1). Conditioned air is supplied toward the occupants’ breathing zone to create comfortable conditions and/or to reduce the concentration of pollutants. These zones may have air that is cleaner than the surrounding air, and should generally be compliant with ASHRAE Standard 62.1. In localized ventilation systems, air is supplied through one of the following devices:

Figure 1. Localized Ventilation Systems

In hot workplaces that have few work areas, it is likely impractical to maintain a comfortable environment in the entire space. However, environmentally controlled cabins, individual cooling, and spot cooling and extraction can improve working conditions in occupied areas. Certain applications require minimum distances from supplied air (and natural ventilation points) to ensure airflow in ventilated hoods and cabinets is not affected.

Environmentally controlled cabins (e.g., operating cabs, pulpits, control rooms, enclosures) can provide thermal comfort, and, when pressurized with a dedicated clean-air supply (either dedicated source or through effective filtration), can improve air quality in occupational environments. There usually are significant economic benefits to properly designing, installing, and maintaining worker-protective environmental enclosures.

Spot cooling, probably the most popular method of improving the thermal environment, can be provided by radiation (changing mean radiant temperature), convection (changing air velocity and/or air supply temperatures), or both. Spot-cooling equipment is fixed at the workstation, whereas in individual cooling, the worker wears the equipment.

Local exhaust ventilation (spot extraction) is another method to remove excess heat from a process or source of high temperature, and should be the first step considered for energy saving over spot cooling.

Ventilation of locker rooms, toilets, and shower spaces is important in industrial facilities to remove odor and reduce humidity. In some industries, control of workroom contamination requires prevention of both ingestion and inhalation routes of exposure, so adequate hygienic facilities, including appropriate ventilation, may be required in locker rooms, changing rooms, showers, lunchrooms, and break rooms. State, provincial, and local regulations should be consulted early in design.

Supply air may be introduced through doors or wall grilles. In some cases, workplace air may be so contaminated that filtration or a dedicated source of clean supply air is required. When control of workroom contaminants is inadequate or not feasible, reduce employee exposure by positively pressurizing locker rooms, lunchrooms, and break rooms to minimize the level of contamination in those areas. Treated supply air may be ineffective in some applications because it can introduce drafts and outdoor contamination or result in excessive condensation in a humid indoor environment.

When mechanical ventilation of supply air is used, the supply system should have adequate ducting and air distribution devices, such as diffusers or grilles, to distribute air throughout the area.

In locker rooms, take exhaust primarily from the toilet and shower spaces as needed and secondarily from the lockers. Exhausting from the room’s open ceiling areas should be a last option. ASHRAE Standard 62.1 and many local mechanical codes provide requirements for these areas.

Roof ventilators are heat escape ports located high in a building and should be properly enclosed for weathertightness (Goodfellow 1985). Stack effect and some wind induction are the motive forces for gravity- (buoyancy-) driven operation of continuous and round ventilators. Round ventilators can be equipped with a fan barrel and motor, allowing gravity or forced ventilation operation.

Many ventilator designs are available, including the low ventilator, which consists of a stack fan with a rain hood, and a ventilator with a split butterfly closure that floats open to discharge air and closes by a counterweight. Both use minimum enclosures and have little or no gravity capacity. Split butterfly dampers tend to increase fan airflow noise and are subject to damage from slamming during strong winds. Because noise is frequently a problem in powered roof ventilators, the manufacturer’s sound rating should be reviewed. Sound attenuators should be installed where required to meet the design sound ratings.

Continuous ventilation monitors remove substantial, concentrated heat loads most effectively. One type, the streamlined continuous ventilator, is efficient, weathertight, and designed to prevent backdraft; it usually has dampers that may be closed in winter to conserve building heat. Its capacity is limited only by the available roof area and the proper location and sizing of low-level air inlets. Continuous ventilation to achieve a slight pressure above the surrounding atmosphere (referred to as pressurization by NFPA) also can be used to reduce or declassify the electrical classification of enclosed spaces. Typically, reductions from class I, zone 1 or division 1 to class I, zone 2 or division 2 can be achieved by following the recommendations of NFPA Standard 496. This allows using general-purpose electrical devices instead of zone 2 or division 2 devices, or using zone 2 or division 2 electrical devices instead of zone 1 or division 1 devices, which (1) greatly reduces the cost of electrical equipment and (2) provides a sound alternative when particular devices are not available for the higher (more volatile) electrical area classifications.

Gravity ventilators, also highly effective, have low operating costs, do not generate noise, and are self-regulating (i.e., higher heat release increases airflow through the ventilators). Gravity ventilators can be affected by environmental conditions and thus should only be used for heat control rather than for the control of gaseous or aerosol contaminants. Care must be taken to ensure positive pressure at the ventilators, particularly during the heating season. Otherwise, outside air will enter the ventilators.

Next in order of heat removal capacity are (1) round gravity or wind-band ventilators, (2) round gravity ventilators with fan and motor added, (3) low-hood powered ventilators, and (4) vertical upblast powered ventilators. The shroud for the vertical upblast design has a peripheral baffle to deflect air upward instead of downward. Vertical discharge is highly desirable to reduce roof damage caused by hot air if it contains condensable oil or solvent vapor. Ventilators with direct-connected motors are desirable to avoid belt maintenance. Round gravity ventilators are applicable for warehouses with light heat loads and for manufacturing areas with high roofs and light loads.

Streamlined continuous ventilators must operate effectively without mechanical power. To ensure ventilator performance, sufficient low-level openings must be provided for incoming air; insufficient inlet area and significant space air currents are the most common reasons gravity roof ventilators malfunction. A positive supply of air around hot equipment may be necessary in large buildings where external wall inlets are remote from the equipment. Chapter 16 of the 2017 ASHRAE Handbook—Fundamentals has additional information on ventilation and infiltration.

The cost of electrical power for mechanical ventilation over that of roof ventilators can be offset by the advantage of constant airflow. Mechanical ventilation can also create the pressure differential necessary for good airflow, even with small inlets. Inlets should be sized correctly to avoid infiltration and other problems caused by high negative pressure in the building. Often, a mechanical system is justified to supply enough makeup air to maintain the work area under positive pressure.

Roof ventilators can comprise either mechanically operated openings or fan-powered mechanical exhaust. Operator-assisted openings or dampers are usually used in shops with high ceilings, and must be installed when natural ventilation is used to provide air to the space.

Many industrial processes release large amounts of heat and moisture to the environment. In such environments, it may not be economically feasible to maintain comfort conditions (ASHRAE Standard 55), particularly during summer. Comfortable conditions are not physiologically necessary: the body must be in thermal balance with the environment, but this can occur at temperature and humidity conditions well above the normal comfort zone. In areas where heat and moisture generated by a process are low to moderate, comfort conditions may not have to be provided if personnel exposures are infrequent and brief. In such cases, ventilation may be the only control necessary to prevent excessive physiological heat stress.

The engineer must distinguish between control needs for hot/dry industrial areas and warm/moist conditions. In hot/dry areas, a process gives off only sensible (primarily convective and radiant) heat without adding moisture to the air. This increases the heat load on exposed workers, but the rate of cooling by evaporation of perspiration may not be significantly reduced. Body heat equilibrium may be maintained, but could cause excessive perspiration. Hot/dry work situations occur around furnaces, forges, metal-extruding and rolling mills, glass-forming machines, etc.

In warm/moist conditions, a wet process may generate a significant latent heat load. The rise in sensible heat load on workers may be insignificant, but the increased moisture content of the air can seriously reduce cooling by evaporation of perspiration, making warm/moist conditions potentially more hazardous than hot/dry. Typical warm/moist operations are found in textile mills, laundries, dye houses, and deep mines, where water is used extensively for dust control.

Industrial heat load is also affected by local climate. Solar heat gain and elevated outdoor temperatures increase the heat load at the workplace, but may be insignificant compared to process heat generated locally. The moisture content of outdoor air is an important factor that can affect hot/dry work situations by restricting an individual’s evaporative cooling. For warm/moist working environments, solar heat gain and elevated outdoor temperatures are even more important because moisture contributed by outdoor air is insignificant compared to that released by the process.

Both ASHRAE Standard 55 and International Organization for Standardization (ISO) Standard 7730 specify thermal comfort conditions.

Methods for evaluating the general thermal state of the body both in comfort conditions and under heat and cold stress are based on analysis of the heat balance for the human body, as discussed in Chapter 9 of the 2017 ASHRAE Handbook—Fundamentals. A person may find the thermal environment unacceptable or intolerable because of local effects on the body caused by asymmetric radiation, air velocity, vertical air temperature differences, or contact with hot or cold surfaces (floors, machinery, tools, etc.).

Another heat stress indicator for evaluating an environment’s heat stress potential is the wet-bulb globe temperature (WBGT), defined as follows:

(1)

(2)

where

Coefficients in Equations (1) and (2) represent the fractional contributions of the component temperatures.

Exposure limits for heat stress for different levels of physical activity are shown in Figure 2 (NIOSH 1986), which depicts the allowable work regime (in terms of rest periods and work periods each hour) for different levels of work over a range of WBGT. When applying Figure 2, assume that the rest area has the same WBGT as the work area. The curves are valid for workers acclimatized to heat. ASHRAE Standard 62.1 provides some metabolic rates for different activities that can be used with Figure 2. Refer to NIOSH (1986) for recommended WBGT limits for nonacclimatized workers.

Figure 2. Recommended Heat Stress Exposure Limits for Heat-Acclimatized Workers [Adapted from NIOSH (1986)]

The WBGT index is an international standard (ISO Standards 7243, 7730, and 7933) for evaluating hot environments. The WBGT index and activity levels should be evaluated on 1 h mean values; that is, WBGT and activity are measured and estimated as time-weighted averages on a 1 h basis for continuous work, or on a 2 h basis when exposure is intermittent. Although recommended by NIOSH, the WBGT has not been accepted as the sole legal standard by the Occupational Safety and Health Administration (OSHA), and it may not apply for non-U.S. jurisdictions. The WBGT is generally used in conjunction with other methods to determine heat stress.

Although Figure 2 is useful for evaluating heat stress exposure limits, it is of limited use for control purposes or for evaluation of comfort. Air velocity and psychrometric wet-bulb measurements are usually needed to specify proper controls, and are only measured indirectly in WBGT determinations. Information on other useful tools, including the heat stress index (HSI), can be found in Chapter 9 of the 2017 ASHRAE Handbook—Fundamentals and in ISO Standards 7243, 7730, and 7933.

The thermal relationship between humans and their environment is determined by four independent environmental variables:

Together with the rate of internal heat production (metabolic rate) and clothing variables, these factors may combine in various ways to create different degrees of heat stress. The HSI is defined as the percent of the skin that is wetted by perspiration:

(3)

where

and incorporates relative contributions of metabolism, radiant heat gain (or loss), convective heat gain (or loss), and evaporative (perspiration) heat gain (or loss). For supplemental information on evaluation and control of heat stress using methods such as reduction of radiation, changes in work/rest pattern, spot cooling, and cooling vests and suits, refer to ACGIH (2013), Brief et al. (1983), Caplan (1980), NIOSH (1986), and Ontario MOL (2009).

Control at Source. Heat exposure can be reduced by providing sufficient insulation to hot equipment or locating it outdoors or in zones with general or local exhaust ventilation, covering steaming water tanks, providing covered drains for direct removal of hot water, and maintaining tight joints and valves where steam may escape.

Local Exhaust Ventilation. Local exhaust ventilation removes heated air generated by a hot process and/or gases emitted by process equipment, while removing a minimum of air from the surrounding space. Local exhaust systems, including heat exposure control using overhead canopy hoods, are discussed in detail in Chapter 32 and McKernan et al. (2014).

Radiation Shielding. In some industries, the major environmental heat load is radiant heat from hot objects and surfaces, such as furnaces, ovens, furnace flues and stacks, boilers, molten metal, hot ingots, castings, and forgings. Because air temperature has no significant effect on radiant heat flow, ventilation is of little help in controlling such exposure. The only effective control is to reduce the amount of radiant heat impinging on the workers by insulating or placing radiation shields around the source.

Radiation shields are effective in the following forms:

If the shield is a good reflector, it remains relatively cool in severe radiant heat. Bright or highly polished tinplate, stainless steel, and ordinary flat or corrugated aluminum sheets are efficient and durable. Foil-faced plasterboard, although less durable, reflects well on one side. To be efficient, however, the reflective shield must remain bright. Radiation shields are much more efficient when used in multiple layers; they should reflect the radiant heat back to the primary source, where the resulting hot gases can be removed by local exhaust. However, unless the shield completely surrounds the primary source, some infrared energy is reflected into the cooler surroundings and possibly into an occupied area. The direction of reflected heat should be studied to ensure proper shielding installation.

Spot Cooling. If the workplace is located near a source of radiant heat that cannot be entirely controlled by radiation shielding, spot cooling can be used. See Chapter 20 in the 2017 ASHRAE Handbook—Fundamentals and data from spot-cooling diffuser manufacturers for further information.