Synergist - March 2012

Strictly speaking, extraneous airflow is not make-up air but includes airflow produced by cooling fans, windows and doors open to the outside or airflow from movement of persons, equipment or opening of doors. The air speeds produced by these extraneous sources are often higher than the hood capture velocity, which may result in less effective capture and increased exposure. The speed of air eddies from a brisk walker can reach more than 250 feet per minute. Employees in the area play an important role in managing these extraneous air sources.

In a few situations where the outside air is at an acceptable temperature and humidity and openings can be provided for flow of air into, through and out of the area—such as a large dilution system—disturbance of the LEV is not an issue if there is effective dilution. It may even be possible to turn off the LEV.

Recommended Reading

Journal of Occupational and Environmental Hygiene: “Flow Characteristics and Spillage Mechanisms of Wall-Mounted and Jet-Isolated Range Hoods Subject to Influence from Cross Draft” (January 2012).

Hemeon’s Plant and Process Ventilation, 3rd Edition, Chapters 9 and 10 (1999).

monoxide) that rely on natural drafting for exhaust; reduced effectiveness of general exhaust ventilation equipment operating with low static pressure fans; difficulty opening and closing doors with a potential for slamming of doors; and impairment of LEV systems at the higher end of negative building pressures.1

when insufficient make-up air to the enclosure causes the enclosure to be strongly negative pressured. In some configurations, the coated web leaves the enclosure through a slot opening that is not connected directly to a ventilated enclosure (e.g., a curing oven). The negative pressure of the enclosure causes air to rush in through the web slot exit, stripping vapor from the coated web and bringing it back into the enclosure, resulting in elevated vapor exposure levels. Adjusting the exhaust and make-up air to make the enclosure just slightly negative will minimize this concern.

Management of Change It is common to add LEV capacity to existing spaces that are served by an HVAC unit. Where and how the make-up air will be supplied is an important consideration, whether the LEV is a new design or a modification of an existing system. Additional HVAC capacity may be needed. Any potential impact on performance of existing LEV should be evaluated using a management of change (MOC) procedure. Examples include installation or modifications of spot cooling or heating, seasonal adjustments to HVAC or other supply systems, changes to ventilation enclosures, changes to employee or process activities near a hood and changes to the building’s internal structure.

Employee Training

Employees play an important role in the effective day-to-day operation of LEV systems. It may be obvious to employees working at a hood that doing anything to impede the flow of air will reduce hood performance and result in higher exposure. The influence that employees have on make-up and extraneous air may be less obvious. The workers whose exposure is controlled by LEV (the operators) need to be aware of the impact of make-up air on the functioning of the LEV and the impacts of air blowing onto both ventilated and unventilated sources, such as solvent coating on films and webs.

Spread the Word

Make-up air is needed for the effective functioning of LEV and to establish differential pressures between rooms and other spaces. It is also apparent that airflow from make-up air and extraneous sources can render ineffective an otherwise effective hood. All LEV installations need to be evaluated for potential impacts of make-up and extraneous airflow in the design phase, during changes as part of MOC and as part of an effective ventilation maintenance program. Hood users and those who benefit from hood use must understand the potential adverse effects of airflow on the hood; ultimately, this knowledge will help minimize make-up air problems.

Safety Issues

In situations where there is insufficient make-up air in a building containing LEV, negative building pressure can have serious consequences. Some of the adverse effects, in order of increasingly negative pressure, include simple discomfort of workers associated with drafts near windows and doors at lower negative pressures; reduced effectiveness of stack ventilation relying on natural drafting; back drafting of combustion emissions (with potential exposure to carbon

Examples

A problem often encountered near workstations involving high-speed application of solvent coating onto wide surfaces is the loss of vapor from coating. This is due to turbulence and impact of air or cleaving of the vapor from the space near the coated surface by a fixed object in the web path. For example, consider a coater workstation enclosed by flexible curtains on all sides but open on the top, with an air-conditioning diffuser blowing into the general coating area for operator comfort. This creates turbulence at the coating source and on the coated web, resulting in unacceptable exposures. Re-direction of the diffuser and an upgrade of the LEV at the source can reduce exposures to acceptable levels.

Another issue experienced with enclosed solvent coating workstations is

Timothy M. Johnson, CIH, CSP, is an Industrial Hygiene Specialist with 3M Safety and Industrial Hygiene in St. Paul, Minn. He can be reached at (651) 737-3497 or tmjohnson@mmm.com.