6 - Make-Up Air

Abstract

Make-up air is introduced into the work area from another area, or from the outside, to replace air that is removed by exhaust systems. Unless air is introduced to replace the air lost by exhaust systems and the process-generated air flow, the work area will develop a negative pressure. This pressure will cause problems in the operation of the ventilation system. Down drafts in stacks and flues will result from the air flowing from the higher pressure outside to the lower pressure in the work area. There will be a reduced airflow in the local and dilution exhaust systems because of the increased static pressure, and the fan would have to operate against that. Negative pressures within the work area will cause air to enter through cracks in the doors and windows of the building. Insufficient make-up air can cause potential hazards in the workplace. The improper operation of local exhaust and dilution systems because of inadequate make-up air can cause a hazardous buildup of contaminants. Cross drafts can cause contaminants to be carried away from hoods into the surrounding work environment. Carbon monoxide from back drafts on heaters can also accumulate. The negative pressure present in the area would cause outward-opening doors to be difficult to open and would result in these doors slamming shut quickly. This can present a potential safety hazard to the workers.