In my line of work, I come across a lot of areas that have a “ventilation system” that intends to reduce exposure and control flammable gases/vapors. The vast majority of these systems are “sized” properly, meaning they move enough air to meet the code/standard they were built to; however, one aspect of the system we find is that over time, changes have been made that no bring into question the “design” of the system. In this article, I will take us on a ride through the International Mechanical Code (IMC) sections that define how our “make-up air” must be designed. I am using the IMC as that is the most common design used in the USA.
First, let’s look at Section 1105, which applies to Ammonia Engine/Machinery Rooms. Specifically, let’s look at 1105.6.2 Makeup Air…
(emphasis by me)
1105.6.2 Makeup air
Provisions shall be made for makeup air to replace that being exhausted. Openings for makeup air shall be located to avoid intake of exhaust air. Supply and exhaust ducts to the machinery room shall not serve any other area, shall be constructed in accordance with Chapter 5 and shall be covered with corrosion-resistant screen of not less than 1/4-inch (6.4 mm) mesh.
So from here, we go to Chapter 5, specifically Section 510.6.5…
510.6.5 Makeup air
Makeup air shall be provided at a rate approximately equal to the rate that air is exhausted by the hazardous exhaust system. Makeup air intakes shall be located in accordance with Section 401.4.
Now we go to Chapter 4, specifically Section 401.4…
401.4 Intake opening location
Air intake openings shall comply with all of the following:
- Intake openings shall be located not less than 10 feet (3048 mm) from lot lines or buildings on the same lot.
- Mechanical and gravity outdoor air intake openings shall be located not less than 10 feet (3048 mm) horizontally from any hazardous or noxious contaminant source, such as vents, streets, alleys, parking lots and loading docks, except as specified in Item 3 or Section 501.3.1. Outdoor air intake openings shall be permitted to be located less than 10 feet (3048 mm) horizontally from streets, alleys, parking lots and loading docks provided that the openings are located not less than 25 feet (7620 mm) vertically above such locations. Where openings front on a street or public way, the distance shall be measured from the closest edge of the street or public way.
- Intake openings shall be located not less than 3 feet (914 mm) BELOW contaminant sources where such sources are located within 10 feet (3048 mm) of the opening.
- Intake openings on structures in flood hazard areas shall be at or above the elevation required by Section 1612 of the International Building Code for utilities and attendant equipment.
This section addresses intake openings and Section 501.3.1 addresses exhaust openings. These two sections must be applied in harmony because they both can affect the separation between intakes and exhaust openings. To prevent the introduction of contaminants into the ventilation air of a building:
- Item 1 requires a MINIMUM SEPARATION OF 10′ BETWEEN OUTDOOR AIR INTAKE OPENINGS AND ANY LOT LINES OR BUILDINGS on the same lot
- Item 2 addresses powered intakes, (mechanical) and gravity (nonmechanical) intakes and REQUIRE SEPARATION FROM POTENTIALLY HARMFUL CONTAMINANT SOURCES INCLUDING CHIMNEYS AND VENTS, PLUMBING VENTS AND AREAS WHERE MOTOR VEHICLES OPERATE. The 10′ distance would be measured from the closest edge of a street, alley, parking lot and loading dock.
- Item 3 addresses those cases where the required 10′ separation CANNOT be met. If the 10′ horizontal separation required in Item 2 cannot be achieved, the intake could be located AT LEAST 3′ below the contaminant source. The code assumes that the contaminants likely to be present are buoyant in air because of their temperature or specific gravity and they will rise above and away from the intake opening.
See Sections 501.3 and 501.3.1 and consider the fact that an existing intake opening can be impacted by the installation of a new exhaust termination, as well as the fact that an existing exhaust termination can affect the location of a newly installed intake opening.
401.5 Intake opening protection
Air intake openings that terminate outdoors shall be protected with corrosion-resistant screens, louvers or grilles. Openings in louvers, grilles and screens shall be sized in accordance with Table 401.5, and shall be protected against local weather conditions. Louvers that protect air intake openings in structures located in hurricane-prone regions, as defined in the International Building Code, shall comply with AMCA 550. Outdoor air intake openings located in exterior walls shall meet the provisions for exterior wall opening protectives in accordance with the International Building Code.
This section aims to protect intake openings, other than windows and doors, from penetration by precipitation, small animals, vermin, and debris. The opening sizes for nonresidential intake opening protection are larger than 1/4 inch (6.35 mm) up to a maximum of 1 inch, as dictated by Table 401.5. The opening size in louvers, grilles, and screens must be large enough to inhibit blockage by debris and to prevent significant resistance to airflow, and yet must be small enough to keep out what is intended to be kept out. A screen of the stated mesh sizes would restrict the passage of rodents and large insects and would be resistant to blockage by lint, debris and plant fibers. Louvers in hurricane-prone areas must be resistant to wind-driven rain, hence the requirement to comply with AMCA 500.