Fume and process exhaust

Kitchen and dust systems are the largest part of Keelson's exhaust work, but not the only part. Welding shops need fume extraction. Finishing rooms need spray booths. Hospitals and schools have laboratories with fume hoods, and hospitals have isolation rooms whose exhaust must never come back into the building. Each of these has its own hood type, its own airflow rules and, often, its own duct materials, because the exhaust may be corrosive or hazardous. This lesson covers the process exhausts Keelson builds most often, using the Eastgate Hospital addition and Keelson's own welding bay as the examples.

Welding fume

Welding fume is a cloud of very fine metal oxide particles formed when vaporized metal condenses in the air above the arc, together with gases such as ozone and nitrogen oxides. Its composition depends on the base metal, filler and coatings. Galvanized steel gives zinc oxide fume, which causes metal fume fever. Stainless steel gives fume containing chromium, some of it in the hexavalent form, which is a recognized carcinogen with a very low exposure limit, as SMW-230 explained. Mild steel fume contains iron and manganese, and manganese exposure is linked to neurological effects. Because the particles are so fine, welding fume behaves almost like a gas: it rises with the hot plume and then spreads through the shop.

Local exhaust is the preferred control. Fume extraction arms, articulated arms with a small hood on the end, are positioned close to the arc, within about one hood diameter, and connected to a fan and filter unit. Downdraft and backdraft tables pull fume away from the welder through a perforated tabletop or a slotted back panel. Fume extraction guns draw fume through a shroud around the welding gun nozzle itself. For each, the airflow is set by the hood size and the capture distance, using the principles of Lesson 6, and the shielding gas must not be disturbed: too much air at the arc pulls the shielding gas away and causes porosity in the weld.

Keelson's own welding bay, where Kai welded the Brine & Ember grease duct, now has three fume arms and a backdraft table on a cartridge collector, installed after a hygienist measured chromium exposure during stainless welding. The cartridge collector suits welding fume because the particles are fine, dry and not fibrous, and it returns no air to the shop: the cleaned air is exhausted outdoors because cartridge filters do not remove the gases.

# Fume arm airflow by distance (flanged hood, Dalla Valle with 0.75 factor)
V = 0.5          # capture velocity at the arc, m/s
A = 0.03          # hood face area, m2
for x in (0.15, 0.25, 0.40):   # distance from hood to arc, m
    q = 0.75 * V * (10 * x**2 + A) * 1000
    print(f"hood {x} m from the arc: {q:.0f} L/s")

hood 0.15 m from the arc: 96 L/s
hood 0.25 m from the arc: 246 L/s
hood 0.4 m from the arc: 611 L/s

The arms are sized for the hood about 250 mm from the arc. Aggie's rule in the bay is to move the arm every time the welding position moves, because a hood left 400 mm away captures only a fraction of the fume at the same airflow.

Spray booths

Spraying paint, lacquer or stain releases solvent vapour and overspray mist, both flammable and both harmful to breathe. Spray finishing must be done in a spray booth or spray room built to NFPA 33, Spray Application Using Flammable or Combustible Materials, as referenced by the fire codes. A spray booth is an enclosing hood: a three-sided or fully enclosed steel box with filters that catch the overspray and an exhaust fan that draws air through the open face or the doors at an average velocity high enough to keep vapour and mist inside. The exhaust system must run whenever spraying is done, the spray gun must be interlocked so it cannot operate without the exhaust, and the electrical equipment inside and near the booth must be suitable for the hazardous location.

For the Ironbark finishing room, the hygienist and Tomasz recommended a listed open-face spray booth for the occasional sprayed finishes, separate from the slot-hooded wiping bench. Its exhaust duct is steel, runs directly to the outdoors with no connection to the dust system, and discharges above the roof away from intakes. Booth ducts collect overspray residue and must have access for cleaning, much like grease ducts, and the booth filters are changed on a schedule and stored as a fire hazard until removal.

Spray booth exhaust must never share a duct with dust collection or kitchen exhaust. Mixing solvent residue with wood dust or grease creates a fuel that is easier to ignite than either alone, and the protection systems for each are different.

Laboratory fume hoods

A laboratory fume hood is a ventilated enclosure in which people handle chemicals. It has a work surface, a movable front sash of safety glass, and an exhaust connection at the top, usually with internal baffles that spread the airflow across the opening. Air is drawn in through the open sash face at a controlled face velocity, sweeps across the work and leaves through the exhaust. The main guidance for laboratory ventilation in North America is ANSI/ASSP Z9.5, Laboratory Ventilation, together with ASHRAE Standard 110, which describes a containment test using a tracer gas and a mannequin. Face velocities commonly used in practice fall around 0.4 to 0.5 m/s, but the standard puts more weight on proven containment than on any single face velocity, because a hood can have a good face velocity and still spill if it is badly placed or disturbed by supply air.

The hood's exhaust airflow is the face velocity times the open sash area. A hood with the sash fully open needs much more air than one with the sash lowered to a working height. Constant-volume hoods exhaust the same airflow whatever the sash position, using a bypass opening that opens as the sash closes. Variable-air-volume hoods measure the sash position or the face velocity and vary the exhaust to hold the face velocity constant, which saves a great deal of energy in a building with many hoods. Either way the lab's supply air is controlled to keep the lab negative to the corridor.

# Fume hood exhaust from sash opening and face velocity
width = 1.5           # sash opening width, m
v_face = 0.5          # face velocity, m/s
for h in (0.45, 0.70):        # sash opening heights, m
    q = width * h * v_face * 1000
    print(f"sash at {h} m: exhaust {q:.0f} L/s")
hoods = 6
print(f"six hoods with sashes at working height instead of full open save about {hoods * (width * (0.70 - 0.45) * v_face * 1000):.0f} L/s")

sash at 0.45 m: exhaust 338 L/s
sash at 0.7 m: exhaust 525 L/s
six hoods with sashes at working height instead of full open save about 1125 L/s

The Eastgate addition has a small clinical laboratory with six VAV fume hoods. Keelson installs the exhaust ducts to each hood, the hood exhaust valves supplied by the controls contractor, and the exhaust risers to two high-plume roof fans. The commissioning authority will test each hood with the ASHRAE 110 method before the lab is occupied, and the TAB group from SMW-330 will verify the airflows and the lab's negative pressure.

Laboratory exhaust stacks

Laboratory exhaust must leave the building fast and high enough to clear the roof and avoid being drawn back in through air intakes, windows or neighbouring buildings. Common practice uses fans that discharge vertically at high velocity through a stack well above the roof; high-plume induced-flow fans mix in extra outdoor air at the fan to raise the plume further without a tall stack. The stack height, exit velocity and location are set by the engineer, sometimes with wind studies for large labs. Rain caps that deflect the plume downward are not used. Keelson builds and supports the stacks to resist wind and installs the fans so they can be serviced from the roof.

Corrosive exhaust and duct materials

Some exhaust streams attack ordinary galvanized steel: acid fumes from labs and plating, chlorine and moisture from pools, chemical vapours from some processes. The engineer chooses the duct material to suit. Type 316 stainless resists many lab chemicals and chlorides better than type 304. Fibreglass-reinforced plastic resists acids and chlorides well and is common for lab and scrubber exhaust, though it has fire limitations that the code and the listing address. PVC-coated galvanized steel, with a plastic coating inside and out, suits many corrosive fumes at moderate temperatures. Some specialty exhausts have their own rules: perchloric acid fume hoods, for example, need dedicated stainless ducts with washdown systems, because perchloric acid residues can become explosive.

Isolation room exhaust at Eastgate

Airborne infection isolation rooms keep patients with airborne infections, such as tuberculosis, in negative pressure so air flows into the room from the corridor and out through a dedicated exhaust. The CSA standard for health care ventilation, CSA Z317.2, and the hospital's infection prevention team set the requirements. The exhaust from these rooms runs in its own duct, is discharged above the roof away from intakes and people, and in some designs passes through high-efficiency particulate filters before discharge. The TAB group balanced these rooms in SMW-330; Keelson's installation job is to keep the duct tight, because a leak in a positive-pressure section after the fan would push potentially infectious air into a shaft or a ceiling.

The design puts the exhaust fans on the roof at the end of each duct so every metre inside the building stays under suction. Kai's crew seals the exhaust risers to the same standard as supply duct, and the roof stacks are located where the hospital's mechanical engineer placed them, well away from the air handler intakes.

Commissioning fume exhaust

Fume systems are judged by how well they protect people, so they are tested that way. Laboratory fume hoods are checked when installed and at regular intervals afterwards, usually yearly: the face velocity is measured across a grid on the open sash, smoke is released at the face to show that it flows smoothly into the hood with no spillage, and the hood's flow monitor is calibrated so it alarms when airflow falls. For a full performance test, ASHRAE 110 releases a tracer gas inside the hood and measures how much reaches a mannequin standing in front of it. Eastgate's lab manager keeps the test results on a label on each hood, and a hood that fails is tagged out of service until it is fixed.

Spray booths and welding systems are checked by measuring airflow at the booth face or the hood and comparing it with the design, and by watching for overspray or fume escaping into the room. Fume arms lose capture quickly as they are moved away from the work, so an airflow reading at the arm hood tells only part of the story; watching where the plume goes during real welding tells the rest.

The isolation rooms at Eastgate have their own checks. Their negative pressure relative to the corridor is shown on a room pressure monitor at the door and verified by the hospital's engineering staff on a regular schedule, and the exhaust fans serving them are on emergency power so the rooms stay negative during a utility failure. Keelson's part at commissioning was to show that the ducts were sealed, the fans ran in the right direction and the dampers moved as the controls commanded.