Question 3 of 7: Purposes and Types of Ventilation Systems, and Their Limitations

Question 3: Purposes and Types of Ventilation Systems, and Their Limitations (20 marks: 7/7/6)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) The Four Purposes of a Ventilation System

• Supply breathable air. Ventilation replaces air used up by occupants and combustion processes with outdoor air, maintaining an adequate oxygen concentration and preventing the buildup of carbon dioxide and other metabolic by-products — the most basic life-support function of any ventilation system.
• Remove or dilute airborne contaminants. Dust, fumes, vapours, gases, and mists generated by industrial processes are diluted with clean supply air (general/dilution ventilation) or captured directly at their source (local exhaust ventilation, developed in part ii) so that worker exposure stays below the applicable occupational exposure limit.
• Control temperature and humidity. Ventilation removes excess process/equipment heat and moisture and helps maintain thermal comfort and workable humidity levels — both for worker comfort/productivity and, in some processes, for product/material quality and the control of mould or corrosion.
• Control odour and maintain pressure relationships between spaces. Ventilation dilutes or exhausts unpleasant or hazardous odours, and (via balanced supply/exhaust airflow) can establish a positive or negative pressure differential between spaces — e.g., keeping a clean assembly area at slightly positive pressure relative to a dirty process area, or a paint booth at negative pressure relative to the surrounding plant — so contaminated air is contained and does not migrate to areas it was not generated in.

These four purposes are interdependent in a real system: the same airflow that supplies breathable air and controls temperature is also the vehicle that dilutes or removes contaminants and establishes the pressure relationships that keep them contained, so a ventilation system is sized and balanced to satisfy all four simultaneously, not just the loudest single requirement.

(ii) The Two Types of Mechanical Ventilation, Selecting Between Them, and a Comparison Chart

The two types of mechanical ventilation commonly used in an industrial workplace are general (dilution) ventilation and local exhaust ventilation (LEV).

• General (dilution) ventilation supplies clean outdoor air to, and exhausts room air from, the workspace as a whole, mixing with and diluting whatever contaminant is present in the room air down to an acceptable concentration. It treats the contaminant only after it has already dispersed into the general workroom atmosphere.
• Local exhaust ventilation (LEV) captures a contaminant at or very near the point it is generated — via a hood, capture duct, air-cleaning device, fan, and stack — and removes it before it has a chance to disperse into the general workroom air or reach a worker's breathing zone.

Which type is best for a given workplace is determined by several factors read together, not by a single rule:

• Toxicity of the contaminant — a highly toxic contaminant (even in small quantity) needs LEV, since dilution ventilation still allows the worker to breathe air at some non-zero concentration on the way to being diluted; a low-toxicity contaminant can often be adequately managed by dilution alone.
• Generation rate and quantity — a high generation rate would require an impractically large dilution airflow (and matching makeup-air/heating or cooling load) to bring room concentration down; LEV controls the same quantity at far lower total airflow because it captures the contaminant in a small volume before it spreads.
• Number and location of sources — a single fixed point source (a welding station, a solvent tank) is well suited to LEV; many small, widely scattered sources across a large floor area are usually more practical to manage with dilution ventilation, since fitting LEV to every source would be prohibitively costly.
• Distance of the source from the worker's breathing zone — a source close to the breathing zone (bench-scale work) benefits most from LEV, since capture happens before the worker's own breathing zone is reached.
• Flammability/explosivity — a source capable of generating a flammable vapour concentration is usually captured at source (LEV) rather than allowed to disperse and accumulate in the general room air, where an ignition source anywhere in the room becomes a hazard.

(iii) Limitations of Any Ventilation System

• Controls the inhalation route only. A ventilation system does nothing to control skin/eye contact or ingestion exposure routes — those still require separate controls (PPE, containment, hygiene practices) regardless of how well the ventilation is designed.
• Cannot exceed its designed capacity. If the actual contaminant generation rate exceeds the airflow/capture rate the system was sized for — a process upset, a spill, or an unplanned increase in production — the system will not maintain a safe concentration; it manages exposure within a designed envelope, it does not eliminate the hazard at its source.
• Effectiveness depends on correct design and undisturbed operation. Hood placement, capture velocity, and duct sizing must be correct for the actual contaminant and source geometry; cross-drafts, open doors, or a worker's own body blocking the airflow path between the source and the hood can defeat an otherwise correctly designed system.
• Controls degrade without maintenance (the same principle already noted for hazard controls generally in Question 2(i)) — loaded filters, duct leaks, and worn fan bearings reduce airflow gradually and often without an obvious symptom, so a system that was adequate when commissioned can silently become inadequate.
• A fan or power failure removes protection with no visible warning unless the system is fitted with airflow/fault monitoring and an alarm or interlock (as with the LEL-interlocked oven purge cycle used in Question 7) — a worker has no inherent way of knowing the ventilation has stopped protecting them.
• Recirculated or improperly exhausted air can relocate rather than remove the hazard — exhaust that is not adequately filtered before recirculation, or that is discharged too close to a fresh-air intake, can reintroduce the same contaminant elsewhere in the building.
• Not a substitute for respiratory protection in an IDLH or oxygen-deficient atmosphere. For immediately-dangerous-to-life-or-health conditions or confined/oxygen-deficient spaces, ventilation alone is not considered adequate protection and supplied-air respiratory protection is still required regardless of the ventilation system's design capacity.

These limitations are the reason ventilation is classified as an engineering control (Question 2(iii)), not as elimination or substitution: it manages an existing hazard's exposure rather than removing the hazard from the workplace, so it remains only as effective as its design, maintenance, and monitoring are kept.