CIH Engineering Controls and Ventilation 4 — Questions and Answers
Question 1: A wet scrubber used in an LEV system primarily controls contaminants by:
- Electrostatically attracting particles to collection plates
- Physically impacting or absorbing particles and gases into water droplets (Correct answer)
- Filtering particles through a fibrous media
- Centrifugally separating particles by density
Correct answer: Physically impacting or absorbing particles and gases into water droplets
Wet scrubbers use water droplets to capture contaminants through impaction, diffusion, and absorption, making them effective for both particulates and soluble gases.
Question 2: Which parameter is most critical when sizing ductwork to prevent particulate settling in horizontal runs?
- Static pressure loss
- Minimum transport velocity (Correct answer)
- Duct diameter
- Aspect ratio of the duct
Correct answer: Minimum transport velocity
Minimum transport velocity must be maintained in all duct segments to keep particles airborne and prevent settling, plugging, and fire hazards.
Question 3: In the context of industrial hygiene engineering controls, 'process substitution' is classified as which tier in the hierarchy of controls?
- Administrative control
- PPE
- Engineering control — elimination/substitution (Correct answer)
- Engineering control — isolation
Correct answer: Engineering control — elimination/substitution
Substitution (replacing a hazardous process or material) is a type of elimination/substitution engineering control and sits near the top of the hierarchy.
Question 4: The 'capture velocity' needed for a welding fume hood should typically be in which range?
- 25–50 fpm
- 75–100 fpm
- 100–200 fpm (Correct answer)
- 500–2,000 fpm
Correct answer: 100–200 fpm
Welding fumes are light with some thermal lift, requiring capture velocities of approximately 100–200 fpm at the point of generation.
Question 5: When a ventilation system is tested and actual airflow is 15% below design, the FIRST corrective step should be:
- Immediately increase fan speed
- Diagnose the cause by measuring static pressures throughout the system (Correct answer)
- Add a booster fan
- Install a larger exhaust fan
Correct answer: Diagnose the cause by measuring static pressures throughout the system
Systematic static pressure measurements identify whether the deficiency is due to obstructions, leakage, damper settings, or an undersized fan before any corrective action is taken.
Question 6: Which equation correctly expresses the relationship between total pressure (TP), static pressure (SP), and velocity pressure (VP) at any point in a duct?
- TP = SP − VP
- TP = SP + VP (Correct answer)
- TP = SP × VP
- TP = SP / VP
Correct answer: TP = SP + VP
Total pressure equals the sum of static pressure and velocity pressure at any cross-section in a duct system.
Question 7: An enclosing hood is preferred over a receiving hood for a grinding operation primarily because:
- It requires lower exhaust volumes
- It physically surrounds the source, preventing contaminants from escaping into the work zone (Correct answer)
- It is less expensive to fabricate
- It allows better visual access to the work piece
Correct answer: It physically surrounds the source, preventing contaminants from escaping into the work zone
An enclosing hood surrounds the emission source, containing contaminants within the enclosure before exhausting them, making it more effective than an open receiving hood.
CIH Engineering Controls and Ventilation 4
A wet scrubber used in an LEV system primarily controls contaminants by: