CIH Engineering Controls and Ventilation 5 — Questions and Answers
Question 1: What effect does increasing duct length have on a ventilation system's total system resistance?
- Decreases resistance proportionally
- Has no significant effect on resistance
- Increases resistance proportionally (Correct answer)
- Increases resistance as the square of duct length
Correct answer: Increases resistance proportionally
Friction losses accumulate along duct length, so total system resistance increases proportionally with duct length, reducing airflow if the fan curve remains constant.
Question 2: When evaluating makeup air requirements for a local exhaust ventilation system, the primary concern is:
- Maintaining a slight negative pressure in the facility
- Ensuring sufficient replacement air to prevent excessive negative pressure that reduces LEV performance (Correct answer)
- Maximizing energy efficiency by minimizing makeup air
- Conditioning makeup air to match process temperatures
Correct answer: Ensuring sufficient replacement air to prevent excessive negative pressure that reduces LEV performance
Insufficient makeup air creates excessive negative pressure (building depressurization), which can reduce exhaust hood airflow below design levels and draw contaminants back into the space.
Question 3: The collection efficiency of a cyclone separator is primarily determined by:
- The inlet gas temperature and humidity
- Particle size, gas velocity, and cyclone diameter (Correct answer)
- The type of material lining the cyclone walls
- The height-to-diameter ratio of the outlet tube
Correct answer: Particle size, gas velocity, and cyclone diameter
Cyclone efficiency depends on centrifugal forces, which are governed by particle mass (size/density), tangential gas velocity, and the radius of the cyclone body.
Question 4: In industrial hygiene, 'engineering controls' are preferred over administrative controls and PPE primarily because:
- They are always less expensive to implement
- They reduce or eliminate the hazard without relying on worker behavior or compliance (Correct answer)
- They satisfy OSHA requirements automatically
- They are easier to document for recordkeeping purposes
Correct answer: They reduce or eliminate the hazard without relying on worker behavior or compliance
Engineering controls modify the work environment or process to reduce exposure at the source, eliminating reliance on human behavior that can fail or be inconsistent.
Question 5: A Pitot tube traverse is performed in an industrial duct to determine:
- The concentration of contaminants in the exhaust stream
- The average air velocity and volumetric flow rate in the duct (Correct answer)
- The static pressure drop across a filter or scrubber
- The fan motor efficiency and power consumption
Correct answer: The average air velocity and volumetric flow rate in the duct
A Pitot tube traverse measures velocity pressures at multiple points across the duct cross-section, which are used to calculate average velocity and total airflow (CFM).
Question 6: Which type of fan is most appropriate for an LEV system handling air with high concentrations of abrasive particulates?
- Forward-curved centrifugal fan
- Axial (propeller) fan
- Radial blade (paddle wheel) centrifugal fan (Correct answer)
- Backward-inclined centrifugal fan
Correct answer: Radial blade (paddle wheel) centrifugal fan
Radial (paddle wheel) fans have simple, flat blades that resist abrasion and buildup, making them the preferred choice for dust-laden or sticky exhaust streams.
Question 7: The 'aspect ratio' of a rectangular duct cross-section is defined as:
- The ratio of duct length to duct perimeter
- The ratio of the longer side to the shorter side of the cross-section (Correct answer)
- The ratio of duct area to duct hydraulic diameter
- The ratio of velocity pressure to static pressure
Correct answer: The ratio of the longer side to the shorter side of the cross-section
Aspect ratio is the ratio of the long dimension to the short dimension of a rectangular duct; high aspect ratios increase friction losses compared to round or near-square ducts.
CIH Engineering Controls and Ventilation 5
What effect does increasing duct length have on a ventilation system's total system resistance?