CIH - Certified Industrial Hygienist Engineering Controls and Ventilation Questions and Answers — Questions and Answers
Question 1: An industrial hygienist is designing a local exhaust ventilation (LEV) system for a process that releases contaminant with low velocity into moderately still air. According to ACGIH's 'Industrial Ventilation: A Manual of Recommended Practice,' which of the following capture velocity ranges would be most appropriate for the hood design?
- 50 - 100 fpm (Correct answer)
- 200 - 500 fpm
- 500 - 1000 fpm
- 1000 - 2000 fpm
Correct answer: 50 - 100 fpm
According to the ACGIH Industrial Ventilation Manual, a capture velocity of 50-100 fpm is recommended for contaminants released with low velocity into moderately still air, such as evaporation from tanks or degreasing operations. Higher velocities are needed for more energetic release conditions.
Question 2: A paint spray booth has an opening that is 7 feet high and 10 feet wide. To maintain worker protection, the design specification requires an average face velocity of 125 feet per minute (fpm). What is the required airflow rate (Q) in cubic feet per minute (CFM)?
- 1,750 CFM
- 5,600 CFM
- 8,750 CFM (Correct answer)
- 12,500 CFM
Correct answer: 8,750 CFM
The required airflow rate (Q) is calculated using the formula Q = V × A, where V is the face velocity and A is the cross-sectional area of the opening. The area (A) is 7 ft × 10 ft = 70 sq ft. Therefore, Q = 125 fpm × 70 sq ft = 8,750 CFM.
Question 3: Which of the following describes the primary function of a flange on an unflanged local exhaust hood?
- To increase the structural rigidity of the ductwork connection.
- To create a barrier that prevents air from being drawn from behind the hood, improving capture efficiency. (Correct answer)
- To lower the transport velocity within the duct to save energy.
- To act as a pre-filter for large particulate matter.
Correct answer: To create a barrier that prevents air from being drawn from behind the hood, improving capture efficiency.
A flange on a local exhaust hood serves as a plane of symmetry, effectively blocking airflow from the zone behind the hood. This forces more air to be drawn from the contaminant source in front of the hood, thereby increasing its capture efficiency for the same amount of airflow.
Question 4: According to the hierarchy of controls, which of the following is considered the most effective method for controlling workplace hazards?
- Personal Protective Equipment (PPE)
- Administrative Controls
- Engineering Controls
- Elimination or Substitution (Correct answer)
Correct answer: Elimination or Substitution
The hierarchy of controls, as recognized by NIOSH and OSHA, ranks hazard control methods by effectiveness. Elimination (physically removing the hazard) and Substitution (replacing the hazard with a less hazardous alternative) are at the top of the hierarchy and are considered the most effective control strategies.
Question 5: A pitot tube traverse in a circular duct measures an average velocity pressure (VP) of 0.64 inches of water gauge (w.g.). Assuming standard air density, what is the approximate air velocity (V) in the duct?
- 2560 fpm
- 3204 fpm (Correct answer)
- 4005 fpm
- 5006 fpm
Correct answer: 3204 fpm
The relationship between velocity pressure and velocity for standard air is given by the formula V = 4005 * sqrt(VP). In this case, V = 4005 * sqrt(0.64). The square root of 0.64 is 0.8. Therefore, V = 4005 * 0.8 = 3204 fpm.
Question 6: An industrial hygienist is evaluating an LEV system where workers are still being overexposed to a chemical vapor despite the system running. The capture velocity at the source is adequate, but there are visible dust deposits in the horizontal duct runs. What is the most likely cause of this issue?
- The fan is not powerful enough.
- The hood is improperly designed.
- The transport velocity in the duct is too low. (Correct answer)
- The air cleaning device is clogged.
Correct answer: The transport velocity in the duct is too low.
The presence of settled material in the ductwork is a classic sign of insufficient transport velocity. The airflow must be fast enough to keep the contaminant entrained in the airstream and prevent it from settling out due to gravity. Even with good capture velocity, low transport velocity will cause the system to fail.
CIH - Certified Industrial Hygienist Engineering Controls and Ventilation Questions and Answers
An industrial hygienist is designing a local exhaust ventilation (LEV) system for a process that releases contaminant with low velocity into moderately still air.
According to ACGIH's 'Industrial Ventilation: A Manual of Recommended Practice,' which of the following capture velocity ranges would be most appropriate for the hood design?