SH5107: A Plain Duct Hood Installed at a Corner of a Workbench is Designed to Capture: Industrial Ventilation Assignment, NUS, Singapore

Assignment 3 – Exhaust Hoods

Question 1

A plain duct hood installed at a corner of a workbench is designed to capture airborne contaminants generated at a distance X meters from the hood opening. The adjacent sides of the workbench are installed with two baffles (see sketch below). Given: Surface area of a sphere S = 4 π Radius²

1. If the required capture velocity is V m/s, what is the theoretical airflow rate Q m³/s (cms) expressed in terms of X and V
2. If X = 40 cm and V = 0.5 m/s, determine the airflow rate Q in cubic meter per second (cms)

Question 2

A free-hanging plain exhaust duct opening is positioned close to a process where styrene vapor is evaporating into still air from adhesive applied to cement plastic pipes. The duct (diameter 36 cm or 14 inches) is connected to a fan operating at 0.8 m³/s (1,694 cfm). Given: An adequate capture velocity needed for evaporating solvent into still air is 0.5 m/s (100 fpm).

1. How close should the exhaust duct opening be placed from the source of evaporation in order to adequately capture the evaporated vapor?
2. If a flange is provided at the exhaust duct opening, how close should the flanged duct opening be placed from the source in order to adequately capture the evaporated vapor?

Question 3

A simple exhaust hood of irregular shape is constructed for a specific process. The diameter of the duct connected to the hood is 20 cm. The airflow rate measured in the duct is 20 m³/min and the hood static pressure is found to be -15 mm w.g.

1. Determine the entry loss factor and the coefficient of entry for the hood.
2. If the flow rate is increased to 30 m³/min, what will be the hood static pressure?

Question 4

A rectangular hole of 2” by 3” (5.08 cm by 7.62 cm) is made in the side wall of an exhaust duct. The static pressure at the site of the hole is –2.70” w.g. (-68.58 mm w.g. or 672 Pa) Estimate the airflow rate into the opening. Assume STP and the entry loss factor for an orifice is 1.8.

Question 5

A plenum lateral slot hood (see sketch) is provided on an open surface degreasing tank 2m x 1m (6.6 ft x 3.3 ft). The required capture velocity is 0.5 m/s (or 0.5 m³/s/m² of the tank surface area). Given: f_slot = 1.78 and f_hood is 0.5

1. Determine the airflow rate.
2. Size the slot (i.e. determine the width of the slot) to give exactly 10 m/s (2,000 fpm) slot velocity.
3. What is the hood static pressure if the duct velocity is 12 m/s (2,362 fpm).