Leakage detection and prevention in vacuum systems
CF lab extraction hood
1. Coalfields laboratory extraction hood report
Extraction hood specifications:
- Hood entrance area: 0.74m2 (length 1.145m; width 0.645m)
- Extraction velocity at entrance (capture velocity): 2.9m/s
- Extraction quantity: 2.15m3/s
Principles of hood designs:
•Enclose the operation as much as possible to reduce the rate of airflow needed to control
the contaminant.
•Locate the hood in such a manner that the contaminant is moved away from the operator’s
breathing zone.
•Locate and shape the hood in such a manner that the contaminant will be thrown into the
hood by the initial velocity (of the contaminant).
•Solvent vapours are generally at the same density as air (1.2 kg/m3), therefore attempt to
capture these at source.
•Locate the hood as close to the source of the contaminant as possible.
•Design the hood in such a way as not interfere with the worker.
2. • The following slide shows the current layout in the lab, the next slide shows the
remedial action that must be taken (both pulverisers moved to directly underneath the
hood); and the slide after that shows the future layout of the hood (size of hood to be
increased, and hood to extend to over the crusher as well)
• The capture velocity of the hood (2.9m/s) is sufficient to extract the contaminant,
however, with the project of increasing the hood size, the capture velocity must be
increased (fan speed increase) to maintain a capture velocity that will be enough to
extract the dust effectively.
• In terms of a lab employee being sampled for personal dust, the employee must wear
the gravimetric dust sampling pump on him/her for the duration of the shift. He/she
must not at any point during the shift, take off the sampler and put it anywhere else.
• The supervisor must check the sampled person 3 times during the shift.
• The sampled employee must not tamper with the instrument (pump). If there is
evidence of tampering, action will be taken by the VOHE department against the
department head.
6. Velocity range (m/s) Type of velocity Operation or Hood type Typical emission and Environmental conditions
0.12-0.25 Random mixing Any Random motion of air in industrial operations
0.25-0.50 Capture Degreasing Release of emissions at no velocity into still air (no cross drafts
0.35-0.60 Face Lab fume hoods, drying ovens Emission source is enclosed
Capture Soldering
Release of emissions at no velocity into air with low cross drafts (both less than
capture velocity)
Capture Open surface tanks For use with area approach to design; at edge of tank
0.60-0.75 Face High toxicity lab hood
Release of emissions at no velocity into air with low cross drafts (both less than
capture velocity)
0.5-1.0 Capture
Spray booths, welding, container, filling,
picking, low speed conveyor transfer
For use with ANSI approach to open surface tank hood design
Control Open surface tanks
1.0-2.5 Capture Crushers, conveyor loading, shallow booths Release of emissions at modest cross drafts (both less than capture velocity
Control Canopy hoods For use with ANSI canopy hood, use lower end of range
2.5-10.0 Capture Grinding, tumbling
Release of emission at high velocities into air with high cross drafts (both less than
capture velocity)
6.0 Slot Lab fume hoods Typical commercial lab fume hoods
7.5-10.0 Slot Slotted hoods, general
To ensure inform flow through all slots, the plenum velocity should be designed
for 1/4 to 1/3 of slot velocity
Table of typical hood designs and specifications
ANSI: American National Standards Institute
Plenum velocity: The velocity at the hood exhaust; helps distribute airflow evenly across the hood face
