Fume hoods and local exhaust ventilation: what protects the operator and what does not
By Vvon Engineering Team | Published | Updated
Face velocity, containment testing, ducted against filtered recirculating hoods, and the failures a hood cannot save you from. What to write into a tender so the hood you receive is the hood you specified.
A fume hood’s containment depends on the installed system, including the hood, duct, fan, discharge and room air supply. Request commissioning evidence for the actual room and working configuration; a factory report alone cannot demonstrate installed performance.
Face velocity is a proxy, not the goal
The goal is containment: contaminant generated inside the hood stays inside it and does not reach the operator's breathing zone. Face velocity became the specification because it is cheap to measure and containment is not. It remains a useful indicator, provided you remember what it does not tell you.
Too little face velocity lets contaminant escape at the sash opening. Too much is also a defect, because a fast inward stream separates around the operator's body and forms a vortex at the sash that pulls contaminant back out. Above a certain velocity, adding more makes containment worse while making the energy bill and the noise worse as well. Design practice for a conventional hood sits in a fairly narrow band, and your supplier should state the design face velocity and the design sash height together, because one is meaningless without the other.
When you measure, measure on a grid across the sash opening at the design height with a calibrated thermal anemometer, and record every reading rather than just the average. A hood with an acceptable average and a wide spread between the highest and lowest readings is a hood with a bad baffle setting or an obstruction, and the average is hiding it. The spread is the more informative number.
Containment testing: the test that actually means something
Type testing to EN 14175 part 3 is done in the manufacturer's laboratory on that model of hood, using a tracer gas released inside the hood and sampled in front of a mannequin. It proves the design. Ask for the report and check that it covers the model and sash configuration you are buying, not a related one.
On-site testing to EN 14175 part 4 is done on your hood, in your room, after installation. It proves the installation. This is the report that matters and it is the one most commonly missing.
The ASHRAE 110 tracer gas method reports results in three conditions that are worth understanding even if you specify the European standard: as manufactured, as installed and as used. 'As used' means with the equipment, bottles and clutter that are actually in the hood, and it is routinely the worst of the three. A hood full of apparatus is a different hood.
Airflow visualisation. A suitable smoke method can reveal disturbed or escaping flow, but it is qualitative and must be performed safely by competent personnel. Agree its use and frequency in the hood’s inspection procedure.
Airflow monitor on every hood. A visual and audible low-flow alarm at the sash is the only thing standing between a belt failure on the roof and an operator who has no way of knowing. Treat it as part of the hood, not an accessory.
Ducted or filtered recirculating
Ductless hoods are attractive in Dhaka for real reasons: rented floors where you have no rights to a riser or a roof, buildings with no spare shaft, and refurbishments where a duct route simply does not exist. They are a legitimate answer to a narrow question, and a poor answer to a broad one.
Consideration
Ducted hood
Filtered recirculating hood
Chemistry it suits
The specified chemistry and process within the entire system’s limits; specialised hazards may need a dedicated hood
Only the chemicals, quantities and loading conditions validated for the filter system
Where the contaminant goes
To a designed discharge with treatment where required
Captured by the approved media while it remains effective
Installation
Duct, fan, safe discharge, make-up air and services as designed
Site suitability, room ventilation, filter monitoring and utilities still need review
Running cost
Fan energy plus conditioning the make-up air, which is the bigger figure in this climate
Filter replacement, and filters have a shelf life as well as a service life
Failure detection
Airflow alarms and performance checks; not every fault is visually evident
Manufacturer-approved filter/loading checks and alarms; airflow alone does not prove capture
Who has to own it
Building maintenance, on a planned schedule
A named person tracking usage, breakthrough tests and filter dates
A recirculating hood requires an approved chemical and loading list, a compatible filter and an effective inspection/replacement method. Changing research chemistry demands a fresh review. Specialised work such as heated perchloric acid, radionuclides or gases poorly retained by a filter must not be assigned to a generic filtered enclosure.
What a fume hood cannot protect you from
It does not protect the sample. A fume hood protects the operator by pulling room air inwards across unfiltered work. A biological safety cabinet protects operator and sample; a laminar flow bench protects only the sample and blows air at the operator. These three look similar from the corridor and are not interchangeable. Using a laminar flow bench for chemical work puts the contaminant straight into the operator's face.
It does not contain an explosion or a serious fire. A sash is a splash and spatter guard. Pressure work needs a blast shield and a hood rated for the duty.
It is not a chemical store. Keep the operating space and baffles clear. Store chemicals in compatible, appropriately rated cabinets; vent a cabinet only where the manufacturer and fire-safety design permit it.
It does not work with the sash raised. The design sash height is part of the specification. Above it, face velocity falls and containment goes with it. Sash stops and a clearly marked working height are cheap.
It depends on coordinated room ventilation. Design the loss-of-power and loss-of-make-up-air response with the ventilation engineer. The safe action may involve process shutdown, standby supply or another controlled response; do not automatically reduce extract while hazardous generation continues.
It does not handle perchloric acid without a wash-down hood and dedicated ductwork, and it does not make radioactive work safe without the additional controls that work requires.
Commissioning, and the annual test after that
Balance the room to its design. Verify the required pressure relationship, supply and extract under normal and failure conditions; do not adopt a universal negative-pressure value for every laboratory.
Check the discharge before you accept the installation. The stack should discharge upwards at a velocity high enough to carry the plume clear of the roof, and it must be away from fresh air intakes, opening windows and neighbouring roofs. In Dhaka the roof is shared with condensers, water tanks and often a caretaker's quarters, so this needs to be drawn and agreed rather than assumed.
Set and record the baffles. Then mark the setting, because somebody will move it.
Measure face velocity on a grid at the design sash height and record every reading, then run a smoke test and a tracer gas containment test to EN 14175 part 4 with the hood in its as-used state.
Prove the airflow monitor by shutting the fan down and confirming the alarm is audible and visible at the hood, and that a low-flow condition is reported to whoever is responsible for the building.
Label each hood with its design sash height, the date of the last test and the date of the next. A sticker on the sash frame does more for daily safety than a folder in an office.
Set a documented periodic test programme. Follow applicable local requirements, the adopted test standard, risk assessment and manufacturer instructions. Review performance after changes to fans, ducts, room ventilation or connected hoods.
Writing the tender so you get the hood you specified
Describe the required system and acceptance evidence clearly so all bidders price the same scope. Follow the institution’s current procurement process and state any justified compatibility or safety requirement.
An EN 14175 part 3 type test report for the offered model, from a third party, submitted with the bid rather than promised after award.
An on-site containment test to EN 14175 part 4 named as a handover deliverable, with the certificate as a condition of the final payment.
Match construction to the complete chemistry. Confirm liner, worktop, seals, duct and fan compatibility, temperatures and cleaning. Hydrofluoric acid, heated perchloric acid and radioactive work each need a specific engineered assessment.
An airflow monitor with visual and audible alarm on every hood, plus the volt-free contact needed to report to the building management system.
Fan, ductwork, discharge stack, make-up air and any exhaust treatment in the same package as the hood. Splitting them across packages is how a laboratory ends up with hoods, a fan and nobody responsible for the fact that they do not work together.
Twelve months of filter or media where a filtered hood is offered, plus the breakthrough test method.
Operator training and a written test schedule handed over at commissioning, naming who tests, how often and against which standard.
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