Ductless fume hoods
Re circulating fume cupboards
Re-circulating fume cupboards / fume hoods are used mainly for educational purposes or where the design of a laboratory building does not allow for the fitting of external ducting. They are sometimes known as ductless fume cabinets.
This type of fume cabinet usually has a fan fitted on the top of the hood or sometimes beneath the work area. The principle is simple. Air is drawn in through the front opening of the fume cabinet, circulated through the work area and then passed through a filter. The air this then expelled back into the laboratory environment via the fan.
The althernative to a ductless fume hood is a ducted fume cabinet.
Methods of filtration in a fume cabinet
Pre-filtration
The air is passed through a filter to remove any noxious chemicals. The first stage of the filter is typically an open cell foam physical barrier, which stops big particles from getting through. Open cell foam filters are generally quite cheap and will last up to six months depending on usage.
Main filtration
After pre-filtration, the air is drawn through a layer of activated charcoal. Activated charcoal is very efficient at removing most harmful fumes. The exception is Ammonia and carbon monoxide.
If the experimental work undertaken in the cabinet will result in ammonia or carbon monoxide fumes, additional filters can be added to remove these.
The main filter of a fume cupboard will last for about two years, depending on the level of use.
Advantages of re-circulating fume cupboards / fume hoods
- No ducting is required
- Heated air is not expelled from the laboratory
- Noxious fumes are not expelled into the atmosphere.
Disadvantages of re-circulating fume cupboards / fume hoods
- The units need regular maintenance – the filters must be checked and replaced
- Filters are not perfect and there is a greater risk of contamination than with ducted version
- The extractor fan can be noisy
Read more:
What are fume hoods?
Fume cupboard technical terms and definitions
How do fume cupboards work?
What are fume cupboards used for?
Tips and advice of ducted fume cupboards
What are ventilated benches?
What are clean benches?
Health and Safety issues concerning fume cupboards
