Should I use a ducted or ductless fume cupboard?

Ductless fume hoods are the cost-efficient alternative to installing a fully ducted unit. Ducted units provide the ultimate extraction performance, as well as the ability to have services (gas & water) on hand.

Here we provide a guide on when a ductless fume hood is most appropriate.

Design

Fume hoods provide a safe negative-pressure environment in which to handle materials, in doing so removing exposure to hazardous materials within the lab. Traditionally, ducted fume hoods are designed to extract air from the workspace and vent it outside of the building, removing the air completely through a centralised ducting system. Running costs are high, in particular because of the energy requirements to run a high-volume fan system. Because of the volume of air being taken from the building, replacing the air through an HVAC system means higher flow rates and consequently higher cost to run.

Ductless fume hoods consist of a fan at the top of the unit which draws air through the front of the chamber, through a carbon filter and sometimes a HEPA or ULPA filter and recirculating back into the ambient air. Normally they are much smaller in size than an inbuilt system with the most common sizes being 1200mm wide or 900mm wide.

They are uniquely portable: not bound to a ducting system, only run on 240-volt general power outlets, and lightweight. Many are installed with a portable trolley for flexibility.

Like a ducted hood, ductless units have a sash at the front, in some cases with sensors so that the fan can operate at the speed appropriate to the sash height in order to maintain the same flow rate (air speed).

Common filter applications

HEPA (High Efficiency Particulate Air) filters, depending on the grade, will remove particulate matter from the air down to 0.2 microns in diameter, or 0.0002mm – about the size of small bacteria. A HEPA filter would be recommended when working with powders, mould, fungus aerosols, dust, soil and other fine particulate matter.

Activated Carbon filters consist of pellets of activated carbon kept in a mesh casing and are the most common filter, suitable for removing Volatile Organic Compounds (VOCs) from the air. Through the Van der Waals forces and the porosity of the carbon, VOCs are retained through adsorption and prevented from leaving the filter. Some fume hoods will include VOC sensors which will indicate when the levels are high and thus the filter needs replacing.

Standard fume hoods will normally only contain a carbon filter; however, a combination of both types is common.

Appropriate use cases

Volatile organic compounds

Because carbon filters are good only for VOCs, this means inorganic compounds are not safe to use in a ductless unit. Here is a list of common inorganics used in the laboratory:

• Hydrochloric acid (HCl)
• Sulfuric acid (H­­₂SO₄)
• Sodium hydroxide (NaOH)
• Potassium nitrate (KNO₃)
• Ammonium nitrate (NH₄NO₃)
• Phosphoric acid (H₃PO₄)

Always check the chemical SDS to determine the hazards when handling the material and also the necessary precautions.

Light chemical handling

If you are engaged in chemical analysis and manipulation, ductless fume hoods provide a secure environment to handle these in.

Education/institutions

Ductless fume hoods are ideal for demonstration of experiments that require fume extraction, or just for added safety.

Inappropriate use cases

Heating

Ductless fume cupboards are not ideal for heating chemicals as this can affect the efficiency of the carbon filter. These are designed for simple, light chemical handling and are not suitable for heating or boiling.

High corrosives

Substances like nitric or perchloric acids must not be used in a ductless fume hood, as the fumes need to be extracted and exhausted to the outside of the building. Often with high corrosives, a scrubber is required to remove the majority of harmful substances from the exhaust air, which can’t be achieved with a ductless unit.

High volume chemicals

If you are using a high volume and large variety of chemicals, a ducted unit is recommended as it is able to handle the continuous and varied volume of chemical fumes.

Others:

• When regulatory requirements demand
• Long-term/intensive use
• Increased airflow requirements
• Handling pathogens/biohazardous materials (biosafety cabinet required)

Westlab ductless fume hoods

Westlab carry an inventory of high-quality ductless fume cabinets at great prices. Our Value-line and Pro-line fume cupboards can be customised with accessories and features.

We lead the market with a same-day dispatch guarantee for all orders placed before 2PM AEDT/AEST.

Reach out to our team for a demo at our Experience Centre!

* This article does not constitute professional advice and is only intended to be a guide. The responsibility lies on the end user to select the most appropriate product.