Constant Volume Bypass Fume Hood

What is the Constant Volume Bypass Fume Hood?

A constant-volume bypass fume hood exhausts a fixed airflow and uses a bypass opening above the sash to keep face velocity within limits as the sash moves. It is the traditional design for laboratories with simple exhaust systems.

Overview

The hood draws a constant exhaust volume, typically 600–1500 m3/h depending on width, while the bypass airfoil above the sash allows air to enter as the sash lowers, limiting face velocity rise to acceptable levels. Standard widths are 1200, 1500 and 1800 mm, with work-zone height 1100–1300 mm and sash opening up to 800 mm. Liners are epoxy resin, fibre-reinforced cement or stainless steel, with a ceramic or epoxy work surface. The sash is vertical-rising tempered safety glass with counterweight. The design is simple and robust, but energy use is high because the building conditions the full exhaust volume at all times. Service fittings for water, gas and electricity are located on the side posts.

How does it work?

A ducted fume hood creates a controlled inward airflow through the sash opening so contaminants generated inside are carried to the exhaust rather than into the room. Face velocity, typically 0.4–0.6 m/s at the sash, is produced by an exhaust fan located on the roof or in the duct. A baffle system at the rear distributes air across slots at the top, middle and bottom to avoid dead zones. A bypass airfoil keeps flow smooth as the sash moves. Variable air volume (VAV) hoods modulate exhaust as the sash opens or closes, saving energy while holding face velocity. The work surface, liner and sash are chemical resistant. Airflow monitors, alarms and lighting complete the hood, and the building make-up air system replaces exhausted air.

What are the key features?

• Fixed exhaust volume with bypass airfoil
• Face velocity held within limits as sash moves
• Epoxy, cement or stainless steel liner options
• Vertical-rising tempered glass sash with counterweight
• Services on side posts
• Simple controls with low maintenance

What are the technical specifications?

A typical constant volume bypass fume hood configuration offers width of 1200 / 1500 / 1800 mm, face velocity of 0.4–0.6 m/s, exhaust volume of 600–1500 m3/h, sash opening of Up to 800 mm, work-zone height of 1100–1300 mm and liner material of Epoxy, FRC or stainless steel. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Constant Volume Bypass Fume Hood is used in general chemistry, sample preparation, teaching laboratories and acid handling. Across the wider Ducted Fume Hoods class, laboratories also deploy it for chemistry laboratories, pharmaceutical research and materials testing.

Which standards and certifications apply?

Relevant standards and methods include EN 14175-2 (fume cupboards, safety and performance), EN 14175-3 (type test methods), EN 14175-4 (on-site test methods), ASHRAE 110 (method of testing performance of laboratory fume hoods), GB/T 34269 and ISO 14644-1. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

Commission each hood with a face velocity survey and, where required, a containment test such as EN 14175-3 or ASHRAE 110. Place it away from doors, walkways and supply diffusers, since cross-drafts above 0.15 m/s degrade containment. Keep the sash at or below the marked working height, and avoid blocking baffles with equipment. Check airflow monitors, alarms and exhaust fans at least every 6–12 months, and clean the liner regularly. Plan make-up air so the room does not become negatively pressured. Train users and label the hood with its tested face velocity and date, and do not use it for storage of chemicals beyond what the work requires.

How do I select the right Constant Volume Bypass Fume Hood?

Choose a constant-volume bypass hood for straightforward laboratories where energy use is less critical and exhaust capacity is sufficient. For large hood counts, consider VAV to cut operating costs. RFQ: width, liner material, sash type, exhaust duct size and the chemicals to be handled.

How does it compare with other Ducted Fume Hoods variants?

Select the hood by the chemicals and process. Choose width, commonly 1200, 1500 or 1800 mm, to match equipment and operators, and check that depth and height suit tall apparatus. Decide between constant volume, bypass and VAV designs, taking the building's exhaust capacity and energy costs into account. Specify liner material: fibre-reinforced cement or epoxy for general use, stainless steel or polypropylene for acids. Consider sash type, vertical or combination, face velocity monitoring, service fittings and base cabinet for storage. Confirm whether perchloric acid or radioisotope work requires a specialised hood instead. Coordinate duct size, fan capacity and make-up air with the building engineer, and plan for containment testing.