How would you recommend Fan Positioning for a Ducted Fume Cupboard?
The fan is normally positioned so that as much (preferably all) of the ducting inside the building is at negative pressure, ensuring that if there is a leak in the duct within the building, clean air will be sucked into the duct rather than fumes being pushed out.
Fans are normally located outside the building and on new buildings it is best practise to position on a level section of the roof.
The duct outlet moves fumes away from the building. Where the outlet should be positioned is a complex task as many factors must be considered.
On some large industrial projects significant time and funds are spent establishing where the stacks should be fixed, the height and appropriate discharge velocity.
Every project is different, but it is not unusual to have stack heights of 2.0m and discharge velocities between 10 and 12 m/sec.
What is the ideal Duct Diameter?
Duct diameter is vitally important. Too large a diameter and the flow velocity will be too low to properly transit the fumes and too small a diameter will give excessive velocity generating noise, high pressure drops and system inefficiency.
Ducted fume cupboards are typically between 1000mm and 1200mm wide using 200mm diameter ducting.
Which the best material for Duct?
PVC is the most common ducting material attached to a fume cupboard. Polypropylene duct is chemically resistant but is much more expensive and normally only used in high acid content industrial applications. Metal duct is not suitable as it will react with some of the chemicals used.
How do I best set the Flow Velocity on a Ducted Fume Cupboard?
A Ducted Fume Cupboard is normally commission at an inflow velocity of 0.45 m/sec. The flow in the duct needs to be altered to allow this inflow to be achieved.
To accurately set the flow use an inverter to alter the frequency of the electricity to the fan ensuring the fan runs at the required speed. The system will also be quieter and more energy efficient as the fan is only doing the work it needs to rather than having to overcome an induced pressure drop.
Air that is being removed from the room via the Fume Cupboard and Ducting System will need to be replaced. If not, enough air is available into the room, flow through the cupboard will diminish and the cupboard’s low airflow alarm will sound.
For more information please call Monmouth Scientific on 01278 458090 and discuss your requirements with one of our experienced Technical Sales Team.