Plenum Versus Tapered Ducting System
Ducting system in the ventilation system is an important component. This is because it carries the highest fraction in the overall capital investment. A wrong design in sizing the duct can affect the overall capital investment so much. The ducting sizing is also responsible in finalizing the sizing of the fan which dictates the operation cost based on the power consumption of the motor.
For a particulate aerosol or condensing type of hood, a poorly sized ducting system can create havoc in overall operation by either chocking the system or even create a potential fire hazard.
Ducting are also usually an area where the system providers will deliberately under design as to be competitive among their peers when its comes to closing a sales. End user whom are not experience in detecting the high usage of transport velocity causing high amperage operation, would be easily fooled by low capital investment duped with high operation cost, in the motor amp draw back and frequent wear and tear. That is why understanding the ducting system will be very crucial in system design assessment.
There are two type of Ducting system. They are Tapered and Plenum.
Tapered system maintains the transport velocity to ensure dust, mist and condensable vapours do not settle along the low velocity ducting areas.
Plenum system provides a low pressure loss path for air flows from the various branches and maintains a balance exhaust in all duct branches. Plenum enables connection of multiples branches with balance exhaust generation among the multiple exhaust registers / hoods.
Duct Delivery System, Options of Plenum Versus Tapered Routing
Advantages of plenum system compared to tapered system
Plenum requires testing and balancing done if branches are added or removed subjective the fan ability to cater for the total flow rate and fan static pressure.
System modification can be done by removing branches and manipulating fan flow rate using its performance curve to meet the demand of newly revised flow rate and system static pressure.
The plenum can work as settling or condensing chamber similar to knock out drum (to remove moisture) as pre-air cleaning device. However, proper removal mechanism must be adopted.
Note : The settling chamber or pre-cleaning device is limited in usage for sticky and combustion material. The removal of sticky material will be difficult. The collection of the contaminant in the plenum can post fire hazard.
Factors in plenum design
Flow rate distribution in each branches
Removal of settlement or condensation
Higher cost of installation due to plenum design but lower cost of operation as the pressure is controlled much lower
Friction estimation on long duct
Factors in tapered design
Eddies and turbulence caused by high transport velocities at junction entries causing regular preventive maintenance and abrasion issues.
Higher transport velocities for settling or condensing contaminant causing higher cost of operation as overall system static pressure will be much higher.
To counter the energy usage, streamlined components such as long radius elbows, branch entry of minimal angle, long run entry into the fan rather minimizing system effect by avoiding abrupt elbow entry. This approach may minimize operation cost and increase initial installation but in the long run it is a economical approach.
