Selection of duct velocities
LOCAL EXHAUST VENTILATION SYSTEM means to exhaust the local exhaust ventilation system at the local point of generation. The design approach is taken by capitalizing on the phenomena of the emission source, dispersion along the pathway driven by draft, and the adjacent presence of the recipient's breathing zone. The hood of an LEV system which is the geometrical shape suiting the contaminant dispersion energy, phase, and buoyancy is to be designed to either enclose or convey inwards the contaminant.
The hood needs to be made in a geographical shape covering the entire plane of emission. The air displacement which is being created by the negative pressure generated by the hood needs to be focussed on the entire plane or radius of emission generation and it’s dispersion energy.
Once that is done, the captured contaminant is to be channeled into the ducting. The usage of energy to create air movement outside the hood or towards the hood is completely different from a scenario of channeling through contaminant-laden air in a much enclosed and narrower passage. There is more resistance to overcome such as duct overall size, wall friction, fittings friction, and system effect. The physical properties of the contaminant have also influenced the velocity. Too much velocity may cause turbulence, noise, vibration, abrasion, and rupture. Too low of velocity will cause settlement, scaling, condensation, or even imbalance in multiple branches. Improper sizing will either increase the cost of installation unnecessarily or it will save the capital Investment but the fan will be oversized incurring higher power. This actually will end up with customers having higher costs of operation which eventually will defeat the actual intention to have a cost-effective project. As to enable an effective system running within optimized power consumption, the selection of duct velocities is equivalently important to the hood design basis.
Selection of Duct Velocities is depending on the:-
1. Type of contaminant especially particulate aerosols which can settle at low velocity
2. Phase change as some chemicals may condensate at lower velocity causing dripping at the connection seam.
3. Noise level generated in the system may not be conducive to the working environment and may even become a noise hazard exceeding 82 dB(A).
4. Lower annual operating cost especially for the system which is not affected by low transport velocity such as non-condensing vapor and gaseous.
HOW TO AVOID DESIGNING DUCT SIZE PURELY BASED ON THEORETICAL NUMBERS WHICH MAY NOT BE ACCURATE IN TERMS OF SETTLEMENT AND CONDENSATION PATTERN? WHAT IF THE DUCT DESIGN BE MADE FURTHER OPTIMIZED REDUCING THE OVERALL OPERATION COST?
Yeah, you read it right. Aim for lower operating costs and not installation costs alone. The installation cost of an LEV or a GV system is purely depending on the transport velocity number. However, most of the time the physical properties of the contaminant are not really known in terms of the pattern of settlement of condensation in comparison to the range chosen or the common range adopted in design. The issue with the problem is not only an oversized motor or oversized ducting but poor periodical baseline and periodical LEV GV assessment stating the duct performance has failed and needs improvement when in actual fact there is nothing wrong with the ducting performance. All these setbacks and repercussions can be avoided if we just follow these simple steps.
DESIGN THE DUCTING BASED ON STANDARDS THE OBSERVE THE SETTLEMENT OR CONDENSATION PATTERN BY ADDING A VIEWPORT / CLEAN-OUT PORT
Start designing the duct based on the standard range of velocity and ensure add viewports or clean out ports along the ducting.
Periodically inspect the internals of the duct to assess the settlement or condensation pattern. Optional duct and operating costs can be determined for other duct velocities for comparison. If the settlement or condensation does not take place, the transport velocities can be maintained or further reduced if the duct enlargement cost is cheaper than the overall operating cost of the fan motor power consumption. Similarly, if the settlement and condensation are imminent, an increase in the transport velocities can be done.
The practice above can be done by also keeping in mind that the optimum economic velocity will normally range from about 2,000 fpm to 4,000 fpm while a velocity of 2,500 to 3,000 fpm will result in an equivalent total annual cost that approximately equals the true optimum.
