4 things to know about duct transport velocity in LEV Systems.
The baseline duct transport velocity value must be made to meet the design criterion. However periodically the ducting internal must be inspected to monitor the scaling and settlement. In order to do this, insist your system installer add viewports along the ducting system. If we were to refer to preventive checks manuals such as ACGIH Industrial Manual for Operation and Maintenance, or JKKP guidelines to conduct LEV or GV System assessment, the need to inspect the internal walls is clearly emphasized. However, it is never put into practice, neither by the system installer nor demanded by the engineering department of the end users.
Duct transport velocity above recommended value can be chosen as the design basis, but the high static pressure incurred by the high transport velocity must be calculated in the sizing of the fan. For example, in the laboratory ventilation system settings, if the thermoplastic ducting is chosen to convey vapor type of contaminant from the hood to chimney, at transport velocity above 2000 fpm to make the ducting cost cheaper, the static pressure incurred must be added to the total system static pressure. As long the fan can deliver the flow rate at these rated system static pressure, it is applicable. However, do calculate the noise generated by the high transport velocity and its reveberation along the ducting to ensure the overall noise level in the enclosure is within the comfortable and safe level.
Low transport velocity may cause settlement, scaling, and condensation. However, the keyword is MAY. If a hygiene technician concludes in his or her’s periodical assessment report that you will need to rectify the low transport velocity, you must insist to perform the internal wall inspection first. Most importantly never easily conclude to upgrade ducting system as it is the most expensive corrective action to be taken and most of the time it is never needed. Do not merely fall prey to transport velocity numbers to spend money on a system without correlating with the actual condition of the walls. Hire a good ventilation expert to assess the situation and validate the report.
You do not increase the flow rate to resolve low transport velocity but change the size of the ducting accordingly if at all it is needed for systems handling condensating type of vapours or particulate / liquid aerosols. However, take this action upon completion of the internal wall inspection. Local exhaust ventilation system flow rate is only based on hood capture velocity or machine flow rate recommended by the manufacturer. Similarly, general ventilation system flow rate is dependant on air change per hour. Recommendation to increase flow rate to rectify poor transport velocity will cause :-
(i) high capture velocity which will create turbulence in the hood and liberate contaminant away from the hood instead of local removal.
(ii) Increase unnecessary air change per hour in an enclosure wasting power via air-conditioned air wastage.
(iii) Increase noise in the room above recommended level causing nuisance or occupational hazard.
(iv) Imbalance pressurization causing unwanted infiltration and exfiltration from adjacent rooms.
(v) Reduces the air cleaning device efficiency as dwelling time for contaminant removal will be affected.
