Local exhaust ventilation system is not a fixed standard 5 component system but an industrial ventilation alternatives to be used as control mechanics based on concentration.
Lack of engineering acumen and stereotype by the book technician is the best description to be given to most of the registered industrial hygiene technicians nowadays.
Equipped with 5 days course experience and a competency certificate, they are running amok dictating all form of verdicts on system performance. In fact, instead of helping client to troubleshoot systems to meet back it’s peak baseline performance, they are doing policing works by findings faults. Some of the recent ridiculous experiences I have had with recent technicians whom I have engaged which are as follows :-
Conducting baseline testing while the bellow / vibration control components added before fan inlet found was observed to have collapsed. Explanation : Fan inlet and outlet are zone of highest energy. When there are obstruction at these area such as a collapsed bellow, a test must not be carried out as the system performance will not be representable for a baseline. The technician should have waited till the bellow is repaired before conducting the test.
Taking velocity pressure reading at location of 2 times diameter before and after elbow. Explanation : Many technicians do not understand the variance between static pressure and velocity pressure clearly . One cannot take velocity pressure reading at a region with high turbulence. Static pressure reading cannot be taken at the short spool as to make differential pressure pattern however the velocity pressure which is taken to calculate velocity needs to be in a laminar flow plane.
Recommending fan flow rate upgrade when then hood capture velocity is within criterion but the transport velocity is lower than the limit. Explanation : Flow rate of the system is purely defined by hood capture velocity, face opening, capture distance, or air change or any design basis of velocity application based on the objective. The basis is purely to be focused at the exhaust point. Flow rate enhancement cannot be done to overcome low transport velocity as it will create imbalance at the hood velocity, air cleaning velocity and will overall mess up the fan duty point. What needs to be done here is a physical check of the settlement or scaling in the duct and make adjustment to the duct size if the settlement or scaling is imminent. If it is not then a new criterion must be established.
Underutilising system static pressure measurement. Explanation : Static pressure measurement provides the pattern of differential pressure across resistance. The periodic reading can be used to gauge increase of resistance across elbows due to settlement, increase of resistance at damper due to best gate coating or failure, increase of resistance across air cleaning device due to clogging of filter face area and etc. This periodical reading must be thoroughly analysed and compared to narrow down hot spots which caused causes progressive increase of overall static pressure. Proper management of this date can trigger effective corrective or preventive maintenance to enable the system to run within acceptable range of system flow rate at all times.
Using high capture velocity without studying the impact on room pressure, make up inadequacy and air conditioned air wastage. Explanation : Regardless of standard text book giving a generic range of capture velocity let say about 100 to 200 fpm, a hygiene technician must equipped themselves with additional knowledge of recommended parameters from design perspective from reference such as ACGIH Industrial Ventilation Design Manual or NFPA or etc. These reference provides the need to run the velocity based on the airborne hazard dispersion energy, vapour pressure and etc. A balance must be struck between optimum capture velocity and power saving efforts. An oversize hood flow rate will not only increase the cooling load but will be running a higher ampere fan, consuming unwanted electric continuously.
Conducting assessment using self derived criterion without referring to design basis especially on how the fan duty point (flow rate and system static pressures) was established. Explanation : Any ventilation system is design to function based on the operation of the fan which is operating solely based on the duty point which is flow rate and system static pressure. This numbers are derived based on designer selection basis of the ranges such as air changes and capture velocity. One cannot figure this out or seem without referring to the design document. Selecting self derived criterion may wrong deem the system inadequate when in actual fact the system was designed to run at lower range of flow rate. In the event the design documents are not available, additional assessment such as static area monitoring, duct internal inspection, heat stress assessment and etc can be done to dictate the adequacy of the system and a reverse engineering design measure can be taken.
