Common errors in assessing the performance of an enclosed hood for an LEV System.
The enclosure type of hood provides the best control for preventing air contaminants from entering the breathing zone of a worker. This is because an enclosure encapsulates the airborne emission at the point of generation and prevents its liberation via the narrowed walls. However, the limited volume of any enclosure will need activation of the ventilation system for continuous removal. In layman's terms this means, there has to be movement of air inside the enclosure conveying the contaminant.
Regardless of a hood being enclosed it has air movement inside. Velocity is the control factor in conveying the airborne contaminant from the enclosed point of origin to the air cleaning device and chimney. Enclosed hood must be assessed for its velocity performance. Taking the duct transport velocity and evaluating the performance of the hood is wrong. One must know what was the design capture velocity if the enclosure.
An adequate amount of flow rate must be provided to enable the negative pressure from the duct will generate air displacement pulling the air from outside the hood into the enclosure and convey the airborne contamination away from the point of generation. The question at this point is, what is the proper way to determine the adequate flow rate ? What will be the accurate way to measure the performance of the capture efficiency ?
Any local ventilation system design performance testing will be needing accurate sampling strategy and assessment criterion. Acceptable standards such as ACGIH Industrial Ventilation Manual stipulate recommended criterion. However these standard does not clearly define the approach. As such, most of the Industrial Hygiene Technician with minimal knowledge on design approach, tend to make major errors by taking ducting velocity or even declaring enclosure is a good form of LEV system without any ventilation assessment at the hood.
The common errors made in ignoring the hood’s internal velocity can be easily rectified by grasping the fundamental that a ventilation system is all about motions of air. The intensity of the air motion versus the suspension of the airborne contaminant is what dictates the performance of any LEV system. This is also the case when the hood or the aspiration point is fully enclosed. This means when one designs an enclosure, the cross-section of the enclosure should be taken as the plane of air movement based on the opening and emission point. The ducting must be placed in a suitable location to create negative pressure so that the variance of pressure in the enclosure will create air displacement from an opening to the ducting. Baffles and plenum must be added to enable homogenous airflow. Uniformity of airflow in the enclosure is very crucial to ensure the complete removal of the suspended airborne hazards. The selection of the capture velocity will be depending on the energy dispersion of the process. Enclosure hoods must be tested based on the design flow rate to determine the actual velocity across the cross-section. An enclosure hood must not be deemed effective without an accurate assessment.
The conservation of energy dictates that the air flow rate entering the hood is equivalent to the air moving into the ducting. This fundamental must be used to derive the enclose hood performance approach. The accurate flow rate which can be determined based on data extracted from the ducting can be used to calculate the velocity across the enclosure. The assessment of the homogenous flow across the enclosure must be made based on how the inlet duct is connected especially to check if it has a proper tapering, baffle plate, inlet access of air into the enclosure, and internal energy from the process which could create an imbalance in the internal draft.
As to conclude, any enclosure of a hood or machinery being connected to a ventilation system undergoes movement of air while conveying the suspended airborne hazards. These movements of air must be quantified for removal efficiency so that the hazard will not backflow into the process area in the event its generation of more than the removal rate.