THE DEFICIENCY OF CAPTURE VELOCITY LEV SYSTEM DESIGN METHOD IS THAT IT CANNOT PREDICT THE ACTUAL EFFICIENCY OF WORKERS EXPOSURE REDUCTION .
Yes you read that right. The current practice of design based on capture velocity is very empirical and it does not determine the efficiency in airborne concentration removal or reduction by the LEV System.
THE CAPTURE VELOCITY METHOD is the common approach in designing the engineering control system specifically the local exhaust ventilation systems.
Capture velocity is determined based on the minimum disruptive air flow that can reasonably be attained. There are two type o hoods which are exterior and enclosures.
The exterior hood is located as close as possible to the source and is shaped to control the area of contaminant release.
For enclosures where contaminants from the process are released inside the hood, the air flow required for contaminant control is simply calculated by multiplying the inward air velocity needed to prevent escape by the area of openings into the enclosure.
Regardless of its common usage, the capture velocity method has always been questioned for various reasons. The capture velocity method does not take directly into consideration the quantified amount of emission at point source, the effect of contaminant source momentum energised by thermal, mechanical and chemical energies, cross drafts or other air disturbances. It is purely based on common practice and cross draft at the emission plane.
There are also difficulties when the capture velocity method is applied to large contaminant sources. The centreline velocity formulas do not account for sources located off the hood centre line. As such there will be issues such as condensation and settlement when the air properties are not of standard air.
There are difficulties also when velocity fields are determined for more complex exhaust openings. Multiple exhaust opening disrupt the air flow direction from one point to another, causing difficulties on efforts to determine the air flow pattern from emission point to the hood while being away from the breathing zone to ensure minimal introduction to the zone which could mitigate the route of entry for contaminant into workers body.
WHAT IS THE IMPACT OF THE CURRENT CAPTURE VELOCITY DESIGN METHOD DEFICIENCY.
Most of the time, the capture velocity Dewika method will oversize the system. Oversized system will have these impacts on overall operations :-
High exhaust flow rate which will require higher make-up air rate.
High exhaust flow rate which will need more cooled / air-conditioned air as the make-up air.
High exhaust flow rate crates extremely high negative pressure in room. Failing to add adequate make-up air system will have impact on fan efficiency and overall enclosure infiltration and exfiltration.
High exhaust flow rate will need higher fan motor amps draw to operate, causing over capital investment for installation and high operation of expenditures.
High exhaust flow rate will have higher rate of wear and tear.
High exhaust flow rate can be a concern for noise at the fan, ducting and exhaust hoods, and extreme draft across the room.
High exhaust flow rate may cause eddy current based on position of the worker.
HOW TO OVERCOME THIS SETBACK
While ACGIH and other professional bodies work on the design improvements continuously, the designer and the end user team can tweak some design and operation settings to close the gaps identified above. They are as follow :-
Quantify the emission concentration of the airborne hazard’s source.
Assess the area and personal exposure trend without the engineering control equipment.
Choose the criterion of capture velocity taking both room cross drafts and emission quality.
During commissioning, conduct several test (check the concentration while the ventilation system is operating). Set the exhaust flow rate based on concentration range . Objective is to be bare minimum so that the engineering control will be deemed as power efficient.
Set the make-up air based on exhaust flow rate and room pressurisation (maintain room pressure within <15%).
Pick the right designer or system provider for your system to address the factors highlight above.
