A local exhaust ventilation system serving several hoods is a connected air network rather than a collection of independent extraction points. Every hood, branch duct, junction, damper, air cleaner, main duct and fan forms part of the same pressure and airflow relationship. Sharjah OSHJ-GL-12 Local Exhaust Ventilation, Version 1 Rev 0, September 2021, issued by the Sharjah Prevention and Safety Authority under the Occupational Safety and Health System in Sharjah, addresses this directly by stating that where an LEV system has several inlets, balancing is required to provide suitable airflow at each inlet. The four sibling resources cover respirable crystalline silica, industrial hygiene including ventilation and LEV assessment from the occupational hygiene angle, VOCs and indoor chemicals including adhesives, sealants and solvents, and construction dust, while this page remains confined to balancing LEV as engineering plant.
A branch connected to an LEV system does not receive airflow independently of the other branches. The fan has a fixed characteristic, while the duct network presents resistance that changes according to the arrangement and condition of the connected paths. Air therefore distributes itself through the available branches according to their relative resistance and the pressure produced by the fan at the system operating point. A short, wide or relatively unrestricted branch may take a greater share of the available airflow than a longer, narrower or more obstructed branch.
Adding another hood or branch creates an additional path through which air can flow. That path competes with the branches already connected because the fan does not automatically create whatever additional airflow the expanded network requires. The overall operating point may change, but the practical result can be reduced airflow through existing hoods. A system that performed adequately before modification may therefore become unbalanced even though the fan, air cleaner and original ductwork remain physically unchanged.
The effect may not be visually obvious. One hood may continue to draw strongly while another receives too little airflow to perform its intended function. A hood close to the main duct may dominate the network, while a distant branch experiences reduced flow. The system may still sound normal and may still produce a convincing indication at the easiest test point, yet the distribution across the complete network may no longer be suitable.
A balancing damper changes the resistance of the branch in which it is installed. Partly closing a damper increases resistance and generally reduces the proportion of airflow travelling through that branch. This may allow a greater proportion of the available airflow to pass through other branches. Opening a damper reduces branch resistance and may increase airflow through that path, but the resulting redistribution can reduce airflow elsewhere.
A damper does not create additional total air. It reallocates the airflow and pressure available from the fan and connected network. Closing a dominant branch may improve another hood, but it can also increase losses, alter main-duct conditions or move the system to a different operating point. Opening every damper fully does not guarantee adequate performance at every hood, because the lowest-resistance paths may still take airflow at the expense of the others.
Balancing is therefore not simply a matter of placing damper handles at visually similar positions. Two branches with dampers set at the same angle may have very different lengths, diameters, fittings, hood losses and contamination levels. Suitable settings must be established from measurements across the system. The final arrangement must reflect the actual relationship between the hoods, branches, main duct, air cleaner and fan rather than the apparent position of individual controls.
A single measurement cannot establish that every inlet in a branched LEV system receives suitable airflow. A main-duct reading may indicate that a substantial quantity of air is moving through the system, but it does not show how that air is divided between the connected branches. A strong flow at one hood may conceal inadequate flow at another. An acceptable reading near the fan may also coexist with an unfavourable pressure condition in a remote branch.
Sharjah OSHJ-GL-12 Section 5.5.3 explains the underlying problem: "Where there are several inlets to an LEV system, balancing will be required to ensure that there is a suitable air flow at each inlet. Without balancing, one inlet may have an excessive air flow at the expense of others which are then not adequate." The Sharjah Prevention and Safety Authority Guideline sets minimum acceptable requirements under the Occupational Safety and Health System in Sharjah. It does not treat overall fan operation as proof that every hood is effective.
Sharjah OSHJ-GL-12 Section 5.6.2.2 provides the practical measurement structure by requiring air velocities to be measured at suitable test points identified in the system documentation, including hood faces, branch ducts and the main duct. The same section requires static pressure measurements at suitable documented test points, including all hoods, ducting, across the air cleaner and the fan. This per-hood and per-branch structure allows imbalance to be located rather than hidden inside one representative figure.
Measurements at hood faces help characterise conditions at individual inlets. Branch-duct measurements show how airflow is distributed through separate paths. Main-duct measurements provide information about the combined system, while static pressure measurements help identify where resistance or pressure conditions differ from the documented arrangement. None of these locations replaces the others in a system with multiple inlets.
A balanced system depends on damper positions remaining consistent with the measured and documented configuration. Where a damper is adjusted after testing, the airflow distribution may change even though no hood, duct or fan has been replaced. An operator seeking stronger extraction at one workstation may open a local damper and unintentionally deprive another branch of airflow. A maintenance worker may close a damper to reduce noise or draught and leave the system operating outside its documented balance.
An adjustment becomes particularly problematic when it is untraceable. Without a record of the original setting, the reason for the change, the person making it and the measurements taken afterwards, the documented balance no longer describes the operating plant. A previous set of hood, branch and main-duct results cannot be relied upon if the resistance of one or more branches has subsequently been altered.
Damper settings may therefore need physical identification appropriate to the plant, supported by system documentation showing the intended arrangement. The supplied UAE material does not prescribe a particular marking method, locking device or change-control form. Those details must not be presented as regulatory requirements. The regulatory point supported by Sharjah OSHJ-GL-12 is that several inlets require balancing and that suitable documented test points include hood faces, branches and the main duct.
A controlled adjustment is not necessarily prohibited. System alterations, changed processes, replaced hoods or modified branches may make rebalancing necessary. The essential engineering issue is that the new distribution must be assessed across the connected network. Restoring one hood without checking the remaining inlets can simply transfer the deficiency to another location.
Balancing begins with an understanding of the system as installed. The documented hood arrangement, branch routes, duct sizes, damper locations, air-cleaner position, main duct and fan should correspond with the physical plant. Missing branches, unrecorded modifications, damaged flexible connections, blocked ducts or changed hood openings can all affect distribution. Sharjah OSHJ-GL-12 requires the relevant velocity and static-pressure test points to be indicated in the system documentation, making the documentation part of the examination structure.
Where results differ between branches, damper position is only one possible cause. A restricted branch, contaminated duct, obstructed hood, damaged connection or altered process enclosure may create additional resistance. Conversely, a disconnected duct or unintended opening may reduce resistance and allow one path to take an excessive share of the available airflow. Measurements across the air cleaner and fan can help place branch results within the wider system condition.
The balancing process should not be reduced to repeated damper movement without recording the corresponding measurements. Each change affects the network, and an apparent improvement at one inlet may be accompanied by deterioration elsewhere. Measurements at the documented hood, branch and main-duct points provide the evidence needed to determine whether the revised distribution remains suitable throughout the system.
A system may also be incapable of achieving suitable airflow at every inlet through damper adjustment alone. Where the fan cannot provide the required performance against the resistance of the complete network, restricting one branch merely redistributes an insufficient total. The located UAE material does not prescribe the engineering remedy for every such condition. It does, however, prevent an adequate result at one inlet from being treated as proof that all inlets are adequate.
Sharjah OSHJ-GL-12 is the located UAE source that expressly addresses balancing several LEV inlets. As a Sharjah Prevention and Safety Authority Guideline, it sets minimum acceptable requirements under the Occupational Safety and Health System in Sharjah. Section 5.5.3 establishes the need for balancing, while Section 5.6.2.2 requires velocity and static-pressure measurements across the documented hood, branch and wider-system locations.
ADPHC Code of Practice CoP 52.0 Local Exhaust Ventilation, Version 4.1, dated 27 February 2026, is issued by the Abu Dhabi Public Health Centre under ADOSH-SF and is mandatory for employers in the Emirate of Abu Dhabi. The word "damper" appears once in the whole Code. Its single appearance is in Section 3.4(a), where laboratory HVAC must be designed to close automatically or, as a minimum, dampers must be capable of local control to avoid toxic-material ingress into HVAC systems. That provision concerns laboratory HVAC protection and does not establish requirements for balancing LEV branches.
No UAE authority located sets a balancing tolerance, a permitted percentage difference between branches or a per-hood pass criterion of any kind. Sharjah OSHJ-GL-12 requires suitable airflow at each inlet and measurements at the documented system locations, but it supplies no numerical balancing tolerance. ADPHC CoP 52.0 contains no general LEV balancing provision. A tolerance imported from another jurisdiction or selected from external guidance must not be represented as a UAE regulatory value.
Sharjah OSHJ-GL-12 is issued by the Sharjah Prevention and Safety Authority and is a Guideline setting minimum acceptable requirements in the Emirate of Sharjah. ADPHC Codes of Practice are mandatory for employers in the Emirate of Abu Dhabi.
Sharjah OSHJ-GL-12 Sections 5.5.3 and 5.6.2.2; ADPHC CoP 52.0 Version 4.1, dated 27 February 2026.
No. Sharjah OSHJ-GL-12 Local Exhaust Ventilation, Version 1 Rev 0, September 2021, issued by the Sharjah Prevention and Safety Authority under the Occupational Safety and Health System in Sharjah, states in Section 5.5.3 that several inlets require balancing so that suitable airflow is available at each inlet, because one inlet may otherwise receive excessive airflow at the expense of others. Section 5.6.2.2 requires air-velocity measurements at documented test points including hood faces, branch ducts and the main duct, together with static-pressure measurements at documented points including all hoods, ducting, across the air cleaner and the fan. A result from one hood or one main-duct point cannot establish the condition of every connected branch.
Opening a damper reduces resistance in that branch and may increase its share of the available airflow, but it does not itself create additional total air. The change may reduce airflow through other connected branches. Sharjah OSHJ-GL-12, a Sharjah Prevention and Safety Authority Guideline setting minimum acceptable requirements under the Occupational Safety and Health System in Sharjah, requires balancing where several inlets are present and requires measurements at documented hood, branch, main-duct, air-cleaner and fan locations. The Guideline does not state that damper adjustment alone can correct every system deficiency.
No UAE authority located sets a balancing tolerance, a permitted percentage difference between branches or a per-hood pass criterion. Sharjah OSHJ-GL-12 Section 5.5.3 requires suitable airflow at every inlet and Section 5.6.2.2 requires measurements across the documented network, but the Sharjah Prevention and Safety Authority Guideline states no numerical tolerance. ADPHC CoP 52.0, issued by the Abu Dhabi Public Health Centre under ADOSH-SF and mandatory for employers in the Emirate of Abu Dhabi, uses the word "damper" only in Section 3.4(a) concerning local control of laboratory HVAC to prevent toxic-material ingress, not LEV balancing. Any externally selected tolerance must therefore be identified as an external engineering criterion rather than a value established by the located UAE sources.
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