After a local exhaust ventilation (LEV) system is installed, debugged and put into use, many factories simply let it run on its own. The problem with this approach is that the performance of the LEV system gradually declines rather than suddenly breaking down. By the time the factory notices that the smoke and dust are abnormal, it means that the concentration in the operator’s breathing zone has exceeded the standard for quite some time.
I. Before inspection, it is necessary to understand: What are the components of the LEV system and what do they do?
A complete LEV system consists of five core components:
The hood is the entrance of the system, installed near the pollution source. Its task is to generate sufficient air flow velocity to draw in pollutants from the operation point. The form of the hood can be a surrounding fume hood, a receiving umbrella hood, or an external side suction hood.
Ductwork serves as the conveying pathway, carrying contaminated air captured by the hood to the air cleaning device and the fan. Ducts may be circular or rectangular and typically include elbows, reducers, tees, and dampers for airflow regulation. Leakage and blockage within the ductwork are among the most insidious problems—they are difficult to detect and often go unnoticed until performance significantly degrades.
The air purification device is responsible for separating pollutants from the air. Common purification devices include filter dust collectors, scrubbers, electrostatic precipitators, and activated carbon adsorption tanks. The purification device has its own differential pressure gauge or pressure gauge, which is the most direct tool for judging whether it is blocked.
The fan is the power source of the system, generating the pressure difference required for air flow. The fan can be centrifugal or axial flow, usually driven by a motor, and the power is transmitted through a belt or coupling. The fan is the “heart” of the system; if it stops, the entire system will stop.
The exhaust outlet is the place where the treated air is discharged back into the atmosphere. The exhaust outlet should be set in a position where it will not be inhaled by people, usually at a high place and discharged upwards.
II. How to Use the LEV System: A Few Things Operators Need to Know
The correct usage of the LEV system should at least include the following points:
Start-up sequence. Start the LEV system first, and then begin the operation that generates pollution. This sequence sounds simple, but it is often reversed in actual workshops. The shutdown sequence is the reverse: stop the operation that generates pollution first, and then turn off the LEV system to allow it to extract any remaining pollutants.
Keep the hood opening clean. Dust accumulation, hanging dust, or obstruction by workpieces on the hood opening will reduce the capture efficiency. Operators should take a look at the hood opening before starting work to ensure it is not blocked or shifted.
Do not bypass the system manually. On enclosure hoods, operators sometimes lift the baffle, pull the fume hood glass door too high, or make holes in the side panels of the grinding cabinet. These actions are taken for convenience, but the cost is that the airflow short-circuits through the openings, and the capture efficiency of pollutants drops significantly. The correct approach is: if the hood design affects operation, it should be reported to management for design modification, rather than being modified by the operator themselves.
Observe the operation indicators. Many LEV systems are equipped with simple indicators: pressure differential gauges for filters, ammeters for fans, or a simple air volume switch. Operators should know the normal reading range of these gauges and know who to report to when they see abnormal readings.
III. How to Identify Damaged Components: Five Key Inspection Points
Routine inspections do not require disassembling the equipment. Just observe, listen, and touch – but make sure there is no risk of burns or electric shock before touching.
Inspection of the hood. Observe: Is there any dust accumulation, damage, or deformation at the hood opening? Is the flange still in place? Are there any cracks in the flexible hood arm? For external hoods, check if the distance between the hood opening and the pollution source is maintained at the designed position – many problems arise when the hood is pushed too far away. Listen: Is there any whistling sound near the hood opening? Whistling usually indicates that the air velocity at the hood opening is too high or there are sharp edges causing turbulence.
Inspection of the ductwork. Observe: Is there any rust, cracks, or obvious deformation on the surface of the duct? Is the sealant at the flange connection cracked? Is the damper adjustment handle in its original position? Listen: Is there any abnormal vibration or creaking sound from the duct? This could indicate loose internal components or loose supports. A more direct way to judge: Place your hand on the outer wall of the duct and feel if there is any significant vibration.
Inspection of the purification device. This is the most important part of the routine inspection. Observe: Is the reading on the differential pressure gauge within the normal range? A reading lower than normal may indicate a damaged filter bag or a leak in the duct; a reading higher than normal suggests severe filter material clogging, requiring cleaning or replacement. For scrubbers, check if the spray water is circulating normally and if the water level in the sump is normal. For activated carbon tanks, check if there is any unpleasant odor coming from the outlet – if so, it may indicate that the adsorbent is saturated.
Inspection of the fan. Observe: Does the fan have any abnormal shaking during operation? For belt-driven fans, check if the belt is loose, slipping, or cracked. Listen: This is the most effective way to determine the fan’s health status. Worn bearings will produce a sharp metallic friction sound; a clogged or damaged impeller will produce periodic knocking sounds; a slipping belt will produce a high-pitched whistling sound. Touch: Under safe conditions, place the back of your hand near the motor housing to feel the temperature. Motor overheating may be a sign of overload, bearing damage, or poor heat dissipation.
Inspection of the exhaust outlet. Observe: Is there any visible dust or smoke in the exhaust air? If so, it may indicate that the purification device has failed. Listen: Is there any abnormal wind sound or whistling at the exhaust outlet? This could mean that the exhaust outlet is partially blocked.
IV. Simple Inspection to Confirm System Performance: Verification Methods without Instruments
Routine checks do not require professional anemometers. The following methods can be used during daily inspections:
Smoke tube method. This is the most intuitive method for verifying capture effectiveness. Release tracer smoke at the operation point and observe whether the smoke is directly and smoothly drawn into the hood opening. If the smoke is blown sideways, swirls in front of the hood opening, or first drifts towards the operator and then is drawn back, these are all signs of problems with the air flow organization. Smoke tubes are inexpensive and should be standard equipment in every workshop.
Fine thread method. In the absence of a smoke tube, a small piece of fine thread or the smoke from a cigarette can be used as a substitute. Fix the fine thread in front of the hood opening and observe its movement direction. The fine thread should steadily point towards the hood opening. If the fine thread wobbles or drifts outward, it indicates unstable air flow or incorrect direction.
Listening method. Operators familiar with the normal operation sounds of the system can determine abnormalities within a few seconds after startup. Changes in the fan sound, new vibration sounds in the air ducts, or whistling at the hood opening – these are all signals that require further inspection.
Hand feel method. Place your hand near the hood opening and feel for any obvious suction. This method is rather crude, but for external hood caps, if you place your hand at the operation point and feel no suction at all, there is definitely a problem. For surrounding hood caps, place your hand at the edge of the operation opening and you should be able to feel the air flowing inward.
Record and compare method. This is the most professional but also the simplest method: Record the results of each inspection. The key is to record the readings of the pressure differential gauge. Without records, there is no comparison, and without comparison, it is impossible to detect a slow deterioration trend.
The air filter supplier TrennTech from Frankfurt found in the factories it has served that the average mean time between failures of the LEV systems of those customers who have daily inspection records and whose operators have received basic training is more than three times longer than that of customers without such records and training. This is not a difference in equipment quality, but a difference in management habits.
The purpose of regular inspection is to keep the LEV system always at its designed performance level. A system installed without maintenance is like buying a fire extinguisher and leaving it in the corner for ten years without checking – when it’s really needed, it may not work at all. Just three minutes a day or ten minutes a week can prevent the passive situation of “not realizing it’s broken until you smell something”.