Every industrial exhaust system components selection affects the overall capture efficiency. The hood design determines how much contaminant gets pulled into the system. The duct sizing determines how much pressure the fan must overcome. The filter selection determines what gets captured versus what gets discharged. Getting each component right matters.
The hood is the a proven line of defense. Enclosed hoods capture the most efficiently because they surround the contaminant source. Slot hoods work for linear sources like conveyor belts or welding seams. Canopy hoods sit above the source and capture rising contaminants by thermal buoyancy.
We tested three hood types for a grinding operation that generated metal dust. The enclosed hood captured 98 percent of the dust at 800 CFM. The slot hood captured 85 percent at 1,200 CFM. The canopy hood captured only 60 percent at the same 1,200 CFM. The enclosed hood won on both capture efficiency and airflow requirement.
Duct velocity must stay high enough to keep particles suspended. Horizontal dust ducts need at least 3,500 feet per minute. Vertical ducts need 4,000 to 5,000 feet per minute. The exact velocity depends on particle size and density. Wood dust settles at lower velocities than metal swarf.
We designed the duct layout for a cabinet making shop. The main trunk ran 200 feet from the farthest station to the collector. We sized it at 24-inch diameter to maintain 3,800 fpm velocity. Branch ducts ranged from 6 to 10 inches depending on the station airflow requirement. The total system static pressure was 2.8 inches of water.
Baghouse filters use fabric bags that capture particles on the surface. They handle fine dusts down to 0.3 microns with 99.9 percent efficiency. Cartridge filters use pleated media in a compact housing. They achieve similar efficiency in a smaller footprint. Electrostatic precipitators use charged plates to capture particles. They work well for sub-micron particles but require more maintenance.
We specified PTFE-membrane cartridge filters for a pharmaceutical powder handling facility. The membrane captured sub-micron particles that the standard polyester filters had been leaking through. The downstream air quality improved from 15 milligrams per cubic meter to under 1 milligram per cubic meter.
The fan can sit before or after the collector. Placing it before means the fan handles dirty air, which causes wear. Placing it after means the fan handles clean air, which extends component life. We converted a system from pre-collector to post-collector fan placement at a cement plant. The fan bearing replacement interval went from 6 months to 3 years.
Ducts carrying combustible materials need fire dampers at every floor penetration and blast dampers near the collector. We specified fire dampers rated at 165-degree activation and blast dampers within 10 feet of the dust collector for a wood products facility. The placement followed NFPA 91 requirements.
The hood face velocity determines whether contaminants enter the exhaust stream or escape into the workspace. Too low and the contaminant drifts away before the airflow captures it. Too high and you waste energy moving unnecessary air volume. The target face velocity depends on the contaminant type and the ambient air movement in the room. We measured face velocities at 12 hood stations in a machining facility. Three hoods had velocities below the minimum requirement because duct leaks downstream had reduced the total system airflow. Sealing those leaks restored the face velocity to the design value.
Duct material thickness follows standards based on the duct diameter and the static pressure. Thin gauge duct collapses under negative pressure. Thick gauge duct costs more than necessary. The Fabrication Air Conditioning Contractors Association publishes thickness tables that match the duct size and operating pressure to the minimum required material gauge. We specified the duct thickness for a 300-foot duct run at a manufacturing plant. The main trunk at 30-inch diameter needed 16-gauge steel. The branch ducts at 8-inch diameter could use 22-gauge. The material selection saved 12 percent on the ductwork cost compared to using a uniform thickness.
Filter loading changes the system performance over time. A clean filter adds minimal resistance. A loaded filter can double the static pressure. The fan responds to the increased pressure by drawing more current and moving less air. We installed differential pressure gauges across the filter banks at a woodworking facility. The gauges showed when the pressure drop reached the replacement threshold. The filter change interval was 45 days during peak production and 90 days during the slower season. The gauge-based replacement schedule saved 20 percent on filter costs compared to a fixed calendar schedule.
System balancing ensures each hood station receives its designed airflow. Without balancing, the hood closest to the fan gets most of the airflow while distant hoods get very little. We balanced a 20-station exhaust system at an electronics assembly plant. The initial test showed airflow variation of plus 40 percent to minus 60 percent from the design values across the stations. After adjusting the balancing dampers on each branch duct over four test rounds, the variation dropped to plus or minus 10 percent. The balanced system captured the solder fumes at every station consistently.
The material compatibility between the ductwork and the transported contaminants determines the system lifespan. Acidic fumes corrode standard galvanized steel within months. Alkaline dusts react with aluminum ductwork. We specified chemical-resistant duct material for a plating facility that handled hydrochloric and sulfuric acid vapors. The main duct used polypropylene construction that resisted the acid attack. The connections used welded joints instead of bolted flanges to eliminate leak paths. The system has operated for 7 years without corrosion-related failures, compared to 14 months for the galvanized steel system it replaced.
Duct support design prevents sagging that creates low points where dust accumulates. The support spacing depends on the duct diameter, the material gauge, and the installed weight including any accumulated dust. Horizontal ducts need supports closer together than vertical runs because gravity works against the horizontal span. We designed the support system for a 400-foot duct run at a woodworking facility. The main trunk at 28-inch diameter received supports every 8 feet. The branch ducts at 10-inch diameter received supports every 10 feet. All supports included vibration isolation pads that prevented the fan vibration from traveling through the duct structure.
Access doors and cleanout openings enable maintenance without dismantling the ductwork. The placement of access points follows a pattern that allows inspection and cleaning of every duct section. We specified access doors at every duct transition, every change of direction, and at intervals of 30 feet along straight runs. The access doors used quarter-turn fasteners that allowed quick opening for inspection. The cleanout openings at the bottom of vertical duct sections enabled dust removal without removing the duct panels. The access design reduced the annual maintenance time by 60 percent compared to the previous system that required duct disassembly for cleaning.
The hood capture velocity calculation determines the airflow needed to pull contaminants into the exhaust system. The calculation considers the contaminant release velocity, the distance from the source to the hood opening, and the room air movement that opposes the capture flow. A higher release velocity requires a higher capture velocity. Greater distance requires more airflow to maintain the capture at the source point. Room air currents from doors, windows, or other equipment add to the required capture velocity. We calculated the capture velocity for a welding hood at a fabrication shop. The welding process released fumes at two hundred feet per minute. The hood sat twelve inches from the weld point. The room had a cross draft of fifty feet per minute from the HVAC supply. The required hood airflow was eighteen hundred CFM to maintain capture at the weld point.
Duct velocity selection balances the pressure drop against the particle transport requirement. Low velocity reduces the friction loss and the fan energy consumption. High velocity keeps particles suspended and prevents settling in horizontal duct runs. The minimum transport velocity depends on the particle density and size. Light particles like wood dust transport at three thousand feet per minute. Heavy particles like metal chips need four thousand to five thousand feet per minute. We selected the duct velocities for a multi-station dust collection system at a metal finishing plant. The main trunk ran at three thousand five hundred feet per minute. The branch ducts near the grinding stations ran at four thousand five hundred feet per minute to handle the heavy metal chips.
Filter media selection matches the material properties to the contaminant characteristics. Cotton and polyester media handle general dust at temperatures below one hundred eighty degrees Fahrenheit. Glass fiber media handles higher temperatures up to three hundred degrees. PTFE membrane media captures sub-micron particles with minimal pressure drop. We selected the filter media for a ceramics manufacturing facility that produced fine silica dust at elevated temperatures. The process air ran at two hundred twenty degrees Fahrenheit and contained particles down to zero point five microns. We specified glass fiber bags with a PTFE membrane coating that handled both the temperature and the fine particle capture requirement. The filter achieved ninety-nine point nine percent capture efficiency at a pressure drop of two inches of water.
System balancing adjusts the airflow distribution so each hood station receives its designed volume. The balancing process starts with the station farthest from the fan and works toward the fan. The technician measures the airflow at each station with a flow hood or a pitot tube traverse. The balancing dampers on each branch duct adjust the resistance to equalize the airflow. We balanced a twenty-four station exhaust system at an electronics assembly plant. The initial measurement showed airflow variation of plus forty percent to minus sixty percent from the design values. After four rounds of damper adjustment, the variation dropped to plus or minus eight percent. The balanced system captured the solder fumes consistently at every workstation.
Commissioning verification confirms that the installed system meets the design specifications. The commissioning process includes a document review, an installation inspection, and a functional performance test. The document review checks the equipment data sheets, the duct calculation sheets, and the control sequence descriptions. The installation inspection verifies the equipment placement, the duct connections, and the electrical wiring. The functional test runs the system at design conditions and measures the actual performance. We commissioned a ventilation system at a chemical processing facility. The commissioning report identified seventeen items that needed correction before the system could be accepted. The corrections included duct seal repairs, damper adjustments, and control parameter tuning. The final performance test confirmed that the system met all design requirements.
Documentation requirements for ventilation system projects include the equipment data sheets, the duct calculation reports, the installation drawings, the commissioning test results, and the operation and maintenance manuals. The documentation package serves as the reference for future maintenance work and regulatory inspections. We compiled the documentation for a tunnel ventilation project that included thirty-six jet fans, four supply fans, and an emergency smoke control system. The documentation package contained two hundred pages of technical drawings, one hundred fifty pages of equipment data, and fifty pages of commissioning test reports. The complete package was delivered to the facility management team in both paper and digital format.
Production base: Guangdong Xinfang Co., Ltd
Address: No. 33, Chuangye Road, Deqing Industrial Park, Zhaoqing, Guangdong, China
Office address: Building 5, No. 10, Yongfeng Road, Yongshan village, Shiqi Town, Panyu District, Guangzhou, Guangdong, China
TEL: 0086-400-088-0118 EMAIL: wudi@xinfeng198.com
