Kitchen exhaust systems are the most demanding part of a building's ventilation. Unlike standard AC ducts, these must handle high-velocity grease-laden air, extreme heat, and the constant risk of internal fires. In Malaysia, these systems are strictly governed by BOMBA and JKR standards to prevent kitchen fires from spreading through the rest of the structure.
Because grease is acidic and highly flammable, standard thin-gauge galvanized iron is often insufficient. High-performance kitchen ducts are built to these specifications:
Stainless Steel (316L/304): The preferred choice for durability and hygiene. It resists the corrosive effects of moisture and cleaning chemicals.
Heavy-Gauge Steel: Minimum thickness of 1.2mm (18 Gauge) or 1.6mm (16 Gauge) is used to ensure the duct can withstand an internal "Grease Fire" without collapsing.
Liquid-Tight Construction: All longitudinal seams and transverse joints must be continuously welded (usually TIG or MIG). Using flange gaskets or sealants alone is a fire risk, as grease will eventually leak through.
If a fire starts in the deep fryer, the exhaust duct becomes a horizontal chimney. To prevent this heat from igniting the ceiling or nearby services, two methods are used:
Fire-Rated Spray/Wrap: Specialized intumescent coatings or ceramic fiber wraps that provide a 2-hour or 4-hour fire rating.
The Insulation Standard: The insulation must be non-combustible. It ensures that the external surface of the duct stays below 140 degrees C even if the internal fire exceeds 1000 degrees C.
Clearance to Combustibles: A mandatory "Air Gap" (typically 450mm for uninsulated ducts) must be maintained from any flammable building materials.
A kitchen duct is only safe if it can be cleaned. Accumulated grease is the primary "fuel" for commercial building fires.
Grease Filters (Baffles): These are the first line of defense in the hood, designed to strip grease out of the air before it enters the ductwork.
Access Panels: Liquid-tight, fire-rated access doors must be installed at every change of direction (elbows) and every 3 meters of horizontal run to allow for manual scraping and steam cleaning.
Slope and Drainage: Horizontal runs should be sloped toward the hood or a dedicated "Grease Trap" to prevent oil from pooling in the bottom of the duct.
Kitchen exhaust air is "heavy." To keep grease particles entrained in the air stream, specific velocities must be maintained:
Minimum Velocity: Usually set at 7.5 to 9 meters per second. If the velocity drops too low, grease drops out of the air and coats the duct walls.
Static Pressure Demand: Because of the heavy-duty filters and long vertical risers, kitchen fans (typically High-Limit Centrifugal Upblast fans) must handle much higher static pressures than standard AC fans.
Make-Up Air (MUA): For every 1000 CFM of air exhausted, roughly 80% to 90% must be replaced with fresh "Make-Up Air" to prevent the kitchen from becoming a vacuum, which causes doors to whistle and stoves to burn inefficiently.
To pass local municipal (DBKL/MBPJ) and BOMBA inspections, the following are required:
Cleaning Certificates: Proof of professional degreasing every 6 to 12 months.
Fire Damper Exclusion: Fire dampers are generally prohibited in kitchen exhaust trunks because they can trap grease; instead, the entire duct must be fire-rated.
VFD Interlock: The exhaust fan and make-up air fan should be interlocked so they ramp up and down together, maintaining the kitchen's air balance.
Malaysia
