Architects and engineers have two options when specifying the design and construction of kitchen hood exhaust ducts. They can use field-fabricated or factory-built grease ducts.
But, which option is the best?
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Field-fabricated ducts are a dated solution. The reason why is very simple, safety.
"factory-built" grease ducts are designed and submitted to a rigorous series of tests that ultimately give them UL listing.
UL - 1978 Standard for Grease Ducts covers factory-built grease duct assemblies, and requires manufacturers have grease duct tested to receive UL listing. The tests are performed to guarantee the safety and reliability of the duct for such a hazardous application. Some of the tests that factory-built grease ducts are submitted to are:
- Abnormal Fire
- Joint Leakage and Torsion
- Vertical support
- Side impact
- Longitudinal force
- Load test for grease duct elbows
Based on this evaluation, it is natural to say that Factory-built ducts are safer to install, safer during operations, and safer to maintain.
But wait, there's more:
- Factory-built duct can resist greater internal temperature than the field-assembled option.
- Has a shape that allows the grease to flow down with less resistance than a field-built rectangular duct, which prevents pooling of the grease, a major hazard.
- Are not welded during installation. Welding in itself can be dangerous, especially if the access to the duct is limited. The quality of field-fabricated ducts relies a lot on the welder’s work, while a factory-built duct is manufactured in a controlled environment, making the final product precise and repeatable.
The advantages of factory-built ducts over field-fabricated ducts don’t stop there:
With 1/16” built-in slope, a cylindrical system provides “equivalent” draining capability for grease to a similar sized field-assembled rectangular system at ¼” per foot slope.
- Lower airflow resistance
- Easier to clean
- Superior corrosion resistance
- Lighter weight
- Aesthetic protective metal outer wall
- Lifetime warranty
- Factory design / layout assistance
- Single inspection (field fabricated options usually require multiple)
In case of a fire with a field-fabricated grease duct, the owner must shut down operation, inspect and replace damaged fire wrap or other enclosures. Sections of the duct must be removed and the system needs to be cleaned. Plus, more downtime and expenses to fabricate new sections of duct with fire protective enclosures.
This entire process can take several months. Time that building owners usually cannot afford.
Factory-assembled grease ducts, in the case of a fire, have a significant shorter downtime. Sealant may need to be replaced, and the duct will need to be inspected, but will most likely survive for further use.
Factory-build Grease Duct Maintenance
Factory-built products offer many specialty components designed to help maintain the duct. Components of a system will include access panels, nozzle sections, and fan adapters for instance, which also allows maintenance staff to clean the system with ease.
Even though factory assembled products present a higher initial material cost, the effectiveness and safety is far beyond superior. You should also consider how safety can be a factor into the cost of an equipment.