Security bars in grease duct

[earshavewalls]

I have a plan for a large kitchen that is demonstrating backdraft dampers and security bars at the roof level for ALL exhaust fans, including grease fans. I could not locate a specific code section (California Mechanical Code, 2010, based on 2009 UMC) as to bars passing through grease duct, but I was able to find 512.1: "Dampers shall not be installed in exhaust duct systems."

Then, 512.1.1 states: "Where specifically listed for such use or where required as part of a listed device or system, dampers in exhaust ducts or exhaust duct systems shall be permitted."

I have not seen any listed grease exhaust fans that used backdraft dampers or dampers of any kind, for the exhaust. As to the security bars (at 6" OC), I am not certain how to proceed. Since the bars will definitely restrict air flow and will also no doubt accumulate grease, I am leaning toward not permitting this, but.......I still have no code to go by and I do NOT wish to attempt to enforce my opinion. I suppose they could clean the bars occasionally since they are at the roof level, and the fan is hinged for cleaning, but I am also concerned with restriction of the flow of air.

Am I concerned about something that I should not be concerned about? Please let me know if I am going in the wrong direction on this.

Thanks in advance for any advice or input on this.

Wayne

 

[TheCommish]

If it is refrenced try NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations

 

[cda]

For the bars

This covers joints, but eludes to grease build up imc 2009

506.3.2.1 Duct joint types. Duct joints shall be butt joints, welded flange joints with a maximum flange depth of 1/2 inch (12.7 mm) or overlapping duct joints of either the telescoping or bell type. Overlapping joints shall be installed to prevent ledges and obstructions from collecting grease or interfering with gravity drainage to the intended collection point. The difference between the inside cross-sectional dimensions of overlapping sections of duct shall not exceed 1/4 inch (6 mm). The length of overlap for overlapping duct joints shall not exceed 2 inches (51 mm).

506.3.2.1 Duct joint types. Duct joints shall be butt joints, welded flange joints with a maximum flange depth of 1/2 inch (12.7 mm) or overlapping duct joints of either the telescoping or bell type. Overlapping joints shall be installed to prevent ledges and obstructions from collecting grease or interfering with gravity drainage to the intended collection point. The difference between the inside cross-sectional dimensions of overlapping sections oo duct shall not exceed 1/4 inch (6 mm). The length of overlap for overlapping duct joints shall not exceed 2 inches (51 mm).

Will look at 96

 

[fireguy]

The only place I have seen bars in the Type I duct was a medium security prison. They looked to be 3/4 pipe, welded to a frame of flat stock. The whole assembly was either welded or bolted into the sheetmetal. The bars were about 12" apart, so the bars did not affect the air movement. If you were concerned, the pipe could be replaced with flat stock oriented so the flat was vertical. We required the prison to make sure the exhaust fans were loosened so my crew could move the fans with 2 people. I think of 6 or 8 fans, 5 were still bolted down when we arrived. That saved lots of times, I just marked the invoice "Not all ducts were accessable. Those ducts were not cleaned"I was told that within 2 hours of the inmates moving hte fans, everyone in the prison knew the ducts could not be used for escapes.

 

[Builder Bob]

The bars should not be a factor in the performance of the unit........it will be a factor in performing maintenance. Either access panels above and below or other arrangements need to be made to ease maint. operations. Also, they may have to clean the hood more often.

 

[Gregg Harris]

The bars should not be a factor in the performance of the unit........it will be a factor in performing maintenance. Either access panels above and below or other arrangements need to be made to ease maint. operations. Also, they may have to clean the hood more often.

I agree with the first part of the sentence, "bars should not be a factor" but they will interfere with the performance by creating static and turbulence that will interfere with the laminar flow of the air and safety of the exhaust.

Any object in the path of the air will cause a static pressure loss that would need to be overcome by the exhaust motor and accumulation of grease that will build up inside the duct where they are attached.

cda sighted 506.3.2.1 that shows the intent not to have grease build up inside the duct by ledges etc..

 

[fireguy]

Increases in static pressure caused by bends in the duct can be lessened by using 45° bends instead of 90° bends. That also allows a better cleaning job with a spinner nozzle on a pressure washing hose.

If access panels are used, they are of no use unless they can be accessed. I have seen access panels that cannot be used because they are installed over equipment that cannot be moved. Or work table placement may not allow ladder access to the panels. And if gas piping or HVAC ducts are in front of access panels, there is no access. Or access panels are located on the bottom of ducts. Those panels are useless, because of leakage. But poor placement is allowed by ICC rules. Again, the NFPA standards are better, because of the input of those involved in cleaning ducts. That may change because of IKECA involvment in writing the ICC codes.

If the bars are no further into the duct than 2 feet, the bars can be cleaned with a 4 foot pressure wand. Or the obstructions can be cleaned with a stainless steel pad. Flat bars would cause less turbulance than round pipe. I am not sure how important disturbing the laminar flow is or the increase in turbulance is. I have seen grease build-up on top of conduit and suppression system piping, but normally that build up is easily removed.

The building official could also contact local exhaust cleaners for their input as to service needs of the duct system.