Ask most commercial kitchen operators when they last had their exhaust system professionally cleaned, and chances are they’ll be able to rattle off a date. But when you ask them when the duct behind the canopy was last properly looked at, well, usually there is just silence. This bit of the system is a real blind spot; hidden from view, it runs from the canopy plenum down to the rooftop fan through all sorts of bends and horizontal bits and fan housings, and it’s exactly here that grease, the stuff that can cause duct fires, tends to build up the fastest. It’s also the bit where virtually all filter-only cleaning programmes come to an end.
How Grease Makes its Way Through the System?
When you’re cooking, the heat breaks down grease into vapour which rises up into the exhaust stream over the cooking line. This vapour makes its way into the canopy, passes through the grease filters and then carries on into the duct. Every time it hits a surface, a thin layer of condensate forms and sticks. After a few sessions of cooking, all these layers start to add up and soon the duct is covered in them, which narrows the duct down, reduces airflow and increases the likelihood of a fire. This is why kitchen exhaust cleaning Glenn Innes services cover the full duct run, not just the visible filters.
A commercial kitchen exhaust system, when it’s just going about its normal business, is actually providing all the ingredients you need for a fire: a load of grease as fuel, some oxygen from the airflow and something to ignite it in the form of your cooking equipment below. So even if you get a single heat surge from a commercial fryer, you could be looking at a fire without even any other issues.
What a Real Duct Fire Is Actually Going to Do?
A duct fire is going to start making its way upwards through the duct towards the exhaust fan at roof level. According to the AIRAH Best Practice Guideline on Commercial Kitchen Exhaust Management, fires that are being fed by all that polymerised grease residue can get up to a thousand degrees Celsius in just a few minutes. At that sort of temperature, it’s not hard to see how radiant heat can start a fire in any combustible materials it comes into contact with in the ceiling voids, wall cavities and roof structures nearby.
And the worst bit is, the fire can start making its way into these surrounding building elements before your kitchen suppression system even has a chance to kick in, because suppression systems are only designed to deal with fires in the cooking zone below, not the ducts above.
The Airflow Problem That Makes Everything so Much Worse
As you get more and more grease building up and the duct gets narrower, the exhaust fan has to work harder to suck everything out. And because the airflow at the canopy is reduced, more cooking vapour escapes and deposits itself a bit further along in the system at an even faster rate, which just accelerates the whole cycle. And if the fan is running hot because it’s got to work so hard, you can also end up with an ignition risk if any of that grease residue gets into the motor casing.
Issues a Complete Clean Must Resolve
Professional servicing of a full system includes:
- Canopy interior and exterior, including plenum chamber behind filter rack visible in the front of the system.
- Grease filters either cleaned to a residue-free state or replaced at the time of servicing.
- Full duct route from the canopy attachment to the exhaust fan, including bends, elbows and horizontal areas where grease builds up, not drains.
- Exhaust fan housing, fan blades, motor housing and roof discharge.
A servicing report listing only conditions of filters and canopy area means the duct has not been evaluated, not that it has been cleaned.
The Access Issue That Makes the Difference Between Compliance and Incompleteness
In many commercial buildings, ducts in place do not include access panels necessary for a complete clean-up. During pre-servicing evaluation, the competent contractor identifies such issues and performs installation of proper access panels. The contractor who recognises access issues and continues without resolving them provides partial service and prepares the report that does not reflect the condition of the system.
For multi-storey or high-rise facilities, rope or elevated work platform access is often necessary to clean certain areas of the ducts and the roof fan. Operators must be sure that prior to hiring any contractor, he has appropriate equipment and certifications for the particular building. Most hazardous grease accumulations form in those areas of the ducts that are the hardest to access.
