In our last article, we used a hypothetical restaurant – The Harbour Lane Bistro – to estimate the amount of grease that might be created in a kitchen and what happens to it.
A Quick Review…
The Harbour Lane Bistro is a busy, mid-sized restaurant in inner-city Sydney serving 100 meals, six nights a week. This generates 6.5 kilograms of airborne grease every week. The kitchen includes a well-designed and diligently maintained exhaust system. The exhaust system operates at 90% efficiency and the baffle filters at 85% efficiency. This means that some 700 grams of grease is not ‘captured’ by the exhaust system and settles in the kitchen.
Of the grease-laden air ‘inhaled’ by the system, the filters will capture a further 4.9 kilograms of grease per week.
This leaves us with a balance of 900 grams of grease that passes through the filters every week.
The purpose of this article is to explore what happens to this grease.
The Other Side of the Filters…
The Harbour Lane Bistro has 9 metres of ductwork between the canopy and a roof-mounted exhaust fan. The ductwork is standard 300mm x 500mm rectangular section. The total interior surface area of the ductwork is 14.4 square metres.
To achieve a target air velocity of between 9 and 12 metres per second, the fan would need to generate an airflow of between 80–110 cubic metres per minute.
The air in the duct would be laden with airborne grease and would be turbulent and fast moving. Any of these grease particles that made contact with the ductwork would adhere and start to accumulate.
This raises the next critical question.
How Much Grease Sticks to the Duct and How Much is Ejected by the Fan?
The best industry estimates are that one-third of the grease will be ejected by the fan and two-thirds will accumulate in the duct.
We know that 900 grams of grease per week passes through the filters, so we can expect that 300 grams will be vented, and 600 grams will accumulate in the ductwork.
Grease Accumulation on the Roof of The Harbour Lane Bistro
Fine grease mist and vapour that passes through the filters and ducts and escapes into the external environment will often cool and condense near the outlet or settle on nearby surfaces due to reduced airflow velocity.
To estimate the radius from the exhaust fan where 300 grams of grease per week might settle and accumulate, let’s assume that grease particles are dispersed and settle evenly in a cone-shaped pattern centred on the exhaust fan outlet, which is common for typical ventilation systems.
The mathematics would suggest that this grease would settle and accumulate within a radius of approximately 10 meters from the fan. However, actual conditions such as wind, obstacles, and surface types can influence this distribution, potentially spreading grease further or concentrating it in specific areas.
This grease accumulation around the exhaust outlet requires regular inspections and routine cleaning to manage environmental impact and ensure safe roof access.
Grease Accumulation in the Ductwork
Grease build-up is not evenly distributed
While grease build-up will happen everywhere in the system, the build-up of grease is not evenly distributed, and air velocity, air turbulence, gravity and centrifugal forces are all factors that can influence the rate of grease accumulation in a duct.
For example, airflow slows in horizontal ducts, allowing heavier grease particles to settle. Airflow is disrupted at bends, creating turbulence that allows grease to deposit on the inner surfaces. Grease can settle on fan blades due to centrifugal forces. Vertical ducts are generally less prone to grease build-up due to gravity and airflow momentum.
In addition, the location of build-up is critical. For example, heavy accumulation at a spot within the hood plenum, immediately above the heat source would present a far higher risk than an accumulation at a bend many meters away from the heat source.
Depth of Accumulation
We know that The Harbour Lane Bistro has 9 metres of 300mm x 500mm rectangular ductwork with internal surface area of 14.4 square metres, and the high efficiency exhaust system is well maintained. Despite this, and as we’ve seen, 600 grams of grease will accumulate in the ductwork every week.
If the grease were evenly distributed, 600 grams would result in 45 microns of grease accumulation throughout the system every week. Over a three-month period, the amount of grease coating the interior surfaces of the duct would weigh almost 8 kilograms and could coat every surface to a depth of almost 600 microns, and there is no doubt that the high points would significantly exceed this.
The next question is this: At what point does grease accumulation pose a significant risk to the operation of a commercial kitchen? This is the subject of Part 5…