Phoenix Journal · Extraction
A converted shipping container makes a superb commercial kitchen - until the sealed steel box strangles the extraction, the fan screams and the grease has nowhere to go. Here is how to design and clean it properly.
Case postmortem
The unit looked perfect on the delivery lorry - a smart converted shipping container, stainless throughout, a canopy, a fan on the roof and a fresh food hygiene rating waiting to be earned.
By the middle of the first Friday service it was a different story. The two chefs working the line were soaked through. Smoke from the chargrill was rolling out from under the canopy instead of being pulled up into it, hanging at head height and drifting toward the serving hatch. Someone propped the back door open to get some air moving, which helped for ten minutes and then made things worse. The extract fan was screaming on the roof, loud enough that the flats above the car park had already rung the landlord twice. Nothing was technically broken. Every part was new. And yet the ventilation simply was not doing its job.
This is one of the most common calls we take about container and modular kitchens. The equipment is rarely faulty. The system was never designed to behave as a system in the first place - it was a collection of correct-looking parts bolted into a very small, very sealed steel box, and a steel box behaves nothing like a normal kitchen.
When we stripped the job back, the failures were textbook, and they all fed each other.
The first and biggest problem was make-up air, or rather the total absence of it. A commercial extract system pulls a large volume of air out of the room every second. That air has to be replaced, or the kitchen turns into a sealed drum. Current UK practice is to mechanically supply roughly 80–90% of the volume you extract, with the small remainder allowed to arrive through natural infiltration. In a masonry building, doors and gaps let enough air leak in to paper over a bad design. A shipping container has almost no infiltration at all - it is a welded steel box built to keep weather out across an ocean. With no supply air, the kitchen went sharply negatively pressurised, the fan found itself trying to pull air out of a vacuum, and the canopy stopped capturing. That is why the smoke was spilling out rather than going up. Propping the door open was the staff instinctively trying to give the fan something to breathe.
The second problem was the ductwork run. Because space is so tight in a container, the duct had been taken on the shortest, ugliest path possible - undersized, with two tight bends straight off the canopy and up through the roof. Undersized duct raises air velocity, and high velocity is exactly what turns an extract fan into the scream the neighbours were complaining about. It also strangles the airflow the system was rated for.
The third problem was the one that worried us most for the longer term. There were no access panels anywhere in the run. The whole point of a compliant grease duct is that you can open it and clean it. This one was sealed like a submarine, which meant that the grease the chargrill was already throwing off had nowhere to be inspected or removed. In a box that small, with that much open-flame cooking, grease was going to build in the duct far faster than anyone was planning to check it.
None of this needed the container ripping apart. It needed the ventilation treated as one designed system rather than a shopping list of parts. The reference points for that in the UK are two BESA specifications: DW/172, the specification for kitchen ventilation systems that governs how extract and grease ductwork should be designed, sized and constructed; and TR19® Grease, which governs how that ductwork is then cleaned and verified through its life. Get a modular kitchen right against both and it will run cool, quiet and clean. Here is the order we worked through it.
That 200 micron figure is worth holding onto, because it is the number a fire investigation will reach for. TR19® Grease requires that grease deposits are kept below a mean average of 200 microns across the system - about half the thickness of a business card. Deposits are measured with a wet film thickness test or a deposit thickness test, recorded with before-and-after photographs from matching positions, and signed off by a technician holding the BESA Grease Hygiene Technician qualification. In a full-size kitchen you have some margin. In a container running open flame in a tiny sealed volume, grease reaches that threshold quickly, which is exactly why the access panels and the three-month schedule are not optional extras.
The trap with a container or modular unit is that it feels finished when it arrives. A fixed kitchen is commissioned on site, in place, and problems show up during that process. A modular kitchen is often built in a workshop, signed off there, and delivered as a sealed product - so the first time anyone tests it against a real service, in a real yard, with real make-up air conditions, is the day it opens. Every weakness lands at once, in front of paying customers, exactly as it did on that first Friday.
The legal backdrop does not soften for a small unit either. The Regulatory Reform (Fire Safety) Order 2005 puts a duty on the responsible person to manage fire risk, and an ungreased, un-inspectable duct is a live fire risk regardless of how new the steel is. Most insurers now ask to see TR19® Grease certification after a duct fire, and if you cannot produce it, cover can be refused. The Health and Safety at Work Act sits behind the working conditions - a kitchen where staff cannot breathe and cannot hear each other is not a compliant place of work. None of that changes because the building started life as a shipping container.
If you are commissioning a modular kitchen rather than rescuing one, the sequence is the same but far cheaper. Fix the heat load and the extract volume the cooking line will actually generate. Size the make-up air to it. Give the duct room to run at a sensible velocity, with gentle bends and access panels built in from the workshop. Treat noise as a design condition wherever there are neighbours. Then set the cleaning schedule against how hard the kitchen will work, and keep the verification reports from day one. Done in that order, a container kitchen is a genuinely excellent way to trade. Done as a box of correct-looking parts, it is a hot, loud, non-compliant afternoon waiting to happen.
Questions
Yes. Neither standard has a lower size limit, and the law behind them does not shrink for a small unit. If you are cooking commercially, the extract duct must be designed to the BESA DW/172 specification and cleaned and verified to TR19 Grease, exactly as it would be in a full-size fitted kitchen. If anything the risk is higher in a container, because a small sealed volume with open-flame cooking builds grease and heat far faster.
Usually not. The most common cause by a wide margin is a lack of make-up air. A container has almost no natural infiltration, so if there is no mechanical supply air the kitchen becomes negatively pressurised and the fan cannot pull against the vacuum. Current practice is to mechanically replace around 80 to 90 per cent of the air you extract. Fix the supply air and the fan you already have will usually capture properly.
It depends on how hard the kitchen works, following the TR19 Grease usage bands. Heavy use of 12 to 16 hours a day is cleaned every three months, moderate use every six months, and light use every twelve. Most container kitchens running a grill or fryer through the day fall into heavy use. Each clean should come with a post-clean verification report and before-and-after photographs so you can prove compliance to an insurer or fire officer.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.