Kitchen extractor advice sought

[mrrusty]

Look, I'm not a ventilation specialist, I am however a specialist in air movement in pipes for central vacuuming and pneumatic transport, hence the advice to talk to a specialist in this field. Input the parameters in to AI and see the answer!

There are several challenges - enough flow to push vertically, enough air speed to mitigate condensation risk, insulation again to mitigate condensation, maintenance access to both flue and (probably) an in-line fan that will become covered in cooking grease over time. In a normal centrifugal extractor the motor isn't in the airstream, but I doubt a centrifugal fan will have enough waft for this application.

In my field, the fall-off in flow of centrifugal blowers and vacuum pumps against back-pressure is beneficial. With flow dropping to zero at around 3-400mb backpressure, it keeps systems away from the pressure regs which start at 500mb.

Back pressure increases with the square of airspeed, which is why in a ventilation system, with slow air speeds, back pressure is negligible.

 

[blackworx]

From my own experience with building a 4 inch sawdust and chip extraction system into my workshop, whose longest run is close on 10 metres and has multiple bends, including a couple of metres of corrugated flex hose, and which has no problems carrying particles significantly larger and heavier than water vapour and grease droplets, I'm hoping a 5 inch, more or less straight, vertical duct with a couple of bends and a much more powerful blower should be ok without the need to spend potentially thousands on a contractor.

Yes I know the two situations are not directly comparable (for one thing the dust extraction system works with negative pressure not positive) the temperature gradient is a consideration and the air stream is wet and relatively cool rather than dry and hot, as it would be from a solid fuel stove. And I get that contracting pros would be the gold standard, however what I need is a safe and functional solution, not a gold standard one. After all I asked the question on DIYnot forums for a reason .

Really grateful for all the replies, thank you, and I now have a significant amount of food for thought as well as actions I will need to take to ensure that whatever I do is safe. At this point I'm pretty much decided on the second option. The first option's ruled out not for perfomance reasons but because it requires six inch ductwork (which I'm doubtful of being able to fit all the way down the chimney) and cannot work easily with a condensate trap. It also has a couple of practical drawbacks unrelated to the extraction itself.

So to make sure what I do works as expected and is safe, I'll...

• ...get a vane anemometer (can be had for £30-40) so after install I'll be able to check the air speed and make a volumetric calculation at the exhaust point to ensure it's within spec.
  
  
• ...check grease buildup rates in the duct over time (I have a borescope) and assess whether it's likely to become an issue. As I said earlier I think the likelihood of buildup to the point of potential for grease fires, as well as the risk of one actually being able to ignite, is minimal, but I won't rule it out. Despite being in my view a negligible risk - mainly because we're not running a commercial kitchen - it's clearly still the biggest single safety consideration.
  
  
• ...install labyrinth grease filters at the kitchen end to trap the majority of the aerosolised grease.
  
  
• ...install a condensate trap and re-evaporator
  
  
• ...ensure the blower runs for a few minutes extra each time to clear the duct of as much vapour and grease as possible. I'm considering adding/maintaining an inline humidistat near the top of the flue to control this automatically.
  
  
• ...ensure the blower itself is regularly cleaned of grease. The one I specced is easily disassembled for cleaning, plus it has its own additional grease trap built in.

Since I couldn't find any solid accounts/experiences of anyone doing anything like this to completion, I'll post an update once I'm done (or have decided to abandon the idea ).

 

[mrrusty]

I hope you've found all the comments helpful. It sounds like you have your head around this challenge, and like you, I like to find solutions. I'm 100% sure it can be made to work - it's just getting all the factors in play worked out.

FWIW, in a different world, I have installed 160mm tube vertically from a manufacturing Pharmacy in a hospital (which was slightly pressurised for environmental reasons), which ran vertically to outside, and we inadvertently built a very good condenser that created what looked like buckets of water thrown across the floor overnight!

 

[blackworx]

maintenance access to both flue and (probably) an in-line fan that will become covered in cooking grease over time. In a normal centrifugal extractor the motor isn't in the airstream, but I doubt a centrifugal fan will have enough waft for this application.

It actually is a centrifugal fan, which I was a little sceptical about myself, however it is purpose designed for the application.

According to the spec sheet the fan will still move 300m3/hr at around 420Pa static pressure. From my amateur Googling it seems friction losses tend to dominate when moving air, but a theoretically frictionless 5m vertical duct would create about 60Pa static pressure, so I should have a lot of headroom for friction losses (which I am minimising anyway)?

Maintenance access to the fan and duct won't be an issue. The blower is at the kitchen end, inside the hood, and is designed to be disassembled for cleaning.

 

[blackworx]

I hope you've found all the comments helpful. It sounds like you have your head around this challenge, and like you, I like to find solutions. I'm 100% sure it can be made to work - it's just getting all the factors in play worked out.

FWIW, in a different world, I have installed 160mm tube vertically from a manufacturing Pharmacy in a hospital (which was slightly pressurised for environmental reasons), which ran vertically to outside, and we inadvertently built a very good condenser that created what looked like buckets of water thrown across the floor overnight!

oh dear, I hope to god I don't end up building one of those. Or at least that the evaporator I install can cope with whatever does come back down the duct.

 

[mrrusty]

Yes - was connected to one of these https://www.quirepace.co.uk/wp-content/uploads/2023/07/Nurse_carrier_543.png and we ended up having to install a valve to block of the service tube when not in use to stop the chimneying effect of the air being pushed through all the time by the pressurised room!

From my quick look, I think if that fan really can maintain 300m3/hr against 420Pa you shouldn't have any problem with flow - you only need 110-120 m3/hr. That should give you an airspeed approaching 2 m/s which hopefully is enough to get it outside before it cools enough to condense.

Have fun!

 

[JohnD]

It actually is a centrifugal fan, which I was a little sceptical about myself, however it is purpose designed for the application.

According to the spec sheet the fan will still move 300m3/hr at around 420Pa static pressure. From my amateur Googling it seems friction losses tend to dominate when moving air, but a thoretically frictionless 5m vertical duct would create about 60Pa static pressure, so I should have a lot of headroom for friction losses (which I am minimising anyway)?

Maintenance access to the fan and duct won't be an issue. The blower is at the kitchen end, inside the hood, and is designed to be disassembled for cleaning.

View attachment 409806

When they are built into commercial kitchens, there is usually a metal mesh grease trap under the hood. Needs to be in sections that can be washed daily. As it is full width, the pressure and airspeed are low and it is not easily clogged

 

[blackworx]

When they are built into commercial kitchens, there is usually a metal mesh grease trap under the hood. Needs to be in sections that can be washed daily. As it is full width, the pressure and airspeed are low and it is not easily clogged

According to my OH in her experience it's more often labyrinth than mesh, but yeah same idea - force the grease-laden air to (repeatedly, if possible) hit some sort of solid surface that will catch grease before it gets into the duct, and make that surface easily removable to allow frequent cleaning.