basic commercial kitchen fan / ventilation, advice on how to improve please

[vde-electrician]

hi
basically we have a small commercial kitchen, we have an open hood/ canopy (approx 1.5m wide), no filters or anything within it (above the gas hobs), then a solid duct goes around 3m up through 1st floor and fan on top of roof (type where fan is directly on top of duct),
The fan is working - but there is grease running down the back of the wall and grease is laying on adjacent surfaces - so can't be very effective.

Would installing a 2nd fan help? for example leaving the one on the roof and installing a fan within the hood and a grease filter below it? or just putting a new hood in with an in built fan (like a domestic one)?

appreciate any advice

 

[davesparks]

When was it last cleaned?

 

[vde-electrician]

a while back i think, but would grease build up etc in the duct make a massive difference. do you think the idea of a second fan would help?

 

[shanky887614]

a while back i think, but would grease build up etc in the duct make a massive difference. do you think the idea of a second fan would help?

maybe maybe not, your going to need to get someone to look at it to tell you for definite.

filters before the duct would be best (washable metal mesh, can be cleaned as part of the nightly wipe down)

 

[davesparks]

a while back i think, but would grease build up etc in the duct make a massive difference. do you think the idea of a second fan would help?

Yes grease build up will make a difference, it builds up on the impeller and slows the fan down plus changes the efficiency.
Or if this has only recently started happening since it was last cleaned then it may have been accidentally damaged.
Is there enough air getting in to the room?

 

[vde-electrician]

would a small domestic type extraction work well with it do you think, give it a helping handf?, then would extract through the duct and pulled up with the roof fan? not sure if mixing fans with different flow rates cause problems

 

[shanky887614]

Yes grease build up will make a difference, it builds up on the impeller and slows the fan down plus changes the efficiency.
Or if this has only recently started happening since it was last cleaned then it may have been accidentally damaged.
Is there enough air getting in to the room?

like dave says make sure there is enough ventilation, ideally you want the room to be positive pressure so that all the crap gets sucked up by the fan

(google positive pressure pc fans, it talks about dust but its the same principle)

 

[shanky887614]

would a small domestic type extraction work well with it do you think, give it a helping handf?, then would extract through the duct and pulled up with the roof fan? not sure if mixing fans with different flow rates cause problems

what he means is one blowing air into the kitchen while the extract fan sucks it out

 

[vde-electrician]

i see, think they have a window open, but not always - your thinking it will be difficult for the flow and harder for the fan?

 

[Lagentium]

Agree with above, I've seen it when a fan isn't working efficiently due to it creating a negative pressure, due to nomairmbeing drawn into a room to replace it.

 

[shanky887614]

Agree with above, I've seen it when a fan isn't working efficiently due to it creating a negative pressure, due to nomairmbeing drawn into a room to replace it.

to add to lag.

hospitals keep the corridors at positive pressure to the wards so the airborne viruses etc are contained to the wards, this is more important in isolation wards etc. (ive worked on some bms for isolation wards before)

 

[davesparks]

i see, think they have a window open, but not always - your thinking it will be difficult for the flow and harder for the fan?

Difficult? Try impossible! It will not extract any faster than replacement air can be drawn in to the room

 

[Marvo]

You should pull the HVACR spec for the building or premises and see if the hood is moving the correct design volume using an anemometer. As a rule of thumb you should see a face velocity of between .5 and .6 m/s but check the spec to confirm. If the volume is low check the fan is the specified size and check there's no fire dampers in the ducting that are closed or partially closed.

I'd strongly suggest you don't just 'upgrade' the fan or jimmy the system to get more extraction without considering the knock-on effect this is going to have to the balance surrounding systems such as air-con and supply air/make-up air. In commercial buildings this is a specialist job with distinct health and safety implications.

like dave says make sure there is enough ventilation, ideally you want the room to be positive pressure so that all the crap gets sucked up by the fan

(google positive pressure pc fans, it talks about dust but its the same principle)

You should never have more make-up or supply air into a kitchen that the extract is removing. Kitchens should never be at positive pressure because fires will spread very quickly and people die. Corridors, stairwells and anywhere else on an escape route would be positive pressure or their may be a separate zoned pressurization system between the fire alarm and the BMS.