Kitchen MUA heat calc

[emalsyd]

Looked a small restaurant kitchen yesterday. The MUA is provided by a un-tempered supply fan directly into the front of the hood so the MUA blows into the hood.

The owner wants to add cooling to the kitchen. Do I have to take the MUA into my calculations or is it negated since it goes right up the hood?

 

[moideen]

what you mean MUA......

 

[emalsyd]

Make up air, in the kitchen

 

[IRstuff]

"is it negated since it goes right up the hood"

If that were true, your MUA wouldn't even be working correctly and there would be zero MUA for the kitchen and it could partly explain the need for cooling for the kitchen.

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[GT-EGR]

The answer is kindof. Do your cooling load calc and then size your cooling unit to provide that cooling capacity - but be aware that the hood likely exhausts more then the makeup air unit provides - so you may have to bring that outside air quantity through your new cooling system (or it’s indirectly coming in your space some other way) in which case it’s still a load on your cooling coil.

 

[Emrelug84]

Hi Emalysd,

Make-up air is usually supplied the edge of the hood to prevent the smoke release to the kitchen area. Theoretically, all make-up air should be exhausted by the hood. However, it is hard to say that since there may be turbulence in make-up air flow and some of air is released to the kitchen area. In my opinion, a safety margin should be taken into the consideration.
Regards,

 

[dbill74]

Make-up air is usually supplied the edge of the hood to prevent the smoke release to the kitchen area.

Not really. The negative pressure from the hood fans will do that just fine.
The main reason for having MUA in or at the hood is to reduce the amount of outside air introduced through the comfort cooling units.

Can you ignore the MUA? No. It's going to have an affect one way or another. How you take it into account will depend on how and where it is introduced into the kitchen.

 

[IRstuff]

"The main reason for having MUA in or at the hood is to reduce the amount of outside air introduced through the comfort cooling units."

This seems counter-intuitive wrt actually moving smoke or fumes out of the room. Every cfm of OA is one less cfm of smoke removed.

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[dbill74]

MUA is OA.

The trick is that the hood MUA is NEVER equal to or greater than the hood exhaust, it's been awhile since I designed a kitchen, I believe the rule of thumb is that hood MUA is 75-80% of hood exhaust. You end up with an area of negative pressure under the hood which is where your smoke and fumes are located. This negative area will keep smoke and fumes under the hood out of the rest of the kitchen.

To maintain a positive building pressurization you still need to bring enough outside air into the building through your main HVAC system(s) to make up the remaining 20-25% plus enough for toilet exhausts and any other code mandated exhaust in sufficient quantity.

For example assume a small restaurant with one hood and two toilets and 50 seats.
Hood exhausts 5,000 CFM, toilets 200 CFM for a total 5,200 CFM EA.
Outside air requirement is going to be 1,000 CFM (let's keep the math simple here).
You have two options:
1) provide 4,000 CFM unconditioned MUA at the hood, and 1,500+ CFM through your main HVAC units
2) introduce 5,500+ CFM through your main HVAC units.

I hope I don't have to explain what the difference in OA will have on sizing the main HVAC units.

There is no code or guide that prohibits MUA from being heated or cooled.

Not all hoods have or need MUA. A lot of small shops will have a small hood over a grille and run with 1,000 CFM exhaust or less. In these cases there is often no need for MUA as the requisite amount of outside air for the rest of the kitchen is sufficient to keep the place positive while still controlling smoke and fumes.