Restaurant/Kitchen Exhaust

[BronYrAur]

I'm reviewing a set of drawings for a restaurant. I have little experience in this area. The plan calls for an exhaust hood in the kitchen with makeup air unit. The problem is that only 60% of the exhaust air is being made up. I spoke to the hood manufacturer and was told that due to turbulance, etc. no more air can be made up through the hood. This job is in Chicago, and the local code I believe calls for 400 cfm per linear foot of hood. So my exhaust is 8,400 CFM and only 5040 in being made up.

I assume my only option at this point is to bring in the air through my main AHU and transfer it into the kitchen so that it can be exhausted. The drawings don't show this and in fact show another exhaust fan as part of the AHU return.

Flow arrows actually show air being transfered out of the kitchen into the main return/exhaust fan of the AHU. I'm raising the red flag on this, but i just wanted to see if I'm missing something here. All of the kitchen hood exhaust needs to be made up, correct? Is is OK to transfer air from the dining area to the kitchen? If so, what's the best way - transfer grilles in the wall?

 

[AbbyNormal]

400 CFM per lineal foot is high like a chinese place or something, but an allowance of only 60% make up air at the hood sounds low to me, however you have to go by what the mfg says.

You do want the kitchen to be a slight negative so perhaps you can bring in a fair amount of the outstanding make up air through the kitchen hvac.

You can also bring in even more air from the dining room hvac. Now be careful when you look at how air transfers from the kitchen and dining room. You do not want a jet of air, coming into the kitchen through a pass through opening, that lines up with that hood. You do not want the momentum of that air going under the hood its going to push the fumes out from under the hood.

Maybe a ceiling transfer system with a jumper duct. You do not want the transfer duct terminating near the hood with air directed at the hood.

Same thing with kitchen AC, you do not want diffusers throwing air at the hood. The cold air falls down the hood and can entrain fumes to spill



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[EEJaime]

I am electrical but our in-house Mech guys say that the 60%make up air you are talking about is what the compensating hood is capable of. You need to make up the remainder of air from your A/C, Make-up air fan or transfer from your dining room. You need to be negative with respect to your dining room to minimize smoke migration, but the remainder of the air needs to be made up from somewhere.

 

[AbbyNormal]

compensating hood is a short circuit hood?



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[EEJaime]

"Compensating Hood – a hood designed to introduce outside make-up air through an integrated section of the hood with little or no thermal conditioning. This design will typically provide 60-80% of the required make-up air through four basic discharge methods: short-circuit, face-discharge, back-discharge, and down-discharge.
These methods may also be combined, such as a face and down discharge arrangement."

This is from a ventilation guide we use here in Southern California, A little dated, (2000), but very informative.

http://www.co.san-diego.ca.us/deh/food/pdf/publications_ventilationguideline.pdf.

 

[AbbyNormal]

big difference between short circuit and a supply plenum at the hood



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[hollandhvac]

I have seen in dining area and kitchen design for an oil company.

We used a transfer fan and ducted it to take air from the dining area to the kitchen. In the kitchen we distributed it not to disturb the airflow near the hood.

Also with the same company I have seen a large transfer grille between dining area and kitchen, so this is other possibility. Here I would be aware of noise from kitchen into restaurant.

As already said you need to keep the kitchen area under negative pressure.
The main flows of air are a short circuit air flow in the hood itself, meaning it comes from the directly into the hood. ( see page 3 and 4 of my attachment) and a flow from the dining area or in your case the restaurant. On the right side of page 4 there is a phone number where you can get a full copy of the HVCA DW/171 . And if you can put the copy here as well.

 

[giap1]

Normally the air balance is done as follows:
KX Exhaust Fan:100%
Make-up Air: 80% of hood requirement (between 250-400 CFM/feet, refer to ASHRAE 2007-Applications for kitchen equipment)
Air flow from Dining:10%
A/C supply air:10%
If you have ventilation requirements for commercial kitchen
(2-3 CFM/sqft in some states) you may add them as a part of the make up air or not (it depends of what type of hood you're using. Greenheck has very good guidelines for this.

 

[DrRTU]

City of Chicago code requires 50 cfm/ft2 for low heat (oven & dishwasher), 75 cfm/ft2 for medium heat (range & flat top grilles), 100 cfm/ft2 for low heat (charbroiler & fryers) and 200 cfm/ft2 for fuel heat (wood & charcoal). At a max of 60% into the hood, it sounds like you have a compensating hood. I would prefer to use a plenum supply. You get better capture at a lower exhaust and makeup rate. Make sure you do not go below the required Chicago exhaust flows. I normally keep the make-up supply 10% less then the total exhaust and transfer the balance from the dining or kitchen AHU.