Diversity Factor for Kitchen Exhaust Duct 1

[iamjustanewbie]

Hi

I have a Centalized Kitchen Exhaust Duct. Is it okay to put a "Diversity Factor" on the the Total Exhaust rate? And what percentage?

Thanks so much

 

[dbill74]

It depends. What is the application, residential/commercial? How many hoods/inlets are on the system? A discussion with the client is in order to get an accurate understanding of how the exhaust system is intended to be used.

With kitchen hoods, a minimum velocity needs to be maintained. I would not do anything as complicated as VFD on the fans, doing so would allow the velocity in the ducts to drop and depending on what the air is transporting, could cause 'stuff' to fall out of the air creating a maintenance nightmare.

Bill

 

[iamjustanewbie]

Hi Bill

The Centralized Kitchen Exhaust Fan is serving ten (10) Commercial Food Retails.

Thanks so much

 

[dbill74]

Sounds like a food court. In this case, you should plan on providing exhaust to all simultaneously.
But then if it is a food court and your providing exhaust to hoods, there are code requirements regarding duct construction, insulation from other building components and sloping the duct so grease does not accumulate.
If my assumption of a food court (or similar) is correct, then having individual exhaust systems for each vendor is the better design as each vendor/counter will have different exhaust requirements. The exhaust requirements will change if one vendor moves out and another moves in. Separate systems are cheaper and easier to modify/replace without affecting adjacent vendor's business.

 

[berkshire]

Read up on " Grease duct" and "Hot table" exhaust standards for your region.
B.E.

You are judged not by what you know, but by what you can do.

 

[iamjustanewbie]

Hi Bill

The design is to provide Centralized Kitchen Exhaust Duct w/ Centralized Kitchen Exhaust Fan because of limited means of termination point. The Architect does not want to terminate all of those individual Kitchen Exhaust Duct to Roof because it will cause ten (10) openings. The team just want to have a Centralized System passing through a shaft. Because of large Total Exhaust rate, and the possibility that not all of those ten (10) Food Retails will meet their individual peak load, I am planning to put a Diversity Factor to lower the fan capacity.

Hi berk

I am reading Kitchen Ventilation Section of ASHRAE Handbook - HVAC Application.

Thanks

 

[cdxx139]

How do you balance the system when some of the kitchen hoods are off? How do you make sure enough airflow is being pulled from the 7 hoods in operation, and it doesn't pull from the 3 hoods not in operation?

Components like dampers and pressure transducers are not allowed in grease duct for fear of grease buildup.
You also should research NFPA 96.

As for size, I believe you have to meet code minimums for ALL kitchens without diversity. Last time I did a kitchen, they the duct sizing requirement to 500 fpm from 2200 fpm?

If this is a developer, he may want 10 different fans, so he can charge the electrical required for the fan (and associated makeup air) back to each kitchen client. I single system would require him to cover the expense and maintenance. 10 systems, you can push the cost and maintenance back to the individual renting the kitchen.

If you go with individual systems, and want to get fancy, look at a Melink system. Higher first cost, but you see operations savings, if that is important to the owner.

Good Luck

knowledge is power

 

[317069]

if we have 10 hoods with one central exhaust fan, how can we consider a hood is on or off, if the fan is running all of them are in service regardless if there is a cooking operation or not under any of them.
central fan is not a good idea, do not use diversity factor, who wants a central fan he has to pay for cfm not to reduce it.

 

[willard3]

Reducing velocity in grease ducts/cooking hoods using diversity factors is bad design and, depending upon where you are, a code violation.

 

[dbill74]

Kuya,
I get what the architect is wanting, and why. However, what he is asking is not smart, is more difficult to do than he thinks; individual systems are the better option.

To be able to apply a diversity factor you need to ensure one or more hoods are "off". To do this you need dampers and as CD mentioned, dampers are not permitted in ductwork per code. Therefore your fan(s) have to be sized to exhaust all hoods simultaneously.

If you haven't already read through your governing code, if that is ICC Int'l Mechanical Code, Section 506 and 507 are what you need to get familiar with.

Code issues aside, how are you going to size a single fan for 10 kitchen hoods? Consider the hood exhaust requirements for Taco Bell versus KFC versus McDonald's versus Panda Express. NONE of these retailers use the all the same equipment for cooking which means each retailer has different hood exhaust requirements. Even if you know who the first 10 retailers are, they can change in 5 or 10 years completely invalidating your design and would require the exhaust for all 10 retailers be reviewed and modified. This means a new fan and ductwork; during future renovations none of the retailers are going to be able to fix food. Ask yourself and the architect what those retailers or food court manager will think of that. (It won't go over well.)

Thinking back on my own experiences with food court retailer renovations, the retailers will want to bring in their own hoods, fans, etc. so they have the exhaust that meets their needs. If this central system is part of the "shell design", my recommendations is to remove it from the scope of work.

CDxx139 has a valid point about being able to individually meter each retailer's power usage including running hood fans.

As you read through the responses here I hope you get the sense that what the architect is asking for is not going to work. It is your job as the engineer to convince him of this. I understand the architect want to minimize the number of roof penetrations, they tend to leak. If you need another option, going out an exterior wall with a sidewall grease fan.

Bill

 

[iamjustanewbie]

Thanks for all your reply. It helped me a lot.

Thanks so much

 

[cdxx139]

keep us updated on how it works out

knowledge is power

 

[SAK9]

What you need to do is document the pros and cons of the two systems and send to the owner.There is no case for a common fan as you can not turn down the system.You will need to run all the 10 hoods even if you need only one.It is very unlikely that all of them will be in use at the same time.You can do with a common makeup air system though but will need to be variable flow based on a pressure differential controller that operates to keep the kitchen negative.

 

[iamjustanewbie]

I do understand the complexity of a multiple-hood system. But the main problem here is the termination point of the KED for those ten (10) Food Retails. This is a Shell & Core 10-storey building. Business Process Outsourcing (BPO) at 4th-10th Floor with Retails at the Ground Floor. Second & Third Floor are parking floors. Facade termination is not an option because of several reasons: (1)Future Expansion on the Retail side; (2) other sides are walkways; (3)Grease on the wall. Shafts are very limited in size. We don't have the details of the tenant's Kitchen Exhaust requirement yet. That's why I consider the hood to be a Single Island - Heavy Cooking. By the way, aside from the Centralized Kitchen Exhaust Fan, there is also a dedicated Exhaust Fan at each retail's hood.

As per Kitchen Ventilation section of the ASHRAE Handbook - HVAC Application, multiple-hood system can be done by having "bleed ducts and/or balancing dampers". I am quite confuse with the phrase or balancing dampers.

Thanks

 

[cdxx139]

Very interesting, I do not remember reading that section of the chapter.

So to re-answer your original question. You cant take a diversity, because you always have to meet code minimum flow velocity in the duct. NO MATTER WHAT! From memory the new code minimum is 500 fpm. I would design the main shaft at 500 fpm for 5 kitchens at light cooking, and 5 kitchens at medium cooking. Size the fan for all 10 kitchens with heavy cooking and put a VFD, so they can balance down. In my opinion, this will maximize the developers future options.

The balancing dampers they are talking about is at the hoods not in the duct. This needs to be provided by the hood manufacture and need to be UL listed to be allowed in the hood. I forgot about these, I used these on my first kitchen project, when I used one fan for two hoods with different exhaust requirements. I should have designed the duct to be equal friction but I didn't, and the manufacturer saved me with these hood sliding dampers.

The ASHRAE chapter also adds an interesting option of a "bleed duct", where if the main drops below the 500 fpm minimum, you bleed outdoor untreated air into the duct to raise the velocity above 500 fpm. I never heard of this. The concept is done with laboratory fans, but never thought to use it here. And you can add a damper in this system, because it is not considered part of the grease duct even though it connect to it. Maybe even pulling that bypass from the parking garage on the 2nd floor?

Thank you teaching me something today



knowledge is power

 

[cdxx139]

"By the way, aside from the Centralized Kitchen Exhaust Fan, there is also a dedicated Exhaust Fan at each retail's hood."

Im not sure about this. If you can reduce airflow in the Centralized Kitchen Exhaust Fan when the retail hood exhaust fan is off, then great, but I don't think you can put a pressure transducer in the grease duct to measure change in pressure and reduce main fan flow. If you cant how else do you reduce the VFD? And when a retail hood fan turns off, what stops the central fan from pulling some air through it. Also then you have fan in series.

I just don't know.





knowledge is power

 

[hafiz88a]

Hi All,

In my country we used to design centralize kitchen exhaust fan for our shopping mall.

The centralize fan is connected to a main shaft and branch out to each tenants lot.

Each tenants hood will have its own booster fan exhausting to the main shaft, while the centralize fan will clear all the pressure buildup in the main shaft.

Centralize fan is equipped with VSD. The main shaft will have a short branch to locate a pressure gauge; so that grease won't accumulate on the gauge.

Any comments on the above system?

Thank you.

 

[cdxx139]

It sounds ideal for energy. How do you design the short branch to locate a pressure gauge; so that grease won't accumulate on the gauge?


knowledge is power