Overview:

Kitchen Hoods & their operation are one of the most misunderstood pieces of equipment in the HVAC industry.

I’ve been messing with kitchen hoods for quite some time.  The one and only consistency between all of them … that the owners of kitchen hood have no idea on how they are supposed to operate.

With almost 100% certainty and accuracy, all owners believe that all smoke is supposed to get sucked out immediately, quickly, and absolutely.  Under no circumstances, is ANY SMOKE, WHAT-SO-EVER, SUPPOSED TO SWIRL AROUND IN THE HOOD.

Additionally, all hood owners believe that if the above is not happening, they need more exhaust.

In this post – we will be covering BUILDING PRESSURE, SUPPLY & EXHAUST BALANCE / IMBALANCE, OUTSIDE MAKE-UP AIR NOT FROM HVAC, OUTSIDE MAKE-UP AIR FROM HVAC. KITCHEN HOOD TEST & BALANCE, AND CAPTURE AND CONTAINMENT.

Finally, we will end with spending less on everything … ENERGY USAGE & EFFICIENCY.

Building Pressure:

Believe it or not, building pressure plays a major role in kitchen hood performance.  This has to be one of the most overlooked aspects of kitchen hood performance. Most people (owners and technicians alike) understand very well the issues surrounding the immediate hood itself, but, since air is invisible, no one thinks to look “beyond the hood”.

This will make sense to most, although it is often overlooked … the harder the fan has to suck on the hood, the less air it’s going to move … PERIOD.

Every fan operates under the influence of static pressure differential.  The greater the difference, the harder it is to move the air. You will also notice on a fan curve / performance data of the exhaust fan in question, the higher the static, the less the air moves … WHY IS THIS IMPORTANT?

BECAUSE: If the building is under a gross negative pressure, the exhaust fan has to work against that pressure to move the air.

The easiest way to put this is, if the building pressure is zero, and the fan has a design static of 0.50 IWC (inches of water column), the fan / hood differential is 0.50 IWC.  If the building is also under a negative pressure of 0.50 IWC, this adds to the 0.50 created from the fan, and now the differential is 1.00 IWC.

One of the fan laws relating air flow in CFM (cubic feet per minute) VS STATIC PRESSURE … CFM decreases as the square root of STATIC PRESSURE … OK – forget the math … what this means is when the pressure differential goes up, the air flow goes down. PERIOD

Supply & Exhaust Balance:

If you are “all-in” & subscribing to the statements made above in the “Building Pressure” section, it should be easy to understand that if you want to increase the amount of exhaust effectiveness you have (AKA “C&C” or Capture and Containment), you need to increase the supply!

Almost EVERY building that has a kitchen exhaust hood has issues with building depressurization, which I am calling “Supply / Exhaust Imbalance”.

If static pressure differential decreases air flow, and building depressurization (negative pressure) increases that static pressure differential, then, mathematically speaking, adding supply to the building will decrease the negative pressure (make the building more positive or close to zero pressure), and, consequently, decrease the total static pressure differential.

Looking at it another way, try and imagine you are running with a rope tied to you and someone else who can’t run as fast as you can …

It will be harder for you to move as fast as you can because you have to literally “drag” the other person forward with you.  If this person could run as fast as you … it would be easier to move as quickly as you can, because you are not being “held back” by anything. Does that make sense?

It is this very resistance that explains what we are trying to state here … which, most simply put, it is easier to remove (exhaust) kitchen heat and smoke (known as cooking effluent) when you are supplying the same amount of air you are trying to exhaust (or close to it).

To conclude this section, you can’t just suck everything out of your kitchen and not add it back and expect performance and efficiency.  Where do you think all of your heating and air are going if you have a building that is under a negative pressure?  You will have severe “infiltration” (sucking unconditioned air into your building), and will have to pay to “re-condition” it via either heating or cooling … what do you think that is costing you?

Outside Make-Up Air not from HVAC:

When replacing the exhausted kitchen air, a majority of it (about 80%) will come from a directly purposed make up air system (MUA).  So if your hood exhausts 4000 CFM, you will get 80% / 3200 CFM from the MUA and the other 20% / 800 CFM from the HVAC System(s).

The best thing to do with this air is to condition / temper it (heated or cooled) so that the kitchen environment doesn’t suffer (although this is not mandatory, and there are several installations that do not condition the MUA).

The idea is not to bring the kitchen into what is known as “the comfort zone” (68 – 74* F … what all heating and cooling is designed to do), but to temper the air so that the kitchen environment is livable.  Having said that, it is not necessary to design / size these units the way you would heating and cooling systems.

The kitchen hood needs to draw so much air UP the hood to work properly.  Interference of this natural occurrence by your make up air will cause interference with the way it works and its “capture and containment”, otherwise know as C&C.

Making a long story short, if the velocity of the air around the hood exceeds a certain design threshold, it could interfere or have an effect on the C&C of the kitchen hood.  You’ll recognize this by plumes of smoke / steam rolling in the hood while some rolls out from under the hood area.

Outside Make-Up Air from HVAC:

In order for outside make-up air to be utilized from the HVAC system(s), the fan MUST BE ON!

You will not be able to get your make-up air from the building’s HVAC systems if the fans are not running.  The resistance you need to overcome just to get the air to the hood is not possible.

What I’ve run into the most is … the economizers are simply not working, set incorrectly, or disconnected.  All of these will throw the kitchen out of balance and influence hood performance and capture & containment.

Kitchen Hood Test & Balance:

It is important to note, that when performing a TAB (test & balance) on a kitchen hood, that everything (all pieces of equipment) be on, in normal operating condition / position, and operating properly.  Influences such as building pressurization, crosswinds around the hood, typical door positions entering kitchens, etc. must all be the way they will be normally, and the way they were designed and installed to work.

In other words, if you have outside air requirements for all HVAC systems and they are not met or dysfunctional, your system will not be in the state it needs to be in to properly test the kitchen equipment.

This may sound odd … but many times there have been situations of operational conditions that change. I.E. – someone turned a fan off or placed it in AUTO during the course of your test / visit.  I’m not suggesting that no one can move during your testing, just make sure all of the mechanical equipment is in the proper operating condition / position.

Capture & Containment:

Capture & Containment (C&C) is likely the most misunderstood operational condition of virtually every hood (or should I say … hood owner?).

The velocity of air delivered in front of the hood (called skirt or apron) has a profound influence on a hood’s capture & containment (C&C).  Make-Up Air (typically conditioned) is supplied to the front of the hood via an auxiliary supply plenum to provide an air curtain of protection to the hood’s C&C area.

Additionally, the velocity / turbulence of the air moving around the hood has a significant impact on the kitchen hood’s ability to remove the cooking effluent.  Some hoods have partitions on the side to reduce the influence of crosswinds near the hood & increase C&C.

Several kitchen owners and operators have fans blowing about the kitchen due to comfort issues or complaints, as well as providing air movement to keep the hotter areas more tolerable. THIS SHOULD NOT BE DONE!

None of them understand what a properly designed kitchen is supposed to look like, work like, and feel like.

More on this topic and others will be coming in future posts.  Check back regularly for updates in this area.

 

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