I read Greenheck, but Also ASHRAE didn't say what greenheck says.
some of designers push 50% of make up air directly into the hood and they temper the rest 50% before pushing it into the space, some of them push 80% directly into the hood nothing in the space. it is the best way to have a trouble as the greenheck said.
but ASHRAE says that "atleast 20% of the makeup air should come from the space" (put a line under atleast)
let us hav a look from both sides, greenheck and ASHRAE
Greenheck adivse to not use it at all, but many people still use it in small restaurants, it is not logic to push a 4000cfm (for a 14' hood) for example in a 10'X8' kitchen area in a small restaurant using make air unit with heater which it cost thousends of dollars either for the installation or for the operation.
if we go with ASHRAE and push up to 80% directly into the hood, so why we do that? the hood will exhaust actually only 20% (800cfm)from the space, if this ratio is good enough for operation, why don't we install a 800 cfm hood exhaust fan with no makeup air system instead of installing 4000cfm hood exhaust fan with makeup air system, this will save also on the fan and exhaust duct.
How do you fit a 14 foot long hood in a 10 ft x 8 ft room?
The exhaust rate is determined to get good capture from the appliances. Some installations work to be approximately 100 CFM per square foot of hood area. The rate depends on the type of hood. Again look to Greenheck or Halton for more information.
On a hood you are trying to contain the effluent from the cooking operation. The make-up hoods are trying to create an air curtain that remains in the proximity of the hood such that the make up air doesn't affect the occupants and need not be conditioned.
So...you generally need around 100 cfm per square foot exhausted, and therefore the make-up air needs to match that amount.
The answer has to be that it will work substantially poorer and less efficiently:
> There's less air being pulled from the room itself, so odors may linger longer, and cooking on burners near the edge of the cooktop may result in more odors getting into the room
> The reduction in hot air being pulled in may cause aerosolized grease and oil to be condensed onto surfaces the system wasn't designed for.
From a purely systems engineering perspective, a short-circuit hood clearly is an attempt to circumvent a known requirement. Apparently, that requirement is too poorly written to prevent usage of short-circuits.
I'm saying, I've never used one, I don't know what the conditions and entrainment of the grease actually would be. Greenheck and Halton have done studies on hoods, they would be your best resource.
If you were talking lab hood, don't even waste your time unless you don't get high outside humidity. The site also discusses how to prevent short circuiting.
I am talking about commercial kitchen hood.
it is not for a specific project, it is a general discussion about this type of hood, I heard many different opinions about this type of hood
some of them says don't use it, in this case the alternative soulution is expensive, some of them say it is ok, in this situation the performance is a question.
the idea I want to discus is two situations:
1-let us say weh have 2000 cfm hood exhasut fan, and let say also the make up air is 2000 cfm too.
80% (1600)of make up air goes into the hood, and 400 cfm goes into the kitchen space.
2- let us say we reduce the make up air from 2000 cfm to 400 cfm and replace the 2000 cfm hood exhaust fan by 400 cfm fan
is the performance the same(or a little different) in both cases?.
I think you may run into problems with IMC and NFPA 96.
I do not believe that the 400 CFM, for same hood, UL labeled or not, would offer the same capture and containment, if that is what you mean by performance.
mentioned number are just an example.(forget any code, UL or air tempering for now)
we have 2 bitza ovens same brand and model for example
- we installed the first one under a compensate hood which has 2000 cfm exhaust fan and 2000 cfm make up air unit, 80% of this make up air(1600 cfm) goes into the hood and the rest 400cfm goes into the space
- we installed the second oven under a dry hood wich has 400 cfm exhaust fan and 400 cfm MAU goes into the space
I think you have a misunderstanding of the hoods. The 1600 CFM isn't blown directly to the exhaust inlet. It is blown into the space as near to the exhaust as possible while maintaining containment. The intent is to minimize the effect on the space and reduce the required tempering of the make up air.
The second situation that you are describing is not the same. Your exhaust air velocity and entrainment at the hood would be severely compromised.
Captive Aire (and others I presume) have done studies and lab tests pinpointing the ideal location for make-up air to a Type I hood. One of the best methods so far, is with a supply plenum attached to front of the hood. This allows make-up air (usually ~80% of exhaust)to be supplied at the hood, but not directly into it, allowing proper entrainment of the grease/smoke laden vapors rising off of the cooking surfaces.
To answer the original question, no, a short circuit hood is not recommended, and may be interpreted as against code (not 100% sure about that one) Short-circuiting a hood causes inefficient entrainment (the make-up air is supplied too close to the cooking surfaces, causing some of the grease exhaust to billow out of the hood)
In scenarios where a supply plenum is not provided, make-up air may be supplied close to the hood via perforated diffusers, or other supply devices that do not confer a directional flow to the make-up air, other than straight down.
In all cases, tempered (heated) make-up air is required. Some people still try to get away with just using an evap. cooler to supply mua, codes are more restrictive now.
Huh? Voltage doesn't have anything to do with what is being discussed in this particular topic.
"Short-circuit" in this case, refers to blowing the make-up air right inside the canopy of the grease hood, sometimes the plenum or supply is inches away from the baffled filters.
The route of the make-up air is too short, too direct, (hence, "short circuit") and causes entrainment issues.
A psp (perforated supply plenum) should ideally be placed on the outside of the canopy, and blow straight down.
