Hi —
We are trying to vent a 30″ gas grill located on a semi-open porch in a condo. The existing ductwork consists of a straight run of about 8′ of 6″ x 8″ galvanized duct tied into a large copper roof jack. We currently have a 50″ wide hood installed with an internal 1200 cfm fan. The system is simply not removing enough of the smoke from the grill. The original vent hood and grill worked quite well together, but were replaced mostly for asthetic reasons. What’s wrong?
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Replies
There is no way you are getting 1200 cfm through that size pipe. The blower is probably rated 1200 cfm at 0" of static pressure. You may be getting a third to a half of the rated cfm through that pipe. I had to use 12" round pipe to get a measured 1000 cfm at the discharge grill.
I c&c (capture and contain) all of the smoke from my indoor grill but just barely. My hood is 54" and is 30" abofe the grill. C&C is easier to accomplish with less arflow if the hood is located closer to the cooking surface. The best route is a shallower hood at 18" above the cooking surface. Most architects want the hood 36" above the surface or even higher. That simply won't work without massive amounts of air.
Of course if the hood is located indoors (I know yours isn't) the exhaust air must come from somewhere. The more air that you exhaust, the greater the load on the air conditioning air due to forced infiltration of outdoor air. We recommend piped makeup air at a rate of 50% of the exhaust air volume to help offset the AC penalty and reduse infiltration through the building envelope.
This is actually a complicated issue that is rarely done right. In large custom homes that are attempting to provide a commercial kitchen of sorts, the hood is usually a failure. It may C&C the steam from boiling water, but fails to capture the effluent produced by frying grilling or heavy saute'ing.
As for your solution, here are the possibilities. Up the pipe size, lower the hood, add side baffles to reduce turbulence from air currents, or the worst option is to replace the blower with one that has a better fan curve for your large static pressures.
Most architects want the hood 36" above the surface or even higher. That simply won't work without massive amounts of air.
Silly archies, we (customers want, really) want to be able to 'see' around their kitchen hoods when they are not running (nigh unto 80% of the time). Gets to be tiring, fighting the (all too often losing) battle of "your range needs nnnn CFM, and we need a smaller volume to do that" battle with some one with their heart set on (or already bought) fancy commercial range unit. Then, a susbtitute is usually sought out, in a huge rush after getting the numbers for the gas & electric install for the fool things <tired sigh>.
Of course if the hood is located indoors (I know yours isn't) the exhaust air must come from somewhere.
I've seen a kitchen or two with feed air behind grills & fryers. It's inelegant at best. I've seen some residential kitchens that probably needed a pop-up unit running in reverse to feed the extravagant hoods miles over the cooking surface . . . Occupational hazard of my occupation not being around (sorry Bubba)
Ray is right about the airflow being way too much for that ductwork. If you tried to put 1200 cfm through 6x8 duct, plus roof jack, the pressure drop would be in the order of 4" water column pressure. The fan doesn't put that out. The fan puts out 1200 cfm at 0" back pressure. And maybe 500 cfm at 1/2" of pressure.
SWAG of where the curves (fan performance and pressure loss in ductwork) meet: about 300 cfm.
For 1000 cfm, 8 feet of straight 10" round duct would do, if there was no cap on it. With a free-flowing cap of some sorts, 12" might work.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks Ray.Since increasing our duct size is not really an option, would I benefit by replacing the existing internal blower with an inline blower that's rated somewhere around 400cfm?
An inline blower won't help and you must remember the risk of a grease fire. My father in law was a mechanical contractor. His blower was not rated for grease. He lost his entire home to a grease fire in his hood vent. Be careful.
Up the duct size. You can put an external blower at a cost but if you get one that pulls the volume that you want, the noise will be excessive.
In your first message you said, " the worst option is to replace the blower with one that has a better fan curve for your large static pressures." That's probably our only option. How do I determine which blower would be best? I've got a Grainger catalog, but I'm not familiar with all of the different specs.
'How do I determine which blower would be best? I've got a Grainger catalog. . ."
You draw a little graph. Get some graph paper. Put "inches water column" on one axis and "cfm" on the other.
The pressure drop curve for 8 feet of 6"x8", plus entrance effects plus the roof jack will be about (I haven't seen the hood or the roof jack):
100 cfm - 0.02" w.c.
200 cfm - 0.10" w.c.
300 cfm - 0.23" w.c.
400 cfm - 0.43" w.c.
500 cfm - 0.66" w.c.
750 cfm - 1.5" w.c.
Then read the specs of the blower from the Grainger's catalog. Plot the cfm/pressure points for the blower. Where the curves intersect is where that blower/duct combo will operate. After plotting a few, you'll notice that the best performing ones hit max flow at about the same pressure. Subsequently, just plot the two points nearest that pressure to save time.
Of the blowers that move the most air, look at the noise, cost, installation options, etc and pick one.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for such a complete response, Dave. I'll try that this weekend and see what I come up with.
Well, I plotted the curve for the 6 x 8 duct and plotted a few different blower assemblies. Turns out only one blower curve actually intersects the duct curve. Seems like a reasonable blower for the application -- same RPM and HP as the old motor. Thanks for all your help. By the way, how did you calculate the water pressure values you posted?
I've collected versus pressure drop versus air charts over the years. One from Lamson Blowers plus some others I'm not recalling just now. And I've made up my own excel tables for pressure drop per hundred feet of pipe.The other reference you need or at least have a feel for is the effects of bends, entrances and exits. In large ductwork, they predominate over straight pipe. For instance a tight 90 has the same pressure drop as 30 pipe diameters of straight pipe. So a 90 in 6" x 8" is like 18 feet of straight pipe. Turning through a tee would be twice that (more disruption to the flow). Just googled on it. Table 4 at this URL has some values: http://muextension.missouri.edu/explore/envqual/eq0380.htmDavid Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for all your help, David. I ordered the blower today and we'll probably get the whole thing up and running next week. Hope it works. I'll let you (and any other interested parties) know how it goes.
Is there a filter in the new hood? If so, how does the hood work with the filter removed?
Thanks Matt.We've tried with the filters in and out. Same results.
"The original vent hood and grill worked quite well together, but were replaced mostly for asthetic reasons. What's wrong?"
The answer to your question lies in the working equipment that was removed.