The effect wind/weather has on a BBQ smoker 2

[gregla]

I'm new to the site/forum so I hope I'm in the right place for this topic. I'm starting to research an article I'd like to write on the effects of wind on hot steel.

Specifically, the effect wind/weather has on a BBQ smoker. I've been in the IT industry for about 25 years, but I also own a BBQ catering business. I have a large trailer mounted smoker and have learned over the years to be very mindful of the weather when I have to cook for an event.

I know how to try and compensate for weather conditions, but would like some scientific facts about whats really going on.

Formulas to calculate heat loss etc. I can give general recommendations now, but would like to boil it down to a few sentences to wrap up the article/advice. For example: A certain size smoker constructed of X gauge steel will take X amount of time to heat to X temp. Taking into consideration, ambient temperature, wind, rain, snow, ice etc.

I need to keep my recommendations simple and easy, but back it up with facts.

Any guidance, advice or resources I could use would be very helpful.

Thanks
Greg

Greg

http://www.halobbq.com

 

[silentbobo941]

This is actually much more complex than it seems. The guiding formula to use in an energy balance Q[sub]in[/sub] = Q[sub]out[/sub] where Q is heat energy. It can be broken down as follows:

Q[sub]geneated[/sub] = Q[sub]conduction[/sub] + Q[sub]radiation[/sub] + Q[sub]convection[/sub]

The burning of your fuel is the input heat energy (Q[sub]geneated[/sub]), while the output heat energy is lost to convection, radiation, and conduction. The easiest of these is conduction, which would be heat lost through direct contact. Heat is conducted out from the hot part and down into the ground (or whatever else is touching it). In reality though, that heat is probably lost due to convection before that, so conduction should be minimal. You can probably safely ignore it as assume Q[sub]conduction[/sub] = 0

The second way heat is lost is due to radiation, which is dependent on the ambient temperature of the surroundings, how well the surroundings reflect or absorb radiation, and geometry of everything around. Because of this, the formula will change drastically from each application. Here is a little bit on radiation:

http://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html

The third way heat is lost is through convection. Convection rate is dependent on many factors that include material properties and airspeed. The formula for convective heat loss is:
q[sup]dot[/sup] = hA(T[sub]s[/sub] - T[sub]A[/sub])

where q[sup]dot[/sup] is the rate of heat transfer (which is energy/time), h is the convection coefficient, A is surface area of convection, Ts is temperature of the hot surface, and TA is ambient air temperature. Unfortunately, the convection coefficient is very hard to determine analytically. It will change based on wind speed and some other factors. Here is a little bit on convection:

http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

Heat transfer is highly non-linear. Basically, it would be extremely difficult to make an analytical model of the bbq without doing multiple experiments or simulations to determine a statistical trend.

 

[MintJulep]

Rain has a single phase change, snow has two.

They will suck away lots of heat.

 

[gruntguru]

So for something as hot as a BBQ or smoker in an outdoor environment, simply shielding the device from wind and rain (ie a secondary enclosure - even a close-fitting one) would produce the biggest improvement in temperature consistency. Insulation would take temperature consistency and fuel efficiency to the next level but at a higher cost and possible interference with the way the device functions.

je suis charlie

 

[gregla]

Thanks for the advice and replies

It is complex given all the variables. I'll read over some of the resources posted in the replies and try to come up with some concise recommendations.

The info in the replies/resources once filtered through my unlearned mind may actually help me keep it simple for the readers.

Thanks!

Greg

http://www.halobbq.com

 

[IRstuff]

dang, I don't suppose you can air ship some ribs to California? The ribs and brisket on your website do look mighty good. ;-)

One addition to gruntguru's missive is that insulation will also make your operation slightly more efficient, and faster to get to operating temperature, which might be worth something.

TTFN
faq731-376
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There is a homework forum hosted by engineering.com:

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[racookpe1978]

If natural draft stove/burner:
1. The Inlet face (inlet opening) of the burner air supply (the opening that the air flows through to get to the damper and then the fire) MUST face the wind to be most efficient.
If the opening is downwind (shielded from the moving air) then the burner air will have to flex and twist around and change directions - many times! - to get pulled into the burner, then burn, then go through the bbq oven racks and meat then up through the chimney then out into the air again. Each twist creates friction (resistance) and reduces flow and reduces the heat you are your fuel bill for. (Nat gas, propane, wood, pine, or oak or mesquite - doesn't matter all fuel is expensive.
2. The chimney should be as high as practical to get the best draft through it from below.
3. Hot air rises (obviously) so your "burned fuel flow" of smoke through the grates and smoker and into the exhaust should always be rising. Unless you want a deliberately slower smoker section. Then add more baffles and more resistance in the smoker - but expect those surfaces to trap contaminates and grease.)
4. Forced draft requires a fan or something like the old locomotives "blastpipe" that induces more air flow through the firetubes by redirecting exhaust steam You're not making steam so this doesn't really count. If you want forced draft, you need an electric supply and a controller and a fan and a supply duct and filter. Makes things more complicated - particularly electric power if you are on a trailer away from the house plugs. But forced draft sucks the combustionair in, so you don't care how you park the trailer to get the wind to add to your convection.

 

[EdStainless]

You can make some approximations. Call the cooker a cylinder (2.5' diam), pick a surface temp and air temp (say 350F and 70F) and then calculate heat loss for just natural convection (ignore radiation and conduction) and then recalculate for a cross flow wind (say 20mph).
I did a rough calculation(very rough) and saw a doubling of the heat transfer coefficient, so a doubling of the heat loss.
A simple stand off shield of sheet metal that is supported by baffles that prevent drafting between them would cut the heat loss, and nearly eliminate the effects of a wind (or rain).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[chicopee]

Also you should be interested in the heat input value which would have units in Btu/Hr for propane, butane or natural gas fired appliances or in Kilowatts for electrically heated appliances. This heat input is not the amount of heat that will smoke and heat up your meat. The heat output is what will do the cooking and smoking and that value is lower than heat input. So what good will be derived from knowing the heat input, well two things come to mind, one being the size of the appliance and secondly gaging the cost of the fuel.