Thermal considerations in BBQ

[urbangriller]

Thermal considerations in BBQ

Thermal (Heat) transfer.
In the Lunchbar we had two gas powered grill plates. One was a half inch thick and it took 10 minutes to heat up, the other was an inch thick and took 20 minutes to heat up. Once hot there were the same in terms of heat applied to the bottom and heat radiated from the top. The advantage of the thicker plate was less susceptible to cooling when a cold burger was placed on it.
The 1 inch thick plate had greater thermal mass.

Metal is not an insulator, otherwise you would not be able to cook on it!

Thermal storage.
Thermal mass is useful in maintaining temperature because it takes time to release its stored heat, but correspondingly, it then takes more energy to get it back to temperature.
This is easy to test. Take two BBQ Kettles with the same fuel load, put a large tray of water in one. Place the lids on both kettles, the one with the water pan takes considerably longer to get to peak temperature because it has to heat all that water.

Now take the lids off both kettles for a minute and replace them, the kettle with the waterpan will show a higher temperature but take time to get back to peak temperature, the kettle without the waterpan will show a lower temperature but will come back to temperature quicker. The temperature swing is less when using a waterpan for thermal mass.
Now let’s look at thermal insulation. A ceramic Kamado stores heat in the thermal mass of its shell, ceramics store heat well. A steel Kamado has little thermal mass, it traps the heat inside by being well insulated. Open the lid on them both and you let out the hot air, close the lid and the steel Kamado is slightly slower coming back to temperature because it has none stored in its shell.

So using thermal mass is useful in BBQ to reduce the fuel needed and to keep temperatures stable, you can also do this by adding insulation, like a thermal blanket or double skinning.
You can have thermal mass in the walls of the BBQ, or you can introduce thermal mass by using a waterpan or simply putting bricks inside your BBQ inside your BBQ. A waterpan often aids the cook by adding steam to the cooking environment.

Increasing thermal mass and insulating are great things to do to any BBQ.

Aluminium is great at transmitting heat, cast iron is good as well, we know this from the way BBQ Grates made from these metals work.
Stainless steel is no good at transmitting heat and BBQ plates that are Stainless Steel have a high percentage of ferrous material in them to assist in the heat transfer.

Cheers
Chris

[wedwards]

I'm not trying to be a tool, but doesn't all of that support what Dave was saying in another thread today about grills made with thicker steel are able to perform better in cold and windy conditions? Thermal mass in a grill with thicker walls would be greater and therefore more impervious to ambient temp changes would it not? Obviously you can supplement the natural thermal mass in a grill with things like water pan, bricks, etc, but if the grill was made from thicker steel that might not be necessary.......dunno.....just a thought. Like I said, not trying to be a tool, but just to understand the science of all this, because I like that stuff.


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[Nath]

Wedwards, yes your correct, in theory it should create more thermal mass (heat storage), but this also takes more fuel to get to this point.
It all gets a bit nitty gritty for me sometimes and I forget about just q'in


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[DingoWA]

Thanks Chris. That's very useful to think about. We can only do so much with the walls etc of what we've already got, but we can add things like the waterpan etc to compensate for the limitations of the Q we are using. And of course depending on what we are cooking and how. Hmmm, control of variables.....I like it.

[urbangriller]

I'm not trying to be a tool, but doesn't all of that support what Dave was saying in another thread today about grills made with thicker steel are able to perform better in cold and windy conditions? Thermal mass in a grill with thicker walls would be greater and therefore more impervious to ambient temp changes would it not? Obviously you can supplement the natural thermal mass in a grill with things like water pan, bricks, etc, but if the grill was made from thicker steel that might not be necessary.......dunno.....just a thought. Like I said, not trying to be a tool, but just to understand the science of all this, because I like that stuff.


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Understanding how heat works and transfers is important, and it is a fascinating science as you say. Having greater thermal density does not necessarily equate to greater efficiency, in fact greater thermal density can mean less fuel efficiency.

This is the case with reverse flow offsets, they have greater thermal mass from the internal full length baffle plate, and this helps keep temperatures stable and even from one end of the chamber to the other, but a massive amount of heat is shed before it gets close to the food, and the result is an increase in the fuel needed to keep them at temperature. For me, moveable tuning plates increase the versatility of a cooker by allowing the user to place the heat where they want it, correct use of tuning plates can increase the fuel efficiency of a cooker, particularly when it is only half full of food.

The other thing to consider is the way the thermal mass absorbs, holds and releases its heat. Consider the difference between steel and ceramic. Steel absorbs heat more rapidly than ceramic. But does not store it well, heat enters one side of a hotplate for example but is readily lost out the other side of the plate at almost the same rate. Ceramic takes longer to heat and is correspondingly slow to release the heat it stores. It’s all about how the heat moves through the object.

Insulation increases efficiency vastly by reducing the shedding of heat from the surfaces that are insulated. Effectively, insulation reduces heat loss by providing a thermal barrier between the hotter environment and the cooler one.

Also we need to consider that the meat we are cooking is also a heat sink, the thickness needs to factored in, but as we all know, it takes considerable time to get heat into meat and we know that it stores heat well, this is evidenced by how meat behaves during resting, stored heat in the outer layers continues to travel deep into the meat even though the outer surfaces may be slowly shedding heat. Short temperature fluctuations in the cooking environment do not produce much fluctuation in the heat stored in the meat.

Regards
Chris

[urbangriller]

Some of you will have noticed a vendor posting his marketing statement based “opinion” as fact using a “Chinese” pellet grill as an example, and claiming that heat loss from windy conditions caused it to fluctuate and over heat.

We’ll, here are the real FACTS:

That Pellet Grill was a Traeger, and as anyone who owns a Traeger knows, unless you have the background timing exactly correct it will overrun the set temperature, even on a still day! That’s not a product of heat loss due to windy conditions, it’s the product of a Traeger running the wrong “P” setting for the conditions.

Wind will take heat from any BBQ, but in the case of the pellet grills I know, they will kick in and restore the heat once the temperature drops 3 degrees below the set point. If it is running properly it will stop when it reaches the set temperature. It doesn’t matter if it takes 30 seconds to lose enough heat, or a minute, it will still only drop 3 degrees, and a controller that is set up properly will recover quickly and stop at your target temperature. That’s the way they run, that’s the advantage of them.

Thick walls in an offset are the same, they help maintain even chamber heat, but shed heat like anything else.

The thicker the metal, the more energy needs to go into heating the metal up. Thicker metal will help maintain even heat in your cooking chamber but it will shed heat externally just like any other BBQ and it will take more energy to build that heat back up.

If you really want to minimise temperature fluctuations in any BBQ insulate it! Using internal Water pans and heat sinks like bricks will help greatly.

And on the topic of insulating, here is an image of how one guy insulated his Chargriller…neat and effective job!
More pics here: http://www.barbecuebible.com/board/view ... hp?t=17291



Cheers
Chris

[alimac23]

Nice explanation Chris, I take it that's an advantage of the Akorn, Dragon and BSK is the actual layer of insulation between the metal walls. I know my Akorn holds a solid temp regardless of how cold or windy it is outside.

[Muppet]

Yep, that function with the Treager can actually be quite handy. Allows the cook some adjustment to the smoke ratio. It did my head in to start with but I use it to tailor my cooks these days.

[urbangriller]

Nice explanation Chris, I take it that's an advantage of the Akorn, Dragon and BSK is the actual layer of insulation between the metal walls. I know my Akorn holds a solid temp regardless of how cold or windy it is outside.

Dead right, the insulation does a great job in a metal Kamado, they are quick to heat up and you can cool them fast! They all sit nicely on temp, when you open the lid they lose cooking chamber heat, but quickly get it back because they are not losing heat or having to reheat thick walls. Ceramic stores heat and is slow to heat and slow to cool.

Cheers
Chris

[urbangriller]

Yep, that function with the Treager can actually be quite handy. Allows the cook some adjustment to the smoke ratio. It did my head in to start with but I use it to tailor my cooks these days.

Yes, that's what I like about it as well, but it's a bit to get your head around when you first meet it! I'm not surprised people have temperature issues until tbhey learn what the "P" settings do and how to work them.

Chris