 |
|
|
that lasted approximately 0.8 seconds.
|
We conducted a number of experiments, deliberately trying to create flashbacks in order to a) film them for the benefit of new owners of ceramic charcoal cookers to make them aware of the dangers and b) discover what might contribute to the strength of flashbacks.
What is a flashback? Basically, a flashback can occur when you have a hot fire in the cooker, you cut off the air supply to the fire for a period of time, and then you open the lid allowing oxygen to enter the cooker. The fire is starved for oxygen, having consumed all the oxygen in the cooker while the air supply was cut off. When the lid is opened and oxygen rushes back in, a flashback can occur if there is fuel and enough heat to ignite the flashback. For example, this often happens when cooking steaks using a high temperature sear followed by a dwell. Typically the steaks are seared over a 750 degree fire for several minutes, then the air vents are closed for a "dwell". When the dwell is finished, the lid of the cooker is opened to remove the steaks. The flashback can occur when the lid is opened after the dwell and air rushes into the oxygen starved fire. However, flashbacks have been known to occur at lower temperatures under different conditions. As the experiments will demonstrate, though, there are 3 factors necessary: lump containing volatile organic compounds (VOCs), heat, and a sudden supply of oxygen to a oxygen-starved fire. It should be noted that flashback is more correctly called "backdraft". Here is what a firefighter was kind enough to send us on the subject: "I would encourage you to refer to it as a backdraft, because that is what you created. Many laypersons are vaguely familiar with it due to the movie Backdraft. As a firefighter, I know a backdraft is a situation where a fire develops in a closed compartment, space, room, whatever....the fire builds, temperature rises and fuel is being consumed until the fire uses up the oxygen in the space. At this point the open burning stops. The heat is still present as are the fuel load and other combustible gases. Now, along comes a firefighter or a backyard chef and open a door, window or other opening. Oxygen rushes in. The other 2 components needed for fire are already there (the heat and the fuel). This results in massive eruption of the unburned fuel and gases lingering there. Voila...Backdraft. Same thing firefighters must be wary of. Except in our case, walls could be blown away, structural collapse, deaths, etc.. I only tell you this to help turn the light on in a few more peoples head." Conditions Known To Cause Flashbacks: This list should not be considered the only set of conditions that can cause flashback, but here are the ones that I have experienced:
The Experiments: I started my experiments by adding a small amount of new lump to my cooker to supplement the lump that was left over from a 600 degree fire to cook a pizza. The lump was almost up to the top of the firebox. I started the fire and let it get to 750 degrees.
WARNING: If you don't watch the video of the experiments, you should know that I opened the lid of the cooker by tying a rope to the handle, running it through a pulley overhead and across my deck. I was 15 feet away whenever I opened the lid. I don't encourage anyone to try this at home.
|
Conclusions: Based on what I just observed, old lump that has been heated by a previous fire will not create much of a flashback and smoke alone is insufficient to cause flashback. The key factor is the "freshness" of the lump. If you observe a hot fire made with lots of fresh lump, when the fire is at 700 degrees or so, you will see blue flames spouting from the top opening, jumping 12 to 18 inches. If you do this with old lump that remains from a previous hot fire, you will see very little (if any) of the blue flames. Why? Even though lump is mostly carbon, it still contains a certain amount of volatile organic compounds. These VOC's are driven off by the heat of the fire and can then burn. This is the blue flame you see spouting from the top of the cooker. So, when the lump is fresh and still contains a significant amount of VOC's, VOC's build up in the cooker while the air supply is shut down. Opening the lid allows oxygen into the cooker which allows the VOC's to ignite and the result is the flashback. Lump which has been heated in a previous hot fire contains a lesser amount of VOC's and thus the severity of the flashback is greatly reduced. Eventually as the lump continues to burn, all the VOC's are driven off and no flashback will occur since there are no VOC's to ignite when the oxygen rushes in. The determining factor on the size of a flashback is the amount of VOC's in the lump. Remember, Fresh Lump = Highest Amount Of VOC's = Biggest Flashback = Greatest Danger. But also remember, you can never be really sure as to the amount of VOC's in your current fire, so always be careful!Regretfully, I failed to conduct one further experiment, and I doubt my wife will put up with this foolishness much longer to allow me to try it. Basically, does smoke contribute to the severity of the flashback? I demonstrated that with no VOC's to ignite, there is no flashback, smoke or no smoke. However, I didn't combine smoke and fresh lump (i.e. lot's of VOC's) to see if the smoke augments the flashback. Maybe the next time my wife is out of town....
UPDATE: We recently experienced a flashback with lump that had been burned a cumulative total of over 30 hours in several sessions. This is a new lump (see the review of Wicked Good Charcoal at The Lump Database) which lasts a very very long time. Apparently this lump contains VOC's even after repeated lengthy burns. So let us emphasize that you can never be sure whether or not you are going to have conditions conducive to flashback, so whatever procedure you use to avoid it, use it everytime.
How To Avoid Flashbacks: There are two methods that I have seen for avoiding flashbacks. The first is to burp the cooker by raising the lid an inch or so, let a small puff of flame possibly come out, hold it open for several seconds and then open the lid the rest of the way. I do not recommend this technique. Burping the cooker allows extremely hot gases to flow out of the cooker between the gaskets and this can damage your gasket. (See Information About Melting Gaskets.)
Instead I recommend that you open the bottom vent and the top vent all the way for about 10 seconds before you open the lid. The lid should then be opened slowly about 1 inch for a few seconds and then the lid can be opened all the way. This allows most of the hot gases to burn and exit the cooker before you open the lid and expose the gaskets.
We videotaped the flashbacks which occurred during our experiments and selected the best one to make into a small video so that you could see a real flashback for yourself. We hope this will motivate you to be careful with your cooking.
To view the following WMV video clips, you will need Windows Media Player or a program capable of playing WMV files. The MOV clip requires Quicktime to play. If the clip doesn't start playing when you click on the link, you can also try clicking the right mouse button on the link and then choose "Save Target As..." to save the file on your computer, using Internet Explorer. With Netscape, you can do the same thing and choose "Save Link As...". Once you have it on your computer, you can then start up the appropriate program and open the file.
FLASH.WMV -- 851K larger video clipOr, of course, you can watch the video to the right, courtesy of YouTube.
FLASH2.WMV -- 371K smaller video clip
FLASH4.MOV -- 1.58M Quicktime format. Not very good, but you'll get the idea.
