标签: oven

Heat dissipation is often tied with power usage; that's not news. But sometimes, in our focus on minimising power consumption and thus subsequent dissipation, we forget that there are situations where properly managed dissipation is the whole point of the design.   This became very clear after I read a fascinating article in a recent issue of IEEE Spectrum about the ineffectiveness of the standard home oven. The article, " Recipe for a Better Oven " by Nathan Myhrvold (yes, the former chief scientist at Microsoft) and W. Wayt Gibbs, discussed the many shortcomings of the standard home and even commercial baking oven.   It was both illuminating and somewhat discouraging for many reasons. It's not just that the oven is wasteful in terms of energy use, it's that it also does a poor job despite that inefficiency; whereas I had hoped that the inefficiency was a necessary part of the tradeoff for getting the baking done right.   I already knew that the simple on/off control of the gas line or electric power was a marginal approach. Anyone who has studied any control theory knows that a loop with on/off control, rather than true proportional control and a PID (proportional-integral-derivative) control strategy, is not good at keeping the error between setpoint and actual value small, unless you are willing to switch that on/off valve at a fairly high rate.   While it is true that thermal situations tend to have relatively long time constants and so can accept a somewhat lower cycling rate than some other situations, oven vendors like to keep the valve's cycle time to a few minutes. So it is common to see swings of ±50°F (about ±25°C) around the setpoint. That's just not good enough for many recipes or foods.   The problems of the standard oven design and subsequent performance go well beyond the basic temperature-control loop. Only after I read the article did I realize how much I hadn't known about the implications of oven design and the cooking process.   For example, the heat source doesn't cool the food directly; instead, it heats the oven walls (metal or brick), which then re-radiate the heat to the food. The article explained how this leads to very uneven heat zones in the oven for conventional metal-wall ovens, and numerous other problems. (The article did highlight some ways to improve an oven's performance, which was nice to see.)   These other problem are not trivial, either, as they have to do with how different foods are affected by the radiant heat of the oven walls versus direct heating; not surprisingly, it does make a difference, just as different types of electrical load impedances affect power supply performance. (There are times when I felt the whole discussion of oven idiosyncrasies was akin to the mysteries of black-body radiation , which were finally resolved with the development of quantum theory in the 1920s by Max Planck and others.)   The irony of the oven problem is that for most engineers, the primary concerns regarding heat are twofold: minimizing generation of it in the first place, through the use of low-power circuits and high-efficiency supplies; and maximizing its removal, using convection, conduction, and radiation via use of fans, heat sinks, cold plates, and even advanced active techniques. It takes a 90° or even 180° shift in thinking to begin to understand heat and its effective delivery as an outcome rather than as an obstacle.   Have you ever been faced with a design challenge that required that you reroute your established way of looking at something, to a very different course? Did you realize the differences right away, or did you have to really step back to understand the many inherent assumptions you were making and how they had to be changed as well?

Heat dissipation is typically linked with power usage; that's not news. But sometimes, in our focus on minimising power consumption and thus subsequent dissipation, we forget that there are situations where properly managed dissipation is the whole point of the design.   This became very clear after I read a fascinating article in a recent issue of IEEE Spectrum about the ineffectiveness of the standard home oven. The article, " Recipe for a Better Oven " by Nathan Myhrvold (yes, the former chief scientist at Microsoft) and W. Wayt Gibbs, discussed the many shortcomings of the standard home and even commercial baking oven.   It was both illuminating and somewhat discouraging for many reasons. It's not just that the oven is wasteful in terms of energy use, it's that it also does a poor job despite that inefficiency; whereas I had hoped that the inefficiency was a necessary part of the tradeoff for getting the baking done right.   I already knew that the simple on/off control of the gas line or electric power was a marginal approach. Anyone who has studied any control theory knows that a loop with on/off control, rather than true proportional control and a PID (proportional-integral-derivative) control strategy, is not good at keeping the error between setpoint and actual value small, unless you are willing to switch that on/off valve at a fairly high rate.   While it is true that thermal situations tend to have relatively long time constants and so can accept a somewhat lower cycling rate than some other situations, oven vendors like to keep the valve's cycle time to a few minutes. So it is common to see swings of ±50°F (about ±25°C) around the setpoint. That's just not good enough for many recipes or foods.   The problems of the standard oven design and subsequent performance go well beyond the basic temperature-control loop. Only after I read the article did I realize how much I hadn't known about the implications of oven design and the cooking process.   For example, the heat source doesn't cool the food directly; instead, it heats the oven walls (metal or brick), which then re-radiate the heat to the food. The article explained how this leads to very uneven heat zones in the oven for conventional metal-wall ovens, and numerous other problems. (The article did highlight some ways to improve an oven's performance, which was nice to see.)   These other problem are not trivial, either, as they have to do with how different foods are affected by the radiant heat of the oven walls versus direct heating; not surprisingly, it does make a difference, just as different types of electrical load impedances affect power supply performance. (There are times when I felt the whole discussion of oven idiosyncrasies was akin to the mysteries of black-body radiation , which were finally resolved with the development of quantum theory in the 1920s by Max Planck and others.)   The irony of the oven problem is that for most engineers, the primary concerns regarding heat are twofold: minimizing generation of it in the first place, through the use of low-power circuits and high-efficiency supplies; and maximizing its removal, using convection, conduction, and radiation via use of fans, heat sinks, cold plates, and even advanced active techniques. It takes a 90° or even 180° shift in thinking to begin to understand heat and its effective delivery as an outcome rather than as an obstacle.   Have you ever been faced with a design challenge that required that you reroute your established way of looking at something, to a very different course? Did you realize the differences right away, or did you have to really step back to understand the many inherent assumptions you were making and how they had to be changed as well?

I have a story I've been meaning to tell. When I returned home the evening before Thanksgiving, I discovered my son, Joseph, looking shell shocked. His hair was standing on end. He was panting and covered in sweat, and he gave the impression of having finished a marathon. The closest thing I can think of to give you an idea of what I'm talking about is Beaker from The Muppet Show having a bad day.   For some reason, the first thing that popped into my mind was the expression "Fire in the hole," which is commonly used to alert people that a controlled detonation is about to take place. The first cannons were fired by applying a flaming torch to a small hole packed with gunpowder. "Fire in the hole" was both a command to the man wielding the torch and a warning to anyone in the vicinity. Once Joseph had quieted down and caught his breath, he explained what had happened. The day before, my wife (Gina the Gorgeous) had mentioned that we needed to clean the oven in anticipation of her preparing our Thanksgiving repast. Since I had been broiling quite a few steaks and burgers recently, I told her she should wait for me to give it a good wipe down to remove any grease before she ran the oven's self-cleaning cycle. Did she listen to me? Ha. Why is it that no one ever listens to what I have to say? It's like casting pearls of wisdom before... creatures that don't appreciate such things. The following afternoon, while I was still at work, Gina initiated the oven's self-cleaning function and then merrily sailed out of the door on her way to the grocery store after instructing Joseph to "keep an eye on things." According to Joseph, it wasn't long before thick, black smoke started pouring out of the oven. Unfortunately, he didn't have a clue how to terminate the cycle. First, he opened the front and back doors to get a breeze blowing through the house... only to discover that it was a still and windless day. While Joseph was contemplating his next move, the smoke detector in the hallway connecting the kitchen to the garage started to sound the alarm. Being well acquainted with the ramifications of his mother's exuberant cooking techniques, Joseph immediately dragged a stool under the detector, mounted the stool, and started fanning the detector with a magazine. As soon as that detector had ceased its warbling, the one in the corridor at the other side of the family room took over. Joseph dragged his stool across the house and started fanning that little rascal. Then the detector in the master bedroom took up the call, quickly followed by the detector in Joseph's bedroom. One thing you can say about our house is that it is in no way underequipped on the smoke detector front. To cut a long story short, smoke kept on pouring out of the oven, and Joseph spent much of the next hour or so fanning one detector into submission, only to have the next one grasp the metaphorical baton and run with it. It was as if Joseph had invented a novel version of Whac-A-Mole. Suffice it to say that Joseph is no longer a fan of self-cleaning ovens.  

Engineers employ various laws: Ohm's law, Kirchhoff's law, and the law of gravity, to name just a few. But there's another less rigorous law that I find also applies to many engineering designs and decisions, and that's the law of unintended consequences (LoUiC), along with its closed corollary, the law of unforeseen consequences. Although we can model, simulate, extrapolate, envision, and even speculate on the effects of a design, there are many harder-to-grasp real-world, regulatory, and human factors which affect the actual impact. Consider, for example, the classic Easy-Bake child's oven from Hasbro. This product has been around since 1963 ( here and here ), and uses a standard-base, 100-watt incandescent bulb as its source of heat. It's simple, cost-effective, reliable, and very replaceable: what more could you want in a central element of a product's design? But developments far outside the toy world have intruded onto this venerable design. New regulations going into effect, restricting the availability of low-efficiency light sources (meaning incandescent bulbs) as part of energy-saving mandates, mean that the oven's heat source may not be available. So Hasbro redesigned the oven to use a built-in heating coil (see here and here ). Ironically, there is probably no net energy saving due to this redesign. Yes, the derided light bulb is only about 10% efficient at generating illumination, but that means that 90% of the input power is converted to heat—which is what you really want in this application. So it is actually a pretty efficient at meeting the product's requirement. If you add in the cost of the redesign's materials to accommodate the new heating element (there are safety issues, of course), I'll bet the new design is no better, or possibly worse, that the old design. Further, if the new heating element burns out, the replacement module (if one is available, it's not clear to me from the web page) will add to materials waste—or perhaps the entire oven will be tossed out; just think of the environmental impact of that. I like to keep the law of unintended consequences in mind whenever I say to myself (or hear someone confidently proclaim) "that change is no big deal" or "if we do this , then clearly such-and-such will be the result." The LoUiC reminds me that our ability to fully gauge the effect of our actions, however well-intended, are often not as clear-cut or foreseeable as we'd like to think—and sometimes are even the opposite. A little more humility and a little less confidence are always good. (Just envision what's going to happen when people try to replace the incandescents used for illumination in freezers or regular ovens with those CCFLs or LEDs.) Have you ever been in a situation where a seemingly straightforward design decision had unpleasant, detrimental, or even a contrary impact to what analysis and conventional wisdom "assured" would be the case?