You don’t need special glasses to read this blog. Also, I’m not talking about the kind of 3D you see in a theater. What I’d like to discuss is heat transfer, one of my favorite topics. In our Level I – Thermographic Applications training course we discuss quantifying transfer and the units of measurement we typically assign to levels of heat. We discuss the Calorie and the British Thermal Unit (BTU). We concentrate mostly on the BTU and you should remember we state a BTU is roughly the equivalent of the energy given off from a standard kitchen match when it is lit and burns until it’s consumed.
Sometimes in Level I after I make that statement, I offer a prize to anyone who will absorb a BTU for the learning benefit of the rest of the class. I rarely have any takers. No sane person wants to hold out his or her hand and let me drop a burning match into it. What they might be missing though is the other dimensions of heat transfer.
A BTU is a finite quantity of energy. We describe it with the kitchen match analogy because it makes sense to people, since most of us have seen a wooden kitchen match at some point in our lives. The kitchen match though is one representation of that quantity. Absorbing one BTU by holding on to a burning kitchen match would be painful because of the intensity of the transfer. It’s one BTU applied over a fairly short period of time. However, if you placed your hand into a bucket of water at 101°F and left it in there until the water dropped to 100°F, that wouldn’t hurt at all. Every time you take a shower (assuming you take warm showers) you absorb way more than one BTU, and it’s not painful at all, in fact it’s downright pleasant. The difference is intensity of the transfer. In terms of both the amount of energy transferred and the time period over which the transfer takes place. Wait, that’s only two dimensions.
The third dimension is the area over which the transfer takes place. This is one of the considerations when people think about the application of arc flash PPE. Arc flash rated PPE is rated in kilo-Calories. I have a number of electrical folks in Level I courses who raise their eyebrow when we talk about how many calories there are in a BTU. One BTU is equal to 252 calories; one burning kitchen match is roughly equivalent to 252 calories. If you’ve ever seen those electrical safety videos of arc flash explosions, they’re more violent than a single kitchen match. Many electricians have asked me, sometimes in a whisper on a break during class, “If my shirt is only rated at eight Calories and a tiny match is 252 calories, how on earth am I safe in this shirt?” The answer: area and time.
Arc flash PPE is rated in kilo-Calories, but there is both a time and area consideration as well. The rating is in Calories per square centimeter per second. A square centimeter is roughly the area of the tip of your index finger; pretty small. So a quantity of energy applied in a small area over a short period of time is much more intense than same amount of energy transferred over a larger area over a longer period of time. A kitchen match takes longer than one second to burn away, so 252 calories is a different intensity than eight Calories applied to a small spot in one second.
Thinking Thermally can be complex. Keep your mind open. Remember there’s more to heat transfer than meets the eye. It happens in 3D: the amount, the time and the area.