Perhaps no single factor is more important to man’s evolution than his learning how to generate and control heat. The ancient Greeks realized this and developed an elaborate myth about Prometheus, the fire-bearing Titan god who stole Fire from the gods for the benefit of mankind. For the Romans, the Vestal Virgins were keepers of the sacred fire.
Man, of course, understood the workings of fire long before scientists were able to measure and explain the phenomenon, because that understanding was essential for survival. He needed fire to cook his food and to keep his body warm in winter. How man gained more and more control over heat and utilized it to forge his tools and weapons is fascinating in itself, but what concerns us here is the nature of heat, its effects, and the factors involved in the combustion of wood and the transfer of the resulting heat to the surrounding atmosphere or living space of man.
How Hot Is Heat?
The term “heat” is used in a number of slightly different ways. First, there is the sensation of heat. Our central nervous system is extraordinarily sensitive to heat or the lack of it, at least within a limited range. This range of heat we call “temperature,” which is the objective measurement of the degree of hotness, or just exactly how hot something is. Our perception of the degree of heat in a room can often be influenced by the health of our body at the time or by our experience of heat immediately preceding our entrance into the room. For example, we often feel “chilled” when we are sick with a cold although the room temperature may be as high as 80 degrees Fahrenheit. Likewise, an outdoor storm cellar with a consistent temperature of 55 degrees F will seem quite warm to us when we step into it out of a subzero climate. Our perception of heat, then, is a psychological phenomenon that is relative to other experiences. Temperature is not. It is an absolute measurement of hotness.
We also speak of heat in terms of quantity. A bathtub full of hot water (Fig. 1) contains a much larger quantity of heat than does a shallow washbasin similarly filled. Or, expressed differently, it takes a lot more heat to raise the temperature of the water in a 60-gallon water tank one degree than it does the water in a 20-gallon tank. (It costs a bit more money, too!)
Materials like iron and steel will retain more heat over a longer period of time than will an equal quantity of copper or tin. In fact, it is just this ability to “store” large quantities of heat that makes iron and steel ideal materials for stovemaking. They do not store heat permanently, however. They merely absorb large quantities of heat and then release (radiate) it more slowly than do most other materials.
This radiation, or “radiant heat,” is what warms our skin when we lie in the sun or reddens our face when we sit near a big potbelly stove. Radiation (of the type we are talking about now) is converted into heat when it strikes an absorbing substance, such as our skin. The transfer of heat energy by radiation and absorption is a most important aspect in our consideration of wood-burning stoves.
The sensation of heat is, therefore, the goal to be reached. Measuring heat is a means of determining how efficiently we can attain that goal. Containing heat in large quantities and allowing it to radiate and diffuse is the means to the end.
With an understanding of these basic characteristics of heat, we now turn to the phenomenon of combustion and observe what happens when we ignite a log of wood. It is important to understand combustion because the many types of woodstoves are made to take advantage of the various processes which occur in an actively burning fire.
Vaporization of Water in the Wood
The first stage in the combustion of wood is the vaporization of moisture (water). Moisture content usually represents the ratio of the weight of contained water to the weight of dry wood. “Dry wood” is considered absolutely dry when all water that is not chemically bonded to other elements in the wood is removed. This can be done by drying wood in an oven at the boiling point of water (100 degrees С or 212 degrees F). A freshly felled cypress tree in a swamp in South Carolina may contain 200 pounds of water for each 100 pounds of dry wood. It is thus described as having 200 percent water. More frequently the water content of sap-wood in freshly felled timber amounts to about 100 percent, with the heartwood containing much less. Wood allowed to dry (“season”) in the open air will generally dry until the moisture content equals that of the atmosphere. Brought indoors, the wood may dry further until it reaches 6 or 7 percent water. Even well-seasoned wood will contain as much as 20 percent water.1
As the wood becomes hotter, it loses more and more of its moisture and begins to give off a complex mixture of gases and tar-forming vapors called volatiles, which immediately burst into flame. The ignition temperature of these gases can be anywhere between 800 and 1200 degrees F, but is usually around 1100 degrees F. It has been estimated by scientists that about 40 to 60 percent of the heat-value of wood comes from the burning of these gases.
While the gases continue to flame, the wood goes through complex chemical changes until, as the flames subside, the wood turns into charcoal. These red-hot coals burn with hardly any flame because the volatiles have been mostly burned up. Charcoal, however, needs large quantities of air in order to keep burning, and the coals are so hot that the mass of air is nearly always heated enough to help burn the carbon monoxide gas produced by the coals. This gas turns into carbon dioxide, the same gas we exhale from our lungs and drink in carbonated beverages.
The charcoal stage of the burning cycle lasts longer in hardwoods like oak and hickory than when relatively soft woods such as white pine and spruce are burned. Soft woods, that is, woods of lesser density, will produce longer flames and burn more fiercely in the beginning stages of the fire. It has been discovered, however, that the flaming stage, even in hardwood Fires, can be prolonged by igniting the fire with a more intense heat-source and by allowing the fire access to much more oxygen. The charcoal stage will then be correspondingly briefer.
Designers and users of wood-burning stoves must be aware of these processes if they are to obtain maximum heat from the fuelwood. For example, it is known that in an open fireplace, most of the gases given off in the first stage of combustion are cooled by too much oxygen; therefore, they do not ignite completely and are lost up the chimney (Fig. 4). In the older-model box stoves, the flame-path to the flue is so direct that much of the gas passes up the flue before it can mix with enough oxygen to burn more vigorously. These gases must be heated enough to ignite and to be maintained in flame both by the surrounding flames and an adequate source of secondary (preferably preheated) air.