Technical

Technical Information Fundamentals & Thermodynamic Properties Heat Transfer Fundamentals Radiation is the transfer of heat energy by The specific heat of a substance is defined as electromagnetic (infrared) waves and is very the amount of heat energy required to raise The principles of heat transfer are well different from conduction and . one pound of the material by one degree understood and are briefly described below. Conduction and convection take place when Fahrenheit. Specific heat factors are usually Heat energy is transferred by three basic the material being heated is in direct contact defined as British thermal units per pound per modes. All heating applications involve each with the heat source. In infrared heating, there degree Fahrenheit (Btu/lb/°F). The specific mode to a greater or lesser degree. is no direct contact with the heat source. heat of most materials is constant at only one Infrared energy travels in straight lines temperature and usually varies to some degree • Conduction through space or vacuum (similar to light) and with temperature. Water has a specific heat of • Convection does not produce heat energy until absorbed. 1.0 and absorbs large quantities of heat • Radiation The converted heat energy is then transferred energy. Air, with a specific heat of 0.24, Conduction is the transfer of heat energy in the material by conduction or convection. absorbs considerably less heat energy per through a solid material. Metals such as pound. Radiant Energy (Infrared) Heating copper and aluminum are good conductors of Heat of Fusion or Vaporization — Many heat energy. Glass, ceramics and plastics are materials can change from a solid to a liquid relatively poor conductors of heat energy and to a gas. For the change of state to occur, heat are frequently used as thermal insulators. All energy must be added or released. Water is a gases are poor conductors of heat energy. A prime example in that it changes from a solid combination of expanded glass or ceramic (ice) to a liquid (water) to a gas (steam or fiber filled with air is excellent thermal vapor). If the change is from a solid to a liquid insulation. Typical conduction heating to a gas, heat energy is added. If the change is applications include platen heating (cartridge from a gas to a liquid to a solid, heat energy is heaters), tank heating (strip and ring heaters), released. These energy requirements are pipe tracing and other applications where the All objects above “absolute zero” temperature called the heat of fusion and the heat of heater is in direct contact with the material radiate infrared energy with warmer objects vaporization. They are expressed as Btu per being heated. radiating more energy than cooler objects. pound (Btu/lb). Infrared energy radiating from a hot object Convection is the transfer of heat energy by • Heat of Fusion is the amount of energy (heating element) strikes the surface of a circulation and diffusion of the heated media. required to transform a material from a solid cooler object (work piece), is absorbed and It is the most common method of heating to a liquid (or the reverse) at the same converted to heat energy. Paint drying by fluids or gases and also the most frequent temperature. Water has a heat of fusion of radiant heaters is a typical application of application of electric tubular elements and 143 Btu/lb. assemblies. Fluid or gas in direct contact with infrared heating. The most important principle a heat source is heated by conduction causing in infrared heating is that infrared energy • Heat of Vaporization is the amount of it to expand. The expanded material is less radiates from the source in straight lines and energy required to transform a material from dense or lighter than its surroundings and does not become heat energy until absorbed a liquid to a gas (or the reverse) at the same tends to rise. As it rises, gravity replaces it by the work product. temperature. Water has a high heat of with colder, denser material which is then vaporization, 965 Btu/lb. Water can transfer heated, repeating the cycle. This circulation Thermodynamic Properties large amounts of heat energy in the form of pattern distributes the heat energy throughout condensing steam. All materials have basic physical constants the media. Forced convection uses the same and thermodynamic properties. These Thermal Conductivity is the ability of a principle except that pumps or fans move the constants are used in the evaluation of the material to transmit heat energy by conduc- liquid or gas instead of gravity. materials and in heat energy calculations. The tion. Thermal conductivity is identified as “k” constants and properties most often used are: Convection in a Liquid and is usually expressed in British thermal units per linear inch (or foot) per hour per Liquid • Specific Heat (C ) p square foot of area per degree Fahrenheit. (Btu/in/hr/ft2/°F) or (Btu/ft/hr/ft2/°F). “k” • Heat of Fusion (Hfus) factors are used extensively in comfort heating Electric • Heat of Vaporization (Hvap) applications to rate the effectiveness of Heater • Thermal Conductivity (k) building construction and other materials as . “k” factors are also used in • Thermal Resistivity (R) the calculation of heat losses through pipe and tank insulation. Specific Heat (Quantity of Heat Energy) — All Typical convection heating applications include materials contain or absorb heat energy in Thermal Resistivity or “R” is the inverse of water and oil immersion heating, air heating, differing amounts. The quantity of heat energy thermal conductivity. Insulating materials are gas heating and comfort air heating. or thermal capacity of a particular material is rated by “R” factors. The higher the “R” factor, Technical called its specific heat. the more effective the insulation.

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