Cold Weather Catalog - Chromalox Precision Heat and Control

TechnicalTechnical InformationHeat Transfer Fundamentals & Thermodynamic PropertiesHeat Transfer FundamentalsThe principles of heat transfer are well understoodand are briefl y described below. Heatenergy is transferred by three basic modes. Allheating applications involve each mode to agreater or lesser degree.• Conduction• Convection• RadiationConduction is the transfer of heat energythrough a solid material. Metals such ascopper and aluminum are good conductorsof heat energy. Glass, ceramics and plasticsare relatively poor conductors of heat energyand are frequently used as thermal insulators.All gases are poor conductors of heat energy.A combination of expanded glass or ceramicfi ber fi lled with air is excellent thermal insulation.Typical conduction heating applicationsinclude platen heating (cartridge heaters), tankheating (strip and ring heaters), pipe tracingand other applications where the heater is indirect contact with the material being heated.Convection is the transfer of heat energy bycirculation and diffusion of the heated media.It is the most common method of heatingfl uids or gases and also the most frequentapplication of electric tubular elements andassemblies. Fluid or gas in direct contactwith a heat source is heated by conductioncausing it to expand. The expanded materialis less dense or lighter than its surroundingsand tends to rise. As it rises, gravity replacesit with colder, denser material which is thenheated, repeating the cycle. This circulationpattern distributes the heat energy throughoutthe media. Forced convection uses the sameprinciple except that pumps or fans move theliquid or gas instead of gravity.Convection in a LiquidElectricHeaterLiquidTypical convection heating applications includewater and oil immersion heating, air heating,gas heating and comfort air heating.Radiation is the transfer of heat energy byelectromagnetic (infrared) waves and is verydifferent from conduction and convection.Conduction and convection take place whenthe material being heated is in direct contactwith the heat source. In infrared heating, thereis no direct contact with the heat source. Infraredenergy travels in straight lines throughspace or vacuum (similar to light) and doesnot produce heat energy until absorbed. Theconverted heat energy is then transferred inthe material by conduction or convection.Radiant Energy (Infrared) HeatingAll objects above “absolute zero” temperatureradiate infrared energy with warmer objectsradiating more energy than cooler objects.Infrared energy radiating from a hot object(heating element) strikes the surface of acooler object (work piece), is absorbed andconverted to heat energy. Paint drying by radiantheaters is a typical application of infraredheating. The most important principle ininfrared heating is that infrared energy radiatesfrom the source in straight lines and does notbecome heat energy until absorbed by thework product.Thermodynamic PropertiesAll materials have basic physical constantsand thermodynamic properties. Theseconstants are used in the evaluation of thematerials and in heat energy calculations. Theconstants and properties most often used are:• Specifi c Heat (C p)• Heat of Fusion (H fus)• Heat of Vaporization (H vap)• Thermal Conductivity (k)• Thermal Resistivity (R)Specifi c Heat (Quantity of Heat Energy) — Allmaterials contain or absorb heat energy indiffering amounts. The quantity of heat energyor thermal capacity of a particular material iscalled its specifi c heat.The specifi c heat of a substance is defi ned asthe amount of heat energy required to raise onepound of the material by one degree Fahrenheit.Specifi c heat factors are usually defi nedas British thermal units per pound per degreeFahrenheit (Btu/lb/°F). The specifi c heat ofmost materials is constant at only one temperatureand usually varies to some degree withtemperature. Water has a specifi c heat of 1.0and absorbs large quantities of heat energy. Air,with a specifi c heat of 0.24, absorbs considerablyless heat energy per pound.Heat of Fusion or Vaporization — Manymaterials can change from a solid to a liquidto a gas. For the change of state to occur,heat energy must be added or released. Wateris a prime example in that it changes from asolid (ice) to a liquid (water) to a gas (steamor vapor). If the change is from a solid to aliquid to a gas, heat energy is added. If thechange is from a gas to a liquid to a solid, heatenergy is released. These energy requirementsare called the heat of fusion and the heat ofvaporization. They are expressed as Btu perpound (Btu/lb).• Heat of Fusion is the amount of energyrequired to transform a material from asolid to a liquid (or the reverse) at the sametemperature. Water has a heat of fusion of143 Btu/lb.• Heat of Vaporization is the amount ofenergy required to transform a materialfrom a liquid to a gas (or the reverse) at thesame temperature. Water has a high heat ofvaporization, 965 Btu/lb. Water can transferlarge amounts of heat energy in the form ofcondensing steam.Thermal Conductivity is the ability of a materialto transmit heat energy by conduction.Thermal conductivity is identifi ed as “k” andis usually expressed in British thermal unitsper linear inch (or foot) per hour per squarefoot of area per degree Fahrenheit. (Btu/in/hr/ft 2 /°F) or (Btu/ft/hr/ft 2 /°F). “k” factors are usedextensively in comfort heating applications torate the effectiveness of building constructionand other materials as thermal insulation. “k”factors are also used in the calculation of heatlosses through pipe and tank insulation.Thermal Resistivity or “R” is the inverse ofthermal conductivity. Insulating materials arerated by “R” factors. The higher the “R” factor,the more effective the insulation.TECHNICAL177