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Thermal System Basics
Heat Transfer Fundamentals
  Thermodynamic Properties

All materials have basic physical constants and thermodynamic properties. These constants are used in the evaluation of the materials and in heat energy calculations. The constants and properties most often used are:

Specific Heat (Quantity of Heat Energy) :

All materials contain or absorb heat energy in differing amounts. The quantity of heat energy or thermal capacity of a particular material is called its specific heat. The specific heat of a substance is defined as the amount of heat energy required to raise one pound of the material by one degree Fahrenheit. Specific heat factors are usually defined as British thermal units per pound per degree Fahrenheit (Btu/lb/F). The specific heat of most materials is constant at only one temperature and usually varies to some degree with temperature. Water has a specific heat of 1.0 and absorbs large quantities of heat energy. Air, with a specific heat of 0.24,absorbs considerably less heat energy per pound.

Heat of Fusion or Vaporization :

Many materials can change from a solid to a liquid to a gas. For the change of state to occur, heat energy must be added or released. Water is a prime example in that it changes from a solid (ice) to a liquid (water) to a gas (steam or vapor). If the change is from a solid to a liquid to a gas, heat energy is added. If the change is from a gas to a liquid to a solid, heat energy is released. These energy requirements are called the heat of fusion and the heat of vaporization. They are expressed as Btu per pound (Btu/lb).

Heat of Fusion :

Heat of Fusion is the amount of energy required to transform a material from a solid to a liquid (or the reverse) at the same temperature. Water has a heat of fusion of 143 Btu/lb.

Heat of Vaporization :

Heat of Vaporization is the amount of energy required to transform a material from a liquid to a gas (or the reverse) at the same temperature. Water has a high heat of vaporization, 965 Btu/lb. Water can transfer large amounts of heat energy in the form of condensing steam.

Thermal Conductivity :

Thermal Conductivity is the ability of a material to transmit heat energy by conduction. Thermal conductivity is identified as "k" and is usually expressed in British thermal units per linear inch (or foot) per hour per square foot of area per degree Centigrade. (Btu/in/hr/ft2/C) or (Btu/in/hr/ft2/F). "k" factors are used extensively in comfort heating applications to rate the effectiveness of building construction and other materials as thermal insulation. "k" factors are also used in the calculation of heat losses through pipe and tank insulation.

Thermal Resistivity :

Thermal Resistivity or "R" is the inverse of thermal conductivity. Insulating materials are rated by "R" factors. The higher the "R" factor, the more effective the insulation.

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