Energy

There are two general types of energy: kinetic and potential. Kinetic energy (KE) is the energy of a given mass of material due to its motion relative to another body. Potential energy (PE) is the energy of a given mass of material as a result of the material's position in a force field. Generally, potential forms can be easily converted into kinetic forms and vice versa.

Energy can be transformed from one form to another. For example, a rock resting on the top of a hill has the potential to roll down, or fuel has the potential to be combusted and liberate heat energy. The more common classifications of energy include: mechanical, electrical, electromagnetic, chemical, nuclear, gravitational, and thermal.

Thermal energy is associated with atomic and molecular vibration. It is considered a basic energy form because all other energy forms can be completely converted into thermal energy. But the second law of thermodynamics limits conversion of thermal energy into other forms.

Enthalpy (H or h) is the thermodynamic property defined as:

Where:

U = Internal energy of system P = System pressure V = System volume

Enthalpy is a general measure of the internally stored energy per unit mass. Commonly used units for expressing enthalpy are Btu/lbm and kJ/kg. Internal energy (U) includes all forms of energy of a given system, except its gross kinetic and potential energy. It is associated with the thermodynamic state of the system.

Two key properties that are used to measure energy levels are temperature and pressure:

Temperature is a property that is the measure of the average kinetic energy possessed by the molecules of a substance. The higher the temperature, the greater the kinetic energy or molecular activity of the substance. The common scales of temperature are called the Fahrenheit (F) and Celsius or Centigrade (C) temperatures. These are defined by using the ice point and the boiling point of water at atmospheric pressure at sea level. The ice point is 32°F (0°C) and the boiling point is 212°F (100°C). Fahrenheit temperature can be determined from Centigrade temperature as follows: F = (C x 9/5) + 32. Centigrade temperature can be determined from Fahrenheit temperature as follows: C = (F - 32) x 5/9.

The scales of absolute temperature are called the Rankine (R) and Kelvin (K) temperature scales. These temperature scales are commonly used with steam applications. Both of these temperature scales use absolute zero as a point of origin. Since absolute zero is measured as -459.59°F, Rankine temperature can be determined by adding 459.59 to the Fahrenheit temperature. Similarly, since absolute zero is measured as -273.15°C, Kelvin temperature can be determined by adding 273.15 to the Celsius temperature.

Pressure (p) is the force per unit area exerted on or by a fluid. Pressure, expressed in pounds per square inch (psi), Pascal, or bar, is typically expressed as either absolute pressure, e.g., psi absolute (lbf/in2 abs, or psia), or gauge pressure, e.g., psi gauge (lbf/in2 g or psig).

• Absolute pressure (psia), which is generally used in steam tables and most fluid and thermodynamic equations, is the true force per unit of area expressed as pounds per square inch exerted by a fluid on the wall of the vessel containing it. Standard atmospheric pressure is 14.696 lbf/in2 abs, or 29.92 in. of mercury atmospheric (in. Hg atm) at sea level, or 406.8 inches of water (in. wg). In SI units, standard atmospheric pressure is 1.013 Bar, 101,325 Pascal (P), or 76.0 centimeters of mercury (cm HgA).

• Gauge pressure (psig) is the difference between absolute pressure of a fluid and ambient atmospheric pressure. Since atmospheric pressure at sea level is about 14.7 psia (101,325 P), absolute or true pressure is determined, using approximate values, simply by adding 14.7 (101,325) to gauge pressure.

• Vacuum, or negative gauge pressure, is pressure below atmospheric pressure.

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