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THERMODYNAMIC CONSIDERATIONS
Heat and Temperature
Heat and temperature are often confused as being synonymous terms. Heat is a form of energy, while temperature is a measurement of the intensity of heat.
Heat is a form of energy caused by the activity of a substance’s molecules. The molecules are assumed to always be in constant motion. The motion of the molecules provides heat.
The amount of heat that a process uses (endothermic) or that it gives off (exothermic) tells us a great deal about what is happening in the process. Thus, it is important to be able to measure the intensity of heat.
Temperature is defined as the measurement of the intensity of heat. In practice, temperature is often thought of as the degree of “hotness” or “coldness” of a liquid, solid, or gas. Temperature is frequently measured on a definite scale, such as degrees Celsius or degrees Fahrenheit.
Conduction, Convection and Radiation
Heat Conduction – When a temperature difference occurs in a body, a net transfer of thermal energy takes place from the warmer areas of the body to the cooler areas. The heat transfer tends to produce a state of temperature uniformity. The heat transfer is called conduction.
An example of heat conduction is shown in Fig. in which a rod with bent ends is arranged with each end in a separate vessel of water. One vessel contains heated water, and the other water at a lower temperature. As long as there is a temperature difference, heat will be conducted from the warmer water through the rod to the cooler water. If no more heat is added to the system, a state of equilibrium will be reached between the two vessels and the atmosphere.
Not all materials conduct heat at the same rate. Metals are considered good conductors because they transfer heat faster than other nonmetal materials.
Heat Convection is defined as the transfer of heat energy through the action of a moving fluid (gases and liquids). For example, steam is transported by pipe to a reboiler at a unit’s distillation tower to boil an oil product.
An example of this type of heat transfer is the hot air in the furnace shown in Fig. 2. When heated the air expands and becomes lighter, moving upwards while the heavier cold air falls and takes its place to be heated
Figure Convection in Warm Air Heating
Heat Radiation (Thermal Radiation) is defined as a mode of heat transfer between two bodies of different temperatures. Radiation does not require direct contact between bodies or substances. For example, the sun’s energy warms the earth by radiation. Radiation is not only dependent on a body’s temperature, it is also dependent on the nature of the body’s surface. Dark, rough surfaces radiate heat more than do smooth, light-colored surfaces.
Please read also: TEMPERATURE TRANSMITTER EXPLANATION COMPARISON BETWEEN RTD AND THERMOCOUPLE
Several practical illustrations of radiant heat are shown in Figs. 3, 4 and 5. The lines of radiation and convection around the equipment are clearly visible. The radiant heat flows out in all directions and pursues its direct path even through the air currents.
Figure Bodies both Radiate and Absorb Heat
Figure Heat radiation from a Heater
Figure Heat is Radiated in all Directions
TEMPERATURE MESUREMENT SCALES
The three temperature measurement scales in use today are:
_ Fahrenheit
_ Celsius (also called Centigrade)
_ Kelvin
Figure Temperature Measurement Scales
FAHRENHEIT SCALE
On this scale, 212° represents the boiling point of water and 32° represents the freezing point of water.. Temperatures measured on this scale are given in degrees Fahrenheit (°F).
To convert a temperature from Celsius to Fahrenheit, use the following equation:
-
Convert 100°C to Fahrenheit scale.
CELSIUS SCALE
On this scale, 100° represents the boiling point of water and 0° represents the freezing point of water.. Temperatures measured on this scale are given in degrees Fahrenheit (°C).
To convert a temperature from Fahrenheit to Celsius, use the following equation:
-
Convert -40°F to Centigrade
KELVIN SCALE
This is an absolute scale. Absolute scales are scales that measure temperature relative to absolute zero. Absolute zero is the lowest possible temperature of matter, or the temperature at which molecular activity ceases.
Absolute zero on the Kelvin scale is equal to 0 °K. Increments on the Kelvin scale are equal to increments on the Celsius scale—that is an increase of 1 °K = an increase of 1 °C.
Degree centigrade and degree Kelvin are related by the equation
°K = °C + 273