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I. Basic Concepts – Temperature: Temperature indicates the degree of coldness or heat of a substance; it is a measure of the average kinetic energy of the molecules in that substance. The higher the temperature, the greater the average kinetic energy of the molecular motion. …………In “Oxygen Production Technology”, the numerical representation of temperature on P1 is achieved through a “temperature scale”; thus, the “temperature scale” serves as a measure for determining the temperature of a substance. “A \"temperature scale\" specifies the starting point for temperature and the basic unit used to measure it. Common temperature scales include: 1. Celsius scale (℃). The physical principle underlying this scale is the linear expansion of mercury’s volume as temperature rises; it is defined such that the freezing point of pure water at standard atmospheric pressure is zero degrees Celsius, and its boiling point is 100 degrees Celsius. 2. The Fahrenheit scale (℉) is based on the same physical principles as the Celsius scale; it defines the freezing point of pure water at standard atmospheric pressure as 32 degrees Fahrenheit, and its boiling point as 212 degrees Fahrenheit. 3. The thermodynamic temperature scale (K) is established on the basis of thermodynamics; it overcomes the drawbacks associated with the Celsius and Fahrenheit scales regarding the working substance, and has been designated as the basic temperature scale for international use. Pressure---the force per unit area, always acting perpendicular to the walls of the container. …………In “Oxygen Production Technology” P2, the following are the common units used: 1. Physical atmosphere – the average pressure of the atmosphere at sea level at a latitude of 45° and a temperature of 0°C; it is also known as standard atmospheric pressure. 2. Engineering atmospheric pressure refers to the pressure generated by a force of 1 kg acting on an area of 1 cm2, with the unit being kg.f/cm2. The pressure indicated by a device used to measure pressure is often the difference between the absolute value of the pressure being measured and atmospheric pressure; the actual pressure exerted by the gas inside a container on its walls is known as absolute pressure. The actual pressure of the gas inside a container that is higher than the atmospheric pressure at that time is called gauge pressure; when it is lower than the atmospheric pressure, the difference is referred to as vacuum degree. Relationship among the three: P_abs = P_surface + P_atmosphere; P_abs = P_atmosphere – P_vacuum. Only absolute pressure can indicate the true state of a gas. Flow rate---the amount of gas that passes through in a unit of time. Units used to express the amount of gas fall into two categories: mass per unit volume and volume units. …………\"Principles and Operation of Oxygen Generators\", P3: Heat—is a measure of the change in energy possessed by molecules within a substance. An increase in the kinetic energy of molecular motion indicates an rise in temperature; during this process, molecules absorb heat, with the unit of measurement being \"J\". …………\"Oxygen Production Technology\", P16: To obtain and maintain low temperatures, heat is removed from the low-temperature device to the outside environment; this heat is referred to as cooling capacity. Work—is a measure of energy change, expressed mathematically as: Work = Force × Distance. …………\"Oxygen Production Technology\", P15: Power refers to the work done per unit of time. Internal energy—is the energy contained within a substance; it represents the sum of the kinetic energy resulting from molecular motion and the potential energy arising from interactions between molecules, that is, u_internal = u_kinetic + u_potential. …………\"Principles and Operation of Oxygen Generators\", P6: Internal energy has a certain relationship with temperature, pressure, and specific volume. The kinetic energy of molecular motion is related only to temperature, whereas potential energy depends on the distance between molecules and is related to pressure and specific volume. Enthalpy—represents the total energy of the fluid substance in a flow system. For a unit mass of flowing gas, the work done in driving its flow is p.v; the total energy possessed by the gas should equal the sum of these two quantities, which is denoted by the symbol i or h, where h = U + pv. ………\"Oxygen Production Technology\", P17: The driving work pv during flow; pressure and specific heat capacity are both state functions, thermal energy U is also a state function, therefore enthalpy is as well a state function. Entropy—a state parameter that indicates the directionality of a process, used to measure the inequivalence between two states before and after an irreversible process. …………Page P18 of \"Oxygen Production Technology\" defines ds as dq/T; the increase in entropy is equal to the heat transferred into the system from the outside during a reversible process, divided by the absolute temperature at that time. Saturation temperature, saturation pressure—these are terms used in gas-liquid equilibrium. A closed container is filled with liquid. As the number of vapor molecules in the space increases, the vapor pressure generated also rises. At a certain point, the number of vapor molecules in the space stops increasing; at this stage, the number of molecules leaving the liquid equals the number of molecules returning to the liquid, achieving a dynamic equilibrium, which is referred to as the \"saturated state.\" The pressure exerted by the vapor at this point is called the \"saturation pressure.\" In a saturated state, the temperature corresponding to the saturation pressure is called the “saturation temperature”. …………\"Questions and Answers for Oxygen Producers\", P20: At a certain pressure, the temperature at which a liquid rises to the point where it begins to boil is called the boiling point.