Properties of Vapors - PDF Free Download

Properties of Vapors Topics for Discussion The Pressure/Temperature Relationship Vaporization Condensation Enthalpy

Properties of Vapors Topics for Discussion Entropy Properties of Substances Saturated Fluids Superheated Vapors Liquid-Vapor Mixtures

The Pressure/Temperature Relationship A direct relationship exists between the pressure of a vapor and its saturation temperature; as one increases so does the other. The temperature where a fluid changes from liquid to vapor phases is called its saturation temperature. The saturation temperature of a liquid and its vapor are the same value for a given pressure.

The Pressure/Temperature Relationship Therefore, a liquid-vapor mixture exists as the substance changes from one phase to another. The saturation temperature is the maximum temperature of the liquid phase and the minimum temperature of the vapor phase. Addition of heat to a liquid above its saturation point will induce vaporization, and any reduction of heat to a vapor will induce condensation.

The Pressure/Temperature Relationship The temperature of the fluid (liquid or vapor) remains at the saturation temperature until the substance has completed its phase change. The saturation temperature of a fluid is related to the vapor pressure of the liquid. The vapor pressure is produced by the mass of vapor molecules that diffuse into the space directly above the liquid s surface.

The Pressure/Temperature Relationship The number of vapor molecules that leave the liquid and, therefore the vapor pressure, is a function of the temperature of the liquid. As the temperature of the liquid increases, the number of molecules having sufficient kinetic energy to break free from the liquids surface also increases. This reaction also increases the vapor pressure above the liquid.

The Pressure/Temperature Relationship As the pressure created by the vapor molecules increases on the liquids surface increases, the saturation temperature (boiling point) also increases. Conversely as the vapor pressure on the liquids surface decreases the boiling point also decreases. This relationship makes it possible to regulate the operating temperature of a refrigeration cycle.

The Pressure/Temperature Relationship The pressures(temperatures) in a refrigeration system are regulated by varying the flow of fluid into a coil or by the rate that vapors are drawn from the coil by the compressor. In most refrigeration cycles temperatures are manipulated by a combination of both, varying the flow of a fluid into a coil (metering device), and varying the rate of vapors drawn from the coil (compressor).

Superheated Vapor Once a liquid has been completely vaporized at the saturation temperature corresponding with it s existing vapor pressure, the temperature can then be increased by adding additional thermal energy to the vapor. The energy added to a vapor to raise its temperature above its saturation temperature is called superheat.

Superheated Vapor Before a vapor can be superheated, it must be removed from contact with its liquid phase. This is because the liquid + vapor contact acts as a heat sink absorbing any superheat energy from the vapor. Consequently the vapor and liquid remain at the saturated temperature during latent heat transfer processes.

Subcooled Liquid A similar relationship exists between sensible heat transfer and the saturation temperature of liquids. Before a liquids temperature can be reduced, the liquid must be removed from contact with the vapor. This must be done because the vapor acts as a heat source transferring energy into the liquid.

Subcooled Liquid As a vapor condenses, it transfers its latent heat of vaporization energy into the liquid, maintaining the liquid at its current temperature at the saturated conditions. After all the vapor is condensed, any additional heat transfer from the liquid reduces the liquids temperature below the saturated temperature corresponding to its vapor pressure.

Subcooled Liquid Under the reduced temperature condition the liquid is said to be Subcooled, and is called a subcooled liquid. Subcooling is a sensible heat transfer process because it affects the internal kinetic energy of the liquid s molecules.

Vaporization Vaporization is a process by which a liquid or solid changes phase into its vapor state. Boiling, evaporation and sublimation are all forms of vaporization. The conditions of the substance and its environment determine which of these processes will occur.

Vaporization Boiling only occurs when the temperature of the liquid is equal to its saturated temperature. Evaporation only occurs when the temperature of a liquid is below its saturation temperature. Sublimation is a process by which a solid changes state directly to a vapor. Sublimation only occurs when the temperature and pressure of a solid are below its triple point temperature and pressure.

Phase Equilibrium Diagram The commonality between the three forms of vaporization is the pressure and temperature conditions of the substance. A phase equilibrium diagram is used to determine which forms of vaporization will occur at any given conditions. The diagrams show a substances three states of matter, the fusion and saturation temperatures, the vapor pressures and its triple point.

Phase Equilibrium Diagram The fusion temperature occurs when energy transfers cause a liquid to change state into a solid or a solid into a liquid. The saturation temperature of a substance was discussed in the previous chapter. The vapor pressure of a liquid is the pressure exerted on its surface by the high energy molecules that broke free above the liquid.

Phase Equilibrium Diagram All liquids lose some molecules from their surface as they absorb kinetic energy from the ambient. If resident their mass generates a force upon the liquids surface. This force is called the vapor pressure of the liquid. The vapor pressure of the liquid regulates its boiling characteristics.

Phase Equilibrium Diagram A liquid cannot boil until its vapor pressure equals the pressure of the surrounding ambient atmosphere. Liquids having higher vapor pressures are more volatile than those with lower ones. A high vapor pressure indicates very little energy transfer is needed to produce a large migration of molecules from the liquid s surface.

Phase Equilibrium Diagram The greater the quantity of molecules leaving the liquid phase, the higher the vapor pressure exerted on the liquid s surface. As soon as the vapor pressure equals the local atmospheric pressure, a liquid will begin to boil. This is why the saturation temperature is based on the pressure of its surroundings.

Phase Equilibrium Diagram The triple point of a substance is the condition of temperature and vapor pressure where a substance can simultaneously exist in its solid, liquid and vapor states. At any other conditions one of the phases will not be present with the other two phases.

Boiling Boiling is an intense vaporization process that can only occur when a liquid s vapor pressure is equal to the ambient pressure. As the liquid s temperature is rising, the increase in kinetic energy increases the number of molecules breaking free of the liquid, there in the vapor pressure rises.

Boiling The increase in vapor molecules above the liquid s surface increases the vapor mass, and in turn the vapor pressure increases. Once the vapor pressure equals the surrounding atmospheric pressure, the atmosphere can no longer impede the release of molecules from the liquid and boiling begins.

Boiling To maintain the boiling process, energy in excess of that needed to maintain the vapor pressure must be continuously transferred to the liquid. Boiling is characterized by the considerable agitation and rapid formation of vapor bubbles throughout the volume of liquid. Vapor bubbles breaking free through the liquid propel tiny liquid droplets making vapor visible.

Evaporation Evaporation is a gentle thermodynamic process induced by a slow rate of heat transfer to the liquid from its surroundings. Evaporation does not generate rapid changes in the volume or mass of the affected liquid. As kinetic energy is transferred through a liquid by the collisions, some molecules near the surface have enough velocity to break free the liquid.

Evaporation Evaporation can only occur when the vapor pressure above the liquid is lower than the current saturated conditions of the liquid. The volume of evaporating liquid continually decreases as the molecules break free from the surface and enter the ambient atmosphere.

Evaporation Some of the breaking free vapor molecules collide with other molecules in the atmosphere. The collisions cause a reduction in energy of the vapor molecules where they fall back into solution. When the number of molecules leaving the liquid equals the number returning to the liquid a state of equilibrium occurs.

Rate of Evaporation The amount of turbulence in the atmosphere above an evaporating liquid had a direct correlation to its rate of evaporation. This occurs because the vapor molecules cannot accumulate above the liquid s surface. Also surface area affects the rate of evaporation, the larger the surface exposed to the atmosphere the greater the rate of evaporation.

Cooling Effect of Evaporation An evaporation process will continue as long as heat transfer to the liquid is maintained, and the vapor pressure is below saturation conditions. Thermal energy is needed to replace the energy removed from the liquid, the loss in energy corresponds to a reduction in temperature of the liquid.

Cooling Effect of Evaporation The transfer of heat from the surroundings indicates that evaporation is a cooling process. This is why evaporating moisture from skins surface produces a cooling effect.

The Pressure Temperature Relationship A direct relationship exists between the saturation temperature of a fluid and its ambient pressure. Increasing the pressure of a fluid raises its saturation temperature, and reducing a fluids pressure lowers its saturation temperature.

Sublimation Sublimation is a vaporization process that applies only to solids. It imitates the evaporation of liquids, but occurring at a much slower rate. In sublimation a solid changes directly to a vapor without passing through its liquid phase. As molecules on the surface of the solid absorb enough energy allows them to break their strong crystalline bonds.

Condensation Condensation occurs whenever a saturated vapor is subjected to temperature below its saturation temperature. When condensation occurs while the vapor is confined within a fixed volume, the density and pressure of the vapor decreases. To maintain the condensing process the liquids temperature must be continually reduced.

Condensation Conversely if vapor continually enters the vessel to replace the mass of vapor being condensed and drawn from the vessel. Then the density, pressure and saturation temperature will remain constant. This type of condensation is used in mechanical refrigeration systems.

Enthalpy Enthalpy is a property of a substance that indicates the amount of energy it contains that is available for conversion into heat. The term available means that some of the internal energy of a substance can not be converted to heat. Some of the energy must remain for a substance to maintain it s molecular structure.

Enthalpy This is why enthalpy is a measure of the amount of energy that is available for conversion into heat at the substance s current pressure and temperature. The amount of enthalpy in a substance is based on its datum temperature. The enthalpy datum temperature varies among substances.

Enthalpy Enthalpy is equal to the internal energy stored within the molecules of a substance added to the work energy of its current state. Work energy is calculated by multiplying the pressure of a substance by it s specific volume. As the pressure of a substance increases, its ability to do work also increases because pressure can be converted into work.

Enthalpy Enthalpy is often referred to as the total energy of a substance because it is equal to the sum of its internal energy. In reality enthalpy does not indicate the total energy in substances having a datum temperature above absolute zero. It is better defined as the total amount of available energy that can be transferred to heat.

Entropy Entropy is a measure of the reduction in the availability of energy of a substance that results from an energy transfer process. The first law of thermodynamics states that energy can not be created or destroyed, therefore the quantity of energy in the universe remains fixed at the level that existed at creation.

Entropy The second law of thermodynamics states that no real (nonreversible) process can be 100% efficient in the conversion between energy and work. Consequently the amount of energy available for conversion into work is continually decreasing over time as heat spontaniously transfers from a higher level to a lower level.

Entropy The amount of energy in the universe remains the same, but its ability to be used to do useful work decreases with every heat transfer and work performed. Entropy is a property that is used to measure this decrease in availability of energy that results from a process.

Entropy In thermodynamics, entropy indicates the ordering of the molecules within a substance or the organization of energy within a system. Systems or substances having high values of entropy are more disorganized than those having lower numbers. Solids have lower entropy values due to their crystalline structure, being more organized.

Entropy Liquids have higher levels of entropy than solids due to the molecules more random positions. Vapors have even higher levels of entropy due to their increase in vibration and more randomness. The same analogy can be made when describing energy sources, if it is distributed among larger quantities of molecules its intensity diminishes, increasing its entropy.

Entropy Energy is always becoming unavailable as processes reduce its intensity, spreading it throughout the universe. As the available temperature difference decreases, the thermal energy available for heat engines to convert into useful work also diminishes.

Entropy Therefore any process that generates an increase in entropy reduces the energy available for future processes. For example if 1000 Btu s of energy originally stored within 1000 molecules were transferred to 1 million molecules, the intensity of the energy diminished and its entropy increases. Entropy is an abstract hard to understand concept

Properties of Substances Properties are characteristics of substances that identify their thermodynamic state at a moment in time. Properties of Substances are defined by its pressure, temperature, internal energy, density, specific volume, enthalpy and entropy. Knowing any two properties of a substance will define its thermodynamic state.

Properties of Substances There are two categories of thermodynamic properties called intensive and extensive properties. An intensive property is a characteristic that is independent on the amount or size of the system. Temperature and pressure are intensive properties.

Properties of Substances Extensive properties are dependent on size of the substance or system. Mass and volume are extensive properties of a substance and systems. Of the six properties of a vapor that are important in refrigeration, pressure, temperature and volume are called measurable properties.

Properties of Substances Internal energy, enthalpy and entropy are properties that must be enumerated. They are known as calculated properties, and are usually found on property tables. Two different types of property tables are commonly used for the heat transfer fluids in HVAC/R systems.

Properties of Substances Saturated tables list the properties of fluids at conditions within a range limited by their saturated liquid and vapor temperatures. Superheat tables list the properties of heat transfer vapors listed by temperature or pressure. The properties of saturated liquids and vapors at various conditions are given in.

Properties of Saturated Fluids The properties of saturated liquids and vapors at various conditions are given in saturated vapor tables, the columns are labeled temperature, pressure, density, volume, enthalpy and entropy. In saturated fluids only one temperature can satisfy saturated conditions at a certain pressure, therefore either temperature or pressure is all that is required to determine fluid properties.

Properties of Superheated Vapors A superheated vapor table lists properties of superheated vapors rather than saturated vapor. A superheated vapor can exist at a multitude of different temperatures while remaining at its current pressure. Therefore, two characteristics of a superheated vapor must always be known in order to define its state.

Properties of Liquid-Vapor Mixtures If the liquid is boiling rather than evaporating, the leaving vapor entrains small droplets of saturated liquid and carries them upward. This is called wet vapor, a saturated vapor that is completely free of liquid particles is referred to as a dry vapor. As a fluid vaporizes, the mass of the liquid decreases as the mass of vapor increases.

Properties of Liquid-Vapor Mixtures The quality of a vapor indicates the percentage of the liquid s original mass that has been converted into a saturated vapor. Quality is designated with the letter x. The quality of a liquid-vapor mixture is used to calculate the enthalpy, entropy, specific volume and density properties of a mixture.