Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics, страница 9

Rather, point N is a point of inflection, with the slope being zero. Thispoint is called the critical point, and at the critical point the saturated-liquid and saturated-vaporstates are identical. The temperature, pressure, and specific volume at the critical point are calledthe critical temperature, critical pressure, and critical volume.A constant pressure process greater than the critical pressure is represented by line PQ. Thereis no definite change in phase from liquid to vapor and no definite point at which there is achange from the liquid phase to the vapor phase. For pressures greater than the critical pressure,the substance is usually called a liquid when the temperature is less than the critical temperature(705.47°F) and a vapor or gas when the temperature is greater than the critical temperature.

Inthe figure, line NJFB represents the saturated liquid line, and the line NKGC represents thesaturated vapor line.FusionConsider one further experiment with the piston-cylinder arrangement of Figure 4. Suppose thecylinder contained 1 lbm of ice at 0°F, 14.7 psia. When heat is transferred to the ice, thepressure remains constant, the specific volume increases slightly, and the temperature increasesuntil it reaches 32°F, at which point the ice melts while the temperature remains constant. In thisstate the ice is called a saturated solid. For most substances, the specific volume increases duringthis melting process, but for water the specific volume of the liquid is less than the specificvolume of the solid. This causes ice to float on water. When all the ice is melted, any furtherheat transfer causes an increase in temperature of the liquid.

The process of melting is alsoreferred to as fusion. The heat added to melt ice into a liquid is called the latent heat of fusion.HT-01Page 36Rev. 0ThermodynamicsCHANGE OF PHASESublimationIf the initial pressure of the ice at 0°F is 0.0505 psia, heat transfer to the ice first results in anincrease in temperature to 20°F. At this point, however, the ice passes directly from the solidphase to the vapor phase in the process known as sublimation. Sublimation is a special termused for cases in which the transition between the solid phase and the vapor phase occursdirectly, without passing through the liquid phase.

Further heat transfer would result insuperheating the vapor.Triple PointFinally, consider an initial pressure of the ice of 0.08854 psia. Again, as a result of heat transfer,the temperature will increase until it reaches 32°F. At this point, however, further heat transfermay result in some of the ice becoming vapor and some becoming liquid because it is possibleto have the three phases in equilibrium.

This is called the triple point, defined as the state inwhich all three phases may be present in equilibrium.Figure 8 is a pressure-temperature diagram for water that shows how the solid, liquid, and vaporphases may exist together in equilibrium. Along the sublimation line, the solid and vapor phasesare in equilibrium, along the fusion line, the solid and liquid phases are in equilibrium; and alongthe vaporization line, the liquid and vapor phases are in equilibrium. The only point at whichall three phases may exist in equilibrium is the triple point. The temperature and pressure forthe triple point of water are 32.02°F and 0.08865 psia.

The vaporization line ends at the criticalpoint because there is no distinct change from the liquid phase to the vapor phase above thecritical point.Rev. 0Page 37HT-01CHANGE OF PHASEThermodynamicsFigure 8 Pressure-Temperature DiagramCondensationAll the processes discussed on the preceding pages (vaporization, sublimation, and fusion) occurduring a heat addition to a substance.

If heat is removed from a substance, the opposite of thedescribed processes will occur.As previously described, a heat addition at a constant pressure to a saturated liquid will causethe liquid to evaporate (change phase from liquid to vapor). If heat is removed at a constantpressure from a saturated vapor, condensation will occur and the vapor will change phase toliquid. So the processes of vaporization and condensation are the exact opposite of each other.Similarly, freezing is the opposite process of melting and fusion.

Sublimation also has anopposite process in which a gas goes directly to solid, but this process is not normally referredto with a unique term.SummaryThe important information from this chapter is summarized on the following page.HT-01Page 38Rev. 0ThermodynamicsCHANGE OF PHASEChange of Phase Summary•Classification of PropertiesIntensive properties are independent of mass (temperature,pressure, or any specific property)Extensive properties are a function of the mass of the system(mass, volume)•Rev. 0The following terms were defined in this chapter:Saturation-combination of temperature and pressureat which a mixture of vapor and liquidcan exist at equilibriumSubcooled liquid-a liquid at a temperature below saturationtemperature for its pressureSuperheated vapor -a vapor at a temperature above saturationtemperature for its pressureCritical point-the temperature and pressure above whichthere is no distinction between the liquidand vapor phasesTriple point-the temperature and pressure at which allthree phases can exist in equilibriumVapor pressure curve -a graphical representation of therelationship between temperature andpressure at saturated conditionsQuality-the fraction of the total mass ofa mixture that is in the vaporphaseMoisture Content -the fraction of the total mass ofa mixture that is in the liquidphasePage 39HT-01CHANGE OF PHASEThermodynamicsChange of Phase Summary (Cont.)•The following terms are labels for processes that occur when a substance changesbetween the three phases of matter: solid, liquid, and vapor.Sublimation-change of phase from solid to vaporVaporization-change of phase from liquid to vaporCondensation-change of phase from vapor to liquidFusion or melting -HT-01change of phase from solid to liquidPage 40Rev.

0ThermodynamicsPROPERTY DIAGRAMS AND STEAM TABLESPROPERTY DIAGRAMS AND STEAM TABLESProperty diagrams and steam tables are used in studying the theoretical andactual properties and efficiencies of a given system.EO 1.16Given a Mollier diagram and sufficient information toindicate the state of the fluid, DETERMINE anyunknown properties for the fluid.EO 1.17Given a set of steam tables and sufficient informationto indicate the state of the fluid, DETERMINE anyunknown properties for the fluid.EO 1.18DETERMINE the change in the enthalpy of a fluid asit passes through a system component, given the stateof the fluid at the inlet and outlet of the componentand either steam tables or a Mollier diagram.Property DiagramsThe phases of a substance and the relationships between its properties are most commonly shownon property diagrams.

A large number of different properties have been defined, and there aresome dependencies between properties. For example, at standard atmospheric pressure andtemperature above 212°F, water exists as steam and not a liquid; it exists as a liquid attemperatures between 32°F and 212°F; and, it exists as ice at temperatures below 32°F. Inaddition, the properties of ice, water, and steam are related. Saturated steam at 212°F andstandard atmospheric pressure has a specific volume of 26.8 ft3/lbm. At any other temperatureand pressure, saturated steam has a different specific volume. For example, at 544°F and 1000psia pressure, its specific volume is 0.488 ft3/lbm.There are five basic properties of a substance that are usually shown on property diagrams.These are: pressure (P), temperature (T), specific volume (ν), specific enthalpy (h), and specificentropy (s). When a mixture of two phases, such as water and steam, is involved, a sixthproperty, quality (x), is also used.There are six different types of commonly encountered property diagrams.

These are: PressureTemperature (P-T) diagrams, Pressure-Specific Volume (P-ν) diagrams, Pressure-Enthalpy (P-h)diagrams, Enthalpy-Temperature (h-T) diagrams, Temperature-entropy (T-s) diagrams, andEnthalpy-Entropy (h-s) or Mollier diagrams.Rev. 0Page 41HT-01PROPERTY DIAGRAMS AND STEAM TABLESThermodynamicsPressure-Temperature (P-T) DiagramA P-T diagram is the most common way to show the phases of a substance. Figure 9 is the P-Tdiagram for pure water. A P-T diagram can be constructed for any pure substance. The line thatseparates the solid and vapor phases is called the sublimation line. The line that separates thesolid and liquid phases is called the fusion line. The line that separates the liquid and vaporphases is called the vaporization line.