{"id":1351,"date":"2016-03-09T16:45:36","date_gmt":"2016-03-09T16:45:36","guid":{"rendered":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/?p=1351"},"modified":"2016-03-09T16:45:36","modified_gmt":"2016-03-09T16:45:36","slug":"properties-of-pure-substancesphase-change-processes-of-pure-substances","status":"publish","type":"post","link":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/properties-of-pure-substancesphase-change-processes-of-pure-substances\/","title":{"rendered":"PROPERTIES OF PURE SUBSTANCES:PHASE-CHANGE PROCESSES OF PURE SUBSTANCES"},"content":{"rendered":"<div class=\"kpwnw6a0dce5b6c52c\" ><script type=\"text\/javascript\">\n\tatOptions = {\n\t\t'key' : '61e5902552e2353963d8d2f1bd1f4a8f',\n\t\t'format' : 'iframe',\n\t\t'height' : 250,\n\t\t'width' : 300,\n\t\t'params' : {}\n\t};\n<\/script>\n<script type=\"text\/javascript\" src=\"\/\/www.highperformanceformat.com\/61e5902552e2353963d8d2f1bd1f4a8f\/invoke.js\"><\/script><\/div><style type=\"text\/css\">\r\n@media screen and (min-width: 1201px) {\r\n.kpwnw6a0dce5b6c52c {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 993px) and (max-width: 1200px) {\r\n.kpwnw6a0dce5b6c52c {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 769px) and (max-width: 992px) {\r\n.kpwnw6a0dce5b6c52c {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 768px) and (max-width: 768px) {\r\n.kpwnw6a0dce5b6c52c {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (max-width: 767px) {\r\n.kpwnw6a0dce5b6c52c {\r\ndisplay: block;\r\n}\r\n}\r\n<\/style>\r\n<p align=\"justify\"><font size=\"5\"><b>PHASE-CHANG<\/b><b>E PROCESSES<\/b><b> <\/b><b>O<\/b><b>F PURE SUBSTANCES<\/b><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">There are many practical situations where two phases of a pure substance co- exist in equilibrium. Water exists as a mixture of liquid and vapor in the boiler and the condenser of a steam power plant. The refrigerant turns from liquid to vapor in the freezer of a refrigerator. Even though many home owners con- sider the freezing of water in underground pipes as the most important phase- change process, attention in this section is focused on the liquid and vapor phases and the mixture of these two. As a familiar substance, water will be used to demonstrate the basic principles involved. Remember, however, that all pure substances exhibit the same general behavior.<\/font> <\/p>\n<h3 align=\"justify\"><font size=\"5\"><font style=\"font-weight: bold\">Compressed Liquid and Saturated Liquid<\/font><\/font><\/h3>\n<p align=\"justify\"><font size=\"5\">Consider a piston-cylinder device containing liquid water at 20\u00b0C and 1 atm <\/font><font size=\"5\">pressure (state 1, Fig. 3\u20136). Under these conditions, water exists in the liquid phase, and it is called a <b>compressed liquid, <\/b>or a <b>subcooled liquid, <\/b>meaning<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">that it is <i>no<\/i><i>t about to vaporize. <\/i>Heat is now transferred to the water until its temperature rises to, say, 40\u00b0C. As the temperature rises, the liquid water expands slightly, and so its specific volume increases. To accommodate this ex- pansion, the piston will move up slightly. The pressure in the cylinder remains constant at 1 atm during this process since it depends on the outside barometric pressure and the weight of the piston, both of which are constant. Water is still a compressed liquid at this state since it has not started to vaporize.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">As more heat is transferred, the temperature will keep rising until it reaches 100\u00b0C (state 2, Fig. 3\u20137). At this point water is still a liquid, but any heat addition will cause some of the liquid to vaporize. That is, a phase-change process from liquid to vapor is about to take place. A liquid that is <i>abou<\/i><i>t to vaporize <\/i>is called a <b>saturated liquid. <\/b>Therefore, state 2 is a saturated liquid state.<\/font> <\/p>\n<h3 align=\"justify\"><font size=\"5\"><font style=\"font-weight: bold\">Saturated Vapor and Superheated Vapor<\/font><\/font><\/h3>\n<p align=\"justify\"><font size=\"5\">Once boiling starts, the temperature will stop rising until the liquid is com<\/font><font size=\"5\">pletely vaporized. That is, the temperature will remain constant during the en- tire phase-change process if the pressure is held constant. This can easily be verified by placing a thermometer into boiling water on top of a stove. At sea level (<i>P <\/i>= 1 atm), the thermometer will always read 100\u00b0C if the pan is un- covered or covered with a light lid. During a boiling process, the only change we will observe is a large increase in the volume and a steady decline in the liquid level as a result of more liquid turning to vapor.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0098.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0098\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0098\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0098_thumb.jpg\" width=\"180\" height=\"202\"><\/a><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0099.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0099\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0099\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0099_thumb.jpg\" width=\"142\" height=\"484\"><\/a><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Midway about the vaporization line (state 3, Fig. 3\u20138), the cylinder contains equal amounts of liquid and vapor. As we continue transferring heat, the va- porization process will continue until the last drop of liquid is vaporized (state 4, Fig. 3\u20139). At this point, the entire cylinder is filled with vapor that is on the borderline of the liquid phase. Any heat loss from this vapor will cause some of the vapor to condense (phase change from vapor to liquid). A vapor that is <\/font><font size=\"5\"><i>abou<\/i><i>t to condense <\/i>is called a <b>saturated vapor. <\/b>Therefore, state 4 is a saturated vapor state. A substance at states between 2 and 4 is often referred to as a <b>saturated liquid\u2013vapor mixture <\/b>since the <i>liquid and vapor phases coexist <\/i>in equilibrium at these states.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Once the phase-change process is completed, we are back to a single-phase region again (this time vapor), and further transfer of heat will result in an in- crease in both the temperature and the specific volume (Fig. 3\u201310). At state 5, the temperature of the vapor is, let us say, 300\u00b0C; and if we transfer some heat from the vapor, the temperature may drop somewhat but no condensation will take place as long as the temperature remains above 100\u00b0C (for <i>P <\/i>= 1 atm). A vapor that is <i>no<\/i><i>t about to condense <\/i>(i.e., not a saturated vapor) is called a <b>superheate<\/b><b>d vapor. <\/b>Therefore, water at state 5 is a superheated vapor. This constant-pressure phase-change process as described is illustrated on a <i>T<\/i><i>&#8211;<\/i>u diagram in Fig. 3\u201311.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">If the entire process described here is reversed by cooling the water while maintaining the pressure at the same value, the water will go back to state 1, retracing the same path, and in so doing, the amount of heat released will exactly match the amount of heat added during the heating process.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">In our daily life, water implies liquid water and steam implies water vapor. In thermodynamics, however, both water and steam usually mean only one thing: H2O.<\/font> <\/p>\n<h3 align=\"justify\"><font size=\"5\"><font style=\"font-weight: bold\">Saturation Temperature and Saturation Pressure<\/font><\/font><\/h3>\n<p align=\"justify\"><font size=\"5\">It probably came as no surprise to you that water started to boil at 100\u00b0C.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Strictly speaking, the statement \u201cwater boils at 100\u00b0C\u2019\u2019 is incorrect. The correct statement is \u201cwater boils at 100\u00b0C at 1 atm pressure.\u2019\u2019 The only reason the water started boiling at 100\u00b0C was because we held the pressure constant at 1 atm (101.325 kPa). If the pressure inside the cylinder were raised to 500 kPa by adding weights on top of the piston, the water would start boiling at 151.9\u00b0C. That is, <i>the temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature<\/i>.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">At a given pressure, the temperature at which a pure substance changes phase is called the <b>saturatio<\/b><b>n temperature <\/b><i>T<\/i>sat. Likewise, at a given temper-<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0100.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0100\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0100\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0100_thumb.jpg\" width=\"188\" height=\"484\"><\/a><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">ature, the pressure at which a pure substance changes phase is called the <b>sat<\/b><\/font><font size=\"5\"><b>uratio<\/b><b>n pressure <\/b><i>P<\/i>sat. At a pressure of 101.325 kPa, <i>T<\/i>sat is 100\u00b0C. Conversely, at a temperature of 100\u00b0C, <i>P<\/i>sat is 101.325 kPa.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Saturation tables that list the saturation pressure against the temperature (or the saturation temperature against the pressure) are available for practically all substances. A partial listing of such a table is given in Table 3\u20131 for water. This table indicates that the pressure of water changing phase (boiling or condensing) at 25\u00b0C must be 3.17 kPa, and the pressure of water must be maintained at 3973 kPa (about 40 atm) to have it boil at 250\u00b0C. Also, water can be frozen by dropping its pressure below 0.61 kPa.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">It takes a large amount of energy to melt a solid or vaporize a liquid. The amount of energy absorbed or released during a phase-change process is called the <b>laten<\/b><b>t heat. <\/b>More specifically, the amount of energy absorbed dur- ing melting is called the <b>laten<\/b><b>t heat of fusion <\/b>and is equivalent to the amount of energy released during freezing. Similarly, the amount of energy absorbed during vaporization is called the <b>laten<\/b><b>t heat of vaporization <\/b>and is equivalent to the energy released during condensation. The magnitudes of the latent heats depend on the temperature or pressure at which the phase change occurs. At 1 atm pressure, the latent heat of fusion of water is 333.7 kJ\/kg and the latent heat of vaporization is 2257.1 kJ\/kg.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">During a phase-change process, pressure and temperature are obviously dependent properties, and there is a definite relation between them, that is,<\/font> <\/p><div class=\"bqbwu6a0dce5b6cacc\" ><script type=\"text\/javascript\">\n\tatOptions = {\n\t\t'key' : '0c1eb4c533eaedb7b996f49a5a4983a9',\n\t\t'format' : 'iframe',\n\t\t'height' : 300,\n\t\t'width' : 160,\n\t\t'params' : {}\n\t};\n<\/script>\n<script type=\"text\/javascript\" src=\"\/\/www.highperformanceformat.com\/0c1eb4c533eaedb7b996f49a5a4983a9\/invoke.js\"><\/script><\/div><style type=\"text\/css\">\r\n@media screen and (min-width: 1201px) {\r\n.bqbwu6a0dce5b6cacc {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 993px) and (max-width: 1200px) {\r\n.bqbwu6a0dce5b6cacc {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 769px) and (max-width: 992px) {\r\n.bqbwu6a0dce5b6cacc {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 768px) and (max-width: 768px) {\r\n.bqbwu6a0dce5b6cacc {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (max-width: 767px) {\r\n.bqbwu6a0dce5b6cacc {\r\ndisplay: block;\r\n}\r\n}\r\n<\/style>\r\n<div class=\"slwje6a0dce5b6c69b\" ><script async src=\"https:\/\/pagead2.googlesyndication.com\/pagead\/js\/adsbygoogle.js?client=ca-pub-0778475562755157\"\n     crossorigin=\"anonymous\"><\/script>\n<!-- 300x600 hydraulics-and-pneumatics -->\n<ins class=\"adsbygoogle\"\n     style=\"display:inline-block;width:300px;height:600px\"\n     data-ad-client=\"ca-pub-0778475562755157\"\n     data-ad-slot=\"3735577695\"><\/ins>\n<script>\n     (adsbygoogle = window.adsbygoogle || []).push({});\n<\/script><\/div><style type=\"text\/css\">\r\n@media screen and (min-width: 1201px) {\r\n.slwje6a0dce5b6c69b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 993px) and (max-width: 1200px) {\r\n.slwje6a0dce5b6c69b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 769px) and (max-width: 992px) {\r\n.slwje6a0dce5b6c69b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 768px) and (max-width: 768px) {\r\n.slwje6a0dce5b6c69b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (max-width: 767px) {\r\n.slwje6a0dce5b6c69b {\r\ndisplay: block;\r\n}\r\n}\r\n<\/style>\r\n\n<p align=\"justify\"><font size=\"5\"><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0101.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0101\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0101\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0101_thumb.jpg\" width=\"403\" height=\"209\"><\/a><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0102.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0102\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0102\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0102_thumb.jpg\" width=\"397\" height=\"179\"><\/a><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><i>T<\/i>sat = <i>f<\/i>(<i>P<\/i>sat). A plot of <i>T<\/i>sat versus <i>P<\/i>sat, such as the one given for water in Fig. 3\u201312, is called a <b>liquid\u2013vapor saturation curve. <\/b>A curve of this kind is characteristic of all pure substances.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">It is clear from Fig. 3\u201312 that <i>T<\/i>sat increases with <i>P<\/i>sat. Thus, a substance at higher pressures will boil at higher temperatures. In the kitchen, higher boil- ing temperatures mean shorter cooking times and energy savings. A beef stew, for example, may take 1 to 2 h to cook in a regular pan that operates at 1 atm pressure, but only 20 min in a pressure cooker operating at 3 atm absolute pressure (corresponding boiling temperature: 134\u00b0C).<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">The atmospheric pressure, and thus the boiling temperature of water, de- creases with elevation. Therefore, it takes longer to cook at higher altitudes than it does at sea level (unless a pressure cooker is used). For example, the standard atmospheric pressure at an elevation of 2000 m is 79.50 kPa, which corresponds to a boiling temperature of 93.2\u00b0C as opposed to 100\u00b0C at sea level (zero elevation). The variation of the boiling temperature of water with altitude at standard atmospheric conditions is given in Table 3\u20132. For each 1000 m increase in elevation, the boiling temperature drops by a little over 3\u00b0C. Note that the atmospheric pressure at a location, and thus the boiling temperature, changes slightly with the weather conditions. But the corresponding change in the boiling temperature is no more than about 1\u00b0C.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><b>Som<\/b><b>e Consequences of <i>T<\/i><\/b><b>sa<\/b><b>t <\/b><b>an<\/b><b>d <i>P<\/i><\/b><b>sa<\/b><b>t <\/b><b>Dependence<\/b><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">We mentioned earlier that a substance at a specified pressure will boil at the <\/font><font size=\"5\">saturation temperature corresponding to that pressure. This phenomenon al- lows us to control the boiling temperature of a substance by simply control- ling the pressure, and it has numerous applications in practice. Below we give some examples. In most cases, the natural drive to achieve phase equilibrium by allowing some liquid to evaporate is at work behind the scenes.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Consider a sealed can of <i>liqui<\/i><i>d refrigerant-134a <\/i>in a room at 25\u00b0C. If the can has been in the room long enough, the temperature of the refrigerant in the can will also be 25\u00b0C. Now, if the lid is opened slowly and some refrigerant is allowed to escape, the pressure in the can will start dropping until it reaches the atmospheric pressure. If you are holding the can, you will notice its temperature dropping rapidly, and even ice forming outside the can if the air is hu- mid. A thermometer inserted in the can will register -26\u00b0C when the pressure <\/font><font size=\"5\">drops to 1 atm, which is the saturation temperature of refrigerant-134a at that pressure. The temperature of the liquid refrigerant will remain at -26\u00b0C until the last drop of it vaporizes.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Another aspect of this interesting physical phenomenon is that a liquid can- not vaporize unless it absorbs energy in the amount of the latent heat of va- porization, which is 217 kJ\/kg for refrigerant-134a at 1 atm. Therefore, the rate of vaporization of the refrigerant depends on the rate of heat transfer to the can: the larger the rate of heat transfer, the higher the rate of vaporization. The rate of heat transfer to the can and thus the rate of vaporization of the re- frigerant can be minimized by insulating the can heavily. In the limiting case of no heat transfer, the refrigerant will remain in the can as a liquid at -26\u00b0C indefinitely.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">The boiling temperature of <i>nit<\/i><i>r<\/i><i>oge<\/i><i>n <\/i>at atmospheric pressure is -196\u00b0C (see Table A\u20133<i>a<\/i>). This means the temperature of liquid nitrogen exposed to the at- mosphere must be -196\u00b0C since some nitrogen will be evaporating. The tem- perature of liquid nitrogen will remain constant at -196\u00b0C until it is depleted. For this reason, nitrogen is commonly used in low-temperature scientific stud- ies (such as superconductivity) and cryogenic applications to maintain a test chamber at a constant temperature of -196\u00b0C. This is done by placing the test chamber into a liquid nitrogen bath that is open to the atmosphere. Any heat transfer from the environment to the test section is absorbed by the nitrogen, which evaporates isothermally and keeps the test chamber temperature con- stant at -196\u00b0C (Fig. 3\u201313). The entire test section must be insulated heavily to minimize heat transfer and thus liquid nitrogen consumption. Liquid nitro- gen is also used for medical purposes to burn off unsightly spots on the skin. This is done by soaking a cotton swap in liquid nitrogen and wetting the target area with it. As the nitrogen evaporates, it freezes the affected skin by rapidly absorbing heat from it.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0103.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0103\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0103\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0103_thumb.jpg\" width=\"175\" height=\"209\"><\/a><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0104.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0104\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0104\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0104_thumb.jpg\" width=\"189\" height=\"447\"><\/a><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">based on <i>r<\/i><i>educin<\/i><i>g the pressure <\/i>of the sealed cooling chamber to the saturation pressure at the desired low temperature and evaporating some water from the products to be cooled. The heat of vaporization during evaporation is absorbed from the products, which lowers the product temperature. The satu- ration pressure of water at 0\u00b0C is 0.61 kPa, and the products can be cooled to 0\u00b0C by lowering the pressure to this level. The cooling rate can be increased by lowering the pressure below 0.61 kPa, but this is not desirable because of the danger of freezing and the added cost.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">In vacuum cooling, there are two distinct stages. In the first stage, the prod- ucts at ambient temperature, say at 25\u00b0C, are loaded into the chamber, and the operation begins. The temperature in the chamber remains constant until the <i>saturatio<\/i><i>n pressure <\/i>is reached, which is 3.17 kPa at 25\u00b0C. In the second stage that follows, saturation conditions are maintained inside at progressively <i>lowe<\/i><i>r pressures <\/i>and the corresponding <i>lowe<\/i><i>r temperatures <\/i>until the desired<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">ated cooling, and its use is limited to applications that result in much faster cooling. Products with large surface area per unit mass and a high tendency to release moisture such as <i>lettuc<\/i><i>e <\/i>and <i>spinac<\/i><i>h <\/i>are well-suited for vacuum cool- ing. Products with low surface area to mass ratio are not suitable, especially those that have relatively impervious peels such as tomatoes and cucumbers.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0105.jpg\"><img decoding=\"async\" loading=\"lazy\" style=\"background-image: none; border-bottom: 0px; border-left: 0px; margin: 0px auto; padding-left: 0px; padding-right: 0px; display: block; float: none; border-top: 0px; border-right: 0px; padding-top: 0px\" title=\"PROPERTIES OF PURE SUBSTANCES-0105\" border=\"0\" alt=\"PROPERTIES OF PURE SUBSTANCES-0105\" src=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-content\/uploads\/2016\/03\/PROPERTIES-OF-PURE-SUBSTANCES-0105_thumb.jpg\" width=\"183\" height=\"222\"><\/a><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">Some products such as mushrooms and green peas can be vacuum cooled suc<\/font><font size=\"5\">cessfully by wetting them first.<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">The vacuum cooling just described becomes <b>vacuu<\/b><b>m freezing <\/b>if the vapor pressure in the vacuum chamber is dropped below 0.6 kPa, the saturation<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><strong>Low vapor pressure Evaporation<\/strong><\/font> <\/p>\n<p align=\"justify\"><font size=\"5\">pressure of water at 0\u00b0C. The idea of making ice by using a vacuum pump is nothing new. Dr. William Cullen actually made ice in Scotland in 1775 by evacuating the air in a water tank (Fig. 3\u201315).<\/font> <\/p>\n<p align=\"justify\"><font size=\"5\"><b>Packag<\/b><b>e icing <\/b>is commonly used in small-scale cooling applications to <\/font><font size=\"5\">remove heat and keep the products cool during transit by taking advantage of the large latent heat of fusion of water, but its use is limited to products that are not harmed by contact with ice. Also, ice provides <i>moistu<\/i><i>r<\/i><i>e <\/i>as well as <i>r<\/i><i>efrigeratio<\/i><i>n<\/i>.<\/font><\/p>\n","protected":false},"excerpt":{"rendered":"<p>PHASE-CHANGE PROCESSES OF PURE SUBSTANCES There are many practical situations where two phases of a pure substance co- exist in equilibrium. Water exists as a mixture of liquid and vapor in the boiler and the condenser of a steam power plant. The refrigerant turns from liquid to vapor in the freezer of a refrigerator. Even [&hellip;]<br \/><a href=\"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/properties-of-pure-substancesphase-change-processes-of-pure-substances\/\" class=\"more-link\" >Continue reading&#8230;<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[],"_links":{"self":[{"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/posts\/1351"}],"collection":[{"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/comments?post=1351"}],"version-history":[{"count":1,"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/posts\/1351\/revisions"}],"predecessor-version":[{"id":1352,"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/posts\/1351\/revisions\/1352"}],"wp:attachment":[{"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/media?parent=1351"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/categories?post=1351"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/machineryequipmentonline.com\/hydraulics-and-pneumatics\/wp-json\/wp\/v2\/tags?post=1351"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}