Vapor pressure cryogenic fluids

Cryogenic fluids are usually taken to be those that boil below some specified temperature at 1 atm pressure. In practice, this temperature is arbitrary and depends on the application of interest for example, Scott and McClintock take it to be — 150°C (ca. 123°K), and BelF — 100°F (ca. 200°K). For our purpose, we will consider a cryogenic fluid to be one with a critical temperature below room temperature (ca. 300°K) the vapors of such fluids must therefore be cooled before they can be liquefied by an increase in pressure. 

When only a single phase is present, the system pressure depends on the temperature, the volume, and the quantity of material. Rather large pressures can be produced by completely vaporizing a cryogenic fluid in a closed chamber. For example, using the helium density data of Figure 6, we see that an unyielding container filled initially with liquid at 1 atm (p = 0.125 g/cm ) would experience a pressure of 100 atm at 27°K and about 1000 atm at 270°K if the container were initially half full of liquid, it would experience a pressure of about 50 atm at 27°K (p = 0.0625 g/cm ) and about 400 atm at 270°K. In practice, if the container remained intact, its volume would increase somewhat so that the final density and the pressure would be reduced. 

With few exceptions, thermodynamic property tabulations are calculated from P-V-T meaz.urements and from zero-pressure specific heat values derived from spectroscopic measurements. It should be noted that the zero-pressure (ideal gas) specific heat values, Cp, for the cryogenic fluids are generally known with an uncertainty of less than 3 parts in 10,000 whereas the random deviations of the P-V-T data are of the order of 2 to 5 parts in 1000. The phase boundaries involve a further complication and, consequently, must be defined by additional experimental data. As a minimum requirement, measurements of the vapor pressure are sufficient for the calculation of thermodynamic property differences due to a phase change. This is indicated by the Clapeyron equation, which may be expressed... 

For illustration. Figure 1.2 plots the temperature and pressure ranges over which the four fluids exist as liquids between the saturation curve and triple line. Temperature/ pressure (T/P) plots are used throughout the text to illustrate how the LAD performs as a function of the thermodynamic state of the liquid. For comparison. Table 1.1 lists the four primary cryogenic fluids along with thermophysical properties at the NBP saturation conditions relevant in the current work, such as the saturation temperature, heat of vaporization hfg, liquid density pf, kinematic viscosity v, and surface tension j lv- Clearly, advanced systems are required to store, maintain, and transfer such cold liquids. 

R. B. Jacobs, in reports presented at a previous cryogenic conference and in published NBS bulletins, derives an equation which permits the calculation of the minimum required inlet pressure of a transfer line that will allow single-phase flow. An approximate form of the Clapeyron equation suitable for most fluids is used. The accuracy of this equation can be improved for hydrogen by substituting the vapor pressure equation of hydrogen for the Clapeyron equation. 

A critical flow can be achieved in the transfer of a cryogenic fluid. Under such a situation, the flow rate of the fluid cannot be altered by lowering the downstream pressure as long as the supply pressure remains constant. Such a situation is due to the increased downstream vaporization, which reduces the available cross-sectional area available for liquid transfer. 

The Lockhart-Martinelli correlations, as noted earlier, were developed for estimating the pressure drops associated with two-phase flow at ambient conditions. However, because of the increased vaporization tendency of cryogenic fluids, these correlations have a tendency to underestimate the pressure drop by as much as 10-30% with low-temperature flows. 

Vapor Pressure Thermometry. The pressure exerted by a saturated vapor in equilibrium with its liquid is a very definite function of temperature, and can be used to measure the temperature of the liquid. A number of useful fixed points for several cryogenic fluids are given in Table 8.3. With a good pressure-measuring device, the vapor pressure thermometer is an excellent secondary standard since its temperature response depends upon a physical property of a pure compound or element. Many expressions have been... 

A drawing of the cross section of a Dewar vessel is shown in Figure 2 [1]. It shows the basic elements of a high performance cryogenic storage vessel. A fill and drain line is provided at the bottom of the vessel to transfer fluid in and out of the tank. Liquid can be removed either by pressurization of the inner vessel with a pressurization gas or by a liquid pump. A vapor vent line is located near the top of the vessel to allow vapor formed from heat leak to escape. This line can also be used to introduce a pressurization gas. If pressurization is used to force liquid from the tank, a diffuser is provided to distribute the pressurization gas in the vapor space away from the surface of the cold liquid. This prevents the unwanted condensation of the warm pressurization gas by the cold liquid surface. 

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