Cryogenic applications specifications

The low-temperature thermal conductivity of different materials may differ by many orders of magnitude (see Fig. 3.16). Moreover, the thermal conductivity of a single material, as we have seen, may heavily change because of impurities or defects (see Section 11.4). In cryogenic applications, the choice of a material obviously depends not only on its thermal conductivity but also on other characteristics of the material, such as the specific heat, the thermal contraction and the electrical and mechanical properties [1], For a good thermal conductivity, Cu, Ag and A1 (above IK) are the best metals. Anyway, they all are quite soft especially if annealed. In case of high-purity aluminium [2] and copper (see Section 11.4.3), the thermal conductivities are k 10 T [W/cm K] and k T [W/cm K], respectively. 

We have seen that many electronic components, even not specifically produced for cryogenic applications, can be usefully operated at low temperature some of them retain their room temperature characteristics like NiCr resistors which do not appreciably change their resistance (less than 10% upon cooling to 4K) and show a lower noise at low temperature. Other resistors (as RuOz) and most capacitors change their characteristics with temperature. Mica and polyester film capacitors show a good temperature stability. If capacitors insensitive to temperature are needed, crystalline dielectric or vacuum capacitors must be used. 

Fully cryogenic licjuid service Pilot-operated, soft-seated valves with a type of vaporizer which (relatively) warms up the fluid entering the pilot or high-performance, soft-seated, spring-operated SRVs. The vaporizer and other accessories of a typical cryogenic configuration on a pilot-operated valve keep the pilot warm, which then works on vapour. In any case, these are applications that should be discussed with your SRV supplier. Some suppliers have done extensive tests on cryogenic applications and have experience to share on this specific application. 

In themselves, cryotronic applications are still in the stage of development, or even in research. On the other hand, as long as new designs were not fully mature, no close specifications in the temperature conditioning methods were required. Therefore, more versatile and efficient cryogenic systems, specifically adapted to airborne and space uses, are now in the course of intense development. 

The evacuated powder type insulations considered were perlite, silica-aerogel, and peach pit charcoal. While the evacuated powder provides excellent insulation in certain cryogenic applications, it was not best adapted for this specific application due to very high installation cost. The factors responsible for these high costs are the additional number of filling and vacuum pumping ports which would be needed, plus the additional vacuum manifold which would be necessary for the full length of the transfer line. 

The high specific strength or stiffness, thermal and electrical insulation capabilities, and the fatigue-endurance limits of fiber composites are the chief reasons for their cryogenic applications. The polymeric matrix plays an important part, and for several properties, constitutes the limiting factor. 

As a general rule, they should not be used in steam, air, gas or vapour applications or in the presence of variable backpressure. Nevertheless, there are high-performance small spring-loaded valves which are designed specifically for specific services on gas, with backpressure or under cryogenic conditions. 

Compression power accounts for more than 80% of the total energy required in the production of industrial gases and the liquefaction of natural gas. In order to minimize the cost and maintenance of cryogenic facilities, special care must be exercised to select the appropriate compression system. The three major types of compressors widely used today are reciprocating, centrifugal, and screw. Currently, there is no particular type of compressor that is generally preferred for all applications. The final selection will ultimately depend on the specific application as well as the effect of plant site and existing facilities. 

Many empirical relationships have been developed to aid the optimization of coiled-tube exchangers under ambient conditions. Many of the same relationships are currently being used in low-temperature applications as well. A number of these relationships are tabulated in readily available cryogenic texts. However, no claim is made that these relationships will be more suitable than others for a specific design. This can be verified only by experimental measurements on the heat exchanger. 

The suitability of a water spray for application to releases composed of water-soluble materials is specific to the particular material in question. If applied to cryogenic materials, a water spray can cause the material to heat and violent boiling can result. Therefore, it is prudent to understand the properties of the material in question. Key considerations include the flash point, specific gravity, viscosity, and solubility of the material, the temperature of the water spray, and the temperature of the hazardous material (NFPA 15, 1990). 

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