Critical constants cryogenic fluids

Table 2.4 displays critical constants Tc, Pc, Vc and critical compressibility factor Zc for a number of common gases. (Accurate determination of the critical point is experimentally challenging, and quoted values are generally uncertain in the final decimal.) One can see from the table that many common gases (including N2, 02, and CH4) are actually supercritical fluids ( permanent gases ) under ambient temperature conditions, incapable of liquefaction by any applied pressure whatsoever. (Aspects of cryogenic gas-liquefaction techniques are discussed in Section 3.6.3.)... 

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. 

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