Cryogenic transfer lines

Liquid cryogen transfer lines should be designed so that liquid cannot be trapped in any nonvented part of the system. Experiments in supercritical fluids include high pressure and should be carried out with appropriate protective systems. 

J. Burke, W. Byrnes, A. Post, and F. E. Ruccia, "Pressurized Cool-down of Cryogenic Transfer Line," Advances in Cryogenic Engineering, Vol. 4, K. D. Timmerhaus, (ed.). Plenum Press, Inc., New York (1960). 

A Liquid Air Device for Cooling the Wearer of a Totally Enclosed Liquid Rocket Propellant Handler s Suit (4) 196 Pressurized Cooldown of Cryogenic Transfer Lines (4) 378... 

Transfer of Liquid Hydrogen through Uninsulated Lines (5) 103 Flexibility Considerations for the Design of Cryogenic Transfer Lines (5) 111 LIQUEFIERS [see also COOLING,REFRIGERATORS, REFRIGERATION]... 

It is quite conceivable that in a short, heavy-wall line, the liquid may reach the end of the line before the line wall temperature has reached equilibrium. However, it is felt that the proposed model applies to most conventional cryogenic transfer lines. 

The above method ass imes an infinite heat transfer coefficient on the fluid side of the concentrated mass, and hence tends to overestimate the percentage cooldown. Additional accuracy might be obtained if the method of calculation were refined to account for finite heat transfer coefficients. A refinement of this type might be helpful in those situations in which the heat input from the partial cooldown of concentrated masses is large in comparison with the heat input from the complete cooldown of the distributed mass. However, it is felt that for most conventional cryogenic transfer lines the assumption of an infinite coefficient is adequate to obtain a reasonable approximation of cooldown mass. 

A method for calculating the pressurized cooldown time of cryogenic transfer lines has been developed. This method has been found to be in good agreement with test data obtained over a fairly wide range of operating conditions in a small-scale pressurized transfer system. The analysis presented should prove to be useful for estimation of the time required for the pressurized cooldown of conventional cryogenic transfer lines. 

D. C. Bowersock, R. W. Gardner, and R. C. Reid, "Pressurized cooldown of cryogenic transfer lines," 1958 Cryogenic Engineering Conference Proceedings. 

FLEXIBILITY CONSIDERATIONS FOR THE DESIGN OF CRYOGENIC TRANSFER LINES... 

Cryogenic transfer lines can be grouped into three major categories (1) uninsulated lines, (2) foam-insulated lines, and (3) vacuum-insulated lines. The latter category can be subdivided into evacuated powder-insulated lines, high vacuum-insulated lines, and MLI-insulated lines. 

Frequently, cryogenic transfer lines may be economically insulated with foam such as polystyrene and polyurethane or fibrous material such as glass wool. These insulations when properly applied with a vapor barrier may significantly reduce the heat inleak and prove feasible for use with cryogens at temperatures as low as that of liquid nitrogen. If the insulation is free of condensed air, the steady-state heat flux to the foam or fibrous-insulated line may be determined from... 

Another concern of a cryogenic transfer line designer is thermal contraction because the inner line is cooled from ambient to cryogenic temperatures while the outer line remains at room temperature. The degree of thermal contraction from room temperature to liquid hydrogen temperature for a copper pipe is about 0.033 m for each 10 m of pipe length. 

Fig. 7.32. Bolted-flange joint for cryogenic transfer line.

Fig. 7.33. Schematic of typical bayonet joint for cryogenic transfer line. Legend 1, outer line, vacuum shell 2, line coupling 3, warm temperature O-ring seal 4, static gas leg 5, vacuum insulation space 6, vacuum barriers 7, additional liquid seal and 8, liquid line.

Unfortunately, however, the problem of analytically predicting two-phase flow behavior is complicated by the mass transition, usually from the liquid to the vapor, that results from the heat influx into the pipe and from pressure changes. Moreover, the liquid and vapor are quite frequently not in equilibrium, and this introduces additional variables into the prediction. In attempting to describe two-phase flow in a cryogenic transfer line, one must first realize that the flow in the vapor phase may be laminar or turbulent, and the flow in the liquid phase may be similar to or different from the gas. Further, this flow pattern may be altered by the incline of the pipe and by the heat influx or pressure drop. 

Solubility. This column gives the solubility of the window in water, at room temperature. The values provided are in grams of window material that will dissolve in 100 g of water. Where that information was not readily available, comments on the relative effect of water are provided. Solubility is of interest since water from the air often condenses on dewars or cryogen transfer lines, and it may be difficult to avoid getting some water on the window. 

The risk of bums is normally only experienced when filling a magnet with nitrogen or helium. You need to be protected in case the liquid spills or the transfer line breaks. Protection just means covering up any exposed skin (lab coat, visor and thick gloves are normally sufficient). At all other times, the cryogens are safely in their cans and should stay there unless something catastrophic happens. 

Cryogenic fluid transfer lines are generally classified as one of three types uninsulated, foam-insulated lines, and vacuum-insulated lines. The latter may entail vacuum insulation alone, evacuated powder insulation, or multilayer insulation. A vapor barrier must be applied to the outer surface of foam-insulated transfer lines to minimize the degradation of the insulation that occurs when water vapor and other condensables are permitted to diffuse through the insulation to the cold surface of the lines. 

Two-phase flow is always involved in the cooldown of a transfer line. Since this process is a transient one, several different types of two-phase flow will exist simultaneously along the inlet of the transfer line. Severe pressure and flow oscillations occur as the cold liquid comes in contact with successive warm sections of the line. Such instability continues until the entire transfer line is cooled down and filled with liquid cryogen. 

As stated, one of the reasons for using a vacuum is to remove oxygen from a system so that air-sensitive compounds will not react. If that is your primary concern and you do not need to use cryogenic transfer for movement of compounds within the vacuum line, there may not be a need to equip your lab with an ultrahigh-vac-uum system. 

Figure 3.50. A cryogenic probe installation. The cryogenic bay (A) contains the cold expansion head that generates cold helium gas that passes along the insulated transfer line (B) to the probe. The vacuum line (C) maintains the cryogenic probe vacuum and the base of the probe (D) houses the cold preamplifier stages. At the rear can be seen the high pressure lines (E) that carry helium gas to/from the remote compressor.

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