Transfer line cooldown

Fig. 7.49. Thermodynamic system (bounded by the dashed line) considered in the transfer-line cooldown analysis.

In addition to these limits on the mass flow rate, the minimum and maximum required liquid quantities for cooldown can be calculated in terms of the cooling obtained from the cryogen being transferred. The maximum cooldown quantities, representing the upper limit on liquid requirements for cooldown, are deduced with the assumption that only the latent heat of vaporization is used to cool the transfer line. The minimum amount of liquid necessary to cool the line is calculated by assuming that the vapor formed as a result of transfer line cooldown is in thermal equilibrium with the transfer line. 

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. 

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... 

We have studied the problem of pressurized line cooldown both experimentally and analytically. We constructed a small pressurized transfer system to investigate the physical processes involved in line cooldown and to determine the effects of various system variables on line cooldown time. An analysis was developed from thermodynamic principles so that the test data could be correlated and the range of our experimental results extended. The analysis and test data have been found to be in good agreement. 

Figure 1 shows the small pressurized transfer system that was constructed for the experimental study of line cooldown. This system consists of a vacuum insulated storage tank feeding a transfer line which discharges into a vented receiver tank. 

Further information on the transient situation existing during line cooldown is shown in Figure 6, which is a plot of liquid flow rate entering the transfer line. This data shows that within the first 10 seconds after the transfer line inlet valve is opened, the flow rate rises to approximately twice the steady-state value. 

This surge is followed almost immediately by a reversal of flow, another surge and then a more or less gradual transition to the steady flow rate. Figure 6 shows that the mean inlet flow during most of the line cooldown period is much less than the steady-state flow. This fact illustrates how the flow of liquid into the line, during cooldown, is limited by the ability of the transfer line to vent the cooldown gas. 

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. 

The method of calculating system vent flow is illustrated in Figure 10. The fluid flow model chosen consisted of a transfer line having a distributed impedance due to valves, line friction, etc, with an orifice at the downstream end. During pressurized cooldown, the downstream orifice operates at, or near, choked flow, and compressible fluid flow theory was used to calculate vent flow. 

By combining the above, two equations, one can determine the vent flow for any transfer line configuration as a function of storage tank pressure. The 4 fL/D term used in the above equation was taken to be the time-mean impedance of the line between the liquid front and the orifice. Since, at the end of cooldown the impedance to gas flow is zero, while at the beginning of cooldown, the impedance to gas flow is equal to the total line impedance, the mean value was taken as one-half the total line impedance upstream of the gas restrictor. 

Figure 12 presents a correlation of the test results and analytical calculations. The various points represent individual tests in which the experimental line cooldown was determined by the appearance of liquid at the sight glass the calculated cooldown time was obtained from the cooldown time equation. The 45 degree line represents perfect agreement. The results show that the analysis is remarkably accurate for prediction of the line cooldown time. The data obtained with the exposed line shows somewhat more scatter than does that obtained with the insulated line. We believe that the relative scatter shown for the exposed line is due in part to the difficulty in accurately estimating the ambient heat transfer coefficients. 

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. 

Mean absolute temperature of cooldown gas leaving transfer line... 

Answer by author Our experience has shown that the fastest cooldown is obtained by suddenly admitting the pressurized cryogenic liquid to the transfer line. Of course, in some applications this approach may be impractical due to the necessity for keeping the receiver tank pressure below some acceptable value. 

Answer by author We have not compared the quantity of liquid consumed during line cooldown with the cooldown rate. However, we believe that for a given transfer line, the amount of liquid consumed will increase with the length of the cooldown period. This is because the heat input due to the cooldown of concentrated masses and ambient heat leak will increase with cooldown time. 

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

This chapter will consider various insulation concepts used in such storage systems and briefly review the basic design approach used for conventional storage vessels and transfer lines. The accompanying problems of cooldown and two-phase flow encountered in the operation of these cryogenic systems are included in the discussion. 

All of these systems share to some degree several typical design problems associated with cryogenic liquid transfer. One class of difficulties results from cooling the system down from ambient to cryogenic temperature. Evidence of cooldown is in the form of two-phase flow, thermal contraction, and line bowing. Thermal contraction of a transfer line must not result in contact between the inner and outer lines, a condition most frequently encountered at changes in direction of the transfer lines. Expansion joints, bellows, and U-bends have been employed to solve the problem of thermal contraction. 

In an uninsulated line, cooldown costs are low and a frost due to sublimed water vapor quickly forms on the pipe to provide some insulation. A heat transfer coefficient exists for the boundary layer between the ambient air and the outer piping surface as well as between the inner surface of the tube and the transported cryogen. Consequently, both the frost and the film effects offer resistance to heat influx. 

The design objectives for a vacuum-insulated transfer line are similar to those for a conventional vacuum-insulated dewar. A vacuum-insulated line consists of an inner pipe which carries the cryogenic fluid and an outer concentric pipe which contains the vacuum. The inner line should be made as thin as possible to minimize cooldown losses, and the material of construction selected should be compatible with the cryogen. The outer piping, which may enclose an evacuated powder, MLI, or simply high vacuum, must resist the compression due to atmospheric pressure. The ASA Code for Pressure Piping provides design equations for both the inner and outer lines. The thickness of the inner line is determined by... 

Several spacer designs are illustrated in Fig. 7.31. For transfer lines with nominal diameters up to 0.2 m the fixed square or triangular spacers are adequate. Rollers are typically used as spacers for longer lines because thermal stresses that result in cooldown necessitate that the contact points not be fixed. Stainless steel and low thermal conductivity organic plastics like Teflon are the most commonly used spacer materials. 

Whether it is a result of unavoidable heat inleak to the pipe or the effects of cooldown, two-phase flow is a common occurrence in the transfer of a liquid cryogen. The existence of vapor in a transfer line may critically reduce the carrying capacity of the line therefore, the presence of vapor must be minimized. The reduction of line capacity occurs because the mixture of vapor and liquid, having a lower density than that of the liquid alone, must have a greater velocity in order to achieve the same liquid flow rate. 

The transient nature of the cooldown process is exemplified by Fig. 7.48. As a liquid cryogen enters a warmer transfer line, the initial liquid quickly vaporizes and becomes superheated. The pipe is subsequently cooled by the cold vapor and by the latent heat absorbed by the evaporating liquid hence,... 

Figure 7.50 illustrates the minimum flow rate required in the cooldown of a number of transfer lines for several different cryogens. 

Another concern regarding the cooldown of a transfer line relates to the maximum flow rate that can safely be used. Figure 7.51 shows the cooldown flow rates that should not be exceeded for several cryogenic liquid and piping... 

Fig. 7.51. Maximum allowable cooldown flow rate of LN2 and LH2 in transfer lines constructed of aluminum and stainless steel.

The mass of liquid oxygen required to cool down the transfer line is obtained from an energy balance equating the energy removed from the metal to the energy gained by the exit gas during cooldown. Thus, we have... 

During the cooldown of liquid transfer lines and storage tanks or containers, the occurrence of 2-phase flow, albeit transient, is unavoidable. 

The best way of understanding cooldown is to consider some examples, namely a long transfer line, a tank and a large mass. 

Pipeline transportation of liquid hydrogen is realized on a small scale and short range. Stainless steel is usually taken for the inner line with low heat conduction spacers as a support in the vacuum jacket. The Kennedy Space Center in Florida uses an LH2 and LOX pipeline of 500 m length with an irmer pipe diameter of 0.15 m. Flow rates achieved are up to 250 LH2 per minute and 100 m LOX per minute, respectively [12]. Transfer is realized by applying pressure, no pumps. Major concerns besides heat leakage is the mechanical stress imposed on the irmer line due to contraction / expansion, pressure oscillations upon cooldown, or two-phase flow. 

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