A standardized way of comparing the thermal performance of different pipelines in different sizes is needed. Vendor data for vacuum-insulated piping are typically given in heat leak rate per unit length (watt per meter [W/m]) for a specific diameter pipeline. An overall apparent thermal conductivity – k-value – for actual field installations (k_oafi) is therefore proposed as a more generalized measure for thermal performance comparison and design calculation. The total system includes the inner piping, the insulation material layers, the outer piping, and other items such as spacers and getters. The k_oafi provides a direct correspondence to the k-values reported for insulation materials and illustrates the large difference between ideal multilayer insulation (MLI) and actual MLI performance.

Ambient heat transfer into a cryogenic pipeline comes through several paths including valves, connectors, instrumentation, and insulation. A common type of thermal insulation system is MLI. MLI systems come in many varieties and must be tailored to the specific application. The performance of MLI is known to be sensitive to localized compression effects and trapped residual gases produced by the combined mechanical influences of bending and spacers. Bending-type mechanical effects come from four sources: bending, as in handling and installation; thermal contraction and expansion; line pressure reaction forces; and the weight of the line (sagging). Spacers are employed in the design of vacuum-jacketed lines to keep the inner line concentric within the outer line during manufacturing and to counteract these mechanical effects during operation. Spacers are made from low-thermal-conductivity materials to minimize heat conduction.

In this experimental research study, a section of insulated piping was tested under cryogenic vacuum conditions, including simulated spacers and bending. Several different insulation systems were tested using a 1-meter-long cylindrical cryostat test apparatus. The simulated spacers tests showed significant degradation in the thermal performance of a given insulation system. The results of the simulated spacers test are given in figure 1. The spacer simulation shows a significant increase in the rate of heat transfer for the high-vacuum tests. For C123 in comparison to C124 the k-value increased from 0.09 to 0.15 milliwatt per meter-kelvin (mW/m-K) (a 67-percent increase in heat transfer).

Figure 2 provides a convenient design tool for estimating heat loads (W/m) for different line sizes and different k_oafi. The experimental laboratory data can be compared with manufacturers’ typical data for a 60- x 110-mm line: 2.30 W/m (flexible) and 0.75 W/m (rigid). Converting these typical heat leak values into their thermal conductivity equivalents, we obtain k_oafi of 0.99 mW/m-K (flexible) and 0.32 mW/m-K (rigid). The k_oafi method is being used by the Cryogenics Test Laboratory to provide practical engineering information for specific system designs and applications.

Contact: J.E. Fesmire (James.Fesmire-1@ksc.nasa.gov), YA-C2, (321) 867-7557
Participating Organization: Dynacs Inc. (Dr. S.D. Augustynowicz)