This work involved designing a liquid nitrogen cold-plate heat exchanger with a high thermal mass using code-standard, high-pressure tubing. High thermal mass requires a substantial amount of material, so heat exchangers of this type are usually fabricated from a solid piece of metal (such as copper) with fluid paths machined into the component. However, standard tubing was desired for the fluid path due to its pressure rating and predictability. The key problem was how to embed copper tubing into a larger mass while maintaining good heat transfer properties.
Many tube-style flow-through heat exchangers have the tubes bonded directly to a cold plate. This method is inefficient from a heat transfer perspective due to the minimal surface area in contact between the two parts (i.e., tubes and plate), and also limits the device’s thermal stability. High-thermal-mass heat exchangers are usually one-off, machined pieces that either do not meet the NASA pressure systems requirements, or are exceedingly difficult to bring into compliance.
This innovation embeds a coil of copper tubing within a volume of copper powder that, in turn, is in thermal contact with a cold plate in order to maximize the heat transfer and thermal stability of the heat exchanger. The heat exchanger is composed of six main parts:
- 9-in.-diameter (≈23-cm) G-10 fiberglass epoxy body,
- G-10 fiberglass epoxy base plate,
- Aluminum cold plate (top plate),
- Aerogel blanket ring,
- Aerogel blanket disc,
- Coil of ¼ in. (≈0.6 cm) ASTM B280 copper tubing [roughly 13 ft (≈4 m) worth].
To date, the heat exchanger has been used three times — once for its initial checkout and performance test, and then for testing during the 2013 CIF Icy Regolith project. Each time, the component was cooled down from ambient temperature to approximately 77 K (–321 °F) using liquid nitrogen with no issues. Additional testing is planned to more adequately characterize the unit’s thermal performance.
Peripheral equipment includes a cryostat vacuum chamber and accessories, and vacuum-jacketed flex-throughs to supply liquid nitrogen. No maintenance is required. Reliability has yet to be determined. Using ASME B31.3 and ASTM B280, the maximum allowable working pressure of the tubing was determined to be 1,420 psig (≈9.9 MPa). The intended application pressure range is from 0 to 100 psig (≈101 to 791 kPa). Therefore, the safety factor is 14 (at a minimum).
The instrument maintains a fluid pressure boundary governed by established ASTM and ASME code standards while maximizing both thermal mass and focused heat transfer at the cold plate. It exploits the mechanical properties of aerogel blanket to act as a gasket around the tube penetrations to seal the fine copper powder from leaking out, and as a “spring” to keep the powder compressed inside the heat exchanger and held firmly against the underside of the cold plate (this is critical for heat transfer purposes, to ensure that no air gaps are created between the cold plate and copper powder when the unit is turned right-side-up after the base plate is installed).
This concept also applies to other thermally conductive (or insulative) fill materials. In this sense, the thermal performance of the heat exchanger can actually be “tailored” to a certain degree.
This work was done by Adam Swanger of Kennedy Space Center. KSC-13934