A report discusses a modified design of a Joule-Thomson (JT) refrigerator under development to be incorporated into scientific instrumentation aboard a spacecraft.In most other JT refrigerators (including common household refrigerators), the temperature of the evaporator (the cold stage) is kept within a desired narrow range by turning a compressor on and off as needed. This mode of control is inadequate for the present refrigerator because a JT-refrigerator compressor performs poorly when the flow from its evaporator varies substantially, and this refrigerator is required to maintain adequate cooling power. The proposed design modifications include changes in the arrangement of heat exchangers, addition of a clamp that would afford a controlled heat leak from a warmer to a cooler stage to smooth out temperature fluctuations in the cooler stage, and incorporation of a proportional + integral + derivative (PID) control system that would regulate the heat leak to maintain the temperature of the evaporator within a desired narrow range while keeping the amount of liquid in the evaporator within a very narrow range in order to optimize the performance of the compressor. Novelty lies in combining the temperature- and cooling-power-regulating controls into a single control system.
This work was done by James Borders, David Pearson, and Mauro Prina of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Machinery/Automation category.
NPO-40225
This Brief includes a Technical Support Package (TSP).
Improving Control in a Joule-Thomson Refrigerator
(reference NPO-40225) is currently available for download from the TSP library.
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Overview
The document is a technical support package from NASA's Jet Propulsion Laboratory, summarizing a study on improving the stability and control of the Planck cryocooler, specifically focusing on the Joule-Thomson refrigeration process. The Planck cooler consists of two main components: the sorption compressor and the PACE (Passive Active Cooling Element). The study aims to enhance the interaction between these components, particularly the exchange and accumulation of hydrogen over time, which is crucial for maintaining system stability.
A key aspect of the proposed modifications involves a mechanical redesign of the existing PACE system to improve thermal isolation between the LFI (Low Frequency Instrument) and the cooler's components. This redesign builds on previous successful designs, particularly one by Adriana Sirbi, which demonstrated effective heat balancing in the heat exchanger before the Joule-Thomson expansion valve. The document emphasizes the importance of maintaining a constant temperature in the high-pressure stream exiting the heat exchanger to minimize temperature fluctuations at the cold end.
The study includes a detailed analysis of the heat balance within the system, particularly focusing on the copper clamp used in the heat exchanger. The heat transfer efficiency of the clamp is evaluated under various conditions, revealing that while it is not very efficient in transferring heat to the incoming high-pressure gas, it effectively removes heat from it. The analysis considers different lengths of the copper clamp to assess the impact of heat transfer efficiency on overall system performance.
The document outlines specific requirements and constraints for the system, including temperature ranges for the liquid-vapor mixture, maximum allowable temperatures at the LFI interface, and goals for temperature fluctuations. It also provides data from EBB (Engineering Breadboard) tests, which indicate the performance of the cooler under different operational conditions.
Overall, the document presents a comprehensive overview of the design considerations and modifications proposed to enhance the stability and efficiency of the Planck cryocooler, ensuring it meets the stringent requirements for space applications. The findings aim to contribute to the development of more reliable cooling systems for future missions.
