The Thirty Meter Telescope (TMT) must remove waste heat generated by electronics on the telescope to prevent overheating and reduce their impact on dome seeing. Traditionally, chilled water-glycol cooling loops have been used for this purpose. However, the potential (and realization on existing telescopes) for damage and disruption due to glycol leaks at the telescope top end has prompted a shift to a refrigerant system, where any leaking refrigerant vaporizes immediately. Additionally, a refrigerant system is needed to cool adaptive optics enclosures to -30°C and cool the telescope hydrostatic bearing oil to -21°C, below the operating range of a chilled water system. This paper discusses the refrigerant services infrastructure in development for the observatory, including the cooling requirements, the trade study that led to the choice of carbon dioxide as the refrigerant, the conceptual design, and the concept of operations.
Recent changes to the Montreal Protocol have led to the eventual phase-out of hydrofluorocarbon-based refrigerants due to their high global warming potential (GWP). TMT has thus transitioned from R507 (GWP ~ 3900) to CO2 (GWP =1) as the primary observatory supplied refrigerant. Thus, the cooling system for NFIRAOS (TMT’s first light adaptive optics system) was redesigned to work with this high pressure refrigerant. In this paper, we describe the key refrigeration requirements and present the updated design of the NFIRAOS cooling system, including its overall architecture, main components and safety related features. To de-risk aspects of the design and to assess vibration levels of the system (critical for the AO system performance) a prototype CO2 cooling system was built and characterized. Key measurement results are presented.
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