Masters Theses

Date of Award

12-1990

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

R. J. Krane, J. R. Parsons

Abstract

The recent design of large computers having heat dissipation rates in excess of 1 kW demands research in the area of the possibility of cryogenic cooling systems. Direct immersion liquid cooling of the CPU boards can be implemented in order to maintain low board temperatures and avoid damage to the circuit components, in this paper, the performance and cost of three types of cryogenic cooling systems, the "once-through" system, the saturated pool boiling refrigerated sytem, and the subcooled pool boiling refrigerated system, will be analyzed. In the "once-through" system, the liquid nitrogen, chosen for its liquid temperature range and low cost, is allowed to boil on the CPU boards and is then vented to the surroundings. Such systems are currently in use since many installations requiring the use of large computers have supplies of liquid nitrogen readily available for numerous other applications. The second cooling system which was investigated is the saturated pool boiling system, in this system, the gaseous nitrogen generated from boiling on the CPU boards is condensed by a condenser placed in the vapor space above the liquid nitrogen pool. The required refrigeration is produced by a cryocooler operating on the Stirling cycle. The final cooling system which was analyzed is the subcooled pool boiling system. This system is very similar to the saturated system discussed above with the exception that the condenser is completely submerged in the liquid nitrogen, which is subcooled to a temperature below the saturation temperature of 77 K. Thermal/fluid numerical models used in conjunction with an optimization code, GRG2, were used to predict the performance and cost of the cryogenic cooling systems. The initial cost of the "once-through" system is approximately $41,150. The annual operating cost is approximately $20,540. The initial cost of the saturated pool boiling system is approximately $102,400, while the annual power cost is approximately $16,000. if the liquid nitrogen is subcooled from the saturation temperature of 77 K to 68 K, the uniform annual cost of the cooling system is practically unaffected. However, the critical heat flux for the subcooled system is increased by approximately 40% over the value for the saturated system. This increase in critical heat flux may become significant in preventing thermal failure or degradation of the electronic components if local values of the heat flux exceed approximately 14 W/cm2. However, due to the extremely high initial cost of the refrigerated systems, the "once-through" cryogenic cooling system is recommended if the local heat flux on the CPU board remains below 14 W/cm2.

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