The combination of increased power dissipation and increased packaging density has led to substantial increases in chip and module heat flux in high-end computers. The challenge has been to limit the rise in chip temperature. In the past virtually all-commercial computers were designed to operate at temperatures above the ambient. However researchers have identified the advantages of operating electronics at low temperatures. The primary purpose of low temperature cooling using vapor compression system are faster switching times of semiconductor devices, increased circuit speed due to lower electrical resistance of interconnecting materials, and a reduction in thermally induced failures of devices and components. Achievable performance improvements range from 1 to 3% for every 10°C lower transistor temperature, depending on the doping characteristics of the chip. The current research focuses on IBM's mainframe, which uses a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures. Although performance has been live key driver in the use of this technology, the second major reason for designing a system with low temperature cooling is the improvement achieved in reliability to counteract detrimental effects, which rise as technology is pushed to the extremes. A mathematical model is developed to determine the time constant for expansion valve sensor bulb. This time constant varies with variation in thermo-physical properties of sensor element that is bulb size and bulb liquid. An experimental bench is built to study the effect of variation of evaporator outlet superheat on system performance. The heat load is varied from no load to full load (1KW) to find out the system response at various loads. Experimental investigation is also done to see how the changes in thermo-physical properties of the liquid in sensor bulb of expansion valve affect the overall system performance. Different types of thermostatic expansion valves are tested to investigate that bulb size; bulb constant and bulb location have significant effect on the behavior of the system. Thermal resistance between the bulb and evaporator return line can considerably affect the system stability and by increasing the thermal resistance, the stability can be further increased.
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