Publisher Summary This chapter describes the method for liquid nitrogen precooling of a plate fin heat exchanger in a helium refrigeration cycle. The principal tasks are to permit a rapid cool down to 80 K of the plant, to increase the cooling power of the plant in low temperature operation and to increase the rate of pure liquid production. The advent of aluminum plate fin heat exchangers in the design of helium refrigerators has made this task more complicated because of the potential damage to these heat exchangers. The solution is unique as it uses helium in all parts of the heat exchanger, thus avoiding the problems associated with nitrogen. Helium is circulated in a semi-closed loop through a liquid nitrogen bath and is then sent via a short transfer line to the liquid nitrogen inlet on the main plant. The inlet to the system is tied to the helium return line from the main plant, providing a constant source of low pressure helium gas to the precooler system. This also prevents the intrusion of air into the system because the low-pressure side is always maintained above atmospheric pressure by the main system. It provides the needed cooling power for SLAC E-158 and has the ability to be expanded to give more cooling power should the need arise in the future.