Abstract: A helium charged refrigerator is capable of cooling a refrigerator box to a temperature of 6° C. (43° F.) and the freezer compartment to a temperature of ?9° C. (15° F.) when the ambient is at a temperature of 43° C. (110° F.). The total cubic area cooled is greater than or equal to six cubic feet. The unit includes a condenser, an evaporator, a liquid ammonia tube, and a gas heat exchanger. The liquid ammonia tube includes two vertical sections. The second section is downstream of the first and is noncontiguous with the heat exchanger so that no heat is exchanged between flowable fluids flowing in the second vertical section and in the heat exchanger. The heat exchanger includes modifications to its inner and outer tubes to produce an increase in surface area of the corresponding surface.