The performance characteristics of double stage vortex tubes have been investigated and compared with that for a single stage tube operating at the same inlet conditions. The effect of the diameter ratio of the second stage to the first stage tube (D2/D1) together with the operating pressure ratios for each stage were investigated. The analytical study of the process of energy separation taking place in the double stage tube have shown that the performance of the first stage is
always higher than that of the second stage tube. This conclusion was also verified experimentally. On the other hand, the experimental data have shown that the maximum temperature drop produced by the double stage vortex tube improves at higher inlet pressures (Po) and excels that of conventional single stage tubes when that pressure exceeds 3 bar. Best cooling performance for the double stage tube is achieved when the diameter ratio D2/D1 approaches unity. Also, the optimum value for the intermediate pressure (Pc1) for the double stage tube is expressed by: 0.45, Pct = Po. Experimental results have also indicated that; under all conditions of operation, the refrigerating effect produced by the double stage operation is always less that for the single stage operation.