The work of the processed air expansion in the air separation units (ASU) of medium productivity can be usefully converted into an additional refrigerating capacity in the expander-compressor unit (ECU) of a special design. Such ECU should be made on the basis of a turbo reducer using which it is possible to ensure the most appropriate number of revolutions of the compressor stages (CS) shafts. At the same time to improve ASU it is advisable to apply in them a two-stage air expansion. It stipulates to use in ASU of a three-shaft ECU in which on separate shafts the expander stages ES1 and ES2 are located, and they are connected mechanically with the shaft of CS. A modal and structural optimization of ASU was made as well as three-shaft ECU which was included in its structure. By varying the intermediate pressure P13 (the pressure of the end of the expansion process in the high-pressure expander stage ES1) it was found that when P13=1,9 or 1,7 MPa the number of revolutions of ES1 and ES2 shafts became equal nES1=nES2 and accepted the values 92580 or 86500 rpm depending on the presence or absence of pre-cooling. The equality of ES1 and ES2 shafts rotation speed allows to use two-shaft ECU instead of a three-shaft one in which the two expander stages are placed on the same shaft as their efficiencies are close. The analysis shows that ECU construction with three shafts though it is slightly more complicated it is easier to be implemented, unlike the two-shaft construction with ES1 and ES2 stages on one common shaft. The three-shaft turbo reducer becomes more simple with the equal rotation speed of ES1 and ES2 shafts.