Research on the flow properties of 3He- 4He mi.xtures in our group has resulted in a description of the 3He-iHe flow properties taking into account both mutual friction ruld viscosity . In this paper the effects of these dissipative forces on the operation of 3He circulating dilution refrigerators are analyzed numerically. It will be shown that mutual friction leads to rather severe conditions regarding the diameter and the length of the dilute exit tube of the mixing chamber for a specified flow rate. The limiting temperature in the Single-cycle mode is in the low flow-rate regime determined by viscous dissipation. but at higher flow rates the temperature increases with the molar 3He-flow rate as a result of the increasing influence of mutual friction.