Abstract: A very interesting phenomenon was observed by G.J. Ranque (1931) and R. Hilsch (1945) when a compressed gas is introduced tangentially into a tube. The tube is fitted with a central aperture orifice at one end (this end of the tube being very close to the tangential entry nozzle) and a throttle valve at the other end (this end being a little far off from the entry nozzle). The shape and size of the nozzle is such that the gas attains maximum velocity of emission as it enters into the tube. In the process of movement of the gas inside the tube towards the throttle end, there develops in the spiraling gas a region of high pressure in the peripheral layers and a region of low pressure in the axis of rotation. Thus a hot stream of gas comes out through the throttle end and a cold stream through the orifice. By manipulating the throttle valve, the amount of gas and also the extent of heating and cooling can be controlled. This very simple device with practically no moving parts is sometimes referred to as Ranque tube, Ranque-Hilsch Vortex Tube (RHVT) or Ranque-Hilsch energy separator.