Lubrication of screw machines

Abstract

A screw machine for use with a working fluid with a liquid phase present comprises rotors having meshed, lubricated helical formations. The rotors have an ‘N’ profile as disclosed in WO 97/43550. In use, lubrication of the helical formations of the rotors and optionally of the rotor bearings is achieved substantially exclusively with the liquid phase of the working fluid.

1 . A screw expander for use with wet steam as a working fluid, the expander comprising two or more rotors having meshed, lubricated helical formations, wherein the rotors have an ‘N’ profile as defined herein and, in use, water entrained in the wet steam working fluid is substantially exclusively responsible for lubricating the helical formations of the rotors. 
     
     
         2 . The expander of  claim 1 , wherein the helical formations are coated with a low-friction coating. 
     
     
         3 . The expander of  claim 2 , wherein the coating comprises a carbide phase and a carbon phase. 
     
     
         4 . The expander of any preceding claim, wherein the rotors are supported by at least one bearing that is also substantially exclusively lubricated, in use, by water. 
     
     
         5 . The expander of  claim 4 , wherein the bearing is a hydrodynamic bearing. 
     
     
         6 . The expander of  claim 4 , wherein the bearing is a rolling-element bearing. 
     
     
         7 . The expander of any of  claims 4  to  6 , wherein the bearing is lubricated by water derived from the wet steam working fluid. 
     
     
         8 . The expander of any preceding claim, wherein the rotors are synchronised by synchronising torque transmitted from one rotor to another via their meshed helical formations. 
     
     
         9 . The expander of  claim 8  and having no timing gears acting between the rotors. 
     
     
         10 . The expander of any preceding claim, further comprising a thrust piston acting against axial loads on a rotor. 
     
     
         11 . The expander of  claim 10 , wherein the thrust piston is acted on by pressure of the wet steam working fluid. 
     
     
         12 . The expander of any preceding claim, wherein the lubricating water entrained in the wet steam working fluid is derived from the wet steam entering the expander, substantially without prior addition of further water to the wet steam. 
     
     
         13 . A method of lubricating a screw expander when using wet steam as a working fluid, the expander comprising two or more rotors having meshed, lubricated helical formations, the rotors having an ‘N’ profile as defined herein, wherein the method comprises lubricating the helical formations of the rotors substantially exclusively with water entrained in the wet steam working fluid. 
     
     
         14 . The method of  claim 13 , comprising using water to lubricate a bearing that supports the rotors. 
     
     
         15 . The method of  claim 14 , comprising deriving from the wet steam working fluid the water that lubricates the bearing. 
     
     
         16 . The method of any of  claims 13  to  15 , comprising synchronising the rotors by transmitting synchronising torque from one rotor to another via their meshed helical formations. 
     
     
         17 . The method of any of  claims 13  to  16 , wherein the lubricating water entrained in the wet steam working fluid is derived from the wet steam entering the expander, substantially without prior addition of further water to the wet steam. 
     
     
         18 . A power generator comprising the screw expander of any of  claims 1  to  12 , or having a screw expander operating in accordance with the method of any of  claims 12  to  17 .