Compressor Unit and Use of a Cooling Medium

Abstract

A compressor unit comprises a centrifugal compressor for compressing a gas, having a rotor with one or more compressor impellers, and an electric motor having a stator and a rotor, for driving the rotor of the compressor. The compressor and the electric motor are accommodated in a common gas-tight housing which is provided with a gas inlet and a gas outlet. The rotor of the compressor and the rotor of the electric motor are arranged on a common rotor shaft which is mounted in magnetic bearings. The rotor shaft comprises a single unit and is mounted in two radial magnetic bearings, each in the vicinity of one end of the common rotor shaft, and one axial magnetic bearing, which is arranged in the vicinity of one of the radial bearings.

1 .- 15 . (canceled) 
   
   
       16 . A compressor unit for underwater compression of a pumping medium, comprising:
 a compressor and an electric motor having a stator and a rotor; and   a separate cooling arrangement connected to and cooling the motor stator wherein the cooling arrangement has a cooling medium comprising pentaerythritol tetracarboxylic acid ester.   
   
   
       17 . The compressor unit as claimed in  claim 16 , wherein the carboxylic acid residue is linear and singly methyl-branched. 
   
   
       18 . The compressor unit as claimed in  claim 17 , wherein that the carboxylic acid residue has an average C-number≧7. 
   
   
       19 . The compressor unit as claimed in  claim 18 , wherein the cooling medium circulates in a closed circuit within the cooling arrangement. 
   
   
       20 . The compressor unit as claimed in  claim 16 , wherein the cooling arrangement has a heat exchanger connected to the cooled stator via a feed line and a return line, with the cooling arrangement designed such that the cooling medium circulates between the heat exchanger, the return line, the stator and the feed line. 
   
   
       21 . The compressor unit as claimed in  claim 20 , wherein a pump is provided in the return line and pumps the cooling medium to produce a forced circulation. 
   
   
       22 . The compressor unit as claimed in  claim 21 , wherein the cooling arrangement is designed for the maximum operating pressure. 
   
   
       23 . The compressor unit as claimed in claim  15 , wherein the compressor, the bearings of the compressor unit and the rotor of the motor are connected to and cooled by a further cooling system. 
   
   
       24 . The compressor unit as claimed in  claim 16 , wherein the cooling system has a cooling medium which is the pumping medium. 
   
   
       25 . The compressor unit as claimed in  claim 24 , wherein the pumping medium is natural gas. 
   
   
       26 . The compressor unit as claimed in  claim 25 , wherein the cooling medium flows around the rotor. 
   
   
       27 . A method of cooling a motor stator of an underwater compressor unit, comprising:
 providing a separate cooling arrangement;   connecting the cooling arrangement to and cooling the motor stator with the cooling arrangement wherein the cooling arrangement has a cooling medium comprising pentaerythritol tetracarboxylic acid ester.   
   
   
       28 . The method as claimed in  claim 27 , wherein a carboxylic acid residue of the pentaerythritol tetracarboxylic acid ester is linear and singly methyl-branched. 
   
   
       29 . The method as claimed in  claim 28 , wherein the carboxylic acid residue has an average C number≧7. 
   
   
       30 . The method as claimed in  claim 29 , wherein the cooling arrangement is operated in a forced-circulation mode.