Method of Pumping Gas

Abstract

A method of pumping a gas stream containing a condensable species and a light gas and an apparatus therefore, the method comprising the steps of conveying the gas stream to a multistage vacuum pump comprising an inlet stage and an exhaust stage downstream from the inlet stage, and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than said light gas to both inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump.

1 . A method of pumping a gas stream containing a condensable species and a light gas, the method comprising the steps of: conveying the gas stream to a multistage vacuum pump having an inlet stage and an exhaust stage downstream from the inlet stage; and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas, the purge gas being selected so as to inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump. 
   
   
       2 . The method according to  claim 1  wherein the exhaust stage has a volume at least three times smaller than that of the inlet stage. 
   
   
       3 . (canceled) 
   
   
       4 . The method according to  claim 2  wherein when the light gas has a flow rate of at least 20 slm, the purge gas is added to the gas stream at a rate of at least 10 slm. 
   
   
       5 . A method of pumping a gas stream containing a condensable species and a light gas having a flow rate of at least 20 slm, the method comprising the steps of: conveying the gas stream to a multistage vacuum pump having an inlet stage and an exhaust stage wherein the exhaust stage has a volume at least three times smaller than the inlet stage; and adding to the gas stream upstream of or at the inlet stage a purge gas heavier than said light gas at a rate of at least 10 slm. 
   
   
       6 . The method according to  claim 4  wherein when the light gas has a flow rate of at least 50 slm, the purge gas is added to the gas stream at a rate of at least 20 slm. 
   
   
       7 . The method according to  claim 6  wherein the pump comprises at least three stages. 
   
   
       8 . (canceled) 
   
   
       9 . (canceled) 
   
   
       10 . The method according to  claim 7  further comprising the step of supplying a further purge gas to the pump between stages of the pump. 
   
   
       11 . The method according to  claim 10  wherein when the light gas has a flow rate of at least 50 slm, and the further purge gas is supplied at a flow rate of at least 20 slm. 
   
   
       12 . The method according to  claim 11  wherein the vacuum pump comprises at least one pumping stage which compresses the gas stream. 
   
   
       13 . The method according to  claim 12  wherein said pumping stage is a Northey pumping stages. 
   
   
       14 . The method according to  claim 13  wherein the pump comprises a stator housing a multistage rotor assembly, each stage comprising intermeshing Roots or Northey rotor components. 
   
   
       15 . The method according to  claim 14  wherein the pump comprises a plurality of intermeshing Northey rotor components. 
   
   
       16 . A vacuum pumping arrangement for pumping a gas stream containing a condensable species and a light gas, the pumping arrangement comprising: a multistage vacuum pump having an inlet stage and an exhaust stage downstream from the inlet stage; and a purge gas supply for adding to the gas stream upstream of or at the inlet stage a purge gas wherein the surge gas is heavier than the light gas, the purge gas being selected so as to inhibit migration of the light gas from the exhaust stage towards the inlet stage and to inhibit condensation of the condensable species within the pump. 
   
   
       17 . The vacuum pumping arrangement according to  claim 16  wherein the exhaust stage has a volume at least three times smaller than that of the inlet stage. 
   
   
       18 . (canceled) 
   
   
       19 . The vacuum pumping arrangement according to  claim 17  wherein the purge gas supply is configured to add purge gas to the gas stream at a rate of at least 10 slm when the light gas has a flow rate of at least 20 slm. 
   
   
       20 . A vacuum pumping arrangement for pumping a gas stream containing a condensable species and a light gas having a flow rate of at least 20 slm, the pumping arrangement comprising: a multistage vacuum pump having an inlet stage and an exhaust stage downstream from the inlet stage wherein the exhaust stage comprises a volume at least three times smaller than the volume of the inlet stage; and a purge gas supply for adding to the gas stream upstream of or at the inlet stage a purge gas heavier than the light gas at a rate of at least 10 slm. 
   
   
       21 . The vacuum pumping arrangement according to  claim 20  wherein the purge gas supply is configured to add purge gas to the gas stream at a rate of at least 20 slm when the light gas has a flow rate of at least 50 slm. 
   
   
       22 . The vacuum pumping arrangement according to  claim 21  wherein the pump comprises at least three stages. 
   
   
       23 . (canceled) 
   
   
       24 . The vacuum pumping arrangement according to  claim 22  wherein the purge gas comprises nitrogen. 
   
   
       25 . The vacuum pumping arrangement according to  claim 24  wherein the purge gas supply is configured to supply further purge gas to the pump between stages of the pump. 
   
   
       26 . The vacuum pumping arrangement according to  claim 25  wherein the purge gas supply is configured to supply the further purge gas to the pump at a rate of at least 20 slm when the light gas has a flow rate of at least 50 slm. 
   
   
       27 . The vacuum pumping arrangement according to  claim 26  wherein the vacuum pump comprises at least one pumping stage which compresses the gas stream. 
   
   
       28 . The vacuum pumping arrangement according to  claim 27  wherein said pumping stage is a Northey pumping stages. 
   
   
       29 . A vacuum pumping arrangement according to  claim 28  wherein the pump comprises a stator housing a multistage rotor assembly, each stage comprising intermeshing Roots or Northey rotor components. 
   
   
       30 . The vacuum pumping arrangement according to  claim 29  wherein the pump comprises a plurality of intermeshing Northey rotor components.