System and method for compressing air

Abstract

A compression train system and method for compressing air for an air separation unit is provided. The system includes at least one initial air compressor configured to receive and compress a flow of ambient pressure air and driven by a variable speed motor. The system further includes a plurality of downstream compressors or compression turbo-machines disposed in a series arrangement with the at least one initial compression stage. The desired flow rates to the compression system is controlled by adjusting the speed of the variable speed motor and the position of inlet guide vanes to increase or decrease the flow rate of the ambient pressure air and keep the speed of the variable speed motor above or below a restricted speed zone.

1 . A compression train system for an air separation unit comprising:
 at least one variable speed motor having at least one shaft;   at least one air compressor having at least one initial compression stage and adapted to receive and compress a flow of ambient pressure air and to be driven by the at least one shaft of the variable speed motor; and   a plurality of downstream compressors or compression turbo-machines disposed in a series arrangement with the at least one initial compression stage;   wherein the speed of the variable speed motor is adjusted to increase or decrease the flow rate of the ambient pressure air to the at least one air compressor and the flow rates of the compressed air to the plurality of downstream compressors or compression turbo-machines toward a desired flow rate.   
     
     
         2 . The compression train system of  claim 1 , wherein the speed of the variable speed motor remains situated above or remains situated below a restricted speed zone. 
     
     
         3 . The compression train system of  claim 2 , further comprising inlet guide vanes disposed on the at least one air compressor and wherein the flow rate of the ambient pressure air to the at least one air compressor is further adjusted by adjusting inlet guide vanes to avoid the speed of the variable speed motor being situated within the restricted speed zone. 
     
     
         4 . The compression train system of  claim 1 , wherein the at least one initial compression stage further comprises two compression stages and wherein the two compression stages are driven by the variable speed motor. 
     
     
         5 . The compression train system of  claim 4 , further comprising:
 a first centrifugal compressor is mounted on one end of the shaft of the variable speed motor to form a first of the two stages of compression configured to compress the ambient pressure air to a first pressure; and   a second centrifugal compressor is mounted on the other end of the shaft and in flow communication with the first centrifugal compressor to form a second of the two stages of compression to further compress the air to a second pressure, higher than the first pressure.   
     
     
         6 . The compression train system of  claim 4 , further comprising:
 a first centrifugal compressor is mounted on one end of the shaft of the variable speed motor to form a first of the two stages of compression and configured to compress a first portion of the ambient pressure air to a first pressure; and   a second centrifugal compressor is mounted on the other end of the shaft to form a second of the two stages of compression and configured to compress to also compress a second portion of the ambient pressure air to the first pressure.   
     
     
         7 . The compression train system of  claim 6 , further comprising a common feed through which the first centrifugal compressor and the second centrifugal compressor are fed with the ambient pressure air. 
     
     
         8 . The compression train system of  claim 6 , further comprising a common outlet from which the compressed air is discharged from the first centrifugal compressor and the second centrifugal compressor. 
     
     
         9 . The compression train system of  claim 6 , wherein the speed of the variable speed motor remains situated above or remains situated below a restricted speed zone. 
     
     
         10 . The compression train system of  claim 6 , further comprising inlet guide vanes disposed on the at least one air compressor and wherein the flow rate of the ambient pressure air to the at least one air compressor is further adjusted by adjusting inlet guide vanes to avoid the speed of the variable speed motor being situated within the restricted speed zone. 
     
     
         11 . A method for compressing air, comprising the steps of:
 receiving a flow of ambient pressure air to at least one air compressor having at least one initial compression stage;   driving the at least one air compressor having at least one initial compression stage with a variable speed motor having at least one shaft;   compressing the flow of ambient pressure air in the at least one air compressor thereby producing a flow of compressed air exiting the at least one air compressor at a first pressure;   further compressing the flow of compressed air at the first pressure to a higher pressure in a plurality of downstream compressors or compression turbo-machines disposed in a series arrangement with the at least one initial compression stage;   adjusting the speed of the variable speed motor to increase or decrease the flow rate of the ambient pressure air to the at least one air compressor and the flow rates of the compressed air to the plurality of downstream compressors or compression turbo-machines toward a desired flow rate.   
     
     
         12 . The method of  claim 11 , wherein the step of adjusting the speed of the variable speed motor to increase or decrease the flow rates further comprises adjusting the speed of the variable speed motor while maintaining the speed of the variable speed motor above or below a restricted speed zone. 
     
     
         13 . The method of  claim 12 , wherein the at least one air compressor includes inlet guide vanes and the step of adjusting the speed of the variable speed motor to increase or decrease the flow rates further comprises adjusting the inlet guide vanes to avoid the speed of the variable speed motor being situated within the restricted speed zone. 
     
     
         14 . The method of  claim 11 , wherein the at least one initial compression stage further comprises two compression stages and wherein the two compression stages are driven by the variable speed motor. 
     
     
         15 . The method of  claim 14 , wherein a first centrifugal compressor is mounted on one end of the shaft of the variable speed motor to form a first of the two stages of compression and a second centrifugal compressor is mounted on the other end of the shaft to form a second of the two stages of compression, and wherein the step of compressing the flow of ambient pressure air further comprises the steps of:
 compressing the flow of ambient pressure air in the first centrifugal compressor to the first pressure; and   further compressing the air from the first centrifugal compressor at the first pressure to a second pressure, higher than the first pressure in the second centrifugal compressor.   
     
     
         16 . The method of  claim 14 , wherein a first centrifugal compressor is mounted on one end of the shaft of the variable speed motor to form a first of the two stages of compression and a second centrifugal compressor is mounted on the other end of the shaft to form a second of the two stages of compression, and wherein the step of compressing the flow of ambient pressure air further comprises the steps of:
 compressing a first portion of the flow of ambient pressure air in the first centrifugal compressor to the first pressure; and   compressing a second portion of the flow of ambient pressure air in the second centrifugal compressor to the first pressure.   
     
     
         17 . The method of  claim 16 , wherein the first centrifugal compressor and the second centrifugal compressor are fed with the ambient pressure air from a common feed and the step of receiving a flow of ambient pressure air further comprises receiving a flow of ambient pressure air from the common feed to the first centrifugal compressor and the second centrifugal compressor. 
     
     
         18 . The method of  claim 16 , wherein the first centrifugal compressor and the second centrifugal compressor are discharged at the first pressure to a common outlet and the method further comprises the step of discharging the flow of compressed air exiting the first centrifugal compressor and the second centrifugal compressor to the common outlet. 
     
     
         19 . The method of  claim 16 , wherein the step of adjusting the speed of the variable speed motor to increase or decrease the flow rates further comprises adjusting the speed of the variable speed motor while maintaining the speed of the variable speed motor above or below a restricted speed zone. 
     
     
         20 . The method of  claim 19 , wherein the first centrifugal compressor and the second centrifugal compressor each have inlet guide vanes and the step of adjusting the speed of the variable speed motor to increase or decrease the flow rates further comprises adjusting the inlet guide vanes to avoid the speed of the variable speed motor being situated within the restricted speed zone. 
     
     
         21 . The method of  claim 11 , wherein the at least one air compressor includes inlet guide vanes and wherein, when the desired flow rate is able to be obtained with the speed of the variable speed motor situated above or below the restricted speed zone, the flow rate of the ambient pressure air to the at least one air compressor is controlled by adjusting the speed of the variable speed motor with a speed controller and with the inlet guide vanes set at a 0° swirl angle. 
     
     
         22 . The method of  claim 21 , wherein, when the desired flow rate is not able to be obtained without the speed of the variable speed motor situated within the restricted speed zone, the flow rate of the ambient pressure air to the at least one air compressor is controlled by setting the speed of the variable speed motor with the speed controller at an upper speed, above the restricted speed zone and adjusting the inlet guide vanes to obtain the desired flow rate. 
     
     
         23 . The method of  claim 22 , wherein, when the speed of the variable speed motor is set at the upper speed, above the restricted speed zone and the desired flow rate requires adjusting the speed of the variable speed motor to a speed below the restricted speed zone, the speed of the variable speed motor is decreased in decreasing speed steps until a lower speed, below the restricted speed zone, is obtained and the inlet guide vanes are successively opened during each of said decreasing speed steps such that the inlet guide vanes are set at the 0° swirl angle when the speed of the variable speed motor is at the lower speed, below the restricted speed zone. 
     
     
         24 . The method of  claim 21 , wherein, when the speed of the variable speed motor is set below the restricted speed zone and the desired flow rate requires adjusting the speed of the variable speed motor to a new speed above the restricted speed zone, the speed of the variable speed motor is increased in increasing speed steps until the new speed, above the restricted speed zone, is obtained and the inlet guide vanes are successively closed during each of said increasing speed steps such that the position of the inlet guide vanes required to obtain the desired flow rate will be reached when the new speed, above the restricted speed zone is reached.