Patent Publication Number: US-2005118034-A1

Title: Compressor unit with control system

Description:
This invention relates to a compressor unit for delivering a pressurised gaseous medium at a varying outlet flow volume to a consumer with varying load demands.  
      In particular, the invention concerns a technique to control the output of a compressor unit of the above type including one or more compressor rotors and a control system for providing a continuous high speed and a certain output flow volume of the compressor rotor or rotors. This technique has the objective to keep up a readiness for rapid changes in the load demand from the consumer, without just dumping or throttling away excessive pressure medium to the atmosphere. Load demands may vary widely within a fraction of a second.  
      A characteristic feature of for instance turbo compressors is their relatively slow acceleration and pressure build up if intermittently operated at a reduced speed. This means that in this type of compressors speed regulation is not a very good way to control the output flow volume and to keep up a proper readiness for rapid load demand increases. One way to approach this problem might be to use a pressure medium reservoir of some volume to keep up the pressure medium delivery volume during the acceleration sequence of the compressor rotor or rotors. Another way of solving the readiness problem, if there is no available space for a large enough reservoir, is to operate the compressor rotor or rotors continuously at high speed with a high output flow volume. This technique is well known in prior art systems and means on one hand that the design of the compressor rotor or rotors can be optimised for a particular speed level and provide a very good efficiency.  
      On the other hand the excessive pressure medium has to be dumped or throttled away to the atmosphere at part load demands which means a high power loss and, hence, a poor overall efficiency of the compressor unit.  
      A gas turbine driven turbo compressor unit of this type is previously described in U.S. Pat. No. 4,809,497. In order to have a good readiness to rapidly deliver full output volume, the compressor is continuously operated at full speed, and a reduction in the load demand from a consumer is met by opening up a by-pass line to dump excessive flow volume to the atmosphere. This flow dumping arrangement is provided to prevent surging and a consequent damage risk for the compressor. However, dumping excessive air to the atmosphere via the by-pass line while operating the gas turbine at full power means an undesirable loss of energy and a poor efficiency of the compressor unit.  
      In U.S. Pat. No. 5,117,625 there is previously described a gas turbine driven turbo compressor which is intended to deliver pressure air through a bleed-off line as desired. However, the gas turbine is mechanically connected also to another load in the form of a generator, hydraulic pump etc., and in situations where just a little power is demanded from that load and at the same time a small pressure air volume only is demanded a substantial part of the output volume from the compressor is dumped to the atmosphere as a waste flow through the bleed-off line. This means a considerable energy loss and a poor efficiency of the compressor.  
      In WO 01/27452 there is described a gas turbine engine which is connected to a load and which comprises a turbo compressor which is partly fed with re-circulated exhaust gas from the turbine. At increasing power demand the compressor is fed with an increased amount of fresh air, and the compressor is provided with a number of outlet taps for stepwise bleeding off waste gas to the atmosphere at varying power demands from the load. In order to reduce power losses there is provided an auxiliary turbine which is driven by waste gas from a tap in a high pressure zone of the compressor and which is intended to drive a fresh air compressor for feeding fresh air to the compressor at high power demands. This waste gas recovery arrangement is used at high load demands only, because at low load demand very little fresh air is needed for turbine combustion. This means that at low power demands, most waste gas is dumped to the atmosphere from the compressor without any energy recovery.  
      The main object of the invention is to provide a compressor unit for delivering pressurised gaseous medium with a high degree of readiness for rapid changes in the load demands, wherein the compressor rotor or rotors are continuously operated at high speed and a high outlet flow volume, wherein the waste flow outlet from the compressor rotor or rotors at reduced load demand is used for energy recovery.  
      Another object of the invention is to provide a compressor unit for delivering pressurised gaseous medium with a varying outlet flow volume, while continuously operating the compressor rotor or rotors at high speed thereby delivering a high output flow volume, wherein waste output flow volume from the compressor rotor or rotors at part-load demand is used for driving one or more power recovery turbines mechanically coupled to the drive line of the compressor rotor or rotors for returning mechanical power to the compressor drive line.  
      Further objects and advantages of the invention will appear from the following specification and claims. 
    
    
      Preferred embodiments of the invention are below described in detail with reference to the accompanying drawings.  
      In the drawings  
       FIG. 1  shows a schematic illustration of a compressor unit according to one basic embodiment of the invention.  
       FIG. 2  shows schematically a two-stage compressor unit according to another embodiment of the invention. 
    
    
      The compressor unit illustrated schematically in  FIG. 1  comprises a drive motor M, and a compressor rotor K coupled to the motor M via a drive line N. The drive line N comprises suitably a rotating shaft in one or more coaxial sections. The compressor rotor K is arranged to be fed with air of atmospheric pressure P 1  and deliver pressure air at P 2 . The compressor rotor K is arranged to be driven by the motor M at a constant speed and to deliver a constant flow volume of pressure air. This constant speed level is chosen with respect to the characteristics of the rotor K so as to make the latter operate under its optimum conditions.  
      The compressor unit is intended to be connected to a pressure air consumer (not shown) with a load demand that could be rapidly varied between zero and full output volume of the compressor rotor K. When full capacity output flow volume of the compressor rotor is not demanded by the consumer the excessive part of the flow is diverted to a waste flow circuit via a valve V. This waste flow circuit comprises an power recovery turbine T R  which is mechanically coupled to the drive line N and driven by the waste flow. The actual waste flow volume makes the power recovery turbine T R  deliver power back to the drive line N and to the drive motor M, thereby recovering energy not momentarily needed to keep up with the actual load demand by the consumer. Thereby, the power supply to the motor M could be reduced to a level where the compressor rotor K is just maintained at its predetermined optimum speed level.  
      The waste flow valve V is suitably controlled by the pressure at the output end of the compressor rotor K so as to continuously divert exactly that flow volume to the power recovery turbine which is not demanded by the consumer. A power control unit PCU is connected both to the waste control valve V and to the drive motor M and is intended to continuously adapt the power supply to the motor M to maintain the compressor rotor K at the predetermined optimum speed level.  
      The drive motor M could be of any kind, an electric synchronous motor, a gas turbine etc.  
      The compressor unit illustrated in  FIG. 2  comprises two compressor rotors  10 , 11  arranged in series via an intermediate cooler  12  to form a two-stage compressor. The compressor unit has an air inlet  13  for atmospheric air and a pressure air outlet  14  for delivering pressure air to a consumer (not illustrated) via an air cooler  15 . The consumer may very well be of a type having a varying load demand with rapid changes between a low and a high load demands.  
      The compressor rotors  10 , 11  are connected to two separate drive lines  17 , 18  formed by rotating shafts driven by a gas turbine  19  comprising two rotors  20 , 21  each connected to one of the drive lines  17 , 18 . In some applications the turbine rotors  20 , 21  and the two drive lines  17 , 18  may rotate in opposite directions for obtaining a favourable efficiency of the turbine  19 . To the drive lines  17 , 18  there are mechanically connected two power recovery turbines  22 , 23  which are energised by superfluous waste air from the compressor at part-load demand from the pressure air consumer.  
      The reason for using two separate drive lines may be to provide different optimal speed levels for the two consecutive compressor rotors  10 , 11 .  
      A waste air flow circuit is connected to the pressure air outlet end  14  of the compressor  10 , 11  via a valve  25  and includes branches to both power recovery turbines  22 , 23 . The valve  25  is arranged also to control the air feed to a burner  27  connected to the gas turbine  19 . The air flow diverted to the burner  27  is dependent on the actual load demand. The air flow volume diverted to the power recovering turbines  22 , 23  is the superfluous waste air volume momentarily needed neither by the consumer nor the burner  27 . At a low load demand there will be a large waste flow diverted to the power recovery turbines  22 , 23  and, hence, a substantial power feed back to the drive lines  17 , 18 . Still there will be a certain air flow directed to the burner  27  for energising the turbine  19  and keep up the rotor speed to predetermined level.  
      The burner  27  is supplied with fuel from a fuel supplying unit  28  and with air from the pressure air outlet end  14  of the compressor  10 , 11  via the valve  25 . The air is supplied to the burner  27  via a recuperator  29  which is heated by the exhaust gases from the turbine  19 . The fuel supplying unit  28  also comprises an ignition device for starting up the burner  27 .  
      The drive line  17  connected to the initial stage compressor rotor  20  is also connected to an electric motor  30  which is intended for turbine starting purposes. The motor  30  is energised by an accumulator  31  or via a mains connection.  
      For obtaining a favourable operation of the power recovery turbines  22 , 23  at different flow volumes these turbines are preferably provided with adjustable guide vanes  32 , 33 . Also, the individual waste flow volumes to the power recovery turbines  22 , 23  are controlled by two separate valves  34 , 35  in the waste flow circuit. It may also be suitable to provide the first stage  20  of the gas turbine  19  with adjustable guide vanes (not illustrated) to optimise the turbine operation to different load demands.  
      The guide vanes  32 , 33  and the valves  25 , 34 , 35  are adjusted in response to a number of parameters like the rotor speed, the actual pressure levels P 1 , P 2 , P 3 , P 4 , P 5  and temperatures T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , T 7 , T 8 , T 9 . See  FIG. 2 . Sensors for these parameters are not shown in detail.  
      For accomplishing a proper operation control of the compressor unit there is provided a control system  36 . For clarity reason, the control system  36  is illustrated symbolically without any leads to the different parts of the system. The control system  36  does not in itself form any part of the invention and is not described in further detail.  
      The above described compressor unit could be operated in different ways dependent on the characteristic of the pressure air consumer. Should the consumer be of the kind requiring a very short notice readiness for full flow volume, the compressor should be operated at full speed all the time and recovering the excessive energy via a waste flow through the power recovery turbines  22 , 23 . At low load demands, the waste air flow is large and the energy fed back to the drive lines  15 , 16  and the motor, i.e. the gas turbine, is high and the net spent energy is quite low. Should the load demand increase rapidly to full flow capacity of the compressor, the gas turbine which is rotating at full speed is able to deliver full output flow volume instantaneously.  
      Should the pressure air consumer require full flow volume at a certain delay, it is possible to apply a little lower degree of readiness of the compressor unit. This means that the gas turbine  19  and the compressor rotors  10 , 11  could be operated at a somewhat reduced speed at part-load or no-load demands, for instance at 80-90% of full speed. The delay for obtaining full output flow volume from the compressor would still be very short, but the required power supply to the gas turbine would be further reduced. Still, the waste flow at part-load would be recovered and transformed to mechanical energy by the power recovery turbines  22 , 23 .