Abstract:
A control system and method are provided for the controlling of steam supplies used by a steam turbine driven chiller unit. The steam turbine can receive steam from a high pressure steam source and/or a low pressure steam source depending on the operating mode of the steam turbine. The high pressure steam is used for operating at the steam turbine at rated speed and to provide the breakaway torque when starting the steam turbine. The low pressure steam is used for extending idling of the steam turbine that enables the steam turbine to transition more quickly to rated speed when desired.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to a control system for a chiller unit, and more specifically, to a control system for a steam turbine powered chiller unit that can control inlet steam supply valves for a steam turbine receiving steam from two different steam supplies. 
         [0002]    While most heating, ventilation and air conditioning (HVAC), refrigeration, or chiller systems use electric motors to power the corresponding compressor(s) in the chiller system, some chiller systems have used a steam turbine to power the compressor. These previous steam turbine powered chiller systems were supplied with only a high pressure steam supply required for normal full load operation and had a PLC based panel for use with the steam turbine drive. The panel logic controlled only the remote speed set point of the electronic governor supplied by the turbine manufacture. This stand-alone speed control prevented the customer from safely taking advantage of an available low pressure steam supply during an extended idle period for the chiller system because it was not possible to add to the PLC the adaptive tuning required to handle changes in the motive force when switching between the high and low pressure steam supplies. 
         [0003]    Furthermore, the use of high pressure steam for extended idling would require sufficient cooling water flow through the steam condenser to be maintained to prevent the steam condenser from overheating. Thus, instead of attempting to maintain the sufficient cooling water flow, the chiller system was completely stopped to prevent overheating and to permit the turbine casing to cool down before the next restart. The stopping of the chiller system then resulted in the operator having to perform an extensive manual start up procedure and slow roll warm up before the turbine could be operated at rated speed again. 
         [0004]    Therefore, what is needed is automated inlet steam supply valves for a steam turbine powered chiller unit and a corresponding control system that can control the providing of both low pressure steam and high pressure steam to the steam turbine with the inlet steam supply valves. 
       SUMMARY OF THE INVENTION 
       [0005]    One embodiment of the present invention is directed to a method of starting a steam turbine driven chiller system having a high pressure steam supply and a low pressure steam supply. The method includes the steps of executing a starting sequence for the steam turbine, initiating a slow roll of the steam turbine using the high pressure steam supply, transitioning from the high pressure steam supply to the low pressure steam supply, and slow rolling the steam turbine at a predetermined slow roll speed using the low pressure steam supply. 
         [0006]    Another embodiment of the present invention is directed to a method of initiating an idling mode in a steam turbine driven chiller system having a high pressure steam supply and a low pressure steam supply. The method includes the steps of executing a transition sequence for the steam turbine, initiating an unload cycle for the chiller system, transitioning from the high pressure steam supply to the low pressure steam supply, and slow rolling the steam turbine at a predetermined idling speed using the low pressure steam supply. The steam turbine operates at a rated speed using the high pressure steam supply prior to the transition sequence. The predetermined idling speed is less than the rated speed. 
         [0007]    Still another embodiment of the present invention is directed to a chiller system having a steam system including a high pressure steam supply, a low pressure steam supply, a steam turbine and a steam condenser connected in a steam loop. The chiller system also has a refrigerant system including a compressor, a refrigerant condenser, and an evaporator connected in a refrigerant loop. The compressor is driven by the steam turbine. The chiller system further has a control panel to control operation of both the steam system and the refrigerant system. The control panel includes a control system to operate the steam system in an idling mode using the low pressure steam supply. The idling mode operation results in the steam turbine operating at a predetermined slow roll speed and no substantial output capacity from the refrigerant system. 
         [0008]    One advantage of the present invention is that the starting mode and the return to a standby idling mode of the steam turbine can be controlled remotely by a control system. 
         [0009]    Another advantage of the present invention is that a reduced cooling water flow is required for the steam condenser during an extended idling period of the steam turbine. 
         [0010]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a side view of a chiller unit of the present invention. 
           [0012]      FIG. 2  is a top view of the chiller unit of  FIG. 1 . 
           [0013]      FIG. 3  is a schematic representation of the chiller unit of  FIG. 1 . 
           [0014]      FIG. 4  is a schematic representation of the control system of the chiller unit of  FIG. 1 . 
           [0015]      FIG. 5  is a flowchart of an embodiment of a start-up process for the present invention. 
           [0016]      FIGS. 6 and 7  are a flowchart of an embodiment of a ramp-up to rated speed process for the present invention. 
           [0017]      FIGS. 8 and 9  are a flowchart of an embodiment of a return to idling speed process for the present invention. 
       
    
    
       [0018]    Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    A general system to which the invention is applied is illustrated, by means of example, in  FIGS. 1-3 . As shown, the HVAC, refrigeration, or chiller system  10  includes a compressor  12 , a steam turbine  14 , a refrigerant condenser  16 , a water chiller or evaporator  18 , a steam condenser  20 , an expansion device  22  and a control panel or controller  90 . The operation of the control panel  90  will be discussed in greater detail below. The chiller system  10  further includes a compressor lubrication system (not shown) and a turbine lubrication system (not shown). The conventional liquid chiller system  10  includes many other features that are not shown in  FIGS. 1-3 . These features have been purposely omitted to simplify the drawing for ease of illustration. 
         [0020]    In one embodiment, a “structural frame” permits the stacking or vertical arrangement of major components of the chiller system  10  to provide a prepackaged unit that occupies less floor space with a smaller footprint than a field fabricated unit where the components are arranged horizontally. The structural frame can include a turbine baseplate  26 , a steam condenser baseplate  27 , a plurality of frame members  28 , and tube end sheets  29 . Tube end sheets  29  can provide both the internal support and refrigerant/water separation for the ends of heat exchange tubes (not shown) within refrigerant condenser  16  and evaporator  18 . Frame members  28  are preselected structural components and materials, such as plate steel and tubular supports, that can support the corresponding components of the chiller system  10 . The mounting between compressor  12  and turbine baseplate  26  is preferably a conventional D-flange coupling device that rigidly interconnects the housing of the compressor  12  with the turbine baseplate  26 . In addition, the D-flange coupling device can afford a predictable degree of shaft alignment for the compressor  12  and the steam turbine  14 . 
         [0021]    The structural frame can incorporate a steam turbine  14  in combination with a refrigerant condenser  16 , evaporator  18  and compressor  12  into a pre-packaged unit for installation. The steam condenser  20  and steam condenser baseplate  27  can then be manufactured as a separate unit from the pre-packaged unit and include all necessary interconnections for connection to the pre-packaged unit. The steam condenser  20  and steam condenser baseplate  27  can be field installed above the refrigerant condenser  16  during installation of chiller system  10 . Finally, in another embodiment of the present invention, the main components of the chiller system  10  can be field installed into any suitable or desirable positions. 
         [0022]    In the chiller system  10 , the compressor  12  compresses a refrigerant vapor and delivers it to the refrigerant condenser  16 . The compressor  12  is preferably a centrifugal compressor, however any other suitable type of compressor can be used. The compressor  12  is driven by the steam turbine  14 , which can drive the compressor  12  at either a single speed or at variable speeds. Preferably, the steam turbine  14  is a multistage, variable speed turbine that is capable of operating the compressor  12  at a speed that more closely optimizes the efficiency of the chiller system  10 . More preferably, the steam turbine  14  is capable of driving the compressor  12  at speeds in a range of about 3200 rpm to about 4500 rpm. The steam turbine  14  is preferably supplied with dry saturated steam from one or both of a high pressure steam source  301  and a low pressure steam source  302 . The high pressure steam source  301  can provide steam within a range of about 90 to about 200 psi and the low pressure steam source  302  can provide steam within a range of about 10 to about 20 psi. 
         [0023]    A high pressure inlet steam supply valve  68  can control the flow of steam from the high pressure steam source  301 . Similarly, a low pressure inlet steam supply valve  69  can control the flow of steam from the low pressure steam source  302 . The flow of steam from the high pressure steam source  301  and/or the low pressure steam source  302  to steam turbine  14  can be further modulated by a governor  48  to vary the speed of the steam turbine  14 , and therefore vary the speed of compressor  12  to adjust the capacity of the compressor  12  by providing a greater (or larger) or lesser (or smaller) amount of refrigerant volumetric flow through the compressor  12 . In another embodiment, the steam turbine  14  can drive the compressor  12  at only a single speed and other techniques are needed to adjust the capacity of the compressor  12 , e.g., the use of pre-rotation vanes  80  and/or a hot gas bypass valve  84  (which devices can also be used with a variable speed compressor). 
         [0024]    The refrigerant vapor delivered by the compressor  12  to the refrigerant condenser  16  enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. In a preferred embodiment, the refrigerant vapor delivered to the refrigerant condenser  16  enters into a heat exchange relationship with a fluid, preferably water, flowing through a heat-exchanger coil connected to a cooling tower. The refrigerant vapor in the refrigerant condenser  16  undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid in the heat-exchanger coil. The condensed liquid refrigerant from refrigerant condenser  16  flows through an expansion device  22  to the evaporator  18 . 
         [0025]    The evaporator  18  can include a heat-exchanger coil having a supply line  38  and a return line  40  connected to a cooling load. A secondary liquid, e.g., water, ethylene or propylene glycol mixture, calcium chloride brine or sodium chloride brine, travels into the evaporator  18  via the return line  40  and exits the evaporator  18  via the supply line  38 . The liquid refrigerant in the evaporator  18  enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator  18  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator  18  exits the evaporator  18  and returns to the compressor  12  by a suction line to complete the cycle. It is to be understood that any suitable configuration of refrigerant condenser  16  and evaporator  18  can be used in the chiller system  10 , provided that the appropriate phase change of the refrigerant in the refrigerant condenser  16  and evaporator  18  is obtained. 
         [0026]    At the input or inlet to the compressor  12  from the evaporator  18 , there are one or more pre-rotation vanes (PRV) or inlet guide vanes  80  that control the flow of refrigerant to the compressor  12 , and thereby control the capacity of the compressor  12 . Pre-rotation vanes  80  are positionable to any position between a substantially open position, wherein refrigerant flow is essentially unimpeded into the compressor  12 , and a substantially closed position, wherein refrigerant flow into the compressor  12  is restricted. It is to be understood that in the closed position, pre-rotation vanes  80  may not completely stop the flow of refrigerant into the compressor  12 . An actuator is used to open the pre-rotation vanes  80  to increase the amount of refrigerant to the compressor  12  and thereby increase the cooling capacity of the system  10 . Similarly, the actuator is used to close the pre-rotation vanes  80  to decrease the amount of refrigerant to the compressor  12  and thereby decrease the cooling capacity of the system  10 . The actuator for the pre-rotation vanes  80  can open and close the pre-rotation vanes  80  in either a continuous manner or in a stepped or incremental manner. 
         [0027]    The chiller system  10  can also include a hot gas bypass connection and corresponding valve  84  that connects the high pressure side and the low pressure side of the chiller system  10 . In the embodiment illustrated in  FIG. 3 , the hot gas bypass connection and the hot gas bypass valve  84  connect the refrigerant condenser  16  and the evaporator  18  and bypass the expansion device  22 . In another embodiment, the hot gas bypass connection and hot gas bypass valve  84  can connect the compressor suction line and the compressor discharge line. The hot gas bypass valve  84  is preferably used as a recirculation line for compressor  12  to recirculate refrigerant gas from the discharge of compressor  12 , via refrigerant condenser  16 , to the suction of compressor  12 , via the evaporator  18 . The hot gas bypass valve  84  can be adjusted to any position between a substantially open position, wherein refrigerant flow is essentially unimpeded, and a substantially closed position, wherein refrigerant flow is restricted. The hot gas bypass valve  84  can be opened and closed in either a continuous manner or in a stepped or incremental manner. The opening of the hot gas bypass valve  84  can increase the amount of refrigerant gas supplied to the compressor suction to prevent surge conditions from occurring in compressor  12 . 
         [0028]    With regard to the steam turbine system, the high pressure steam source  301  and the low pressure steam source  302  provide steam to the steam turbine  14 . The steam from the high pressure steam source  301  and the low pressure steam source  302  preferably enters a corresponding moisture separator (not shown) for each steam source. In the moisture separator, moisture-laden steam from the steam source enters and is deflected in a centrifugally downward motion. The entrained moisture in the steam is separated out by a reduction in the velocity of the steam flow. Separated moisture then falls through a moisture outlet and dry saturated steam flows upward and exits through a steam outlet where it flows toward a corresponding inlet steam supply valve. 
         [0029]    The controller  90  automatically positions the high pressure inlet steam supply valve  68  and the low pressure inlet steam supply valve  69  to control the amount of steam that flows toward a governor  48  during the operation of the steam turbine  14 . The governor  48  is located in the steam supply line to regulate steam flow and is preferably located adjacent a steam inlet of steam turbine  14 . The governor or governor valve  48  can be opened or closed in a continuous manner or in a stepped or incremental manner. Steam turbine  14  includes a steam inlet to receive the steam from the high pressure steam source  301  and/or the low pressure steam source  302 . The steam from the high pressure steam source  301  and/or the low pressure steam source  302  flows through the steam inlet and turns a rotatable turbine portion of the steam turbine  14  to extract the energy therefrom to turn a coupler  66  that interconnects the shafts (not shown) of the steam turbine  14  and compressor  12 . After rotating the turbine portion of the steam turbine  14 , the steam then exits the steam turbine  14  through a steam exhaust. 
         [0030]    In a preferred embodiment, the coupler  66  provides for a direct rotational connection between the steam turbine  14  and the compressor  12 . In alternate embodiments, the coupler  66  can include one or more gearing arrangements (or other similar arrangements) to increase or decrease the relative rotational speeds between the steam turbine  14  and the compressor  12 . In addition, one or both of the steam turbine  14  and compressor  12  can also include an internal gearing arrangement connected to the coupler  66  to adjust the relative rotational speeds of the steam turbine  14  or compressor  12 . 
         [0031]    In addition, a turbine steam ring drain solenoid valve  63  is provided to automatically remove any condensate from the steam turbine  14  during the slow roll warm up of the steam turbine  14 . A gland seal steam supply solenoid valve  67  is provided to automatically admit steam to the gland seal supply pressure regulating valve during a slow roll. A steam condenser vacuum pump  65  evacuates the steam condenser and turbine exhaust to a desired vacuum that is required for the steam turbine  14  to produce the power required by the compressor  12 . 
         [0032]    The exhausted steam from the steam turbine  14  flows to the steam condenser  20 . Within the steam condenser  20 , the steam/condensate flow from the steam turbine  14  enters into a heat exchange relationship with cooling water flowing through the steam condenser  20  to cool the steam. Steam condenser  20  includes a hotwell  44  connected to a condensate recirculation system  46 . Condensate recirculation system  46  includes a condensate outlet in the hotwell  44  that can provide or transfer condensate from the hotwell  44  to a condensate pump  62 . From the condensate pump  62 , the condensate is selectively provided to a condensate recirculation inlet of the steam condenser  20  and/or to a condensate return inlet of the high pressure steam source  301  and/or the low pressure steam source  302 . In this manner, the condensate recirculation system  46  can maintain a preselected flow of condensate through the steam condenser  20  and return condensate to the high pressure steam source  301  and/or the low pressure steam source  302  for further generation of steam. 
         [0033]    As discussed above, cooling water from a cooling tower or other source, is preferably routed to the refrigerant condenser  16  by a cooling water supply line  70 . The cooling water is circulated in the refrigerant condenser  16  to absorb heat from the refrigerant gas. The cooling water then exits the refrigerant condenser  16  and is routed or provided to the steam condenser  20 . The cooling water is circulated in the steam condenser  20  to further absorb heat from the steam exhausted from the steam turbine  14 . The cooling water flowing from the steam condenser  20  is directed to the cooling tower by a cooling water return line  76  to reduce the temperature of the cooling water, which then may be returned to the refrigerant condenser  16  to repeat the cycle. 
         [0034]    Typically, the steam condenser  20  operates at a greater temperature than the refrigerant condenser  16 . By routing the cooling water through the refrigerant condenser  16  and then the steam condenser  20 , in a series or serial arrangement, the low temperature cooling water can absorb heat within the refrigerant condenser  16  then be transferred to the steam condenser  20  to absorb additional heat. In a preferred embodiment, this ability to use the cooling water to cool both the refrigerant condenser  16  and the steam condenser  20  can be accomplished by selecting the appropriate refrigerant condenser  16  and steam condenser  20 . The refrigerant condenser  16  is selected such that the outlet cooling water temperature from the refrigerant condenser  16  is lower than the maximum acceptable inlet cooling water temperature for the steam condenser  20 . This series or serial flowpath for condenser (refrigerant and steam) cooling water within the chiller system  10  can reduce the need for multiple supplies of cooling water, and can reduce the total amount of cooling water required for the chiller system. However, it is to be understood that the steam condenser  20  and the refrigerant condenser  16  can have separate cooling water systems and connections to the cooling tower. 
         [0035]    As illustrated in  FIG. 4 , the control panel  90  includes analog to digital (A/D) and digital to analog (D/A) converters, a microprocessor  96 , a non-volatile memory or other memory device  92 , and an interface board  98  to communicate with various sensors and control devices of chiller system  10 . In addition, the control panel  90  can be connected to or incorporate a user interface  94  that permits an operator to interact with the control panel  90 . The operator can select and enter commands for the control panel  90  through the user interface  94 . In addition, the user interface  94  can display messages and information from the control panel  90  regarding the operational status of the chiller system  10  for the operator. The user interface  94  can be located locally to the control panel  90 , such as being mounted on the chiller system  10  or the control panel  90 , or alternatively, the user interface  94  can be located remotely from the control panel  90 , such as being located in a separate control room apart from the chiller system  10 . 
         [0036]    Microprocessor  96  executes or uses a single or central control algorithm or control system to control the chiller system  10  including the compressor  12 , the steam turbine  14 , the steam condenser  20  and the other components of the chiller system  10 . In one embodiment, the control system can be a computer program or software having a series of instructions executable by the microprocessor  96 . In another embodiment, the control system may be implemented and executed using digital and/or analog hardware by those skilled in the art. In still another embodiment, the control panel  90  may incorporate multiple controllers, each performing a discrete function, with a central controller that determines the outputs of control panel  90 . If hardware is used to execute the control algorithm, the corresponding configuration of the control panel  90  can be changed to incorporate the necessary components and to remove any components that may no longer be required. 
         [0037]    The control panel  90  of the chiller system  10  can receive many different sensor inputs from the components of the chiller system  10 . Some examples of sensor inputs to the control panel  90  are provided below, but it is to be understood that the control panel  90  can receive any desired or suitable sensor input from a component of the chiller system  10 . Some inputs to the control panel  90  relating to the compressor  12  can be from a compressor discharge temperature sensor, a compressor oil temperature sensor, a compressor oil supply pressure sensor and a pre-rotation vane position sensor. Some inputs to the control panel  90  relating to the steam turbine  14  can be from a turbine shaft end bearing temperature sensor, a turbine governor end bearing temperature sensor, a turbine inlet steam temperature sensor, a turbine inlet steam pressure sensor, a turbine first stage steam pressure sensor, a turbine exhaust pressure sensor, a turbine speed sensor, and a turbine trip valve status sensor. 
         [0038]    Some inputs to the control panel  90  relating to the steam condenser  20  can be from a hotwell condensate level sensor, a hotwell high level status sensor, and a hotwell low level status sensor. Some inputs to the control panel  90  relating to the refrigerant condenser  16  can be from an entering refrigerant condenser water temperature sensor, a leaving condenser water temperature sensor, a refrigerant liquid temperature sensor, a refrigerant condenser pressure sensor, a subcooler refrigerant liquid level sensor, and a refrigerant condenser water flow sensor. Some inputs to the control panel  90  relating to the evaporator  18  can be from a leaving chilled liquid temperature sensor, a return chilled liquid temperature sensor, an evaporator refrigerant vapor pressure sensor, a refrigerant liquid temperature sensor, and a chilled water flow sensor. In addition, other inputs to the controller  90  include a HVAC&amp;R demand input from a thermostat or other similar temperature control system. 
         [0039]    Furthermore, the control panel  90  of the chiller system  10  can provide or generate many different control signals for the components of the chiller system  10 . Some examples of control signals from the control panel  90  are provided below, but it is to be understood that the control panel  90  can provide any desired or suitable control signal for a component of the chiller system  10 . Some control signals from the control panel  90  can include a turbine shutdown control signal, a compressor oil heater control signal, a variable speed oil pump control signal, a turbine governor valve control signal, a hotwell level control signal, a hot gas bypass valve control signal, a subcooler refrigerant liquid level control signal, a pre-rotation vane position control signal, and steam inlet valve control signals. In addition, the control panel  90  can send a turbine shutdown signal when either the technician has input a shutdown command into the user interface  94 , or when a deviation is detected from a preselected parameter recorded in the memory device  92 . 
         [0040]    The central control algorithm executed by the microprocessor  96  on the control panel  90  preferably includes a startup control program or algorithm to control the startup of the steam turbine  14  and compressor  12 . The startup control program and the integration of controls in the control panel  90  provides for additional protections for individual components in the event of an off-design operating condition in the steam turbine  14  or the chiller system  10 . The startup control program provides automatic shutdown logic and protective functions to protect the chiller system  10  during operation. These protective functions include a pre-lubrication for the compressor  12  and steam turbine  14  to ensure that adequate lubrication is provided prior to rotating the compressor  12  and steam turbine  14 . These protective systems also include a time sharing for redundant equipment such as hotwell pumps and vacuum pumps, wherein equipment are selectively operated in an alternate fashion to provide greater long term reliability. 
         [0041]    In addition, the central control algorithm can maintain selected parameters of the chiller system  10  within preselected ranges. These parameters include turbine speed, chilled liquid outlet temperature, turbine power output, and anti-surge limits for minimum compressor speed and compressor pre-rotation vane position. The central control program employs continuous feedback from sensors monitoring various operational parameters described herein to continuously monitor and change the speed of turbine  14  and compressor  12  in response to changes in system cooling loads. 
         [0042]    The central control algorithm also includes other algorithms and/or software that provide the control panel  90  with a monitoring function of various operational parameters for the chiller system  10  during both startup and routine operation of the chiller system  10 . Undesirable operational parameters, such as low turbine speed, low turbine oil pressure, or low compressor oil pressure, can be programmed into the control panel  90  with a logic function to shutdown the chiller system  10  in the event that undesired, or beyond system design, parameters are detected. Additionally, the central control algorithm has preselected limits for many of the operational parameters of the chiller system  10  and can prevent a technician from manually operating the chiller system  10  outside of these limits. 
         [0043]    In one embodiment of the present invention, the central control algorithm incorporates a governor control system either as a separate program or as a subprogram of the central control algorithm. The governor control system is used to control the positions of the high pressure inlet steam supply valve  68 , the low pressure inlet steam supply valve  69  and the governor valve  48  during the start-up, slow roll and shut down of the compressor  14 . The governor control system can generate the appropriate control signals for the valves in response to system parameters. 
         [0044]      FIG. 5  illustrates an embodiment of an automatic start-up process for the control program of the present invention. The start-up process brings the chiller system  10  out of a shutdown state and starts the turbine  14  slow rolling or idling. The start-up process begins at step  502  with the execution of an initiation sequence for the chiller system  10 . In step  502 , the initiation sequence can include, among other steps, the resetting of the controller logic to clear any safeties that may have been set in the controller logic and the checking of all systems in the chiller system  10  to ensure readiness for operation. In step  504 , the operator is able to select whether the start-up process is to be completed with low pressure steam or with high pressure steam. In a preferred embodiment, if a selection of either low pressure steam or high pressure steam is not made within a predetermined steam selection time period, e.g., about 1 minute, the start-up process uses low pressure steam. 
         [0045]    Condenser water flow to the chiller system  10  (particularly the steam condenser  20 ) is started in step  506 . The condenser water flow is preferably set to a predetermined start-up condenser water flow rate, e.g., about 3000 gpm (gallons per minute). Once the condenser water flow reaches a predetermined minimum start-up condenser water flow rate, e.g., about 2000 gpm, for a predetermined minimum start-up condenser water flow time period, e.g., about 30 seconds, the oil pumps for the chiller system  10  are started and pre-lube and slow roll warm-up sequences are initiated in step  508 . In addition, in step  508 , the steam condenser hotwell pump and vacuum pump can be started after a predetermined pre-lube time period, e.g., about 30 seconds. 
         [0046]    In step  510 , the governor valve  48  is opened by setting a turbine speed setpoint to a predetermined slow roll speed, e.g., about 500 rpm (revolutions per minute) at a predetermined slow roll ramp rate, e.g., about 50 rpm/sec. Once the governor valve  48  has reached a predetermined slow roll governor valve position, e.g., about 5% open, the high pressure inlet steam supply valve  68  is opened and the turbine  14  can begin to slow roll in step  512 . In step  514 , the speed of the turbine  14  is checked to see if it is greater than a predetermined minimum slow roll speed, e.g., about 200 rpm. If the turbine speed is not greater than the predetermined minimum slow roll speed in step  514 , then the governor valve  48  and the high pressure inlet steam supply valve  68  are continued to be opened. However, if the turbine speed is greater than the predetermined minimum slow roll speed in step  514 , then the turbine  14  is considered to be “slow rolling” and the compressor oil cooling system is started in step  516 . 
         [0047]    In one embodiment of the present invention, the compressor oil cooling system is controlled based on the temperature of the thrust bearing oil in order to prevent over cooling of the compressor oil during the extended slow roll and idling periods. The compressor oil cooling system controls the activation and deactivation of both an oil heater and a cooling water supply that supplies cooling water to the compressor oil cooler. The cooling liquid supply is started when the thrust bearing oil temperature is greater than a predetermined maximum cooling supply temperature, e.g., about 155° F., and stopped when the thrust bearing oil temperature decreases below a predetermined minimum cooling supply temperature, e.g., about 140° F. The oil heater is started if the oil temperature is less than a predetermined minimum oil heater temperature, e.g., about 130° F., and stopped if the thrust bearing oil temperature increases above a predetermined maximum oil heater temperature, e.g., about 150° F. 
         [0048]    Finally, in step  518 , the turbine  14  is ramped up to the predetermined slow roll speed. If the low pressure steam option was selected in step  504 , then the turbine  14  is to be slow rolled with low pressure steam. In this case, the slow roll of the turbine  14  is transitioned to low pressure steam after the turbine  14  has been slow rolling for a predetermined minimum slow roll speed time period, e.g., about 4 minutes. To make the transition, the low pressure inlet steam supply valve  69  is opened to a predetermined slow roll LP inlet steam supply valve position, e.g., about 10% open. When the low pressure inlet steam supply valve  69  starts to open, the high pressure inlet steam supply valve  68  is closed. The governor control system then controls the low pressure inlet steam supply valve  69  to maintain the speed at the predetermined slow roll speed. If the high pressure steam option was selected in step  504 , then the turbine  14  is to be slow rolled with high pressure steam. The high pressure steam option is preferably selected when the operator requires the turbine  14  to ramp the chiller up to rated speed as soon as available. 
         [0049]    Once the turbine  14  has reached the predetermined slow roll speed in step  518 , the turbine  14  begins a predetermined slow roll warm up time period, e.g., about 26 minutes, to ensure all condensate is blown out of the inlet piping, the casing is uniformly heated, and the turbine shaft is not bowed due to sitting idle. After the predetermined slow roll warm up time period, if the turbine exhaust pressure is at or below a predetermined slow roll vacuum, e.g., about 24 in. Hg vac., the user interface  94  displays “TURBINE IDLING”. Otherwise, if the turbine exhaust pressure is not below the predetermined slow roll vacuum, the user interface  94  displays “TURBINE IDLING—INSUFF VACUUM”. This warning could indicate a problem with the steam ejectors or an excessive leak requiring investigation by the operator. 
         [0050]    Once the turbine is idling properly after the predetermined warm up time period, if the operator has selected low pressure steam and an idling mode of operation for the turbine  14 , the turbine  14  continues to slow roll with low pressure steam. The governor control system continues to control the low pressure inlet steam supply valve  69  to maintain the turbine speed at the predetermined slow roll speed. The turbine  14  is then ready to ramp up to rated speed as described in  FIGS. 6 and 7  in response to the operator&#39;s command. However, if the operator has selected high pressure steam and a rated speed mode of operation for the turbine  14 , the turbine  14  can proceed directly to ramping up to rated speed as described in  FIGS. 6 and 7 . In one embodiment of the present invention, the governor control system can use a new set of tuning parameters when the vacuum level is below a preselected level, which depends on the steam supply pressure, to prevent instability. 
         [0051]      FIGS. 6 and 7  illustrate an embodiment of the ramp-up to rated speed process for the control program of the present invention. The ramp-up to rated speed process transitions the turbine  14  from a slow rolling or idling speed to an operational speed sufficient to drive the compressor  12  of the chiller system  10 . The process begins at step  602  to determine if the turbine  14  is slow rolling using low pressure steam. If the turbine  14  is slow rolling using low pressure steam, the control proceeds to step  604 . Otherwise, the turbine is slow rolling with high pressure steam and the control proceeds to step  608 . In step  604 , the turbine  14  is transitioned from low pressure steam to high pressure steam. To make the transition, the high pressure inlet steam supply valve  68  is opened to a predetermined slow roll HP inlet steam supply valve position, e.g., about 6% open. When the high pressure inlet steam supply valve  68  starts to open, the low pressure inlet steam supply valve  69  is closed. The governor control system then controls the high pressure inlet steam supply valve  68  to maintain the turbine speed at the predetermined slow roll speed. 
         [0052]    In step  606 , the turbine  14  is slow rolled or idled using high pressure steam for a predetermined HP warm up time period, e.g., about 15 minutes. The high pressure steam slow roll is required to ensure that all turbine components are uniformly heated to the higher temperature before ramping to rated speed. Once the predetermined HP warm up time period expires, the turbine  14  is ready to begin the process of ramping up to rated speed and the control proceed to step  608 . At step  608 , the condenser water flow is then increased to a predetermined ramp-up condenser water flow rate, e.g., about 9400 gpm (gallons per minute). Once the condenser water flow reaches the predetermined ramp-up condenser water flow rate, the evaporator water flow is then set to a predetermined ramp-up evaporator water flow rate, e.g., about 3750 gpm (gallons per minute). Once the condenser and evaporator water flow rates are stable, the control proceeds to step  610 . 
         [0053]    In step  610 , the turbine speed setpoint is set to a predetermined minimum turbine speed, e.g., about 2000 rpm, at a predetermined minimum turbine speed ramp rate, e.g., about 50 rpm/sec. As a result of adjusting the turbine speed setpoint, the governor valve  48  and the high pressure inlet steam supply valve  68  are both further opened by the governor control system. In step  612 , the turbine speed is checked to determine if it is greater than a predetermined ramp up turbine speed, e.g., about 1000 rpm. If the turbine speed is less than the predetermined ramp up turbine speed, the turbine  14  is continued to be accelerated in accordance with step  610 . However, if the turbine speed is greater than the predetermined ramp up turbine speed, then the pre-rotation vanes  80  are opened to a predetermined PRV ramp up position, e.g., about 18% open, in step  614 . 
         [0054]    In step  616 , the turbine speed is checked to determine if it is greater than the predetermined minimum turbine speed. If the turbine speed is less than the predetermined minimum turbine speed, the turbine  14  is continued to be accelerated in accordance with step  610 . However, if the turbine speed is greater than the predetermined minimum turbine speed, then the turbine speed setpoint is set to a predetermined critical speed range turbine speed, e.g., about 2500 rpm, at a predetermined critical speed range turbine speed ramp rate, e.g., about 100 rpm/sec, in step  618 . In step  620 , the turbine speed is checked to determine if it is greater than the predetermined critical speed range turbine speed. If the turbine speed is less than the predetermined critical speed range turbine speed, the turbine  14  is continued to be accelerated in accordance with step  618 . However, if the turbine speed is greater than the predetermined critical speed range turbine speed, then the turbine speed setpoint is set to a predetermined rated turbine speed, e.g., about 3000 rpm, at a predetermined rated turbine speed ramp rate, e.g., about 50 rpm/sec, and the turbine steam ring drain valve  63  is closed in step  622 . 
         [0055]    In step  624 , the turbine speed is checked to determine if it is greater than a predetermined operational turbine speed, e.g., about 2700 rpm. If the turbine speed is less than the predetermined operational turbine speed, the turbine  14  is continued to be accelerated in accordance with step  622 . However, if the turbine speed is greater than the predetermined operational turbine speed, then the elapsed time the turbine has been operating a speed greater than the predetermined operational turbine speed is compared to a predetermined operational turbine speed first time period, e.g., 15 seconds, in step  626 . If the elapsed time is less than the predetermined operational turbine speed first time period, the turbine  14  is continued to be accelerated in accordance with step  622 . However, if the elapsed time is greater than the predetermined operational turbine speed first time period, then the turbine  14  is considered to have reached its minimum rated speed and the user interface  94  displays “System Running” in step  628 . 
         [0056]    In step  630 , the elapsed time the turbine has been operating at a speed greater than the predetermined operational turbine speed is compared to a predetermined operational turbine speed second time period, e.g., 25 seconds. If the elapsed time is less than the predetermined operational turbine speed second time period, the turbine  14  is continued to be operated in accordance with step  622 . However, if the elapsed time is greater than the predetermined operational turbine speed second time period, then the capacity control logic is started in step  632 . When the capacity control logic is started in step  632 , the hot gas bypass valve  84  begins to close and the compressor pre-rotation vanes  80  begin to open. The high pressure inlet steam supply valve  68  is ramped slowly to a fully open position, i.e., 100%, at a predetermined HP inlet steam supply valve opening rate, e.g., 1%/second. If the turbine  14  attempts to speed up with the increased steam flow, the capacity control system closes the governor valve  48  and maintains the turbine speed at the set point dictated by the capacity/anti-surge controls. 
         [0057]      FIGS. 8 and 9  illustrate an embodiment of the return to idling speed process for the control program of the present invention. The return to idling speed process transitions the turbine  14  from an operational or rated speed sufficient to drive the compressor  12  of the chiller system  10  to a slow rolling or idling speed. The process begins at step  802  with the initiation of a predetermined controlled stop time period, e.g., 30 minutes. Next, during the predetermined controlled stop time period, the high pressure inlet steam supply valve  68  is closed at a predetermined HP inlet steam supply valve closing rate, e.g., −2%/second, at step  804 . The high pressure inlet steam supply valve  68  is continued to be closed during the predetermined controlled stop time period until the position of the high pressure inlet steam supply valve  68  permits it to be controlled by the governor control system, i.e., the position of the high pressure inlet steam supply valve  68  is more closed than or at the same position as the determined position by the governor control system for the high pressure inlet steam supply valve  68 . Once the high pressure inlet steam supply valve  68  is under the control of the governor control system, a normal unloading cycle is initiated at step  808 . In the normal unloading cycle, the leaving chilled water temperature setpoint is slowly increased at a preselected rate, e.g., 0.1° F./5 seconds. The turbine speed decreases to a calculated minimum anti-surge RPM, then the compressor pre-rotation vanes  80  are closed to the calculated minimum anti-surge % opening, and finally the hot gas bypass valve  84  is opened. 
         [0058]    The chiller system  10  continues to slowly unload until the hot gas bypass valve  84  is more than a predetermined controlled stop hot gas valve position, e.g., 20% open, or the predetermined controlled stop time period has expired. Once the hot gas bypass valve  84  is more open than the predetermined controlled stop hot gas valve position or the predetermined controlled stop time period has expired, the high pressure inlet steam supply valve  68  is closed in step  812 . In addition, the exhaust of the turbine  14  is opened to atmospheric pressure to slow the turbine  14  down through the critical speed range as rapidly as possible. In step  814 , the turbine speed is checked to determine if it is less than a predetermined controlled stop first turbine speed, e.g., about 2400 rpm. If the turbine speed is greater than the predetermined controlled stop first turbine speed, the turbine  14  is continued to be decelerated in accordance with step  808 . However, if the turbine speed is less than the predetermined controlled stop first turbine speed, then the leaving chilled water temperature setpoint is set to track the leaving chilled water temperature and the compressor pre-rotation vanes  80  are set to a predetermined PRV controlled stop position, e.g., about 18% open, in step  816 . 
         [0059]    Next, in step  818 , the high pressure inlet steam supply valve  68  is checked to see if it is fully closed and the turbine speed is checked to determine if it is less than a predetermined controlled stop second turbine speed, e.g., about 1800 rpm. If both conditions are satisfied in step  818 , the control proceeds to step  820 . Otherwise, the turbine speed is decelerated in accordance with step  816 . In step  820 , the governor control system begins to control the speed of the turbine  14  with the low pressure inlet steam supply valve  69 . In addition, the turbine speed setpoint is set to the predetermined slow roll speed, e.g., about 500 rpm, at a predetermined controlled stop ramp rate, e.g., about −50 rpm/sec. 
         [0060]    Once the turbine speed is less than the predetermined controlled stop second turbine speed, the hot gas bypass valve  84  is fully opened in step  824 . In addition, the vacuum pump of the turbine  14  is started to re-establish a vacuum in the turbine  14 . In step  826 , the turbine speed is checked to determine if it is less than a predetermined ramp down turbine speed, e.g., about 1000 rpm. If the turbine speed is greater than the predetermined ramp down turbine speed, the turbine  14  is continued to be decelerated in accordance with step  820 . However, if the turbine speed is less than the predetermined ramp down turbine speed, then slow roll or idling mode operation is initiated in step  828 . The initiation of the slow roll mode of operation includes the shut down of the evaporator water flow and the setting of the condenser water flow to the predetermined start-up condenser water flow rate. Furthermore, the pre-rotation vanes  84  are fully closed and the user interface displays the message “Turbine Idling”. The turbine  14  is then idled at the predetermined slow roll speed by controlling the low pressure inlet steam supply valve  69  until the operator decides to either shut down the chiller system  10  or ramp up the turbine speed to an operational speed as described above with respect to  FIGS. 6 and 7 . 
         [0061]    In one embodiment of the present invention, if a complete shutdown of the chiller system is required, e.g., in an emergency situation, the above process for returning to idling speed is followed except that the turbine speed is not maintained at the predetermined slow roll speed, but is permitted to coast down to zero. Once the turbine speed reaches a predetermined shut down turbine speed, e.g., 200 rpm, the turbine steam ring drain valve  63  is opened, the condenser water flow is stopped and the hotwell pump(s) are stopped. Finally, the compressor and turbine auxiliary oil pumps are operated for predetermined time periods after the stop of the turbine  14  to prevent damage to the turbine  14  and compressor  12 . 
         [0062]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.