Abstract:
A control system and method for interactive startup and shutdown of a steam turbine driven chiller unit is provided. The chiller unit includes an integrated central control panel to control operation of both the steam turbine system and the refrigerant system. The central control panel has startup control system to assist an operator manually start the steam turbine system and the refrigerant system and a shutdown control system to assist an operator manually shutdown the steam system and the refrigerant system. Both the startup control system and the shutdown control system include logic for performing necessary protective actions and for notifying an operator when to perform required actions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/539,014, filed Jan. 23, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     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 provide assistance to an operator during a manual startup or shutdown of a steam turbine powered chiller unit.  
         [0003]     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 can use a steam turbine to power the compressor. Typically, these steam turbine powered chiller systems have required an excessive amount of fieldwork to install and connect the chiller system to the steam turbine system. Some previous steam turbine driven chiller units or systems have involved the packaging of the steam turbine on the chiller unit and resulted in unique installations requiring field routed piping and instrumentation to link the steam turbine system with the chiller unit to complete the installation.  
         [0004]     In these previous steam turbine driven chillers systems, many of the controls used with the steam turbine and the chiller unit, e.g., steam turbine governor control, pre-rotation vane control, hot gas control, turbine torque limitation control and surge prevention control, were “stand alone” controls that operated independently of the other controls and did not communicate with the other controls. The use of these separate controls resulted in difficult and complex startup and shutdown procedures, as an operator of the steam turbine driven chiller unit had to monitor all of the separate controls and then initiate the appropriate actions on the appropriate controls at the appropriate times to avoid damaging the steam turbine driven chiller unit or having an unnecessary shutdown of the steam turbine driven chiller unit. In addition, the use of these separate controls results in the requirement that the control operations for the steam turbine system be coordinated with the control operations for the chiller unit for a proper startup or shutdown of the steam turbine chiller unit.  
         [0005]     Therefore, what is needed is a control system for a steam turbine powered chiller unit that can assist an operator with a manual startup or shutdown of a steam turbine powered chiller unit and can automatically prevent undesired operational parameters during the manual startup or shutdown of the steam turbine powered chiller unit.  
       SUMMARY OF THE INVENTION  
       [0006]     One embodiment of the present invention is directed to a method of manually starting a chiller system driven by a steam turbine. The method includes entering, by an operator, a start command into a control panel of the chiller system to initiate a steam turbine start sequence and executing the steam turbine start sequence. The method also includes prompting the operator to enter a slow roll command upon completion of the steam turbine start sequence to initiate a steam turbine slow roll mode operation and executing the steam turbine slow roll mode operation in response to entry of the slow roll command by the operator. Finally, the method includes prompting the operator to enter an acceleration command upon completion of the steam turbine slow roll mode operation to initiate a steam turbine acceleration process and executing the steam turbine acceleration process to accelerate the steam turbine to an operational speed in response to entry of the acceleration command by the operator.  
         [0007]     Another embodiment of the present invention is directed to a method of manually shutting down a chiller system driven by a steam turbine. The method includes selecting, by the operator, a stop option on a control panel of the chiller system. The method also includes automatically decreasing an operating speed of the steam turbine to a predetermined minimum turbine speed, automatically closing pre-rotation vanes in the chiller system to a predetermined minimum vane position, and automatically opening a hot gas bypass valve in the chiller system to a fully open position. Finally, the method includes automatically initiating a shutdown process for the steam turbine upon the completion of the steps of automatically decreasing an operating speed of the steam turbine, automatically closing pre-rotation vanes, and automatically opening a hot gas bypass valve and wherein at least one action is required by the operator to complete the initiated shutdown process for the steam turbine.  
         [0008]     Still another embodiment of the present invention is directed to a chiller system having a steam system and a refrigerant system. The steam system includes a steam supply, a steam turbine and a steam condenser connected in a steam loop. The refrigerant system includes a compressor, a refrigerant condenser, and an evaporator connected in a refrigerant loop and the compressor is driven by the steam turbine. The chiller system also includes a central control panel to control operation of both the steam system and the refrigerant system. The central control panel has a startup control system to assist an operator to manually start the steam system and the refrigerant system and a shutdown control system to assist an operator to manually shutdown the steam system and the refrigerant system.  
         [0009]     One advantage of the present invention is a central control system that utilizes the full range of controls for the steam turbine driven chiller system to assist an operator with a manual startup or shutdown of the system while preventing unsafe operation of the system.  
         [0010]     Another advantage of the present invention is improved hot well and vacuum pump controls that provide increased pump reliability and eliminate the need for operator intervention on a primary pump failure.  
         [0011]     Still another advantage of the present invention is that operator actions to reset the steam turbine governor valve logic and initiate the start sequence for the steam turbine governor valve have been eliminated.  
         [0012]     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  
       [0013]      FIG. 1  is a side view of a chiller unit of the present invention.  
         [0014]      FIG. 2  is a top view of the chiller unit of  FIG. 1 .  
         [0015]      FIG. 3  is a schematic representation of the chiller unit of  FIG. 1 .  
         [0016]      FIG. 4  is a schematic representation of the control system of the chiller unit of  FIG. 1 .  
         [0017]      FIGS. 5-7  are a schematic representation of a portion of the startup control logic of the present invention.  
         [0018]      FIG. 8  is a flowchart of an embodiment of a shutdown process for the present invention. 
     
    
       [0019]     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  
       [0020]     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.  
         [0021]     In a preferred 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 compressor  12  with 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 .  
         [0022]     In one embodiment of the present invention, the structural frame incorporates 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  are preferably 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 .  
         [0023]     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 steam turbine  14  can drive the compressor  12  at either a single speed or at variable speeds. Preferably, steam turbine  14  is a multistage, variable speed turbine that is capable of operating compressor  12  at a speed that more closely optimizes the efficiency of the chiller system  10 . More preferably, steam turbine  14  is capable of driving compressor  12  at speeds in a range of about 3200 rpm to about 4500 rpm. The supply of steam to the steam turbine  14  is preferably dry saturated steam within a range of about 90 to about 200 psi. The flow of steam supplied to steam turbine  14  can be 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 by providing a greater or lower amount of refrigerant volumetric flow through the compressor  12 . In another embodiment, the steam turbine  14  can drive the compressor 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 .  
         [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 compressor  12 , and a substantially closed position, wherein refrigerant flow into 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 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 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 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, a steam supply provides steam to the steam turbine  14 . The steam from the steam supply preferably enters a moisture separator  64 . In the moisture separator  64 , moisture-laden steam from the steam supply 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 (not shown) and dry saturated steam flows upward and exits through a steam outlet (not shown) where it flows toward a main steam inlet block valve  69  and a steam inlet slow roll bypass valve  68 . The main steam inlet block valve  69  and a steam inlet slow roll bypass valve  68  can be positioned to control the amount of steam that flows toward a governor  48  during the slow roll ramp up to minimum rated speed at start up. 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 steam supply. The steam from the steam supply 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 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.  
         [0029]     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 .  
         [0030]     In addition, a turbine steam ring drain valve  63  is provided to permit the removal of any condensate from the steam turbine  14  during the slow roll warm up of the steam turbine  14 . A gland seal steam supply valve  67  can be used to admit steam to the gland seal supply pressure regulating valve during a slow roll. A steam condenser vacuum pump  65  evacuates the steam condenser  20  and turbine exhaust to a desired vacuum that is required for the steam turbine  14  to produce the power required by the compressor  12 .  
         [0031]     The exhausted steam from steam turbine  14  flows to steam condenser  20 . Within steam condenser  20 , the steam/condensate flow from the steam turbine  14  enters into a heat exchange relationship with cooling water flowing through 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 steam supply. In this manner, condensate recirculation system  46  can maintain a preselected flow of condensate through steam condenser  20  and return condensate to the steam supply for further generation of steam.  
         [0032]     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 refrigerant condenser  16  to repeat the cycle.  
         [0033]     Typically, the steam condenser  20  operates at a greater temperature than the refrigerant condenser  16 . By routing the cooling water through 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.  
         [0034]     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 .  
         [0035]     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, 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.  
         [0036]     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.  
         [0037]     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 controller  90  include a HVAC&amp;R demand input from a thermostat or other similar temperature control system.  
         [0038]     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 a steam inlet valve control signal. In addition, control panel  90  can send a turbine shutdown signal when either the technician has input a shutdown command into user interface  94 , or when a deviation is detected from a preselected parameter recorded in memory device  92 .  
         [0039]     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 control panel  90  provides for additional protections for individual components in the event of an off-design operating condition in 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 compressor  12  and steam turbine  14  to ensure that adequate lubrication is provided prior to rotating compressor  12  and steam turbine  14 . As detailed below, 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.  
         [0040]     In addition, the central control algorithm can maintain selected parameters of 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.  
         [0041]     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 chiller system  10  during both startup and routine operation of 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.  
         [0042]      FIGS. 5-7 , illustrate an embodiment of a manual startup process for the startup control program of the present invention. In block  802 , an operator selects a manual start mode for chiller system  10 . In block  804 , turbine vibration monitors are checked for availability and that they are not transmitting a signal that would indicate the presence of excessive vibrations. In block  806 , the chiller system  10  is checked to confirm that it is shutdown and not operating. In block  808 , the operator manipulates a switch to reset any previous safety trips. In block  810 , a self-diagnostic check is executed to determine if any fault conditions are present. As an example, a fault condition may be caused by a reading from a sensor that is outside the expected range of values indicating a normal startup condition. If no fault conditions are detected in block  810 , the control logic continues to block  812 . However, if a fault condition is detected in block  810 , the logic proceeds to block  822 . In block  812 , the user interface  94  can display the message “System Ready to Start” or other similar indication, and the logic proceeds to block  816  upon the completion of the operator action in block  814 . In block  814 , the operator starts the chiller system  10  by entering or inputting a start command into the user interface  94  or by selecting a “Start” key, button, switch or option on the control panel  90 . In block  816 , in response to the receipt of both the start command from block  814  and the signal from block  812 , a pre-lubrication of compressor  12  and steam turbine  14  is started for a predetermined pre-lubrication time period, e.g., 50 seconds, by starting a turbine auxiliary oil pump and a condenser water pump. In addition, a compressor oil pump is started after a predetermined oil pump time delay, e.g., 13 seconds. The startup control program logic then proceeds to block  818 . In block  818 , another self-diagnostic check is performed, similar to the self-diagnostic check of block  810 . If there is no detected fault condition, the control logic proceeds to block  820 , if a fault condition is detected, the logic proceeds to block  822 .  
         [0043]     In block  820 , the user interface  94  can display the message “Start Sequence Initiated”, and the logic proceeds to block  824 . In block  822 , safety shutdown logic is initiated where parameters that were identified as a fault condition may be recorded in a retrievable memory for future diagnostics. In block  824 , a determination is made as to whether adequate oil pressures and condenser water flow have been established after a predetermined amount of time, e.g., 45 seconds, after the starting of the pumps in block  816 . If adequate flows are determined to have been established, the logic proceeds to block  826 . If adequate flows are determined to not have been established, the logic proceeds to block  822  for the initiation of the safety shutdown logic. In block  826 , a determination is made as to whether the predetermined pre-lubrication time period started in block  816  has ended. If the determination of block  826  is positive, the control logic proceeds to block  828 , if the determination is negative, the control logic returns to block  818  and proceeds as described above.  
         [0044]     In block  828 , a condensate, or hotwell pump  62  is started and the logic proceeds to block  830 . Preferably, steam condenser  20  includes more than one hotwell pump  62 , and the pump that was idle during the last chiller system  10  operation can be selectively started or the standby pump can be started if the lead pump fails to start. In block  830 , turbine trip and vacuum breaker solenoids are energized, and the logic proceeds to block  831 .  
         [0045]     In block  831 , user interface  94  can display the message “Starting System”, and the logic will proceed to block  832 . In block  832 , a determination is made as to whether the turbine trip valve limit switch is closed, and the logic proceeds to block  834  if the limit switch is closed. If the turbine trip valve limit switch limit switch is open, the logic returns to block  832 .  
         [0046]     In block  834 , the user interface  94  can display the message “Begin Slow Roll” to prompt the operator to manually enter the slow roll command, and the logic proceeds to blocks  836  and  838 . In block  838 , the operator preferably enters a “Begin Slow Roll” command into the user interface  94 , and the logic proceeds to block  844 . After the operator enters the “Begin Slow Roll” command, the operator must open a main inlet steam bypass valve to permit steam to enter steam turbine  14  to start the slow roll of the steam turbine  14 . In block  836 , a determination is made on whether a predetermined amount of time, e.g., 20 minutes, has elapsed since the “Begin Slow Roll” message was displayed in block  834 . If the determination in block  836  is positive, the logic proceeds to block  840 . If the determination in block  836  is negative, the logic returns to block  836 .  
         [0047]     In block  840 , a determination is made on whether the “Begin Slow Roll” command has been entered into user interface  94  by the operator in block  838 . If the determination in block  840  is negative, the logic proceeds to block  842 . In block  842 , the user interface  94  can display the message “Excessive Start Delay”, and the logic proceeds to block  822  for the initiation of the safety shutdown logic. In this manner, the logic of blocks  836 ,  840 , and  842  are used to shutdown the chiller system  10  if a “Begin Slow Roll” command is not entered into the user interface  94  in block  838  within the predetermined amount of time from the notification in block  834  that steam turbine  14  is ready for a slow roll. If the determination in block  840  is positive, the logic proceeds to block  844 .  
         [0048]     In block  844 , a desired speed, SSP 1 , for the slow roll of the steam turbine  14 , and a desired acceleration, or speed ramp rate, RRSP 3  to obtain the desired speed are selected. Preferably, RRSP 3  is set at a first predetermined acceleration rate, e.g., 50 rpm/second, during initial steam turbine startup and SSP 1  is set at a first predetermined turbine speed, e.g., 1000 rpm, although these desired values can be any appropriate values for the particular steam turbine  14  selected. In block  846 , a determination is made on whether the speed of the steam turbine  14  has increased above a first predetermined threshold speed, e.g., about 500 rpm. If the determination in block  846  is negative, the logic returns to block  846 . If the determination in block  846  is positive, the logic proceeds to block  848 .  
         [0049]     In block  848 , user interface  94  can displays the message “Slow Rolling” or other similar indication, to notify the operator that steam turbine  14  is rotating above the predetermined threshold speed from block  846  and the logic proceeds to block  850 . In block  850 , the evaporator low pressure safety shutdown setpoint is increased to a predetermined value, e.g., 30 psig. The evaporator low pressure safety shutdown setpoint is used to shutdown the chiller system  10  when the evaporator pressure decreases to below the setpoint value. In block  852 , oil cooler water solenoid valves are opened to permit the compressor and turbine oil temperature control valves to maintain the bearing oil temperatures at approximately 110 to 120° F., and the logic proceeds to block  856 . In block  856 , a determination is made as to whether the speed of steam turbine  14  has exceeded a second predetermined threshold speed, SSP 5 . Preferably, the second predetermined threshold speed, SSP 5 , is about 1200 rpm. If the determination in block  856  is positive, the logic proceeds to block  857 . In block  857 , a chilled water pump is started, and the logic proceeds to block  859 . In block  859 , a determination is made if a fault condition is present based on the establishment of a chilled water flow. A fault condition is present in block  859 , if adequate chilled water flow has not been established, and the logic proceeds to block  822  for the initiation of the safety shutdown logic. A fault condition is not present in block  859 , if adequate chilled water flow has been established, and the logic proceeds to block  858 .  
         [0050]     If the determinations in block  856  and block  859  are negative, the logic proceeds to block  858 . In block  858 , the speed of steam turbine  14  is maintained at about SSP 1 , and the logic proceeds to block  860 . In block  860 , a determination is made on whether the calculated slow rolling time has elapsed, and the logic proceeds to block  862 . The minimum desired slow rolling time, SRT, (in minutes) is calculated as a function of the number of stages, Ns, of steam turbine  14  and the steam inlet temperature, Ti (° F.), using the following algorithm: 
 
 SRT= 20 +Ns +[( Ti− 350)/50]
 
         [0051]     An exemplary slow roll time calculation for a steam turbine with 7 stages with a steam inlet temperature of about 353° F. would yield a slow roll time of about 27 minutes. Alternately, an abbreviated slow roll time can be used, e.g., 5 minutes, if the present chiller start command from block  814  was initiated within a predetermined amount of time, e.g., 15 minutes, of a previous ramp to rated speed command. Preferably, the user interface  94  displays the remaining slow rolling time for operator notification. In block  862 , the vacuum pump  64  is started, and the logic proceeds to block  864 . In addition, the operator must close the steam turbine casing drain valve so the steam turbine vacuum pump can evacuate the steam turbine  14  in preparation for increasing the speed of steam turbine  14  to a minimum rated speed, as discussed herein.  
         [0052]     In block  864 , a determination is made as to when the steam turbine exhaust pressure decreases below a predetermined setpoint pressure, PSP 6 . If the steam turbine exhaust pressure is above the predetermined setpoint pressure, PSP 6 , the logic proceeds to block  865 . In block  865 , user interface  94  displays the message “Turbine Idling—Insufficient Vacuum”, and the logic will return to block  864  for continuous monitoring of the turbine vacuum. Preferably, PSP 6  is about 5 psia, although PSP 6  can be adjusted by the operator in block  866  to any appropriate amount of vacuum. This vacuum is desirable within steam turbine  14  to ensure that an influx of steam to obtain an operational speed produces sufficient power to accelerate the turbine rapidly through the critical speed range and maintain the speed above the minimum rated speed during stabilization. When the steam turbine exhaust pressure decreases below PSP 6 , the logic proceeds to block  868 .  
         [0053]     In block  868 , the user interface  94  displays the message “Ready to Run” prompting the operator to manually enter a “Run at Rated” command, and the logic will proceed to blocks  870  and  872 . In block  872 , the operator preferably enters a “Run at Rated” command into user interface  94  and the logic proceeds to block  874 . In block  870 , a determination is made on whether the chiller system  10  has been in the “Ready to Run” state for a predetermined time period, TDSP 1 . Preferably, the predetermined time period, TDSP 1 , is determined from manufacturer&#39;s data on maximum desirable slow rolling times. If time TDSP 1  has not elapsed, the logic returns to block  870 . If time TDSP 1  has elapsed, the logic proceeds to block  874 .  
         [0054]     In block  874 , a determination is made on whether the “Run at Rated” command has been entered into user interface  94  by the operator in block  872 . If the determination of block  874  is negative, the logic proceeds to block  876 . In block  876 , the user interface  94  can display the message “Excessive Slow Rolling Trip”, and the logic will proceed to block  822  for the initiation of the safety shutdown logic. In this manner, the logic of blocks  870 ,  872  and  874  are used to ensure that the operator enters a “Run at Rated” command in block  872  within the predetermined amount of time, TDSP 1 , from the “Ready to Run” notification given in block  868 . If the determination of block  874  is positive, the logic proceeds to block  878 .  
         [0055]     In block  878 , the chilled water pump is started if it was not started previously in block  857 , and the logic proceeds to block  880 . In block  880 , the presence of the minimum flow of chilled water is confirmed and then, the speed of steam turbine  14  is increased at a second predetermined acceleration rate, RRSP 4 , to a second predetermined turbine speed, SSP 3 . Preferably, SSP 3  is 4200 rpm and RRSP 4  is 100 rpm/second. This can be accomplished by sending a control signal to the governor valve  48  to rapidly open the governor valve  48 . In addition, the operator must open the main steam inlet block valve to ramp the turbine up to rated speed.  
         [0056]     In block  882 , a determination is made as to whether the speed of steam turbine  14  is greater than the second predetermined threshold speed, SSP 5 . If the speed of steam turbine  14  is greater than SSP 5 , the logic proceeds to block  884 . If the speed of steam turbine  14  is not greater than SSP 5 , the logic returns to block  880 .  
         [0057]     In block  884 , user interface  94  can display the message “Ramping to Rated Speed”, and the logic will proceed to block  886 . In addition, the pre-rotation vanes  80  are positioned at a predetermined minimum position. In block  886 , it is determined whether the speed of steam turbine  14  is greater than a third predetermined threshold speed, SSP 7 . If the speed of steam turbine  14  is greater than SSP 7 , the logic proceeds to block  888 . If the speed of steam turbine  14  is not greater than SSP 7 , the logic returns to block  884 . Preferably, SSP 7  is preselected to be about 3000 rpm, or at a similar speed in which the shaft driven turbine oil pump produces sufficient pressure for lubrication without the steam turbine auxiliary oil pump.  
         [0058]     In block  888 , the steam turbine auxiliary oil pump is stopped and the logic proceeds to block  890 . In block  890 , the oil return and liquid line solenoids are energized and the logic proceeds to block  894 . In block  894 , a determination is made as to whether the speed of steam turbine  14  is greater than or equal to a predetermined minimum rated speed, SSP 2 , for the turbine  14  and compressor  12  combination. Preferably, SSP 2  is about 3200 rpm and is based on the specific steam turbine  14  and compressor  12  used in the chiller system  10 , and stored into control panel  90 . If the determination in block  894  is positive, the logic proceeds to block  896 . If the determination in block  894  is negative, the logic proceeds to block  898 .  
         [0059]     In block  898 , a determination is made on whether the steam turbine  14  has been operating between about 2000 rpm and SSP 2  for more than about 17 seconds. If the determination in block  898  is positive, the logic proceeds to block  900 . In block  900 , user interface  94  can display the message “Turbine Underspeed”, and the logic will proceed to block  822  for the initiation of the safety shutdown logic. If the determination in block  898  is negative, the logic returns to block  894 . In this manner, the steam turbine  14  and compressor  12  can be brought up to a predetermined rated speed, SSP 2 , within a desired amount of time, thereby preventing damage associated with prolonged operations in a critical speed range which is less than the minimum rated speed. It is to be understood that the desired minimum time of operation between 2000 rpm and SSP 2 , of about 17 seconds, is stored in control panel  90  to ensure this critical safety logic remains active.  
         [0060]     In block  896 , user interface  94  can display the message “Turbine Stabilizing” to indicate that the predetermined minimum rated speed, SSP 2 , has been obtained, and the logic will proceed to block  902 . In block  902 , the evaporator low pressure safety shutdown setpoint is then decreased to 25 psig. In addition, in block  902 , a turbine stabilization timer, as discussed below, is started. Thereafter, the logic proceeds to block  904 . In block  904 , a self diagnostic check is performed to detect for any safety faults. If the determination in block  904  is positive, i.e., a safety fault is present, the control logic proceeds to block  822  for the initiation of the safety shutdown logic. If the determination in block  904  is negative, i.e., a safety fault is not present, the control logic proceeds to block  906 . In block  906 , a determination is made on whether a stop command has been entered into user interface  94 . If the determination in block  906  is positive, i.e., a stop command has been entered, the logic proceeds to block  908  to initiate a normal shutdown of chiller system  10 . If the determination in block  906  is negative, i.e., a stop command has not been entered, the logic proceeds to block  910 .  
         [0061]     In block  910 , a determination is made as to when the speed of steam turbine  14  falls below a predetermined speed of about 3100 rpm for greater than a predetermined time of about 10 seconds. If the determination in block  910  is positive, the logic proceeds to block  912  wherein the user interface  94  can display the message “Turbine Underspeed”, and the logic will proceed to block  822  for the initiation of the safety shutdown logic. If the determination in block  910  is negative, the logic proceeds to block  914 .  
         [0062]     In block  914 , a determination is made as to when the speed of steam turbine  14  has been above (SSP 2 −100) rpm (about 3100 rpm) for greater than a second predetermined time period, TDSP 17 . Preferably, TDSP 17  is about 120 seconds, or about 2 minutes. If the determination in block  914  is positive, the logic proceeds to block  916 . If the determination in block  914  is negative, the logic returns to block  904 . This delay of TDSP 17  seconds in block  914  permits the steam turbine speed to stabilize well above the critical speed range of the steam turbine  14  before loading the chiller. It is to be understood that while TDSP 17  is preferably 120 seconds, TDSP 17  can be set or programmed to any suitable timeframe that provides for steam turbine stabilization upon startup. In block  916 , user interface  94  can display the message “System Running” to indicate that the minimum rated speed has been obtained, and the logic will proceed to block  918 .  
         [0063]     In block  918 , automatic control over the capacity of chiller  10  is initiated, and the logic will return to block  904  for continuous monitoring of a safety fault, stop command and turbine underspeed condition. The capacity control logic of control panel  90  can increase, or decrease, the speed of steam turbine  14  to a desired speed, based upon chiller  10  system demand.  
         [0064]     The central control algorithm executed by the microprocessor  96  on the control panel  90  also preferably includes a shutdown control program or algorithm to control the shutdown of the steam turbine  14  and compressor  12 .  FIG. 8  illustrates an embodiment of a manual shutdown process of the present invention. The shutdown process begins at step  1002  with an operator selecting a “System Stop” or “Soft Stop” key, button, switch or option on the control panel  90  or by entering or inputting the “Soft Shutdown” command into the user interface  94 . The user interface  94  can then display the message “System Shutting Down-Speed Decreasing” to the operator. In step  1004 , the speed of the steam turbine  14  is ramped down or decreased to a predetermined minimum turbine speed. In a preferred embodiment, the predetermined minimum turbine speed is the calculated anti-surge minimum speed for the steam turbine  14 . The speed of the steam turbine  14  is evaluated in step  1006  to determine if it is equal to the predetermined minimum turbine speed. If the speed of the steam turbine  14  is equal to the predetermined minimum turbine speed then the process proceeds to step  1008 . Otherwise, the speed of the steam turbine  14  is continued to be decreased or ramped down in step  1004 .  
         [0065]     In step  1008 , the pre-rotation vanes (PRV)  80  are closed to a predetermined minimum vane position in response to the turbine speed being equal to the predetermined minimum turbine speed. In a preferred embodiment, the predetermined minimum vane position is the calculated anti-surge minimum vane position for the pre-rotation vanes  80 . The user interface  94  can then display the message “System Shutting Down-Vanes Closing” to the operator. The position of the pre-rotation vanes  80  is then evaluated in step  1010  to determine if it is equal to the predetermined minimum vane position. If the position of the pre-rotation vanes  80  is equal to the predetermined minimum vane position, then process proceeds to step  1012 . Otherwise, the pre-rotation vanes  80  are continued to be closed in step  1008 .  
         [0066]     In addition, while the pre-rotation vanes  80  are closing in step  1008 , the system head or system pressure differential is also decreasing as a result of the closure of the pre-rotation vanes  80 . The lower system pressure differential can result in a lower value for the predetermined minimum turbine speed and, preferably, in a lower calculated anti-surge minimum speed for steam turbine  14 . In a preferred embodiment, while the pre-rotation vanes  80  are closing in step  1008 , the speed of the steam turbine  14  is also being decreased, as set forth in step  1004 , in response to the reduction in the predetermined minimum turbine speed resulting from the reduction in system pressure differential.  
         [0067]     In step  1012 , the hot gas bypass valve (HGV)  84  is opened to a fully open position (100%) in response to the position of the pre-rotation vanes  80  being equal to the predetermined minimum vane position. In addition, while the hot gas bypass valve  84  is opening in step  1012 , the system head or system pressure differential is decreasing as a result of the opening of the hot gas bypass valve  84 . The lower system pressure differential can result in a lower value for the predetermined minimum turbine speed and, preferably, in a lower calculated anti-surge minimum speed for steam turbine  14 . Also, the lower system pressure differential can result in a lower value for the predetermined minimum vane position and, preferably, in a lower calculated anti-surge minimum vane position. In a preferred embodiment, while the hot gas bypass valve  84  is opening in step  1012 , resulting in a reduction in system pressure differential, the speed of the steam turbine  14  is also being decreased, as set forth in step  1004 , in response to the reduction in the predetermined minimum turbine speed and the pre-rotation vanes are closing, as set forth in step  1008 , in response to the reduction in the predetermined minimum vane position.  
         [0068]     In step  1014  a determination is made on whether the hot gas bypass valve  84  has been opening, as set forth in step  1012 , for more than a predetermined time period. The position of the hot gas bypass valve  84  does not impact the determination in step  1014  and the hot gas bypass valve  84  may not be in the fully opened position upon a determination that the hot gas bypass valve has been opening for more than the predetermined time period. The predetermined time period can be between about 1 minute and about 5 minutes and is preferably about 3 minutes. If the predetermined time period has elapsed, indicating that the hot gas bypass valve  84  has been opening for more than the predetermined time period, the control proceeds to step  1016 . Otherwise, the hot gas bypass valve  84  is continued to be opened in step  1012 . In step  1016 , the turbine shutdown process is initiated to shutdown the steam turbine  14  and the chiller system  10 .  
         [0069]     In one embodiment of the present invention, the steam turbine system shutdown or trip process from step  1016  of  FIG. 8  begins with the de-energizing of the main system run relay in response to the initiation of the turbine trip or shutdown process. The de-energizing of the main system run relay causes the de-energizing of a turbine trip solenoid, which causes a pneumatic turbine trip valve to close. The compressor pre-rotation vanes  80  are closed and the hot gas bypass valve  84  and a subcooler level control valve are opened. In addition, the vacuum pump  65  is stopped and the vacuum breaker solenoid valve is opened to break the vacuum in the exhaust line and more quickly slow the speed of the steam turbine  14 . Furthermore, the speed control set point for the steam turbine  14  is set to 0 RPM which causes the control output signal to the governor valve  48  to decrease to 0% and close the governor valve  48 . The operator must close the main steam inlet block valve  69 .  
         [0070]     Next, when the speed of the steam turbine  14  decreases below 3000 RPM, the oil return and liquid line solenoid valves are deenergized and a turbine auxiliary oil pump is started. During the coast down of the drive train between the steam turbine  14  and the compressor  12 , the compressor oil pump and turbine auxiliary oil pump can continue to operate to maintain lubrication of the corresponding compressor and turbine bearings and a message “System Coastdown” can be displayed on user interface  94 . Once the rotation of the drive train has stopped the message “Compressor Shutdown; Turbine Cooldown” can be displayed on user interface  94 .  
         [0071]     After no rotation has been detected for about 20-50 seconds, the compressor oil pump can stop and the chilled water pump contacts are opened to stop the chilled water pump. The turbine auxiliary oil pump can continue to run for about 20-40 minutes to remove excess heat from the turbine bearings. If the chiller system  10  is not to be restarted in a short time, i.e. the steam turbine  14  can be permitted to cool down, the operator must open the turbine steam ring drain valve  63  and close the gland seal steam supply valve  67 . About 5-7 minutes after tripping the steam turbine  14 , if no rotation is detected, the condenser water and hotwell pumps  44  are stopped and the oil cooler water solenoid valves are de-energized. Finally, about 25-35 minutes after the rotation has stopped, the turbine auxiliary lube oil pump will be stopped and the message “System Shutdown” can be displayed on the user interface  94 . While one embodiment of a steam turbine shutdown process has been described above, it is to be understood that any suitable steam turbine process can be used with the shutdown process of  FIG. 8 .  
         [0072]     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.