Air cooling system and method for a heat recovery steam generator inlet

Abstract

The air cooling system and method for a heat recovery steam generator (HRSG) inlet provides a combined cycle power plant utilizing a powerful fan coupled to ductwork connected to pipes that enter the HRSG inlet duct coupled to an exhaust duct of a Combustion Turbine (CT) for lowering the temperature of the CT exhaust gas provided to the heat recovery steam generator by the CT. The cool air injection system is utilized during low load operation or startup of the CT to ensure that spray water from an inter-stage desuperheater in an HRSG is fully evaporated prior to entering the downstream superheater or reheater. A feedback system includes temperature elements measuring the mix temperature that regulates the cooling air injection rate into the HRSG inlet.

1 . An air cooling method for a heat recovery steam generator (HRSG) having a chamber containing inter-stage attemperators in a desuperheater and a turbine engine exhaust HRSG inlet duct accepting flow of exhaust gas from a heat source, the method comprising the steps of:
 beginning a first cooling air flow sequence when output from the heat source is greater than 20% load in MW and the heat source has an increasing rate of change, the first air cooling sequence injecting the cooling air flow into the heat source exhaust or the HRSG inlet duct; and   terminating the first cooling air flow sequence when the heat source output is greater than 80% load in MW or the heat source exhaust inlet duct temperature is less than the steam set point plus 50° F.   
     
     
         2 . The air cooling method according to  claim 1 , further comprising the step of beginning a second cooling air flow sequence when the heat source output in megawatts is less than 80% and the inlet duct temperature is greater than the steam set point plus 50° F., the second air cooling sequence injecting the cooling air flow into the heat source exhaust or the HRSG inlet duct. 
     
     
         3 . The air cooling method according to  claim 2 , further comprising the step of initiating a changeover from the first cooling sequence to the second cooling sequence when the heat source output remains below 80% and the HRSG inlet duct temperature goes above the steam set point plus 50° F. at any time during the first sequence. 
     
     
         4 . The air cooling method according to  claim 3 , further comprising the steps of:
 transferring a lead temperature control to cooling air after a predetermined time delay;   limiting the minimum speed of a source of the cooling air flow; and   controlling a high-pressure superheater temperature of the HRSG responsive to temperature readings inside the HRSG inlet duct.   
     
     
         5 . The air cooling method according to  claim 4 , further comprising the step of automatic changeover from the second sequence to desuperheater cooling water when the heat source output goes above 80% or the HRSG duct temperature goes below the steam set point plus 50° F. during the second sequence. 
     
     
         6 . The air cooling method according to  claim 5 , further comprising the steps of:
 isolating a transit path of the cooling air flow from entering the HRSG inlet duct;   preventing backflow of exhaust gases from the heat source; and   turning off the cooling air flow source after a predetermined time delay.   
     
     
         7 . The air cooling method according to  claim 6 , further comprising the step of varying a rate of the cooling air flow based on feedback parameters from the HRSG. 
     
     
         8 . An air cooling system for a heat recovery steam generator (HRSG), the HRSG having a chamber containing inter-stage attemperators in a desuperheater and a heat source exhaust to HRSG inlet duct accepting flow of exhaust gas from a heat source, the system comprising:
 means for beginning a first cooling air flow sequence when the heat source output in megawatts is greater than 20% and the heat source has an increasing rate of change, the first air cooling sequence injecting the cooling air flow into the heat source exhaust to HRSG inlet duct; and   means for terminating the first cooling air flow sequence when the heat source output is greater than 80% or the turbine engine exhaust inlet duct temperature is less than the steam set point plus 50° F.   
     
     
         9 . The air cooling system according to  claim 8 , further comprising means for beginning a second cooling air flow sequence when the heat source output is less than 80% MW and the inlet duct temperature is greater than the steam set point plus 50° F., the second air cooling sequence injecting the cooling air flow into the heat source exhaust or the HRSG inlet duct. 
     
     
         10 . The air cooling system for a heat recovery steam generator inlet according to  claim 9 , further comprising means for initiating a changeover from the first cooling sequence to the second cooling sequence when the heat source output remains below 80% and the heat source exhaust to HRSG inlet duct temperature goes above the steam set point plus 50° F. at any time during the first sequence. 
     
     
         11 . The air cooling system according to  claim 10 , further comprising:
 means for transferring a lead temperature control to cooling air after a predetermined time delay;   means for limiting the minimum speed of a source of the cooling air flow; and   means for controlling a high-pressure superheater temperature of the HRSG responsive to temperature readings inside the heat source exhaust to HRSG inlet duct.   
     
     
         12 . The air cooling system according to  claim 11 , further comprising means for automatic changeover from the second sequence to desuperheater cooling water when the heat source output goes above 80% or said HRSG duct temperature going below the steam set point plus 50° F. during the second sequence. 
     
     
         13 . The air cooling system according to  claim 12 , further comprising:
 means for isolating a transit path of the cooling air flow from entering the HRSG inlet duct;   means for preventing backflow of exhaust gases from the heat source; and   means for turning off the cooling air flow source after a predetermined time delay.   
     
     
         14 . The air cooling system according to  claim 13 , further comprising means for varying a rate of the cooling air flow based on feedback parameters from the HRSG.