Process and apparatus for heating feedwater in a heat recovery steam generator

Abstract

A feedwater heater ( 14 ) in a heat recovery steam generator (A,B) lies within a flow of hot exhaust gas. The feedwater heater ( 14 ) converts subcooled feedwater into saturated feedwater water, the temperature of which is only lightly above the acid dew point temperature of the exhaust gas so that corrosive acids do not condense on coils ( 18 ) of the feedwater heater ( 14 ). Yet the temperature of the saturated feedwater lies significantly below the temperature of the exhaust gas at the coils ( 18 ), so that the coils ( 18 ) operate efficiently and require minimal surface area. Pumps ( 26, 28, 30 ) elevate the pressure of the saturated feedwater and direct it into an economizer ( 64, 90 ) where, owing to the increase in pressure, the water is again subcooled. The economizer ( 64, 90 ) elevates the temperature still further and delivers the higher pressure feedwater to evaporators ( 34, 70, 78 ) that convert it into saturated steam that flows on to the superheaters ( 50, 78, 84 ). Higher pressure pegging stem admitted to the feedwater heater ( 14 ) controls the pressure—and temperature—of saturated steam and water in the feedwater heater ( 14 ).

1 . In an HRSG for extracting heat from an exhaust gas that flows through the HRSG and has an acid dew point temperature, and for utilizing that heat to convert subcooled feedwater into steam, the improvement comprising:
 a feedwater heater located in the flow of the exhaust gas, the feedwater heater converting subcooled feedwater at a low temperature into saturated steam and saturated water, with the temperature of the saturated water being above the acid dew point temperature of the exhaust gas;   a feedwater pump receiving water from the feedwater heater and elevating the pressure of the water; and   an economizer located in the flow of the exhaust gas upstream from the feedwater heater and connected to the feedwater pump for receiving feedwater at an elevated pressure from the pump and elevating the temperature of that feedwater.   
     
     
         2 . The combination according to  claim 1  wherein the feed water heater comprises:
 a steam drum into which the subcooled feedwater is directed; 
 a coil located below the steam drum within the flow of the exhaust gas and at its lower and upper ends communicating with the steam drum such that water from the steam drum circulates through coil where some of the water is converted to saturated steam; and 
 a feedwater discharge line connecting the steam drum and the feedwater pump. 
 
     
     
         3 . The combination according to  claim 2  wherein the steam drum is connected to a source of pegging steam for controlling the pressure in the steam drum. 
     
     
         4 . The combination according to  claim 2  and further comprising:
 an initial pump that receives subcooled water at a low pressure and temperature; and 
 a feedwater line connecting the initial pump and the drum of the feedwater heater for directing feedwater from the initial pump into the drum. 
 
     
     
         5 . The combination according to  claim 4  and further comprising an evaporator including a coil located in the flow of exhaust gas upstream from the coil of the feedwater heater and receiving heated feedwater at an elevated pressure produced by the feedwater pump and producing saturated steam; and
 a steam discharge line through which saturated the steam escapes. 
 
     
     
         6 . The combination according to  claim 5  wherein the evaporator further includes a steam drum with which the coil of the evaporator at its upper and lower ends communicates, so that water from the steam drum circulates through the coil and some of it transforms into saturated steam that flows into the steam drum. 
     
     
         7 . The combination according to  claim 6  and further comprising:
 a pegging line that opens into the steam drum of the feedwater heater and communicates with the steam drum of the evaporator; and 
 a pegging valve in the pegging line for controlling the pressure of steam admitted to the steam drum of the feedwater heater through the pegging line. 
 
     
     
         8 . The combination according to  claim 5  and further comprising a superheater located in the flow of the exhaust gas upstream from the coil of the evaporator and connected to the discharge line of the evaporator for converting saturated steam received from the evaporator into superheated steam. 
     
     
         9 . The combination according to  claim 5  wherein the coil of the evaporator is located between the economizer and the coil of the feedwater heater in the flow of the exhaust gas. 
     
     
         10 . The combination according to  claim 5  wherein the economizer is located between the coil of the evaporator and the coil of the feedwater heater in the flow of the exhaust gas, and the feedwater pump forces feedwater from the feedwater heater through the economizer and into the evaporator. 
     
     
         11 . The combination according to  claim 5  and further comprising another evaporator located in the flow of the exhaust gas upstream from the economizer for receiving heated feedwater from the economizer and converting it into saturated steam. 
     
     
         12 . The combination according to  claim 1  wherein the feedwater heater produces saturated water, the temperature of which exceeds the acid dew point temperature by no more than about 15° F. 
     
     
         13 . A process for furnishing liquid water at an elevated temperature and pressure to an evaporator located in a flow of exhaust gas having an acid dew point temperature, said process comprising:
 extracting heat from the flow of exhaust gas to heat subcooled feedwater to a saturation temperature above the acid dew point temperature so as to provide saturated feedwater;   elevating the pressure of the saturated feedwater to produce higher pressure feedwater; and   upstream from the extraction of heat to create saturated feedwater, extracting more heat from the flow of exhaust gas to heat the higher pressure feedwater to a higher temperature.   
     
     
         14 . The process according to  claim 13  and further comprising upstream from the extraction of heat to heat the higher pressure feedwater extracting more heat from the flow of the exhaust gas to convert the higher pressure feedwater into saturated steam. 
     
     
         15 . The process according to  claim 13  wherein extracting heat from the exhaust gas to heat subcooled water comprises:
 introducing the subcooled water into a steam drum; and 
 circulating the water from the steam drum through a coil located in the flow of the exhaust gas such that some of the water converts into saturated steam and both saturated steam and saturated water flow into and occupy the steam drum. 
 
     
     
         16 . The process according to  claim 15  and further comprising controlling the temperature of the saturated steam and water in the feedwater heater by controlling the pressure of the steam in the steam drum. 
     
     
         17 . The process according to  claim 15  and further comprising:
 converting the higher pressure feedwater into higher pressure saturated steam; and 
 controlling the temperature of the saturated water in the steam drum by subjecting it to the higher pressure saturated steam. 
 
     
     
         18 . The process according to  claim 15  wherein the temperature of the saturated steam and water in the steam drum does not exceed the acid dew point temperature by more than about 15° F.