Cascading once through evaporator

1. A steam generator comprising:

a substantially horizontal gas conduit ( 1 ) to guide a heating gas flow ( 2 );

an evaporator unit positioned at least partially in the horizontal gas conduit ( 1 ) for transferring heat from the gas flow to a flow medium which flows through the evaporator unit, in which the evaporator unit comprises at least a first and a second evaporator stage ( 3 , 4 ) which are arranged in a cascade, in which the first evaporator stage ( 3 ) comprises:

a first heat transfer section ( 12 ) having upright positioned heat transfer tubes which are in fluid communication with a first inlet conduit ( 8 ) to supply the flow medium to the heat transfer tubes and a first outlet conduit ( 13 ) to discharge the flow medium from the heat transfer tubes, wherein the first heat transfer section ( 12 ) is bottom fed, which means that the inlet conduit is arranged at a lower region of the heat transfer section, such that during use the flow medium is supplied from beneath via the first inlet conduit ( 8 ) to the heat transfer section ( 12 ) at a lower region and discharged via the first outlet conduit ( 13 ) at an upper region, wherein the first evaporator stage is a single pass evaporator stage in a cascade arrangement, such that a flow medium passes the substantially horizontally flowing heating gas only in upward direction from a bottom inlet to a top outlet of the evaporator stage; and

a first separator ( 14 ) to separate liquid and vapor out of the flow medium coming from the first outlet conduit ( 13 ) wherein the liquid is discharged via a first liquid outlet and wherein the vapor is discharged via a second vapor outlet of the first separator ( 14 ), wherein the second evaporator stage ( 4 ) comprises a second heat transfer section ( 21 ) having upright positioned heat transfer tubes which are in fluid communication with a second inlet conduit ( 17 ) to supply the flow medium to the heat transfer tubes and a second outlet conduit ( 22 ) to discharge the flow medium from the heat transfer tubes, wherein the second heat transfer section ( 21 ) is bottom fed, which means that the inlet conduit is arranged at a lower region of the heat transfer section, such that during use the flow medium is supplied from beneath via the second inlet conduit to the heat transfer section ( 21 ) at a lower region and discharged via the second outlet conduit at an upper region, wherein the liquid outlet of the first separator ( 14 ) is connected to the second inlet conduit ( 17 ) via a first downcomer conduit ( 15 ), wherein the second evaporator stage is a single pass evaporator stage in a cascade arrangement, such that a flow medium passes the substantially horizontally flowing heating gas only in upward direction from a bottom inlet to a top outlet of the evaporator stage, wherein the second evaporator stage ( 4 ) further comprises a second separator ( 23 ) which is in fluid communication connected to the second outlet conduit ( 22 ) to separate liquid and vapor out of the flow medium coming from the second outlet conduit ( 22 ) wherein the liquid is discharged via a second liquid outlet and wherein the vapor is discharged via a second vapor outlet of the second separator ( 23 ).

2. The steam generator according to claim 1 , wherein the heat transfer tubes of a heat transfer section ( 12 , 21 ) are in fluid communication with each other without any choke or restrictor means like valves to throttle a through flow of a heat transfer tube with respect to another heat transfer tube of the heat transfer section.

3. The steam generator according to claim 1 , wherein at least one evaporator stage ( 3 , 4 ) is a once through evaporator stage.

4. The steam generator according to claim 1 , wherein a by-pass conduit ( 39 ) is provided to by-pass a first evaporator stage ( 3 ) and supply flow medium to a more upstream the gas flow arranged evaporator stage ( 4 , 5 ).

5. The steam generator according to claim 1 , wherein the evaporator unit comprises a third evaporator stage ( 5 ) which is in fluid communication connected to the second separator ( 23 ) by a second downcomer conduit ( 24 ).

6. The steam generator according to claim 1 , wherein at least the first and second evaporator stages ( 3 , 4 ) are arranged in a cascade in counter-current flow with respect to the heating gas ( 2 ).

7. The steam generator according to claim 1 , wherein the evaporator unit has a heat transfer section comprising a matrix of heat transfer tubes ( 22 ) having at most five arrays of transversal the gas flow arranged heat transfer tubes.

8. The steam generator according to claim 1 , wherein heat transfer tubes of a heat transfer section ( 12 , 21 , 30 ) in successive arrays downstream the gas conduit have a substantially equal diameter in cross section.

9. The steam generator according to claim 1 , wherein an auxiliary heat transfer section ( 6 ) is arranged upstream the gas flow and in series with an evaporator stage ( 5 ).

10. The steam generator according to claim 1 , wherein a common vapor conduit ( 35 ) is provided for discharging vapor from the evaporator stages ( 3 , 4 , 5 ) to a superheater ( 6 ).

11. A method of generating steam comprising the steps of:

providing a steam generator according to claim 1 ;

supplying a flow medium to an evaporator unit having multiple evaporator stages;

forcing the flow medium through a first heat transfer section ( 12 ) of a first evaporator stage ( 3 ) of the evaporator unit, wherein the flow medium is supplied from beneath via the inlet conduit to the heat transfer section ( 12 ) at a lower region and discharged via the outlet conduit at an upper region;

discharging the flow medium comprising a vapor and a liquid content from the first heat transfer section to a first separator ( 14 ) which is connected to an outlet of the first evaporator stage ( 3 );

supplying the liquid content of the flow medium from the first separator ( 14 ) to a second evaporator stage ( 4 ) having a bottom fed second heat transfer section ( 21 ) via a first downcomer conduit ( 15 );

forcing the flow medium through a second heat transfer section ( 21 ) of the second evaporator stage ( 4 ) of the evaporator unit, wherein the flow medium is supplied from beneath via the inlet conduit to the heat transfer section ( 21 ) at a lower region and discharged via the outlet conduit at an upper region,

wherein the method further comprises the steps of:

discharging the flow medium comprising a vapor and a liquid content from the second heat transfer section of the second evaporator stage ( 4 ) to a second separator ( 23 ) which is connected to an outlet conduit ( 22 ) of the second evaporator stage ( 4 ); and

discharging the vapor content of the flow medium from the first and the second separator ( 14 , 23 ) to a super heater ( 6 ).