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1. (WO1998049493) DRUM-TYPE BOILER WITH ENHANCED CIRCULATION
Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

What is claimed is:
1. An evaporator system, comprising:
a drum;
a gas duct through which hot gas can pass;
first and second evaporator sections located in said gas duct, each of said evaporator sections defining an inlet and an outlet and including heat-absorbing tubes;
a feedwater pump in fluid communication with the inlet of said first evaporator section;
a downcomer from said drum in fluid communication with the inlet of said second evaporator section;
a connecting line providing fluid communication between the outlet of said first evaporator section and said drum; and
a riser providing fluid communication between the outlet of said second evaporator section and said drum, wherein said first and second evaporator sections are connected in parallel to said drum through said connecting line and said riser.

2. An evaporator system as recited in claim 1 , and further comprising an economizer section, including a plurality of tubes located in said gas duct beyond said first and second evaporator sections, said economizer section including an inlet and an outlet, wherein the outlet of said economizer section is in fluid
communication with the inlet of said first evaporator section and the inlet of said economizer section is in fluid communication with said feedwater pump.

3. An evaporator system as recited in claim 2, wherein said feedwater pump pumps water through the economizer and into the first evaporator section.

4. An evaporator system as recited in claim 1 , and further comprising an injector in fluid communication with said connecting line and riser, wherein fluid leaving said connecting line passes through said injector, creating a driving head which draws fluid up said riser.

5. An evaporator system as recited in claim 1 , and further comprising a control valve between said feedwater pump and said first evaporator inlet, for maintaining the water level in said drum.

6. An evaporator system as recited in claim 3, and further comprising a first recirculation line in communication with the outlet of said economizer section and with the inlet of the feedwater pump, and a control valve in said first recirculation line, which can be used to prevent steaming in the economizer section.

7. An evaporator system as recited in claim 6, and further comprising a second recirculation line, in communication with said drum and with the feedwater pump inlet.

8. An evaporator system as recited in claim 1 , wherein said heat absorbing tubes are oriented substantially horizontally.

9. An evaporator system, comprising:
a drum;
a gas duct through which hot gas can pass;
first and second evaporator sections located in said gas duct, each of said evaporator sections defining an inlet and an outlet and including heat-absorbing tubes;
a downcomer from said drum to the inlet of said second evaporator section;
a riser from the outlet of said second evaporator section to said drum; a feedwater pump in communication with the inlet of said first evaporator section; and
a connecting line providing fluid communication from the outlet of said first evaporator section to said drum.

10. An evaporator system as recited in claim 9, wherein said connecting line and said riser merge together to form a combined riser, and further comprising an injector at the point at which the connecting line and riser merge, so that the fluid leaving said first evaporator section passes through said injector, creating a driving head which draws fluid through said second evaporator section and out the combined riser.

11. An evaporator system as recited in claim 9, wherein said heat- absorbing tubes are oriented horizontally.

12. A method for enhancing the circulation in a drum-type waste heat recovery boiler, comprising the steps of:
dividing the heat-absorbing tubes into a first evaporator section and a second evaporator section; wherein said first and second evaporator sections are connected in parallel with the drum;
providing forced circulation in said first evaporator section, using the driving head of a feedwater pump; and
relying primarily on natural circulation in said second evaporator section.

13. A method as recited in claim 12, wherein the heat-absorbing tubes are oriented horizontally.

14. A method as recited in claim 12, and further enhancing the circulation in said second evaporator section by combining the fluids leaving said first and second evaporator sections and passing the fluid leaving said first evaporator section through an injector where the fluids from the first and second evaporator sections are combined, which creates an additional driving head to drive fluid through said second evaporator section.

15. A method as recited in claim 12, and further controlling a valve between said feedwater pump and said first evaporator section in order to maintain the proper water level in said drum.

16. A method as recited in claim 12, and further providing an economizer section, so that water is pumped from said feedwater pump, through said
economizer section, and through said first evaporator section to the drum, and further providing a first recirculation line from the output of said economizer back to the inlet of the feedwater pump and providing a control valve in the first recirculation line, and controlling the recirculation control valve so as to prevent steaming in the economizer.

17. A method as recited in claim 16, and further providing a second recirculation line in communication with the drum and with the feedwater pump inlet, and including a second recirculation line control valve, and controlling said second recirculation line control valve to maintain a desired flow rate through the first evaporator section even while decreasing the level in the drum.

18. An evaporator system as recited in claim 7, wherein said second recirculation line includes two branches:
a first branch which goes to the deaerator and includes a valve; and
a start-up branch, which goes to the inlet of the feedwater pump and includes a valve.

19. In an evaporator system, including a drum, a gas duct through which hot gas can pass, an evaporator located in said gas duct and including heat- absorbing tubes, a feedwater pump, a downcomer providing fluid communication between the drum and the evaporator inlet, and a riser providing fluid
communication between the evaporator outlet and the drum, the improvement comprising:
said evaporator being divided into first and second evaporator sections, each of said evaporator sections defining an inlet and an outlet, wherein said feedwater pump is in fluid communication with the inlet of said first evaporator section and said downcomer is in fluid communication with the inlet of said second evaporator section;
a connecting line providing fluid communication between the outlet of said first evaporator section and said drum; and
a riser in fluid communication between said second evaporator section and said drum.

20. In an evaporator system as recited in claim 19, wherein said connecting line extends from the outlet of the first evaporator section to the riser; and further comprising an injector at the point where the connecting line meets the riser, so that fluid flowing through the connecting line and through the injector helps pull fluid through the second evaporator section.