US3164134A

US3164134A - Apparatus and method for operating a forced flow once-through vapor generator

Info

Publication number: US3164134A
Application number: US238887A
Authority: US
Inventors: Jr Edward L Kochey
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1962-11-20
Filing date: 1962-11-20
Publication date: 1965-01-05
Anticipated expiration: 1982-01-05
Also published as: BE640158A; CH433391A; ES293654A1; GB1052385A

Description

Jan. 5, 1965 E. KOCHEY, JR 3,

APPARATUS AND METHOD FOR OPERATING A FORCED FLOW ONCE-THROUGH VAPOR GENERATOR 2 Sheets-Sheet 1 Filed Nov. 20, 1962 54" X ZQ FINISHING SUPERHEATER AUXILIARY VAPOR SOURCE PRIMARY SUPERHEATER DE'AERATOR ecouomzen PREHEATER INVENTORZ EDWARD L..KOCHEY JR. BY wafi AGENT Jan. 5, 1965 E. L. KocI-IEY, JR 3,164,134

APPARATUS AND METHOD FOR OPERATING A FORCED FLOW ONCE-THROUGH VAPOR GENERATOR 2 Sheets-Sheet 2 Filed Nov. 20, 1962 I FINISHING ./I- SUPERHEATER PRIMARY SUPERHEATER ECONOMIZER I l l I 1 l l l I I l DE'AERATER FIG. 2

INVENTOR. EDWARD L. KOCHEY JR. 3%?

AGENT United States Patent 3,164,134 APPARATUS AND METHGD FOR OPERATING A FORED FLOW ONCE-THROUGH VAPOR GEN- ERATOR Edward L. Koehey, Jr., Colebrook, Conn., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Nov. 20, 1962, Ser. No. 238,887 9 Claims. (Ci. 122-406) The invention relates in general to a forced flow modified once-through vapor generating power plant and more particularly to an apparatus and method for starting up the vapor generator and turbine associated therewith.

It is known in starting up forced flow once-through vapor generators to separate the vapor superheating and turbine portion from the vapor generating portion by a so-called boiler throttling valve. Such separation permits the starting up of the steam generating portion independent and simultaneously with the start-up operation of the turbine with, for instance, vapor being used from an auxiliary source for the latter operation. Such separation also permits the starting up of the vapor generator with only vapor entering the superheater portion whether such vapor originates from an auxiliary source or is obtained by throttling and expansion of the high pressure fluid heated in the vapor generating portion.

In large vapor generators generally two or more superheating sections are employed. In these instances the boiler throttling valve is preferably located downstream of the primary superheater instead of upstream thereof. This permits heating of the working fluid in the vapor generating portion to a higher temperature such as 800 F. before this fluid is expanded to the lower pressure region of the intermediate or final superheater section during a later phase in the start-up procedure. In forced flow modified once-through vapor generators a recirculation circuit is used which recirculates the working fluid around the vapor generating section for the purpose of speedily raising the pressure and temperature thereof to the desired operating values. To obtain accurate measurements of the temperature of the fluid at a point between the primary superheater outlet and the boiler throttling valve, some flow of this fluid must be established away from such point to a lower pressure region.

It is accordingly a primary object of the invention to establish this flow Without destroying the advantages of the recirculation circuit and Without necessitating the feeding of working fluid to the vapor generator or the draining of such fluid from the vapor generating system during a major phase of the start-up operation.

Other objects and advantages will become apparent as the description of illustrative embodiments of the invention proceeds. The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. These claims as Well as the following description of the invention will best be understood when read in conjunction with the accompanying drawing in which:

FIG. 1 is a representation of a vapor power plant system in the form of a flow diagram incorporating the features of the present invention as employed in connection with a vapor generator using vapor from an auxiliary source for starting up the turbine associated with the vapor generator.

FIG. 2 is a flow diagram representing a vapor power.

plant system similar to that depicted in FIG. 1, however illustrating the invention in connection with a vapor generator wherein the vapor for cooling the superheater and starting the turbine is produced by expanding and flashing the high pressure working liquid.

Referring now to the drawing in which like reference characters are used throughout to designate like elements, the diagrammatic representation of FIG. 1 shows a forced flow once-through vapor generator 10. A feed pump 12 is organized to supply working fluid to vapor generator 10 froma source such as de-aerator 14 by Way of conduit 16 and via a fluid preheater 18. To control or shut-ofl the flow of working fluid a feed valve 24) is provided in conduit 16. The working fluid passes through economizer 22, conduit 23, vapor generating section 24, conduit 25, primary superheater 26, conduit 27, finishing superheater 28 and conduit 29 to a point of use such as vapor turbine-30. A valve 31 is provided in conduit 29 to shut-off the flow of the working fluid to turbine 30. After having given up a major portion of its thermal energy, the vapor is condensed in condenser 32 and the condensate returned to de-aerator 14 through conduit 33 via condenser pump 34 and heater 35.

Fuel and air for combustion are supplied to vapor generator 10 by way of burner 36 in any conventional manner. The hot combustion gases produced by the burning of the fuel pass in heat exchange relation overthe heat absorbing surfaces of the vapor. generating section 24,

superheating sections

26 and 28 and economizer 22. Oth er conventional means of supplying heat to the vapor generator may be used in connection with the invention.

In accordance with the invention a source 38 of auxili ary vapor is provided for supplying vapor during the startup operation to primary superheater 26 by way of conduit 40 including valve 42. An important element of the inventive combination herein disclosed is the provision of a recirculating conduit 44 including valves 4-6 and recirculating pump 48 for the recirculation of working fluid from the outlet of vapor generating section 24 to the inlet thereof. In addition an overflow conduit 50' is provided;

including

valves

51 and 52 which connects the outlet of primary superheater 26 to a point of low pressure such as the hot well of condenser 32 or de-aerator 14; Furthermore the invention contemplates the use of a by-pass conduit 53 including valve 54 for lay-passing the turbine during an early phase of the start-up operation. Another important element of the inventive combination is the provision in conduit 25 of a shut-off valve 55 including a throttling valve 56 arranged in conduit 57 which bypasses valve 55. The purpose of bypass valve 56 is to obtain throttling with high pressure drop and/or small flow quantities, while the main shut-off valve 55 is designed for relatively low pressure drops and large flow quantities.

The forced flow modified once-through vapor generator is started up according to the invention with the following general considerations in mind. First as earlier mentioned herein the vapor generating power plant is divided into two main portions. These are separated by a socalled boiler throttling valve 55. With this valve closed it' is possible by use of vapor obtained from an auxiliary source 38 and superheated by passing it through

superheaters

26 and 28, to warm up and roll the turbine long before any vapor has been generated in the vapor generating portion 24 of the generator 10. During the. time the turbine is thus being prepared for full operation with auxiliary vapor, the vapor generator is also being brought up to temperature and pressure with a vaporizable fluid which fluid may have previously been de-aerated also by the use of vapor obtained from auxiliary source 38.

Accordingly the starting up operation of th e forcedflow once-through vapor powerplant commences with the closing of the boiler throttling valve 55 and bypass valve 56, and the opening of boiler extraction or

overflow valves

51 and 52. Vaporizable fluid is pumped from the deaerator through economizer 22 and vapor generating section2l by means of feed pump 12 via feed valve 29. This fluid is first being discharged to a point of lower pressure such as condenser 32 by way of extraction or overflow line Stl and extraction valves Bland 52, while the unit is being fired up. The heated fluid is thus discharged until the socalled clean-up operationof the vapor generator is completed, i.e., until the working fluid has been purged of decontaminants such as iron oxide which may have been present in the Working fluid. At such time feeding of the working fluid is discontinued by closing feed valve Eli and also by closing extraction valve .51, with the latter however being set to open automatically at a predetermined higher pressure. This is necessary in order to allow for overflow due to expansion of the fluid as the fluid is being heated in the'vapor generating section 24.

A second consideration of the present invention is the provision for protecting the tube lined walls 24 of th furnace from overheating while the feed valve 25 is closed. This is accomplished by recirculating the Working fluid from the outlet of the vapor generating section to the inlet thereof by way of conduit 44 and valve 46 and by means of recirculating pump 48. Recirculation of the workingfluid thus serves two purposes. First it provides for eflicient cooling of the tubular furnace surface exposed to radiant heat, thereby permitting a faster increase of the firing r ate. And second, such recirculation simultaneously results in a quicker heating of the fluid since none of the heated fluid is being'discharged to waste. Such discharge and waste of heat would be unavoidable without recirculation since a minimum velocity of the working fluid must be-maintainedin the furnace tubes to prevent overheatingthere'of.

While the vapor generating portion of the unit is thus prepared for full operation in a minimum of time by virtue of the recirculation circuit the turbine is warmed up at the same time and rolled by being supplied with auxiliary vapor from source 38 with this vapor having been superheated in

superheaters

26 and 28. When the fluid contained in vapor generating section 24 and continually being recirculated thereabout has reached a temperature and pressure exceeding the temperature and pressure in the superheater 26 by a predetermined desired amount, valve 56 in bypass 54- is opened with the high temperature fluid expanding into primary'superheater 26, which; leads to the. third and most important consideration of the inventive combination.

This third consideration hinges upon the fact that in V order to accurately measure the temperature of the Working fluid in the conduit 27 upstream of valves 55 and 56 a continuous flow must be established out of conduit 27. It is of primary importance that the pressure and temperature in conduit 2'7be accurately obtained'since the difference in pressure and tempertaure between the fluid or vapor in vapor generating section ZAand that in superheater 26 is chosen to be of such an amount that the throttling action taking place in valve 55 results in a flashing of vapor without any appreciable amount of liquid entering the primary superheater 26. While flow of the working fluid out of conduit 27 could be accomplished by way of conduit 56 and valve Sisuch flow at this time would not be desirable for several reasons of economy. a

First, such flow would lower the eihciency or" operation by discharging a portion of the heated high pressure fluid to a region of low pressure such as the hot well of condenser 32 or the de-aerator 14;. Second, a large portion of the heat containedin'the discharged fluid would bej-lost I if conducted to the condenser. Such heat as well as pressure would have to be added again, to the relatively cold working fluid that would have to be supplied byway of feed conduit-Hand feed pump 12. And third, the time for bringing the working fluid that is recirculating around furnace walls 24 to the desired high temperature and pressure would be unduly prolonged,- thereby partially defeaing the beneficial result of recirculating conduit er.

To avoid the above'disadvantages the invention provides a return conduit as leading from conduit 27 to the recircula-ting conduit 44 or to the inlet of vapor generating section 24. A continuous flow of fluid is thereby established out of the primary superheater 26 without discharging this flow to waste, or appreciably lowering the pressure thereof, or requiring replenishment of hot high pressure fluid by relatively cool working fluid which must be supplied by feed pump 18, or prolonging the time required to heat the working fluid that is recirculating through furnace Wall 24 and to raise it to operating pressure. Thus a continuous flow of fluid is being provided leaving conduit 27. The pressure and temperature of this fluid can conveniently and accurately be measured by means of

devices

64 and 66 respectively and continuously be observed while the vapor generator is prepared for full operation. When the pressure and temperature have reached values which assure a complete flashing into vapor of the working fluid when it is throttled down to the pressure prevailing in finishing superheater 28, valve 56 is cracked open either manually or by the action of control element 68 receiving temperature and/or pressure impulses from measuring devices 66 and d4. Simultaneously with the opening of bypass valve 56 feed valve 20 is gradually being opened to re-establish flow of working fluid into the vapor generator. Also the flow of auxiliary vapor is proportionally reduced by controlling valve 42. The feeding of relatively cool working fluid into economizer 22 and vapor generating section 24 requires a corresponding increase in heat input to the vapor generator 1% Flow of high pressure, high temperature fluid from vapor generating section 24 to primary superheater 2t: continues through valve 56 until the pressure in primary superheater approaches that prevailing in vapor generating section 24*. At such time valve 55 is opened and valve 42 closed to establish normal operation of the vapor generator and associated turbine.

FIG. 2 is a representation of a forced flow vapor generating system wherein the vapor for cooling the superheater 28 and starting turbine 3% is not supplied from an auxiliary source but is produced only by flashing which results from expanding the high pressure liquid leaving primary superheater 26 and valve 56.

The advantages derived from employing return line 6% in accordance with the invention are even greater when used in connection with the vapor generator illustrativciy shown in FIG. 2, than when used in connection with the vapor generator shown in FIG. 1. This is so because in the FIG. 2 embodiment the pressure and temperature of the fluid leaving primary superheater 26 via return line 66 must be closely watched even during the early phase of start-up operation so that steam by flashing can be supplied to the heating surfaces of finishing superheater as early as possible to protect these surfaces from the heat of the combustion gases sweeping thereover. While steam is thus supplied to the superheater 23 via throttle valve 56, turbine valve 31 is closed and by-pass valve 54 is controlled so as to gradually raise the pressure and temperature of the vapor to desired values suitable for rolling and synchronizing turbine 30. With these considerations in mind the start-up procedure or" the power plant depicted in FIG. 2 proceeds in a like manner as that earlier herein described in connection with the power plant shown in FIG. 1.

While specific embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that various changes, modifications, substitutions, additions and omissions may be made therein without departing from the spirit and scope of the invention as set orth in the appended claims.

7 What I claim is:

1. A method of starting up a forced flow modified once- Y through vapor generator having a first heatin g section inarranged for flow of a vaporizable fluid therethrough; the

continuing the feeding of vaporizable fluid to said first section while recirculating said fluid from the outlet of,

said furnace walls to the inlet thereof to maintain the flow velocity in said furnace walls above a predetermined minimum velocity; permitting a second overflow from the outlet of said primary superheater to the inlet of said furnace walls while discontinuing said first overflow; obtaining an indication of the temperature and pressure of said second over-flow; gradually establishing fiow from said primary superheater to said finishing superheater when the temperature and pressure of said second overflow reach predetermined values; and re-establishing a flow of vapor- TlZSJblC fluid to said vapor generator generally proportional to the flow from saidfinishing superheater.

2. A method of starting up a forced flow modified oncethrough steam generator having a first heating section including steam generating furnace walls and primary superheater, and having a second heating section including a finishing superheater, said heating sections being serially arranged for flow of water and steam therethrough; the invention comprising feeding water to said furnace walls and primary superheater only; while blocking the flow from said primary superheater to said finishing superheater heating said water by supplying fuel and air to said steam generator for burning and producing hot combustion gases flowing in heat exchange relation over the heating surfaces thereof including said furnace walls, primary superheater and finishing superheater; permitting a first overflow of heated water due to expansion thereof and for cleaning-up purposes from said first heating section to a point of lower pressure; discontinuing the feeding of water to said first section; while recirculating said water from the outlet of said furnace walls to the inlet thereof to maintain a predetermined :rninimum flow velocity in said furnace walls; permitting a second overflow from the outlet of said primary superheater to the inlet of said furnace walls; obtaining an indication of the temperature iand pressure of said secondoverflow; and gradually esta-blishing flow from said primary superheater to saidfinishing superheater when the temperature and pressure of said second overflow reach predetermined values, and re-establishing a flow of feedwaterto said steam generator generally proportional to the output flow from said steam generator. I i

7 3. A method of starting up a forced flow modified oncethrough steam generator having a first heating section including an economizer, steam generating furnace walls and primary superheater and having a second heating section including a finishing superheater said heating sections being serially arranged for flow of water and steam therethrough; the inventioncomprising feeding water to'said economizer, furnace walls andprimary superheater-only;

heating said feedwater by supplying fuel and air to said steam generator for burning and producing hot combustion gases flowing in *heat exchange relation over the heat ing surfaces of said furnace walls, primarysuperheater,

finishing superheater and, economizer; permitting a first] overflow of heated water due to expansion thereof 'and for cleaning-up purposes from the outlet of said primary superheater to a point of lower pressure; discontinuing .the feeding of water to said first section while recirculating said feedwater from the outlet of said furnace walls to the to said finishing superheater when the temperatureand pressure of said second overflow has reached predetermined values, and re-esta'blishinga flow of feedwater to said steam generator generally proportional to the output flow from said steam generator.

4. A method of starting up a forced flow modified oncethrough vapor generator having a first heating section 1n-- cluding vapor generating furnace walls and a primary superheater, and having a second heating section including a finishing superheater, said heating sections being serially arranged for flow of a vaporizable fluid therethrough; and an auxiliary source of vapor; the invention comprising feeding vaporizable fluid to said first heating section only; feeding vapor from said auxiliary source to said second heating section; heating said vaporizable fluid and said vapor by supplying heat to said first and second heating sections; permitting a first overflow of heated vaporizable fluid from said first heating section to a point of lower pressure; flowing said heated vapor to a point of use; discontinuing the feeding of vaporizable fluid to said first section while recirculating said fluid from the outlet of said furnace walls to the inlet thereof to maintain the flow velocity in said furnace walls above a predetermined minimum velocity; permitting a second overflow from the outlet of said primary superheater to the inlet of said furnace walls 'while'discontinuing said first overflow; obtaining an indication of the temperature and pressure of said second overflow; gradually establishing flow from said primary superheater to said finishing superheater when the temperature and pressure of said second overflow reach predetermined values while discontinuing the feeding of vapor from said auxiliary source; and re-esta-blishing a flow of vaporizable fluid to said steam generatorgenerally proportional to the output flow from said steam generator. 5. In a forced flow modified once-through steam generatorhaving a first'heating section including steam generating furnace'walls and primary superheater, and having a second heating section including a finishing superheater,

means for shutting off flow of water and steam to said second heating section; means: for supplying heat 'to said first and second heatingsections; means for permitting a first overflow of heated water from said first heating section to a point of lower pressure; shut-off means for dis-: continuing the feeding of water to saidfirst section; means for recirculating said heated water from the outlet of said furnacewalls to the inlet thereof to maintain the flow velocity thereof in said furnace walls above a predeter mined minimum velocity; means for permitting a second overflow from the outlet of said primary superheater to the inlet of said furnacewalls; shut-off means for dis continuing said first-overflow; means for obtaining an indication of the temperature and pressure ofsaid-second, overflow; means for gradually establishing flow from said primary superheater to said finishing superheater when the temperature and pressure of said second overflow reach predetermined values, and means for re-establishing a flow of feedwater to saidsteam generator generally proportional to .the'output flow therefrom. a

6; In a forced flow modified once-through steam generator having a first heating section including steam gen erating furnace walls and a primary superheater, and having a second heating section including a finishing superheater, said heating sections being serially arranged for inlet thereof to maintain the flow velocity in said furnace walls above a predetermined minimum velocity; permitting a second overflow from the outlet of said primary superheater to the inlet 'of said furnace walls while dis continuing said first overflow; obtaining an indication of the temperature and pressure of said second overflow;

gradually establishing flow from said primary superheater flow of water and steam therethrough; the combination comprising means for feeding water to said first heating section; means for heating said water by supplying fuel and air to said steam generator for burning and producing hot combustion gases flowing in heat exchange relation over the heating surfaces of said steam generator including said furnace walls, primary superheater, and

finishing superheater means for permitting a first overflow of heated water due to expansion thereof and for cleaning- 7 up purposes from said first heating section'to a pointof lower pressure; shut-01f means for discontinuing the feeding of water to said first section; means" for recirculat ing said water from the outlet ;of said furnace walls to the inlet thereof to maintain a predeterminedminimum flow I indication of the temperature and pressure of said second overflow; means for gradually establishing flow from said primary superheater to said finishing superheater when t the temperature and pressure of said second overflow reach predetermined values, and means form-establishing a flow of feedwater to said steam generator generally proportional tothe output flow therefrom.'

g 7. In a forced flow modified once-through steam gen erator having a first heating'section including an economizer, steam generating furnace walls,'.primary superheater and a second heating section including a finishing superheater said heating sections being serially arranged for flow of Water and steam'therethrough; the combination comprising means for feeding water tosaid first heating section; means for heating said water by supplying fuel and air to said steam generator for burning and producing hot combustion gases flowing in heat exchange relation over the heating surfaces ofsaid furnace walls; primary superheater, finishing superheater and economizer; means for permitting a first overflow of heated water due to expansion thereof and for cleaning-up purposes from the outlet of said primary superheater to a point of lower pressure; shut-off means for discontinuing the feeding of Water to 'said first section; means for recirculating said 7 water from the outlet of "said furnace'walls to the inlet thereof to maintain the flow velocity thereof in said furnace walls above a; predetermined minimum velocity;

means for-'permitti-ng'a'second overflow'from the outlet of said primary Isuperheater to the inlet of "said furnace walls; shut off means for discontinuing said first overflow;

' .ineans for obtaining an indication of the temperature and,

pressure of'saidsec'ond overflow;-means'for gradually establishing'fiowfrom said'primary superheater to said finishing jsuperheater when the temperature and pressure of said second overflow reach predetermined values,'and

means for re-establishing a'flow of feedwater to said steam generator generally proportional to the output flow there- 8. In .a forced flow modified once-through steam generator having a first heating section including steam generating furnace walls and primary superheater, and shaving a second heating section including a finishing superheater,

said heating sections being seriallyarranged for flow of water and steam therethroug'h; anda sourcebf auxiliary steam; the combination'comprising means for feeding waater to said first heating section; means for shutting ofi flow -I of water and steam to said second heatingsection; means for supplying auxiliary steam to said second heating section; means for supplying heat' to said first and second heating sections; means for-"flowing steam; generated in said steam. generator to a point of use; means for permitting a'first overflow'of heated'water from said first heating section to a point of lower pressure; shut-off means, for discontinuing the feeding of water to said first section; means for recirculating said heated water from the outlet ing section; and means for re-establishing a flow of feedwater to said steam generator generally proportional to the output flow therefrom.

9. In a forced flow modified oncethrough steam generator having a first heating section including an economizer, steam generating furnace walls, primary superheater and a second heating section including a finishing superheater said heating sections being serially arranged for fiow of water and steam therethrough; and a source of auxiliary steam, the combination comprising means for feeding water to said first heating section; means for feeding vapor from said auxiliary source to said second heating section; heating-said feedwater to said auxiliary vapor by supplying fuel and air to said steam generator for burning and producing hot combustion gases flowing in heat exchange relation over the heating surfaces of said furnace walls, primary superheater, finishing superheater and economizer; means for permitting a first overflow of heated water due to expansion thereof and for cleaningup purposes from the outlet of said primary superheater to a point of lower pressure; shut-off means for discontinuing the feeding of water to said first section; means for recirculating said feedwater from the outlet of said furnace walls to the inlet thereof tomaintain the flow velocity in said furnace walls above a predetermined minimum ve'olcity; means for permitting a second overflow from the outlet of said primary superheater to the inlet of said furnace walls; shut-off means for discontinuing said first overflow; means for obtaining an indication of the temperature and pressure of said second overflow; means for gradually establishing flow from said primary superheater to said finishing superheater when the temperature and a, pressure of said second overflow reach predetermined and Control System Integration With the Turbine Gen- 1 values; means for discontinuing feeding of vapor from said auxiliary source; and means for re-esta-blishing a flow of feedwater to said steam generator generally proportional to the out'put fiow therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 851,784 Great Britain Oct. 19,1960

' OTHER "REFERENCES I Large Sub and SupercriticalSteam Generator Startup erator,. by Charles Strohmeyer, In, reprint of paper preof said furnace walls to the inlet thereof to maintain the flow velocity thereof in said furnace walls above a pre second-overflow from the outlet of'said primary superdetermined minimum velocity; means for permitting a I sented at American Power Conference, Chicago, lllinois,

March 1962. 1 V

Mitteilungen Number 1, published September 1956 by Dorrwerke Aktiengesellschaft, Ratingen, Germany.

Claims

1. A METHOD OF STARTING UP A FORCED FLOW MODIFIED ONCETHROUGH VAPOR GENERATOR HAVING A FIRST HEATING SECTION INCLUDING VAPOR GENERATING FURNACE WALLS AND A PRIMARY SUPERHEATER AND HAVING A SECOND HEATING SECTION INCLUDING A FINISHING SUPERHEATER SAID HEATING SECTIONS BEING SERIALLY ARRANGED FOR FLOW OF A VAPORIZABLE FLUID THERETHROUGH; THE INVENTION COMPRISING FEEDING VAPORIZABLE FLUID TO SAID FIRST HEATING SECTION ONLY; HEATING SAID VAPORIZABLE FLUID BY SUPPLYING HEAT TO SAID FIRST AND SECOND HEATING SECTIONS; PERMITTING A FIRST OVERFLOW OF HEATED VAPORIZABLE FLUID FROM SAID FIRST HEATING SECTION TO A POINT OF LOWER PRESSURE; DISCONTINUING THE FEEDING OF VAPORIZABLE FLUID TO SAID FIRST SECTION WHILE RECIRCULATING SAID FLUID FROM THE OUTLET OF