CA1054364A - Furnaces - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A furnace for heating one or more bodies in which heated gas is introduced into the furnace chamber through one or more injectors and are conveyed to the bodies in manner involving appreciable resistance to gas flow. In preferred arrangement a continuous metal strip is conveyed through the furnace on a gaseous support cushion, the gas being introduced into the furnace under pressure through injectors which communicate with the lower portion of the furnace. The injectors generate a substantial recirculation of gas within the furnace.

Description

; This invention relates to furnaces of the kind in which a stream of hea-ted gas (which term also includes gas mixtures) is arranged to be directed over a hody or bodies reguired to be heated or to be maintained at a predetermined temperature, and is especially, though not exclusively, con-- cerned with furnaces of this kind designed for heating strip or sheet material, hereinafter referred to simply as strip.
Where the body or bodies are required to be heated to a high temperature, involving a large volume flow of the gas, thermal losses may be considerable, and whilst recirculation of the gas may in some cases provide a way of reducing such thermal losses to some extent, practical difficulties have previously limited the usefulness of this technique. Thus for some appli~
cations the gas may have to negotiate restricted passages providing an appreciable resistance to gas flow, and high power blowers, compressors, or like devices may therefore be needed to impart sufficient pressure energy into the system in order to obtain the required circulation. However there is a limit to the temperature at which such devices as are currently available can operate satisfactorily at least for prolonged periods.
Accordingly, what is being broadly claimed herein is a furnace for the heat treatment of metal strip comprising a furnace chamber with heating means. The furnace has an internal wall structure which defines a flow path for gas recirculating internall~ within the chamber; a strip support surface extending lengthwise through the urnace chamber; a plurality of injectors for communication with a source of gas under pressure located externally of the chamber and operably connected with the furnace to place the interior of the furnace chamber under pressure to promote internal recirculation of gas within the furnace chamber, and means for directing recircula-ting gas over ' / 1- ~
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the-strip support surface to provide a gaseous cushion between the said surface and metal strip travelling through the furnace.
Such an arrangement has the advantaye that the pres-sure energy required for producing a rapid circulation of the gas is imparted to the gas within ~he furnace without the need for a blower, compressor, or other device, hereinafter referred to simply as blower means, being located in the recirculating-flow circuit, so that the blower means can be operated at a temperature which is lower than that of the gas circulated within the furnace chamber, it being found that the energy level of the gas injected into the furnace chamber can readily be arranged to produce a circulation of gas around the :-! -recirculating-flow circuit of between two and six times its own weight. At the same time the recirculation of gas within the furnace has the effect of reducing thermal losses, which could be considerable if the gas, afte~passing over the body ~` or bodies, were allowed to escape freely to the atmosphere.
It is, of course, not possible to avoid a certain amount of gas escaping from the furnace, and an additional quantity of fre~h gas is conveniently admitted to the blower means in order to make up the losses which are incurred.

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5~64 Preferably the ~urnace incorpoxat.e~ one or more : lnternal passage3 for returnlng ga~ which has pas~ed through the furnace chamber to the vicinity of the injactor mean8 r for recirculation through the furnace chamber.
The gas may be hea~ed externally of the furnace and/~r : by means of heaters located withiin the furnace. The principal source of heat to the body or bodies may compri~e the recirculated gas; the heating capacity of the gas may be ~upplemented by direct radiation from, for example, electrical resistance heating elements or gas fired -radlant tubes mo~nted on the furnace walls. AlternatiYe the principal heat energy source to the boay or bodie~
; may be provided by heaters such as electrical resistance heatlng elements located wlthln the furnace chamber.
In some cases a proportion o~ the gas a~ter it has been pas~ed over~he body or bodies may be recirculated to ~ ~
the blower means, and where the temperature of the heated -~ -gas exceeds that with which the ~aid blower means oan deal sat~s~actorily, mean~, such as a heat exchanger, may be provided for cooling the gas after it has left the chamber.
Means may~in some ~a~es~ he provide~ ~or varylng .. ~ . .
the proportions o re-circulated and fre~h gas ~ed into the chalr~er.
For some purpo es heat extracted from the gas may be used to heat up the fre~h ga~ used to make up losses .:
fr~m the ~ystem before it i~ admitted to the blow r means, thereby reduclng still ~urther the thermal losses from the :
~' 3y8tem. :
In some cases, however, the xecirculated and ~resh ga~ may be allowed to mix directly in appropriate proportion~
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before being admitted to the blower mean~, so a~ to reduce the temperature of the former to a value at which the blower means can operate satisfactorily, wi~hout any additional cooling means being required.
e invention has application, fQr example, to furnaces in which the body or bodies are accommodated ln the chamber so that the gas can be directed on to the surface of the body or bodies in the iorm of a multiplicity of ~ts having appreciable resistance to gas flow. Such furnaces are employed forthe jet heating o~ bodies in ~he form of sheet cr strip material where the jets are obtained, for exampla, by forcing the gas under pressure through a ~-suitably apertured plate, commonly known aB a jet plate so that pressure energy need~ to be imparted to the gas strearn to compensate for losses in the circuit which a~e particularly high across the jet plates, and possibly across : .......... . .
`1 any heater or heat-exchange means use for providing ~he .. thenmal energy.
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. However, the inventlon also has application to furnaces ln whlch the bodies are accommodated in the chamber in baskets or trays in a manner invol~ing appr~ciable resistance to gas ~low, so that high pressure energy i~
. required ko force the recirculating gas ~tream through or between the bodies ln order to provide a high and uniform : level o~ heat tr~nsfer.
. For the heat trea~ment of matal strip in particular - the furnace will b~ of considerably length in order to ensure adequate heating of the ~trip during its passage through the ~:
; furnace ahar~er, and in such a aase gas ls pre~erably lnjected lnto the chamber through a pluralit~ of in~ec~or , :
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means spaced apart along the furnac~. A ~i~gle blower means may be co~mon to all the lnjector means, although a ~umber of blowers each ~upplying gas to one or more injector mean~
may be employed. ~.
Where the furnace i~ used for heating metal or other ~trip with the strip arranged to be fed through the furnace chamber substantially hori20ntally or down a ~light incline, the injector means may be arranged to feed the said gas under pressure into the lower part of the chamber o ~hat it passes upwards through the chamber and provides a cushion o~
gas which act~ as a support for the strip on its passage :-through the chamber.
Thus according o the present invention in another ~ ;
aspectJa furnac~ for the continuous heat treatment of metal ~trip~in which the strip is supported on a gaseous cushlon as it travels through the furnace,comprise~ a furnace chamber including a lengthwise-extending strip upport ~urface, injector means for feeding gas under .
pressure into the furnace chamber so as to promote rapid ; .
recirculation of at least a ma~or proportion of the gas . ~ , .
;~ ~ contalned ln the furnace chambex and m~ans for directing ' recircul~ting gas over the strip support surface to provide ;- a gaseous cushlon be~ween said support ~urfa¢e and me al ~trip travelling through tha furnaca~
Preferably, the gas introduced into the ~urnace is den~e to minimise the volume required to support ~he 3trip.
' Light gases such as hydrogen can be employed that would entail .J the use o~ large gas volumes. Addittonally, the gas may ~e selected to provide a protectlve or reduclng atmo~phere .

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wi~hln the furnace cham~er. In one example the yas comprises a mixture of argon and hydrogen and in ano~her a mixture o~ nitrog~n and hydrogen.
The furnace chamber may include a plurality o ~ran~YerSe baffle~ which divide the lower part o~ the chamher into a nwmber of compartments, into each of which is fed ga3 tmder pres ure; the ratP of flow of sas into each compartm~nt may be controlled lndependently of the rate of flow into the othex compartments thereby enabllng the degrea of flo~tation of the strip at any region ofthe furnace to be adju-~ted as desired.
Thi~ i8 of advantaye, for example, during the threadi~g of strip through the ~urnace, or when a change o~ strip thlcknes~ i~ passed through it, as well a~ wh`en local-difficultle~ are experienced with floatation ~or reasons of trip, furn~ce or gas flow deterioration.
In some casesJ different gase~ or gaces at dlfferent ..! :
temperatures may be fed into the furnace either through the sam2 or diferent injector means, and with uch an arrangement the rates of ~low may be controlled, for examp~le, 80 as to produce a variation in the p~oportions of the constituents at dlfferent parts of the.furnace or on a time basl~
A number of ft~naae~ in accordance with the inv~ntion will now be de~cribed by way o e~ample with re~erence to Figt~es 1 to 7 of the accompanying schematlc drawing~, in whlch:- :
Figure~ 1 and 2 lllustrate ln dlagrammatic Sorm a perspective ~iew and a tran~verse cros~-~ection through the f 1 rst ~urnace, Flgure 3 illu~trateis, al80 in diagrammatic orm, a . -- 6 --.

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tran~ver~e sec~ion through the second furnace, ~ igure 4 shows a modified form of the fuxnaae illustrated ln Flgure 3, Flgure 5 lllus~rates a transver~e section through a further form of furnace, and Figures 6 and 7 axe respectively a plan and a ~:
transverse section through a still further form of furnace.
Referring first to Figures 1 and 2, the ~urnace ~
;, illustrated therein is designed to heat a continuously ~ :
` moving met~l ~trlp 1 and comprises an elongated chamber2 :
- through which the ~trip is fed substan~lally horiz~ntalIy, the furnace being closed apart ~rom narrow horlzon~al slots 3 `. at its two ends for the passage of the strip lnto and from the chamber.
Between the two side walls 4,5 of the furnace ~here .. .
iB located an internal wall 6 which extends the length of the furnace adjacent the wall ~, the internal wall ~ and the other side wall 5 defining the furnace cha~er and s~pporting between them a horizontal plate 7 formed with a multiplicity of openings B, the plate being slightly lower than the slots 3 :~
as shown in Figure 2.
The strip is de~igned to be heated by a gas, or gaR mixture fed into chamber by a ~uitable blower 9, means of any aonvenient klnd beingprovided for heating ~he gas to an appropriate temperature, either between the blower and ~;~
the chamber a~ shown at lO or alternatively after its entry ~:
into the chamber.
The gas is arranged to be i~troduced into the lower , ~! part of the furnace under pras~ure through a plurality o~

lnjectorn 11 =paced alorg th; nLde 4 o thc urnacc no that : , ' ' , , ,, ;
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they dlrect the gas into ~he fuxnace chamber between th walls 5,6, the pressuxe of the ga~ being such that it i8 forced upwards through the openiny~ ~ in the plate 7 which cause lt to be directed in the fonm of jet~ on to the 1~ OE
surface of the strip 1.
In addition the rate at which gas i~ lnjected into the furnace produces a pressure drop in the Yicinity of the injectors 11 sufficient ~o cau~e gas, after heating the strip 1, to be recirculated at a high rate from the upper to the lower part of the chamber through the passage 12 provided between the internal wall 6 and the adjacent slde wall 4. By causing ~a~ to be circulated wi~hin the furnace ln thi~ manner thermal losses can be considerably reduced, and the need for locating the blower ln the recirculatlng flow path is avoided.
Some gas will es~ape from the ends of he furnace, and losses are made up by fresh gas fed to ~he blower 9 .:
through inlet 13. ~ ;
In the second furnace illustrated in Figure 3 the exter~al heating means 10 is replaced by internal heater~, ~or example electrical resistance heating elements 14, conveniently fitted to the furnace walls 5,6 beneath the plate 7~. In addition part of the gas ~rom the upper part of the furnace chamber 2 is recirculated through the ~lower 9 thereby reducing the quantity of fresh and unheated nake-up gas required.
Since the temperature o~ the gas stream leaving the chamber 2 will normally be hlgher than that which can be reliably withstood by conventional ~ox~ of blowers a heat exchanger 15 is provided for coollng the gas between the chamber 2 and the blower 9.

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Means (not shown) may be pxovided for varying the proportions of the recirculated and fre~h ga~ fed into the furnace through the.in~ectors 11.
Electric heating elements may additionally or alternatively be placed in the passage 12 as 3hown at 14.
Other forms of heating can alte~natively be employed, ~or example gas burners located within the lower part of the Æurnace chamber.
In the modification of this second furnace which is illustrated in Figure 4 the heat exchanger 15 is used to raise the temperature of the fresh make-up gas beore it is admitted to the blower 9 , thereby reducing still fur~her the thermal losses of the system.
In some case i~ may be po~ible to dispense with ~he heat exchanger completely~ fresh gas at a suitably low temperature being mixed with the recirculated gas in ~:
uitable proportions in order to cool it before lt i5 returned to the blower.
Whilst, in the furnaces illustrated, the strip 1 i~
arranged to be fed through the furnace horizontally it ma~
alternatively be ~ed through the furnace in a vertical or incllned plane, the ~urnaae includlng the positionlng of the injectors 11 being modified accordingly. -H~we~er, in case~ whera the strip i9 arranged to befed horizontally or down a slight lncline the pressure of the gas injected into the furnace may ~e such that in addltion to heating the strip 1t al~o pro~lde~ a cushion of pre~surised gas ~ihich ~upports khe ~trip on its passage through th~
furnace.
In ~uch a ca~e the lower part o.f the chamber o~ a : ' _ 9 _ .

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furnace as shown in any o~ E~igure~ 1 ~o 4 may be ~plit into 3eve~al compartments by a number of transverse vartical wall3, (not shown), 3paced apart a dlstance of the order of two or three feet/ with their upper edges texminating just below the le~l of the slots 3, each o~ the compartment~
being associated with an indi~idual injec~or ll. Such an arrangement prevents local floata~ion being seriously affected by a different sltuation existing elsewhere in the fuxnace, and allows the flow through a particular region to be adjusted, ~or example by controlling the flow of gas thxough a respective injector ll, or by throttling the appropriate region of th~ return duct 12 wh~ch can be divided into a number of separate ~ections by means of tra~æverse walls (not shOWn).
In some cases, h~ated gas ma~ be required to be directed ~n the form of ~ts o~ to both surfaces of the strip 1, and a fuxnace for achie~ing ~hi~ !s shown chematically in Figure 5.
In this furnace the furnace chamber 2 accommodates a pair of horizontal jet plates 7A and 7B located above and below the slots 3, and so axranged that the heated gas from the injectors ll is dlrected on to the ~pper and lower ~urfaces oæthe plates through the hole~ 8 in the plates 7A
and 7B respectively, the rate at which gas is ~ed into the f~rnace khrough the injec~ors producing a recirculation of the gas fxom between the plates back into the ma~n part of the furnace thxough the pa~ags 12, wh~re it is redirected on to the ~trlp through the hole~ in the jet plates, in a manner similar to that of ~he urnaces illustrnted in Flgures 1 to 4. ~:
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The furnace lllus~ra~e~l ln F~.cJures 6 and 7 i3 again de~igned to heat a continuously moving metal ~trip 1 and comprises an elongate chamber 2 thxough which th ~trip i~
fed ln a sub3tantially horlzontal or slightly downwardly lnclined directlon~ More particularly, the furnace includes a water . .:
cooled entry zone 14, a heatlng zone 15 and an exit cooling zone 16. Pressure seals 17 are provided at the entrance to and exit from the furnace in order to mlnimise gas lo~ses to the atmosphere. Strip is fed to the furnace along a floatation table 18 and is withdrawn from the fuxnace over a roller table 19 by means of a pair of pinch rolls 20.
As described with reference to the previous embsdiments, ?
gas under pressure is fed into the heat1ng zone 15 through :~`~the injectors 11, ~he ga5 being compressed in the blower 9 and fed via the heaters 10 to manifold~ 21 in communication wi~h the indivldual injectors.
The lower part of the heating zone 15 is split into ~everal compartments by a pluraliky o~ spaced transverse vertical walls 22. The upper edges of the walls 22 ~ermina~e just below the level of khe entry and exit ports of the ~ .
furnace ~nd each compa~tment is ~upplied with gas under .; :
pre~sure through an individual injector 11 or through a ba~ch of injectors. This arr~ngement prevents local floatation being serlou ly affected by a different situa~ion exi~ting : elsewhere in the furnace, and allows the flow through a particu~r region to be ad~u3ted, for ex~mple by aontrolling , the flow of gas through a re~pective injec~or 11.
; As illustrated in Flgure 7 each injector 11 extends ~ through one re~raator~ lined side w~ll 4 of the ~urnace , ' ~' . .
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to di~charge gas under pre3sure thxough an opening 23 orm~d in the furnace haarth lnto a plenum chamber 24.
The hearth lnclude~ two vextical walls 25,~6 provided at their upper ends with outwardly inclined lips 27 running horizontally along the full length of the furnace.
Electrical heating el~ments 28 are mounted on the side walls of the furnace chamber. Ducting 29 is provided in the upper portion of the side wall 4 to convey ga~ from the ~urnace chamber to an external gas circulation circuit whlch includes a cooler 30, the blower 9, a d~yer 31, heaters 10 and injecto~ manifolds 21. Gas under pressure is fed frQm the manifolds 21 to the injectors 11.
In operation of the furnaces illustrated in ~igures 6 and 7 metal strip i~ admltted to the furnace through the entry seal 17 and lssupported on its passage through the furnace chamber above the lips 27 on cu~hionæ oi gas supplied from the plenum ch~mber 24, In turn the plenum chamber i8 supplied with gas under pre~sure from the :: .
injectors 11. The gas jets issuing from the injectors 11 induce gas already present in the furnace to flow ~hrough the passage 23 into the plenum chambers 24. As indicated by arrows in Figure 7, the gas fram the plenum chamber 24 pas~es between the opposed sur~aces o~ the lips ~7 and the strip 1 and is caused to reairclllate via a side passage 32 back to the ~icinity o~ the injsctors 11 to be returned t~
the plenum chaMber 2~.
A portion of the recirculating gas is withdrawn from the ~urnace thro~gh the ductlng 29, is cooled and then compre~ed, dried and heated before b~ing returned to the furnace by the injector manifolds 21.

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In ~reatments in whlch the ma~s of ga~ re~uired to provide the hea~ to ralse the strlp to the requi~ed temperature and at the same time to support the strip during it5 passage through the furnace axe mutuall~ compatible, no additional heat source other th~n the heat energy contained ln the gas wlll be required. Thu5, in the furnace illustrated, the entire heat energy input to the yas may be dari~ed from the heaters 10 the heater elements 28 either being reme~ed or switched off. In other treatments however, where the throughput of strip through the furnace is large and/ox wh~n the desired strip temperature i8 in exce~s o~ the gas : :
temperature, additional sources of heat are required. In the~e cases, the electrical x~sistance heating elements 28~ :
or gas flred radiant tubes located within the ~urnace chamber provide ~irect radiation heating to the strip.
This situation arises when slntering strip made by roll compaction of metal powders~ where the desired tempra~ure~ :
may in the case of stai~less steel~ fall within the range ~ -1000 to 1400C.
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It will be appreciated that use o the injQctor~

promotes rapid recirculation o~ the gases within the furnace . ,~ .
to provide the læge volum~ of ga~ re~uired to support and heat the ~trip within the furnace both economlcally and ~ ~:
practicahly. These criteria are of particular importance when the nature o the strip being heat kreatad requi~as a ~urnace atmosphere which i8 relatively expensive. For . . .
example, ~tainless st~el strip made by roll compaating stalnles~ steel powder can advantageou~ly be sintered in atmospheres composed o~ mlxture~ of argon and hydrogen , . . .

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by employing in~ectors to feed the ~upport ga~ lnto the lower part of the furnace chamber, the mass of gas :~
delivered to the plsnum chamber 24 being lncreased by virtue of ~he induced internal reGirculation by a factor of between 2:1 and 6:1 compared with the mass of gas fed to ~he injectors. Further signi~icant economies are achieved by recirculating a proportion of the gas after ji it has passed over the trip externally of the furnace to the blower compressor 9 for re-injection to the furnace.

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Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A furnace for the heat treatment of metal strip comprising a furnace chamber with heating means having an internal wall structure defining a flow path for gas recircula-ting internally within the chamber, a strip support surface extending lengthwise through the furnace chamber, a plurality of injectors in communication with a source of gas under pres-sure located externally of said chamber and operably connected with the furnace to place the interior of the furnace chamber under pressure to promote internal recirculation of gas within the furnace chamber and means for directing recircula-ting gas over the strip support surface to provide a gaseous cushion between said surface and metal strip travelling through the furnace.

2. A furnace as claimed in claim 1 wherein the strip support surface extends along the full length of the furnace chamber.

3. A furnace as claimed in claim 1 wherein the strip support surface comprises an apertured plate positioned immediately below the path to be taken by metal strip as it travels through the furnace, the internal wall structure being such that recirculating gas passes upwardly through the apertures of the plate into contact with the undersurface of strip travelling through the furnace chamber.

4. A furnace as claimed in claim 1 wherein the strip support surface comprises two substantially parallel members spaced apart by a distance less than the width of strip to be heat treated within the furnace.

5. A furnace as claimed in claim 4 wherein the members are inclined downwards at a small angle to the horizontal towards their inner edges.

6. A furnace as claimed in claim 1 wherein the strip support surface comprises outwardly inclined walls of a trough extending along substantially the full length of the furnace chamber, the internal wall structure being such that recirculating gas passes into the base of the trough and flows outwardly therefrom over the said inclined walls of the trough.

7. A furnace as claimed in claim 1 wherein the lower portion of the furnace chamber is divided into several compartments by spaced transverse vertical walls having upper edges which terminate below the path to be taken by metal strip as it travels through the furnace.

8. A furnace as claimed in claim 1 wherein a minor proportion of the gas recirculating within the furnace chamber is withdrawn from the chamber via external ducting, is conveyed by the ducting to a blower and is readmitted at a raised pressure to the furnace chamber through the injectors.

9. A furnace as claimed in claim 8 wherein gas withdrawn from the furnace chamber is heated as it passes between the blower and the injectors.

10. A furnace as claimed in claim 8 wherein a quantity of fresh gas is admitted to the blower means in order to make up for gas escaping from the furnace.

11. A furnace as claimed in claim 8 wherein a single blower means is provided to feed gas under pressure to all injectors.

12. A furnace as claimed in claim 8 wherein the blower means comprises a plurality of individual blowers connected one to feed each injector.

13. A furnace as claimed in claim 1 further com-prising a plurality of electric heating elements mounted within the furnace chamber to heat the recirculating gas.

14. A furnace for the continuous heat treatment of metal strip comprising a furnace chamber having an internal wall structure defining a flow path for gas recirculating internally within the chamber, a trough extending along substantially the full length of the furnace chamber and inclu-ding a base and divergent side walls extending upwardly from the base which define strip support surfaces for strip tra-velling through the furnace chamber, a plurality of injectors operable to place the interior of the furnace chamber in com-munication with a source of gas under pressure to promote internal recirculation of gas within the furnace chamber, and means for directing recirculating gas into the trough whereby said gas flows upwardly over the side walls of the trough to provide a gaseous cushion between said walls and metal strip travelling through the furnace.

15. A furnace as claimed in claim 14 wherein the injectors extend through the side walls of the furnace chamber at locations immediately upstream of the means for directing recirculating gas into the trough.

16. A furnace as claimed in claim 14 wherein the internal wall structure includes an upstanding partition member extending lengthwise of the furnace chamber to one side of the trough to define with one side wall of the furnace cham-ber, a channel for directing recirculating gas to the base of the trough.