FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
& THE PATENTS RULES, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13) A COMPACT THERMAL OIL HEATER
THERMAX LIMITED
an Indian Company of D-13, MIDC Industrial Area, R.D. Aga Road, Chinchwad, Pune - 411019, Maharashtra, INDIA
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

Field of the Invention:
The present invention generally relates to heat-exchangers, more particuiariy, the present invention relates to an oil-heater that is fired by solid fuel.
Definitions of the terms used in the specification:
Pass - The term "Pass" used in the specification means a passage of heating media through one or more set of serpentine coils carrying the thermic fluid to be heated. In case of multiple passes more number of serpentine coils set may be arranged for the exchange of heat between two mediums.
Serpentine coils - The term "serpentine coils" used in the specification is a serpentine type arrangement of tubes.
Counter flow - The term "counter-flow" used in the specification is a configuration wherein inlet of hot fluid is in contact with outlet of cold fluid and inlet of cold fluid is in contact with outlet of hot fluid.
Thermic fluid heater - The term "Thermic fluid heater" used in the specification is a type of fired heater in which hot gases produced in the combustion process is used to heat thermal oil.
These definitions are in addition to those expressed in the art.

Background of the Invention:
Conventionally, solid fuel fired oil-heater includes a plurality of circular helical coils, inside which oil to be heated flows. The plurality of circular helical coils carrying the oil to be heated is disposed near a solid fuel combustor, wherein the solid fuel combustor is disposed at a base of the oil-heater. The solid fuel combustor is adapted to burn solid fuel such as coal, wood, pellets etc. to define a combustion zone and generate high temperature combustion products or flue gases. The flue gases emanating from the combustion zone allow for the transfer of heat, thereby heating a continuous flow of oil contained in the circular helical coils disposed near the solid fuel combustor.
The circular helical coils used in the solid fuel fired oil-heater possesses poor heat transfer coefficient and require higher heat transfer areas as is evident from graph illustrated in Figure 1. The graph illustrated in Figure 1 depicts a comparative heat transfer performance of serpentine coil and helical coil under the same set of design parameter and operating conditions (i.e.76.1 mm tube and velocity of 15m/s). As is clear from the graph, there is a significant gap in the heat transfer coefficient of the serpentine coil and helical coil at the same velocity conditions. Also, in case of helical coils the heat transfer coefficient decreases with increase in flow. This is due to the fact that the helical coil requires higher gap at higher flow to maintain same velocity and the performance of the helical coils worsens with increase in gap between the coils.

Furthermore, the helical coils are hollow and occupy more space as the ratio of heat transfer area to volume is very low for helical coils, thereby making the entire structure bulky and difficult to handle. In order to increase the heat transfer, the helical coils may be replaced with serpentine coils that are adapted to provide higher heat transfer coefficient in comparison with helical coils. Also, rectangular helical coils may be used as radiant coils but such coils are required to be separated from the combustion zone by using a refractory wall or by placing the coils within a refractory chamber, thereby making the configuration bulky. Further, the use of the serpentine coils results in difficulty in pass arrangement and limits the heat transfer arrangement inside the oil-heater to a single pass design. The oil heaters with single pass design results in higher flue gas temperature and lower efficiency. In order to increase the heat transfer, multiple passes of the heated flue gases over the oil carrying coils may be provided. However, the multi-pass arrangement of the heated flue gases over the oil carrying coils of the solid fuel fired heaters makes the heaters bulky and difficult to handle, as the multi-pass configuration requires a baffle wall made of refractory material to be placed between each pass.
For example, ihs GB Patent Number 1,418,456 issued on December 17, 1975 discloses a water tube boiler that includes a radiant heating furnace chamber leading to an upright convection pass. The upright convection pass provided with banks of substantially horizontal superheat tubes, upright evaporator tubes are arranged in horizontally spaced groups extending through the superheat tubes between headers. The water tube boiler further includes one or more burners that are mounted in the roof of the furnace chamber and tubes of dividing wall are pulled apart to allow gases to flow

up the convection pass. In another embodiment the furnace chamber is fired by burners mounted in the lower end of a side wall and an outlet at the top of the dividing wall allows gases to flow down the pass. However, the water-tube boiler disclosed herein is complex in structure.
US Patent Number 5,353,749 issued on October 11, 1994 discloses a boiler for converting water to steam. The boiler includes an upper header and a lower header. The boiler further includes a plurality of separate combustion chambers that can be provided and arranged in parallel and boiler tubes extending form the combustion chambers. The boiler tubes are bent generally in a serpentine shape having bases and crests. Base of each adjacent boiler tube is disposed adjacent a base of an adjacent boiler tube. Each crest is spaced from crest of an adjacent boiler tube which provides elongated combustion chambers that are arranged in both vertical and horizontal rows relative to one another. However, the boiler for converting water to steam is bulky and occupies more space.
US Patent Number 7,395,785 issued on July 8, 2008 discloses a design of direct fired heaters which consist of vertically oriented refractory lined enclosures containing tubular heat transfer elements. More particularly, the direct fired heaters include a lower radiant section and upper convection section. The convection section consists of a refractory lined enclosure containing a plurality of closely spaced horizontal tubes. The combustion products passing thru the convection section are relatively low, heat is transferred from the combustion products to the heating coils, and the process fluid flowing through said coils. Several coils may be contained in the convection section, one of which consists of a process coil, the outlet of

which is connected to the radiant section, so that process fluid can be preheated in the convection section, raised to the design temperature required at the inlet of the radiant section, and heated further in said radiant section. However, the direct fired heater disclosed herein is complex in construction.
Although, most of the direct fired heaters utilize both radiation heat transfer and convective heat transfer for heating the process fluid flowing through the heat transfer coils but most of such direct fired heaters are bulky and complex in construction. Further, most of the direct fired heaters disclosed in the prior art documents face difficulty in pass arrangement and has higher stack temperature and low efficiency.
Accordingly there is a need of solid fuel fired oil-heater that utilizes multiple passes of the flue gas over the oil carrying coils inside the oil-heater and still maintains a compact and simpler construction.
Objects of the Invention:
An object of the present invention is to provide an oil-heater fired by solid fuel that utilizes multiple pass design.
Another object of the present invention is to provide an oil-heater that requires minimum floor space area.
Another object of the present invention is to provide an oil-heater of compact construction that is fired by solid fuel.

Still another object of the present invention is to provide an oil heater that utilizes simple and compact arrangement of serpentine coils with multiple pass provision to achieve better thermal performance.
Yet another object of the present invention is tP provide an oil-heater fired by solid fuel that is convenient to handle and transport.
Another object of the present invention is to provide an oil heater comprising an arrangement of heat transfer elements that ensures an enhanced ratio of heat transfer area to volume.
Still another object of the present invention is tP provide an oil-heater fired by solid fuel that exhibits improved heat transfer performance and higher heat transfer efficiency.
Yet another object of the present invention is to provide an oil-heater fired by solid fuel that possesses simplified design and optimal dimensions.
Another object of the present invention is to provide an oil-heater fired by solid fuel that minimizes energy losses and flue gas losses by completely utilizing the heat energy of the flue gases.
Summary of the Invention:
In accordance with the present invention there is provided a compact thermal oil heater. The compact thermal oil heater includes a furnace for combustion of solid fuel, a radiative pass arrangement, a connection means for

connecting the furnace to the radiative pass arrangement, a first convective pass arrangement, a second convective pass arrangement and a secondary membrane panel. The radiative pass arrangement includes a first membrane panel constructed with parallel heat transfer tubes and strips providing enclosure to the combustion chamber and providing radiation heat transfer. The first convective pass arrangement is functionally connected to the radiative pass arrangement and is adapted to receive flue gases leaving the radiative pass arrangement from the operative top of the secondary membrane panel separating the radiative pass arrangement from the first convective pass arrangement. The second convective pass arrangement is functionally connected to the first convective pass arrangement and is adapted to receive the flue gases leaving the first convective pass arrangement.
Preferably, the radiative pass arrangement is disposed just above an operative top portion of the furnace.
Typically, the first membrane panel and the second membrane panel are placed perpendicular to each other.
Specifically, the second membrane panel serves as separating wall between said radiative pass and said first convective pass.
Further, the second membrane panel also serves as separating wall between the first convective pass and the second convective pass.

Typically, the perpendicular arrangement of membrane panels can be also be used for a thermal oil heater with single convective pass.
Generally, a plurality of thin metal strips is provided for connecting tubes of the parallel heat-transfer tubes to define an enclosure serving as the radiative pass.
Typically, the first convective pass arrangement includes a plurality of serpentine convective tubes that are in fluid communication with the radiative pass arrangement.
Additionally, the second convective pass arrangement includes a plurality of serpentine convective tubes in fluid communication with the serpentine convective tubes of the first convective pass arrangement, the second convective pass arrangement further includes an exit port for discarding exhausted flue gases.
Generally, the second convective pass is placed inside the metallic casing with the exit duct.
Typically, a first flow distribution plate is disposed at the operative top portion of the first convective pass arrangement in order to direct the flue gases leaving the radiative pass arrangement to the first convective pass arrangement.
Similarly, a second flow distribution plate is disposed at the bottom portion of the second convective pass arrangement to direct the flue gases leaving
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the first convective pass arrangement to the second convective pass arrangement.
Brief Description of the Accompanying Drawings:
The invention will now be explained in relation to the accompanying drawings, in which:
Figure 1 illustrates a test graph showing comparative heat transfer performance of serpentine coil and helical coil under same set of conditions;
Figure 2 illustrates a perspective view of an oil-heater fired by solid fuel, in accordance with one embodiment of the invention;
Figure 3 illustrates a cut-away perspective view depicting a radiative pass arrangement for the oil-heater of Figure 2;
Figure 4 illustrates a cut-away perspective view depicting a first convective pass arrangement for the oil-heater of Figure 2;
Figure 5 illustrates a partial perspective view depicting a second convective pass arrangement for the oil-heater of Figure 2;
Figure 6(a) illustrates an arrangement of flow-distribution plates disposed at a top portion of the first convective pass arrangement of the oil-heater of Figure 2;

Figure 6(b) illustrates an arrangement of flow distribution plate disposed at a bottom portion of the second convective pass arrangement of the oil heater of Figure 2; and
Figure 7 illustrates a graphical representation depicting variation in flue gas outlet temperature as a function of heater load.
Detailed Description of the Accompanying Drawings:
The invention will now be described with reference to the accompanying drawings which do not limit the scope and ambit of the invention. The description provided is purely by way of example and illustration.
The block diagram and the description hereto are merely illustrative and only exemplify the invention and in no way limit the scope thereof.
Referring to Figure 2 and Figure 3 of the accompanying drawings, an oil-heater 1000 fired by solid fuel is disclosed. Figure 2 of the accompanying drawings illustrates a perspective view of the oil-heater 1000 and Figure 3 of the accompanying drawings illustrates a cut-away perspective view depicting a radiative pass arrangement 200 for the oil-heater 1000. The oil-heater 1000 includes a furnace 100, a radiative pass arrangement 200, a primary membrane panel 202, a secondary membrane panel 208, a first convective pass arrangement 300 and a second convective pass arrangement 400. The radiative pass arrangement 200 of the oil heater 1000 includes a set

of parallel tubes 204 and strips 206 between the tubes defining primary membrane panel 202 and serving as heat transfer surface for radiation.
The oil-heater 1000 of the present invention is based on a three pass system, with one radiative pass and two convective passes. More specifically, the oil to be heated by the oil-heater 1000 is adapted to continuously flow through the heat-transfer tubes, wherein the heat transfer passes includes the radiative pass arrangement 200 and the convective pass arrangements 300 and 400 and the oil inside the heat transfer tubes gets heated by a combination of radiation heat-transfer and convective heat-transfer phenomenon.
The oil-heater 1000 includes the furnace 100, disposed at a base of the oil-heater 1000 and just below the radiative pass arrangement 200 of the oil-heater 1000. The furnace 100 includes the hopper 102 adapted to introduce solid fuel such as coal, wood, pellets etc. inside the furnace 100. The furnace 100 is adapted to carry out combustion of the solid fuel held therein to define a combustion zone and to generate high temperature combustion products or flue gases. The high temperature combustion products or flue gases generated inside the furnace 100 are adapted to enter the radiative pass arrangement 200 of the oil-heater 1000. More specifically, the furnace 100 may include an open top portion, and a connecting channel for connecting the open top portion of the furnace 100 to the radiative pass arrangement 200 of the oil-heater 1000.
The radiative pass arrangement 200 includes a primary membrane panel 202 with a set of parallel tubes 204 and connecting strips 206 for the sealing and

providing radiation heat transfer surface and a pair of header to distribute the thermal oil among the heat transfer tubes. All parallel tubes are connected with each other by means of thin metal strips or fins, to define an enclosure around the combustion zone and to facilitate radiation heat exchange between a flue gas zone of the furnace 100 afld the parallel tubes of the radiative pass arrangement 200. The radiation heat exchange between the flue gas zone of the furnace 100 and the parallel tubes of the radiative pass arrangement 200, results in heating of the oil flowing through the parallel tubes of the radiative pass arrangement 200. The enclosure formed by the parallel tubes of the radiative pass arrangement 200, joined by the fins may be also referred to as a primary membrane panel. The primary membrane panel is separated from the first convective pass 300 of the oil-heater 1000 by a secondary membrane panel 208 wall placed there-between.
The flue gases leaving the radiative pass arrangement 200 enters the first convective pass arrangement 300 of the oil-heater 1000 from top of the secondary membrane panel wall separating the first convective pass arrangement 300 of the oil-heater 1000 and the primary membrane panel. The top portion of the secondary membrane panel 208 does not have strip and provide flue gas passage from the first membrane panel 202 to the first convective pass 300. The flue gases leaving the first convective pass arrangement 300 of the oil-heater 1000 may enter the second convective tube bank 400 from a bottom of the convective tube bank 400 and exit from top of the second convective tube bank 400.
Referring to Figure 4 of the accompanying drawings, the first convective pass arrangement 300 of the oil-heater 1000 is disclosed. Figure 4 of the

accompanying drawings depicts a perspective view of the first convective pass arrangement 300 provided in the oil-heater 1000. The first convective pass arrangement 300 of the oil-heater 1000 includes a plurality of serpentine convective tubes 302, a secondary membrane panel 208 and a plurality of flue gas exit channels 306. The secondary membrane panel 208 serves as casing for the first convective pass.
The flue gases leaving the radiative pass arrangement 200 enters the first convective pass arrangement 300 of the oil-heater 1000 from top of the secondary membrane panel 208 wall separating the first convective pass arrangement 300 of the oil-heater 1000 and the primary membrane panel. The flue gases entering the first convective pass arrangement 300 descends and transfers heat to the oil that is continuously flowing through the serpentine convective tubes 302 disposed inside the first convective pass arrangement 300 (Counter flow arrangement). The serpentine convective tubes 302 of the first convective pass arrangement 300 are enclosed by the secondary membrane panel 208. More particularly, the oil-heater 1000 of the present invention incorporates a split membrane panel that simplifies casing design.
The secondary membrane panel 208 in accordance with the new design acts as a casing for the serpentine convective pass tubes 302 of the first convective pass arrangement 300. The secondary membrane panel 208 acts like a separating wall between radiative pass and first convective pass. Furthermore, the secondary membrane panel 208 is placed perpendicular to the primary membrane panel. The top portion of the secondary membrane panel 208 is free of any fins and provides flue gas channel for facilitating

the entry of the flue gases into the first convective pass arrangement 300, whereas the other section of the secondary membrane panel 208 acts like a separating wall between primary and secondary membrane panel 208. The bottom portion of secondary membrane panel 208 provides flue gas exit channels 306 for facilitating exit of the flue gases from the first convective pass arrangement 300 after losing heat to the oil that is continuously flowing through the serpentine convective tubes 302 of the first convective pass arrangement 300. The convective tubes 302 of the first convective pass arrangement 300 are a set of serpentine tubes arranged in the path of the flue gas so as to maximize the heat transfer. The secondary membrane panel 208 also acts like a separating wall between first and second convective pass.
Referring to Figure 5 of the accompanying drawings, the second convective pass arrangement 400 of the oil heater 1000 is disclosed. Figure 5 of the accompanying drawings depicts a perspective view of the second convective pass arrangement 400 provided in the oil-heater 1000. The second convective pass arrangement 400 includes a plurality of serpentine convective tubes 402 and a metallic casing 403 with an exit opening 404.
The flue gases leaving the first convective pass arrangement 300 from the bottom portion thereof after losing heat to the oil flowing through the convective tubes 302 of the first convective pass arrangement 300 enters the second convective pass arrangement 400. Accordingly, arranging the second convective pass arrangement 400 is easier as it has to handle cold flue gases and a metallic casing 403 with insulation serves the purpose. The flue gases entering the second convective pass arrangement 400 rises inside the second convective pass arrangement 400 and further heats the oil that is

continuously flowing through the serpentine convective tubes 402 of the second convective pass arrangement 400. The exit opening 404 provided at the top of the second convective pass arrangement 400 facilitates exit of the flue gases from the second convective pass arrangement 400 and hence the oil-heater 1000, after losing most of its heat to the oil that is continuously flowing through the serpentine convective tubes 402. More particularly, the oil-heater 1000 of the present invention incorporates a double convective pass arrangement to simplify the layout and to optimize boiler height and improve heat transfer performance.
In case of the double convective pass arrangement, the flue gases entering the second convective pass arrangement 400 is relatively cold, as such the flow area in the second convective pass arrangement 400 may be reduced to maintain flue gas velocity. The flow area in the second convective pass may be reduced by providing uneven number of tubes in the two passes, which results in very high thermal oil velocity in the second convective pass and a low thermal oil velocity in the first convective path. According to an embodiment of the present invention, the number of tubes and flow area for both the convective passes is kept same.
According to another embodiment of the present invention, the split flow of thermal oil for both the passes along with lower number of tubes in the second pass and higher number of tubes in the first pass has been used to maintain thermal oil velocity in both pass without adding in pressure drop. The two pass arrangement and uniform thermal oil flow distribution in addition to the serpentine tube helps to achieve better heat transfer performance and lower stack temperature.
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The heated oil flowing through the serpentine convective tubes 402 of the second convective pass arrangement 400 remains warm for a long period of time while the heat is slowly transferred to a room by conduction and radiation.
Referring to Figure 6(a) and Figure 6(b) of the accompanying drawings, the flue gas guiding arrangement of the oil heater 1000 is disclosed. Figure 6(a) and Figure 6(b) of the accompanying drawings illustrates arrangement of the flow distribution plates used for guiding the flue gases entering the first convective pass arrangement 300 and the second convective pass arrangement 400. The flue gas guiding arrangement includes a first flow distribution plate 502 and a second flow distribution plate 504.
The first flow distribution plate 502 is disposed at the top of the first convective pass arrangement 300 and is adapted to direct the flue gases leaving the radiative pass arrangement 200 to the first convective pass arrangement 300. Similarly, the second flow distribution plate 504 is disposed at the bottom of the second convective pass arrangement 400 and is adapted to direct the flue gases leaving the first convective pass arrangement 300 to the second convective pass arrangement 400.
TEST SET-UP
Experiments and tests were conducted to determine effectiveness and performance characteristics of the compact thermal oil heater of the present invention. The oil to be heated by the compact thermal oil heater was

introduced into the oil heater at different inlet temperatures and the temperature achieved by the flue gas corresponding to different inlet temperatures of the oil entering the compact thermal oil heater were recorded. The following table puts across different values of the outlet temperatures of the flue gases recorded corresponding to different values of inlet temperatures of oil entering the compact thermal oil heater.

: Thermic fluid inlet temperature. Flue gas outlet temperature
210 297.9
220 306.5
230 315.2
240 323.8
As, is clear from the above table the flue gas outlet temperature increases with increase in thermic fluid inlet temperature.
Further, the flue gas outlet temperatures were recorded corresponding to different load conditions. The following table puts across different values of the outlet temperatures of the flue gases recorded corresponding to different load conditions under which the compact thermal oil heater operates:

r .. Load% .1.... Flue gas outlet temperature
100 323.8
90 307.1
80 291.3
70 275.4
60 262
50 251.2
40 244

Figure 7 illustrates a graphical representation depicting variation in flue gas outlet temperature as a function of heater load. As, is clear from the above table and Figure 7, the flue gas outlet temperature increases with increase in load under which the compact thermal oil heater operates.
Technical Advancements and Economic Significance:
The solid fuel fired oil-heater of the present invention is of a compact construction and may be used for process heating applications. The solid fuel fired oil-heater of the present invention utilizes multiple pass design of heat transfer coils. Further, the oil-heater fired by solid fuel is convenient to handle and transport. The arrangement of heat transfer elements used in the oil-heater of the present invention ensures an enhanced ratio of heat transfer area to volume. The solid fuel fired oil-heater of the present invention requires less refractory material. Furthermore, the oil-heater of the present invention exhibits improved heat transfer performance and higher heat transfer efficiency. The oil-heater of the present invention incorporates a simplified casing design and has optimal dimensions. Further the oil-heater of the present invention incorporates two serpentine tube convective passes with different flow areas to maximize heat transfer performance and minimize stack temperature.
The numeral values given of various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the invention and the claims unless there is a statement in the specification to the contrary.

While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We Claim:
1. A compact thermal oil-heater comprising:
o a furnace for combustion of solid fuel;
o a radiative pass arrangement comprising a first membrane panel constructed with heat transfer tubes and strips providing an enclosure to combustion chamber and providing radiation heat transfer;
o a connection means for connecting said furnace to said radiative pass arrangement;
o a secondary membrane panel place perpendicular to first membrane panel serving as an enclosure for first convective pass and separating walls for radiative pass, first convective pass and second convective pass;
o a first convective pass arrangement functionally connected to said radiative pass arrangement and adapted to receive flue gases leaving said radiative pass arrangement from the operative top of a refractory wall of said secondary membrane panel separating said radiative pass arrangement from said first convective pass arrangement; and
o a second convective pass arrangement functionally connected to said first convective pass arrangement and adapted to receive said flue gases leaving said first convective pass arrangement.
2. The compact thermal oil-heater as claimed in Claim 1, wherein said
radiative pass arrangement is disposed just above an operative top portion
of said furnace.

3. The compact thermal oil heater as claimed in claim 1, wherein said first membrane panel and second membrane panel are placed perpendicular to each other.
4. The compact thermal oil heater as claimed in claim 1, wherein said second membrane panel serves as separating wall between radiative pass and first convective pass.
5. The compact thermal oil heater as claimed in claim 1, wherein said second membrane panel also serves as separating wall between said first convective pass and said second convective pass.
6. The compact thermal oil heater as claimed in claim 3, perpendicular arrangement of membrane panels can be also used for a thermal oil heater with single convective pass.
7. The compact thermal oil-heater as claimed in Claim 1, wherein a plurality of thin metal strips are provided for connecting tubes of said parallel heat-transfer tubes to define an enclosure serving as said radiative pass.
8. The compact thermal oil-heater as claimed in Claim 1, wherein said first convective pass arrangement comprises a plurality of serpentine convective tubes that are in fluid communication with said radiative pass arrangement.
9. The compact thermal oil-heater as claimed in Claim 1, wherein said second convective pass arrangement comprises a plurality of serpentine convective tubes in fluid communication with said serpentine convective tubes of said first convective pass arrangement, said second convective pass arrangement further comprising an exit port for discarding exhausted flue gases.

10.The compact thermal oil heater as claimed in Claim 1, wherein second convective pass is placed inside the metallic casing with said exit duct.
11.The compact thermal oil-heater as claimed in Claim 1, wherein a first flow distribution plate is disposed at the operative top portion of said first convective pass arrangement in order to direct flue gases leaving said radiative pass arrangement to said first convective pass arrangement.
12.The compact thermal oil-heater as claimed in Claim 1, wherein a second flow distribution plate is disposed at the bottom portion of said second convective pass arrangement to direct flue gases leaving said first convective pass arrangement to said second convective pass arrangement.