EP1876390A1

EP1876390A1 - Method of producing steam in a gas tube steam boiler and gas tube steam boiler for implementing said method

Info

Publication number: EP1876390A1
Application number: EP06388049A
Authority: EP
Inventors: Bodil Irene Mosekaer Nielsen; Thomas Paarup Pedersen
Original assignee: Aalborg Industries AS
Current assignee: Alfa Laval Aalborg AS
Priority date: 2006-07-05
Filing date: 2006-07-05
Publication date: 2008-01-09
Also published as: KR20090031606A; CN101512223A; JP2009541705A; EP2044365B1; EP2044365A1; DK2044365T3; CN101512223B; KR101009212B1; WO2008003322A1

Abstract

A gas tube steam boiler comprising a heat exchange compartment (2) filled with water forming a water surface (2A), a steam head space (8) above the water surface and delimited by a cylindrical wall (1) with a substantially vertical axis (10) and a top plate (4), a steam outlet (6) communicating with the steam head space (8), gas tubes (3) extending through the heat exchange compartment (2) and the steam head space (8), means for supplying heated gas to the gas tubes (3) for generating a flow of steam from said water surface (2A) by heat exchange between the gas tubes (3) and the water, and a steam flow conduit (7',7") arranged in the steam head space (8) for conducting the flow of steam from the water surface (2A) to the steam outlet (6), at least one of the gas tubes (3) extending transversely through the conduit such that the flow of steam flows in a direction transverse, preferably generally orthogonal, to the gas tubes (3), the configuration of the conduit being such that more than half, preferably substantially all, of the steam in the flow is constrained to flow along a flow path which when projected on a horizontal surface has a length at least equal to the shortest distance between a first point of the wall (1) and a second point of the wall (1) horizontally opposite the first point, said distance, in the case the wall (1) is circular cylindrical, being the diameter D of the circular cylindrical wall.

Description

The present invention relates to a method of producing steam in a gas tube steam boiler and a gas tube steam boiler for implementing said method.
In connection with such a method and apparatus it is of importance to optimise the energy transfer from the gas tubes to the water and steam in order to reduce the overall size of the boiler for a given energy output. Typically, the primary design parameters are involving the input gas temperature, the exhaust gas temperature and the desired maximum energy output, i.e. feed water temperature, steam pressure, temperature and mass flow. Preferably, the steam should be dry or even more preferred superheated.
From US 1,546,665 it is known to provide a gas tube steam boiler with a baffle plate providing an annular opening along the boiler wall, whereby the steam flow from the water surface is restricted to flow from the water surface to the annular opening and from there to the centrally positioned steam outlet. In this construction a major part of the generated steam will only pass the gas tubes over a distance corresponding to approximately 1/2 of the diameter of the gas tube boiler leading to insufficient heat exchange between the gas tubes and the generated steam.
In particular, but not exclusively, for marine boilers, the size of the boiler is of great importance due to limited space available, and accordingly it is an object of the present invention to provide a method and an apparatus of the type in reference that allows reduction of the space requirements of a boiler.
According to the invention this object is achieved by the method comprising the steps of

providing a gas tube steam boiler comprising:
- a heat exchange compartment filled with water forming a water surface,
- a steam head space above said water surface and delimited by a cylindrical wall with a substantially vertical axis and a top plate,
- a steam outlet communicating with said steam head space,
- one or more gas tubes extending through said heat exchange compartment and said steam head space,
supplying heated gas to said gas tubes for generating a flow of steam from said water surface by heat exchange between said gas tubes and said water in said heat exchange compartment,
causing said flow of steam to flow through said steam head space from said water surface to said steam outlet in such a manner that more than half, preferably substantially all, of the steam in said flow in said head space is constrained to flow in a direction transverse, preferably generally orthogonal, to said gas tubes along a flow path which when projected on a horizontal surface has a length at least equal to the shortest distance between a first point of said wall and a second point of said wall horizontally opposite said first point, said distance, in the case said wall is circular cylindrical, being the diameter of said circular cylindrical wall,
such that residual heat in said gas tubes is transferred to said flow of steam.

Hereby, the heat transfer from the gas tubes to the steam in the steam head space is significantly improved, whereby the efficiency of the boiler is increased and at the same time the produced steam has a higher quality by being dried and possibly superheated.
In a currently preferred embodiment of the method according to the invention said horizontal projection of said flow path is at least two times said distance. In this way the contact between the gas tubes and the flow of steam from the water surface to the steam outlet is increased, thus increasing the heat transfer in the steam head space.
In a currently preferred embodiment of the method according to the invention the steam outlet is located at a distance from the axis of the wall, preferably proximate to the wall of the head space, thus restricting the steam flow to flow sideways along all of the gas tubes before leaving through the steam outlet.
In a currently preferred embodiment of the method according to the invention the horizontal projection of the flow path extends from proximate or at a first point on said wall past said axis to proximate or at a second point on said wall opposite said first point and preferably extends back from the second point and at least partways back to said first point. In this way the steam flow is caused to move past all of the gas tubes, preferably at least two times before leaving through the steam outlet steam outlet.
In a currently preferred embodiment of the method according to the invention the steam flow velocity is lower when flowing from the second point towards the first point than when flowing from proximate said first point towards said second point, providing a relative high velocity, preferably approximately 15-30 m/s, more preferably 15-25 m/s over a part of the distance between the water surface and the steam outlet and to pass the gas tubes at a reduced velocity, preferably approximately 10-15 m/s over a subsequent part of the path from the water surface to the steam outlet, whereby the high velocity provides a turbulent flow around the gas tubes securing a high heat transfer and securing that possible droplets of water in the steam will hit the tubes and evaporate.
In a currently preferred embodiment of the method according to the invention the hot gasses for the gas tubes are provided from a combustion chamber, combusting a fuel such as oil, carbon dust, natural gas, etc., however, also hot exhaust gasses from a gas turbine or an internal combustion motor and the like may be used. When using hot gasses from a combustion chamber, said combustion chamber may be provided in contact with the water in the heat exchange compartment, in order to deliver further heating to the water directly from the combustion chamber.
In a second aspect the present invention relates to a gas tube steam boiler comprising

a heat exchange compartment filled with water forming a water surface,
a steam head space above said water surface and delimited by a cylindrical wall with a substantially vertical axis and a top plate,
a steam outlet communicating with said steam head space,
one or more gas tubes extending through said heat exchange compartment and said steam head space,
means for supplying heated gas to said gas tubes for generating a flow of steam from said water surface by heat exchange between said gas tubes and said water in said heat exchange compartment,
a steam flow conduit arranged in said steam head space for conducting said flow of steam from said water surface to said steam outlet, at least one of said gas tubes extending transversely through said conduit such that said flow of steam flows in a direction transverse, preferably generally orthogonal, to said gas tubes, the configuration of said conduit being such that more than half, preferably substantially all, of the steam in said flow is constrained to flow along a flow path which when projected on a horizontal surface has a length at least equal to the shortest distance between a first point of said wall and a second point of said wall horizontally opposite said first point, said distance, in the case said wall is circular cylindrical, being the diameter of said circular cylindrical wall.

This gas tube steam boiler is particularly suited for carrying out the above method.
In a third aspect the present invention relates to a gas tube steam boiler comprising

a heat exchange compartment filled with water forming a water surface,
a steam head space above said water surface and delimited by a circular cylindrical wall with a substantially vertical axis and a top plate,
a steam outlet communicating with said steam head space,
one or more gas tubes extending through said heat exchange compartment and said steam head space,
means for supplying heated gas to said gas tubes for generating a flow of steam from said water surface by heat exchange between said gas tubes and said water in said heat exchange compartment,
a steam flow conduit arranged in said steam head space for conducting said flow of steam from said water surface to said steam outlet and comprising:
a first generally horizontal plate arranged at a vertical distance h1 below said top plate and having a first gap forming edge arranged at a distance from said wall to provide a first steam flow gap between said first plate and said wall.

Preferred embodiments of the gas tube steam boilers in accordance with the second and third aspect of the present invention, the advantages of which will be evident from the following detailed description, are revealed in the subordinate claims 11-21 and 23-31.
In the following the invention will be explained in more detail with reference to the embodiments shown solely by way of example in the drawings, where

Fig. 1 schematically shows a vertical cross sectional view along A-A in a boiler in accordance with a preferred embodiment of the present invention,
Fig 1a schematically shows the boiler in Fig. 1 seen from the top and indicating the lines along which the cross sectional views in Figs. 1 and 2 are taken,
Fig. 2 schematically shows a vertical cross sectional view along B-B perpendicular to the one in Fig. 1,
Fig. 3 schematically shows a perspective view of the baffle plate configuration,
Fig. 4-7 show four different possible configurations of the boiler steam head space in accordance with the present invention.

The gas tube steam boiler shown in Fig. 1 comprises a heat exchange compartment 2 filled with water, said water forming a water surface 2a, and a steam head space 8 above said water surface and delimited by a cylindrical wall 1 with a substantially vertical axis 10, a top plate 4 and a diameter D. A steam outlet 6 is connected to the steam head space 8 and several gas tubes 3 extend through the heat exchange compartment 2 and the steam head space 8. Heated gas flows through the gas tubes 3 in order to generate a flow of steam from the water surface by heat exchange between the gas tubes 3 and the water in the heat exchange compartment 2.
In the embodiment shown in Figs. 1-3 and Fig. 5 the steam flow from the water surface is caused to flow along a conduit delimited by the baffle plates 7', 7", whereby said steam is made to flow in a direction transverse to the gas tubes 3 and all of the steam in the flow is constrained to flow between the two baffle plates 7', 7" in a horizontal direction and back between the upper baffle plate 7" and the top plate 4 in an opposite horizontal direction and out through the steam outlet 6.
As shown in Figs. 2 and 3 the two baffle plates 7', 7" each covers a smaller area than the cross sectional area of the steam head space 5 and the edges of the two baffle plates 7', 7" are connected to vertical plates 9 extending from the top plate 4 down to the lower baffle plate 7' in the vertical direction and in the horizontal direction all the way across between individual positions on the cylindrical wall of the steam head space 5.
The gas tubes 3 extend from the bottom of the boiler, possibly from a combustion chamber 14 as shown in Fig. 2 up through the water filled heat exchange compartment 2 and the steam head space 5, where the gas tubes 3 extend through the plates 7', 7" and through holes in the top plate 4, from where the gas leaves through a gas exhaust 11.
Heat exchange between the gas tubes and the water will produce steam entering the steam head space 5 at the water surface and the plate 7' forces the steam to move to the left as seen in Fig. 1 around a gap forming edge 12 and in between the two plates 7', 7" moving to the right as seen in Fig. 1 and around a gap forming edge 13 and moving between the plate 7' and the top plate 4 to the steam outlet 6. In this way the steam is forced to move in a direction transverse to the gas tubes 3 over a distance corresponding to approximately two times the diameter of the boiler 1. Minor leakage of steam between the openings in the plates 7', 7" and the gas tubes 3 will be no problem as the steam in these small gaps will come very close to the tube walls and be heated up here. In order to provide a sufficient heat exchange between the gas tubes and the flow of steam, more than half of all the steam in the flow in the head space is constrained to flow in a direction transverse, preferably generally orthogonal, to said gas tubes along a flow path, which when projected on a horizontal surface has a length at least equal to the diameter of the circular cylindrical wall. By this feature the present invention is distinguished over the above-mentioned US 1, 546,665 , in which less than half of the steam flows over a distance corresponding to less than 3/4 of the diameter of the circular cylindrical wall.
When the steam leaves the water surface it will typically contain small water droplets, which, by the forced flow across the gas tubes 3 with a relatively high flow velocity, will hit the gas tubes 3 and evaporate, and furthermore the relatively high velocity of the steam will provide a heat exchange between the steam and the gas tubes in the steam head space 5, said heat exchange is increased compared to the traditional boiler construction in which the steam simply moves from the water surface to the steam outlet in a slow flow mainly parallel to the gas tubes 3 in an open steam head space without plate 7', 7". Furthermore, the distance between the water level and the top plate can be reduced, said distance normally being relatively high in order to avoid a high concentration of water droplets in the steam leaving the steam outlet 6.
When the water in the boiler 1 is not boiling, the water level inside the boiler corresponds to the level measured with different traditional equipment, such as pressure difference between two known levels, water level glasses, etc. However, the actual water level seen inside the boiler will rise because of the presence of a large amount of steam bubbles inside the water area, and the water level will be fluctuating more or less, dependent on the heat load and the steam pressure. The highest water level will be around the gas tubes 3 and accordingly the gas tubes are wetted by water up to this water level, which typically is about 250 mm above the measured water level, in the following named water surface in cold condition.
The total height h of the steam head space 5 is subdivided into the distance h1 between the top plate 4 and the first plate 7", the distance h2 between the first plate 7" and the second plate 7', and finally the distance h3 between the second plate 7' and the water surface in cold condition.
In a preferred embodiment the distances h1 and h2 are dimensioned in such a way that the steam velocity between the top plate 4 and the first plate 7" is approximately 10-15 m/s and the steam velocity between the two plates 7' and 7" is approximately 20-30 m/s., i.e. the distance h1 is approximately two times the distance h2. Under all circumstances the distance h3 should be sufficient to secure that during boiling at full load, the actual water level should be approximately 200 mm below the second plate 7'.
In the steam head space, as described above, the flow of the steam perpendicular to the gas tubes 3 is relatively high between the two plates 7', 7", a bit lower between the top plate 4 and the plate 7" and even between the water surface and the plate 7' a certain cross flow relative to the gas tubes 3 will be present. The steam velocity provides a higher heat transfer coefficient from the gas tubes 3 to the steam compared to a traditional situation in which the steam head space 5 transmits steam at a low velocity, substantially parallel to the tubes. Actually, the heat transfer is close to the same heat transfer that exists in the water filled heat exchange compartment 2 in which water is in direct contact with the gas tubes 3.
The produced steam is not only free of water droplets when it leaves the boiler through the steam outlet 6, but it is actually superheated. This superheating ensures that no salts leave the boiler, resulting in that there will be no deposits in the steam pipe from the boiler to the user. Furthermore, a possible steam outlet valve after the steam outlets can be dimensioned to a higher steam velocity, which means that the size of the valve can be reduced, thus saving both space, weight and costs for said steam outlet valve. Furthermore, the total heating surface can be reduced due to increased heat transfer, which means reduced tube length, number of tubes, boiler height and thus both weight and cost price for the boiler.

In the following table process data for at gas tube steam boiler in accordance with the present invention are indicated.

Boiler steam capacity	kg/h	3700
Working pressure of steam	barg	6.5
Saturation point	°C	167
Fuel flow (HFO)	kg/h	250
Flue gas flow	kg/h	3900
Smoke tube dimension	mm	∅18x2
Number of smoke tubes	pcs	300
Total tube length, furnace top plate to boiler top plate	mm	1440
Boiler shell, inside diameter	mm	01276
h1, distance between boiler top plate and upper baffle plate	mm	125
h2, distance between the two baffle plates	mm	75
h3, distance below lower baffle plate and measured water level (Normal Water Level)	mm	450
Boiling up (wet heating surface above measured water level)	mm	250
Wet tube length	mm	1040
Max. steam velocity, 2. pass (between the two baffle plates)	m/s	16
Max. steam velocity, 3. pass (between upper baffle plate and boiler top plate)	m/s	10
Steam space volume load (to measured NWL)	h^-1	1100
Inlet gas temperature (leaving furnace)	°C	1360
Gas velocity, tube inlet	m/s	107
Gas temperature inside smoke tubes at actual water level (upper part of wet heating surface)	°C	550
Outlet gas temperature	°C	400
Outlet steam temperature	°C	220

The following table compares a traditionally dimensioned gas tube steam boiler indicated as standard, and a gas tube steam boiler in accordance with the present invention indicated as new. The two boilers have been designed for having the same steam capacity, thermal efficiency (i.e. same flue gas outlet temperature) and same pressure drop.

Description	Unit	Standard	New
Tube dimension	mm	18x2	18x2
Number of tubes	Pcs	375	300
Wet tube length	mm	1370	1040
Length to normal water level (measured)	mm	1120	790
Steam space to measured water level	mm	840	650
Total tube length	mm	1960	1440
Total tube length in boiler	m	735	432

It can be seen that the direct result of using the present invention is that approximately 40% of the tubes (approximately 240 kg) in the standard boiler design can be saved. Furthermore, the tube length and accordingly the boiler height is shortened by 520 mm, which with a boiler diameter of 1300 and a plate thickness of 12 mm provides a difference in plate weight of approximately 200 kg.
Above the invention has been described in connection with a preferred embodiment, however, many modifications may be envisaged without departing from the following claims. Among such modifications are the omission of the second plate 7' leading to a more simplified construction, but, however, still providing a flow of the steam through the steam head space 5 from the water surface to the steam outlet 6 in such a manner that the majority of the steam in the flow in said head space 5 is constrained to flow in a direction transverse to the gas tubes along a flow path having a length corresponding to the diameter of the boiler 1. This possibility is shown in Fig. 4, and the above described preferred embodiment is schematically shown in Fig. 5. Other possibilities as shown in Fig. 6 and Fig. 7 provide a steam flow from the water surface to the steam outlet restricted by a first plate having a central hole for passing the steam and a second plate above said first plate, said second plate providing an annular opening along the boiler wall, whereby the steam flow is restricted to flow across the gas tubes, first in a radially outward direction and following in a radially inward direction towards the centrally positioned steam outlet 6.
In the Fig. 7 embodiment the steam flow is restricted to a helically flow path by means of a helically shaped plate positioned between a central tube and the outer wall of the boiler 1, said steam flow path again having a length corresponding to at least the distance between two opposite point of the wall of the boiler 1.

Claims

A method of generating steam comprising the steps of:
- providing a gas tube steam boiler comprising:
- a heat exchange compartment filled with water forming a water surface,

- a steam head space above said water surface and delimited by a cylindrical wall with a substantially vertical axis and a top plate,

- a steam outlet communicating with said steam head space,

- one or more gas tubes extending through said heat exchange compartment and said steam head space,

- supplying heated gas to said gas tubes for generating a flow of steam from said water surface by heat exchange between said gas tubes and said water in said heat exchange compartment,

- causing said flow of steam to flow through said steam head space from said water surface to said steam outlet in such a manner that more than half, preferably substantially all, of the steam in said flow in said head space is constrained to flow in a direction transverse, preferably generally orthogonal, to said gas tubes along a flow path which when projected on a horizontal surface has a length at least equal to the shortest distance between a first point of said wall and a second point of said wall horizontally opposite said first point, said distance, in the case said wall is circular cylindrical, being the diameter of said circular cylindrical wall,
such that residual heat in said gas tubes is transferred to said flow of steam.
A method according to claim 1, wherein said residual heat in said gas tubes is transferred to said flow of steam to an extent that all the steam exiting said steam space through said steam outlet is super-heated.
A method according to claim 1 or 2, wherein the length of said horizontal projection of said flow path is at least two times said distance.
A method according to any of the preceding claims, wherein said steam outlet is located at a distance from said axis of said wall, preferably proximate to or in said wall.
A method according to any of the preceding claims, wherein said horizontal projection of said flow path extends from proximate or at a first point on said wall past said axis to proximate or at a second point on said wall opposite said first point.
A method according to claim 5, wherein said horizontal projection of said flow path extends back from said second point and at least partways back to said first point.
A method according to claim 6, wherein the flow velocity of said flow is lower when flowing from said second point towards said first point than when flowing from proximate said first point towards said second point.
A method according to claim 7, wherein the flow velocity of said flow is approximately 10-15 m/s when flowing from said second point towards said first point and preferably approximately 15-30 m/s, more preferably 15-25 m/s when flowing from proximate said first point towards said second point.
A method according to any of the preceding claims, wherein said heated gas is supplied from any of the following sources:
a) a combustion chamber,

b) a gas turbine,

c) an internal combustion motor, and

d) a gas or oilfired engine used e.g. for ships propulsion or electricity production, heating processes, industrial and power plants.
A gas tube steam boiler comprising:
- a heat exchange compartment filled with water forming a water surface,

- a steam head space above said water surface and delimited by a cylindrical wall with a substantially vertical axis and a top plate,

- a steam outlet communicating with said steam head space,

- one or more gas tubes extending through said heat exchange compartment and said steam head space,

- means for supplying heated gas to said gas tubes for generating a flow of steam from said water surface by heat exchange between said gas tubes and said water in said heat exchange compartment,

- a steam flow conduit arranged in said steam head space for conducting said flow of steam from said water surface to said steam outlet, at least one of said gas tubes extending transversely through said conduit such that said flow of steam flows in a direction transverse, preferably generally orthogonal, to said gas tubes, the configuration of said conduit being such that more than half, preferably substantially all, of the steam in said flow is constrained to flow along a flow path which when projected on a horizontal surface has a length at least equal to the shortest distance between a first point of said wall and a second point of said wall horizontally opposite said first point, said distance, in the case said wall is circular cylindrical, being the diameter of said circular cylindrical wall.
A gas tube steam boiler according to claim 10, wherein said steam outlet is located at a distance from said axis of said wall, preferably in said top plate proximate to said wall.
A gas tube steam boiler according to claim 10 or 11, wherein said steam flow conduit comprises a first generally horizontal plate arranged at a vertical distance h1 below said top plate and having a first gap forming edge arranged at a distance from said wall to provide a first steam flow gap between said first plate and said wall.
A gas tube boiler according to claim 12, wherein said steam flow conduit comprises a second generally horizontal plate arranged at a vertical distance h2 below said first plate and having a second gap forming edge arranged at a distance from said wall to provide a second steam flow gap between said second plate and said wall.
A gas tube boiler according to claim 13, wherein said first gap and said second gap are located opposite one another relative to said axis.
A gas tube boiler according to any of the claims 12-14, wherein said steam outlet is located proximate or in said wall opposite said first gap relative to said axis.
A gas tube boiler according to any of the claims 13-15, wherein the distance h1 is larger than the distance h2 such that the flow velocity of said steam flow is lower in the portion of said conduit located between said top plate and said first plate than said flow velocity in the portion of said conduit located between said first and second plates.
A gas tube boiler according to claim 16, wherein all said gas tubes extend through said first and second plates.
A gas tube boiler according to any of the claims 13-17, wherein said water surface in cold condition of said water is located at a vertical distance h below said top plate and said distance h1 is approximately 15-25%, preferably 18-22% of said distance h and said distance h2 is approximately 7-13%, preferably 8-12% of said distance h.
A gas tube boiler according to any of the claims 12-18, wherein said first and second plates have smaller areas than the cross sectional area of said steam head space and the edges of said first and second plates extending from said first and second gap forming edges, respectively, are interconnected by means of vertical plates to form a tube.
A gas tube boiler according to any of the claims 12-19, wherein said first plate has a smaller area than the cross sectional area of said steam head space and the edges of said first plate extending from said first gap forming edges, respectively, are interconnected with the top plate by means of vertical plates to form a tube.
A gas tube boiler according to any of the claims 12-20, wherein said first and second plates are generally rectangular.
A gas tube steam boiler comprising:
- a heat exchange compartment filled with water forming a water surface,

- a steam head space above said water surface and delimited by a circular cylindrical wall with a substantially vertical axis and a top plate,

- a steam outlet communicating with said steam head space,

- one or more gas tubes extending through said heat exchange compartment and said steam head space,

- means for supplying heated gas to said gas tubes for generating a flow of steam from said water surface by heat exchange between said gas tubes and said water in said heat exchange compartment,

- a steam flow conduit arranged in said steam head space for conducting said flow of steam from said water surface to said steam outlet and comprising:

- a first generally horizontal plate arranged at a vertical distance h1 below said top plate and having a first gap forming edge arranged at a distance from said wall to provide a first steam flow gap between said first plate and said wall.
A gas tube boiler according to claim 22, wherein said steam flow conduit comprises a second generally horizontal plate arranged at a vertical distance h2 below said first plate and having a second gap forming edge arranged at a distance from said wall to provide a second steam flow gap between said second plate and said wall.
A gas tube boiler according to claim 23, wherein said first gap and said second gap are located opposite one another relative to said axis.
A gas tube boiler according to any of the claims 22-24, wherein said steam outlet is located proximate or in said wall opposite said first gap relative to said axis.
A gas tube boiler according to any of the claims 23-25, wherein the distance h1 is larger than the distance h2 such that the flow velocity of said steam flow is lower in the portion of said conduit located between said top plate and said first plate than said flow velocity in the portion of said conduit located between said first and second plates.
A gas tube boiler according to any of the claims 23-26, wherein all said gas tubes extend through said first and second plates.
A gas tube boiler according to any of the claims 23-27, wherein said water surface in cold condition of said water is located at a vertical distance h below said top plate and said distance h1 is approximately 15-25%, preferably 18-22% of said distance h and said distance h2 is approximately 7-13%, preferably 8-12% of said distance h.
A gas tube boiler according to any of the claims 22-28, wherein said first and second plates have smaller areas than the cross sectional area of said steam head space and the edges of said first and second plates extending from said first and second gap forming edges, respectively, are interconnected by means of vertical plates to form a tube.
A gas tube boiler according to any of the claims 22-29, wherein said first plate has a smaller area than the cross sectional area of said steam head space and the edges of said first plate extending from said first gap forming edges, respectively, are interconnected with the top plate by means of vertical plates to form a tube.
A gas tube boiler according to any of the claims 22-30, wherein said first and second plates are generally rectangular.