Recuperative heat exchanger

A recuperative heat exchanger for gas-gas heat exchange at a temperature above 700.degree. C., comprising a refractory lined vessel having a vertically extending steel shell closed at top and bottom by respective ends, wherein the space within the vessel is divided into respective top and bottom end chambers and a heat-exchange chamber therebetween by top and bottom apertured refractory plates and the end chambers are connected by a plurality of substantially vertical tubes of refractory ceramic material extending between said plates. To provide good contact of the second heat exchange medium with the tubes, connections for respectively supply and discharge of a first one of the heat-exchange media are provided in said ends, and on one side of said steel shell there are a plurality of connections for supply of the second of the heat-exchange media distributed over a region extending both vertically and circumferentially and on the other side of the steel shell there are a plurality of connections for discharge of the second medium also distributed over a region extending both vertically and circumferentially. The connections for supply and discharge of the second medium being connected via respective manifolds to main supply and discharge conduits respectively.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a recuperative heat exchanger for gas-gas heat 
exchange at temperatures above about 700.degree. C. In this specification, 
heat exchange between a gas and a vapour is included within the scope of 
the term gas-gas heat exchange as well as heat exchange between a gas and 
a gas. The invention particularly relates to a heat exchanger comprising a 
refractory lined vessel having a vertically extending steel shell closed 
at its top and bottom ends by respective ends, wherein the space within 
the vessel is divided into respective top and bottom end chambers and a 
heat-exchange chamber therebetween by top and bottom apertured refractory 
plates and the end chambers are connected by a plurality of substantially 
vertical tubes of refractory ceramic material extending between said 
plates. 
2. Description of the Prior Art 
Heat exchangers for gas-gas heat exchange at very high temperatures are 
known from for example blast-furnace technology. There, heat exchangers of 
the regenerative type are used, in which the heat derived from exhaust 
gases is stored in ceramic material and combustion air for the blast 
furnace process is subsequently pre-heated by passing it through this 
ceramic material. Such exchangers, which are called hot-blast stoves or 
"cowpers", involve very high investment costs for which reason there have 
been frequent searches for a gas-gas heat exchanger which is not of the 
regenerative type but which can be operated as a recuperative heat 
exchanger at temperatures above 700.degree. C. At temperatures of the 
order of 700.degree. to 1250.degree. C., metals are not suitable as a 
construction material for heat exchangers, so that in this temperature 
range recourse has always been made to regenerative heat exchangers of 
ceramic material. 
GB-A-No. 1100036 describes a heat exchanger as set out in the initial 
paragraph above. This recuperative heat exchanger has a large number of 
upright ceramic tubes through which hot combustion gases are passed 
downwardly so as to heat pressurized air passed upwardly between the 
tubes. Each tube is in a number of interconnected sections. The tubes are 
mounted in top and bottom plates within a vessel having domed ends. The 
upper ends of the tubes are sealed to the top plate but can move through 
the top plate to allow differential thermal expansion upon heating up and 
cooling. 
SUMMARY OF THE INVENTION 
The present invention has the object also of providing a construction for a 
recuperative heat exchanger which is suitable for gas-gas heat exchange in 
the temperature range 700.degree. to 1250.degree. C., even when there is a 
significant pressure difference between the heat exchange gases. 
The invention consists in that connections for respectively supply and 
discharge of a first one of the heat-exchange media are provided in said 
ends, on one side of said steel shell there are a plurality of connections 
for supply of the second of the heat-exchange media distributed over a 
region extending both vertically and circumferentially and on the other 
side of the steel shell there are a plurality of connections for discharge 
of the second medium also distributed over a region extending both 
vertical and circumferentially, the connections for supply and discharge 
of the second medium being connected via respective manifolds to main 
supply and discharge conduits respectively. 
In horizontal section, the steel shell is preferably of circular shape, but 
may have another shape, such as square or rectangular. Both ends may be 
domed but a flat bottom end may be preferable, e.g. as is known in blast 
furnaces and cowper stoves. 
The construction of the heat exchanger of the invention is particularly 
suitable for the exchange of heat between two gases which are both already 
at a temperature above 700.degree. C. For example one of the heat exchange 
media can cool from a temperature of about 1225.degree. to about 
930.degree. C., thereby transferring heat to the other heat exchange 
medium to raise its temperature from about 700.degree. C. to about 
1000.degree. C. The heat exchange medium with the highest pressure is 
preferably in this case passed through the vertical tubes. 
However, in many cases an apparatus is required for exchange of heat 
between two gases in which the lower temperature is much lower, e.g. of 
the order of 150.degree.-350.degree. C. In that case there is preferred a 
heat exchanger system consisting of a plurality of heat exchangers 
connected in series in which the heat exchange at the higher temperature 
level occurs in a heat exchanger of the present invention as described 
above, while the other heat exchanger may be of a metal type. Metal heat 
exchangers for gas-gas heat exchange at temperatures up to 
800.degree.-900.degree. C. are available to the expert in the present 
state of the art. It is therefore not necessary to discuss the 
construction of such metal heat exchangers in more detail. 
Refractory ceramic tubes are commercially obtainable, but usually in 
restricted lengths. For this reason, but also to permit differential 
thermal expansion of the tubes, it is recommended that the tubes of the 
heat exchanger are made in sections and extend through the top plate while 
being substantially sealed thereto by means permitting relative vertical 
movement. In modern refractory installations the incorporation of such 
expansion capability is familiar technology, and the dimensional accuracy 
of the ceramic elements used and the clearances with which these move 
relative to each other can be sufficient that an adequately good gas-tight 
fit is obtained. 
This dimensional accuracy can be improved even more when using extra narrow 
tubes in larger embodiments of the heat exchanger, by incorporating 
between the end plates one or more lateral partitions or supporting floors 
with apertures for the tubes. Since the tubes are arranged vertically, 
they will have little tendency to buckle under their own weight. However, 
in order to compensate for the effect of the transverse flow of the second 
heat exchange medium, the partition(s) can support the tubes laterally at 
one or more heights. 
The heat exchanger of the invention can achieve good distribution of the 
flow of the second heat exchange medium within the steel shell so that 
there is the most effective possible flow of this second heat exchange 
medium around the vertical tubes. For this purpose the connections for the 
supply and discharge of the second heat exchange medium in each case are 
distributed vertically and circumferentially over the surface of the steel 
shell. The manifolds preferably each comprise a ring segment connected at 
a central point on one side to the main supply or discharge conduit and 
connected on the other side to the said connctions for supply or discharge 
of the second medium via branches located at points spaced along the 
length of the segment. This construction has a certain similarity with the 
ring conduits for the hot blast in blast furnace structures, although the 
application is wholly different here. 
Depending on the structural form of the heat exchanger and the position of 
the connections, the diameter and the distribution of the diameters 
thereof in the manifolds can be chosen suitably in order to obtain an 
optimal heat transfer to the tubes. 
The refractory lining of the steel shell and the bottom end do not pose any 
special technical problem, since this requires technology similar to that 
of cowpers. Such a problem may however arise with the refractory lining of 
the top end. A suitable construction for this is that the upper end is 
outwardly domed and has its periphery radially outwardly of the said steel 
shell, the upper end having a refractory lining which is a self-supporting 
dome supported at its base radially outwardly of the innermost lining 
layer of the steel shell. A self-supporting dome of this kind is known in 
itself. This construction has the effect that any thermal expansion in the 
innermost layer of lining of the steel shell does not affect the support 
of the domed construction within the top end. 
The stability of the top and bottom plates between which the vertical tubes 
extend is important. These plates should not be affected in the relevant 
temperature range by their own weight or that of the tubes. It is 
conceivable that these plates should be made slightly convex for this 
purpose, but greater safety can be obtained if, as is preferred according 
to the invention, the top and bottom plates comprise metal boxes lined 
exteriorly at both upper and lower sides by refractory material, with the 
interiors of the boxes forming passages for the flow of coolant. If the 
heat eschanger forms part of a system with two or more series-connected 
heat exchangers, in which a relatively cold gas is introduced at the 
coolest end of the series, this cold gas can be used as a coolant for the 
top and bottom plates.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Shown in FIGS. 1 and 2 is a heat exchanger 1 for gas-gas heat exchange 
embodying the present invention, in which a cylindrical steel shell or 
jacket 2 with vertical axis is closed at its top and bottom ends by 
outwardly domed steel ends 3 and 4. A first heat exchange medium is 
introduced from a supply line 5 through a connection in the bottom end 4, 
and this medium leaves the top end 3 via a discharge connection 6. The top 
end 3 has a shape which extends to its periphery which is located radially 
further outwardly than the circumference of the cylindrical shell 2. A 
tapering transitional piece 7 serves to connected it to the cylindrical 
shell 2. 
The second heat exchange medium is introduced via a supply conduit 8 into a 
ring segment 9 which it joins at one side at a central point. Spaced along 
the segment 9 on the other side (inside) are a plurality of branches 10,11 
and 12. By means of connections, these admit the medium into the 
cylindrical vessel 2, so that the second heat exchange medium is 
introduced in the case illustrated at several levels through a total of 
twelve inlet openings, which are thus distributed of a vertically and 
circumferentially extending region of the shell 2. 
In a similar and symmetrical way the second heat exchange medium is 
discharged from the vessel via connections into vertically and 
horizontally spaced branches 15,16,17 and thence into a ring conduit 
segment 14 and a discharge conduit 13. 
Referring to FIG. 4, inside the top and bottom ends 3 and 4 there are open 
chambers 18 and 19 which are separated by a top plate 20 and a bottom 
plate 21 from the chamber within the cylindrical shell 2. The chambers 18 
and 19 are connected together by a set of tubes 24 of which for clarity 
only four are shown in this Figure. The tubes 24 are supported on the 
bottom plate 21 in the apertures thereof and are fixed to it, while they 
project through the apertures of the top plate with some allowance for 
differential expansion. In order to prevent deflection of the tubes 24 
under the influence of the transverse flow of the second heat exchange 
medium, horizontal supporting partitions or floors 22 and 23 are provided 
intermediately between the top and bottom plates, the floors 22,23 having 
apertures in which the tubes 24 can move. 
The steel shell 2, top end 3 and bottom end 4 are entirely lined with 
layers of insulating refractory material 25,26 and 27. In a corresponding 
way the conduits 5,6 and 8 to 17 are lined with a refractory material (not 
shown) in a manner which is known. 
The ceramic tubes 24 are, in the case illustrated, made from a high quality 
refractory material, e.g. silicon carbide. Tubes of this material are 
available in various lengths. In the case of large installations it may in 
some circumstances be recommendable that instead of manufacturing longer 
tubes, the tubes 24 should be assembled from sections which can move 
relatively when there is some expansion, but this should be arranged 
without affecting the gas-tightness. FIG. 5 shows, in this connection, on 
a larger scale how two sections 28,29 of a tube 24 can fit into each other 
while maintaining gas-tightness. 
FIG. 6 shows in section on a larger scale a part of the construction of the 
top plate 20. Principally this plate consists of a box-shaped body 30 
whose interior space is connected to supply and discharge conduits 31 for 
a coolant. The box-shaped body 30 is exteriorly clad on top and bottom 
with layers of refractory material 32. By means of this construction there 
is obtained a cooled rigid construction for the top and bottom plates, in 
which nevertheless the layers of insulating refractory lining 32 ensure 
that the coolant does not have an unnecessarily unfavourable effect on the 
efficiency of the heat exchanger. 
If the ceramic heat exchanger 1 according to FIGS. 1,2,4,5 and 6 forms part 
of an installation in which gases have to be cooled to temperatures 
significantly below 700.degree. C., or gases colder than 700.degree. C. 
have to be heated, FIG. 3 shows a possible series circuit of the heat 
exchanger 1 together with two metal heat exchangers 33 and 34. This series 
circuit is also an embodiment of the invention, in this aspect. For 
illustration a number of temperatures and pressures have been indicated on 
this Figure, for the two heat exchange media. At point A a gas at 
temperature 1223.degree. C. and pressure 1.77 atm. is introduced to the 
ceramic heat exchanger 1 and, after moving from left to right successively 
through the three heat exchangers, is discharged with a temperature of 
351.degree. C. at point B. A cold gas with an initial temperature of 
132.degree. C. and an overpressure of 7.44 atm. is supplied at point C to 
heat exchanger 34, and flows zig-zag in counterflow through the three heat 
exchangers, in order to leave the installation finally at point D with a 
temperature of 1005.degree. C. The pressures and temperature shown are 
purely for illustration and have no significance in themselves. This 
Figure illustrates the possibility of effecting heat exchange between 
gases within a very wide temperature range, the heat exchange at the 
highest temperature level being performed in the new heat exchanger of 
FIG. 1 according to the invention.