High temperature heat exchanger having porous tube sheet portions

A high temperature tube and shell vertically positioned heat exchanger including a plurality of tube sheets dividing the interior of the shell into three consecutive chambers. Ceramic tubes are vertically hung from hemispherical seats in an upper tube sheet and extend downwardly through loose fitting porous inserts which line perforations in the lower tube sheets. A hot fluid flows from the bottom chamber upwardly through the ceramic tubes, a cool fluid flows through the upper chamber across the tubes, and a third fluid is injected into the intermediate chamber at a pressure higher than that of the fluid mediums in the upper and lower chambers. The third, high pressure fluid flows through the porous inserts and into the other chambers forming a dynamic seal which also allows unrestricted axial motion of the tubes and limited laterial motion of the bottom of the tubes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to shell and tube-type heat exchangers, and more 
particularly to a construction utilizing ceramic components transporting 
high temperature fluid mediums. 
2. Description of the Prior Art 
In combined gas turbine-coal gasification generating plant designs, raw 
fuel gas, prior to being burned in the gas turbines, has to be 
sufficiently cleaned in order to remove particulate matter and chemical 
impurities such as H.sub.2 S, COS, HCN, CS.sub.2, HCl, KCl, KOH and 
FE(OH).sub.2 to very low levels, as an impure fuel gas detrimentally 
affects turbine life. Several options for cleaning fuel gas for gas 
turbine applications are available, or are presently being considered. 
For example, the raw fuel gas can be cooled by direct water sprays to a 
temperature below the boiling point of water, at the operating pressure, 
and particulate matter and chemical impurities removed by commercially 
available so-called "wet" processes. Also, raw fuel gas can be cooled in a 
waste heat boiler, with or without a water quench, then cooled further by 
direct water sprays, and particulate matter and chemical impurities 
subsequently removed by commercially available wet processes. Although 
these processes are presently being utilized commercially, they result in 
gross generating system inefficiencies because cooling of gas in these 
systems involves large temperature differences. Also being investigated 
are processes which remove the particulate matter and chemical impurities 
directly from the high temperature fuel gas. However, viable commercial 
technology for high temperature gas cleaning is not presently available. 
Another option is to cool the raw fuel gas by heat exchange with a clean 
fuel gas, followed by further cooling such as by direct water sprays, 
followed by removal of the particulate matter and chemical impurities by 
commercially available processes. This latter process appears to have a 
high potential for widespread use in coal gasification-gas turbine power 
generating plants. However, associated with use of such high temperature 
heat exchangers are a number of concerns including the corrosive effect of 
the chemical impurities in the raw fuel gas on commercially available 
alloys. Additionally, a high temperature heat exchanger is highly 
susceptible to erosion of the heat transfer surfaces by particulate matter 
in the raw fuel gas stream. And, the heat transfer surfaces are further 
subject to fouling by coal tar deposition and cracking. 
The corrosion concerns can be alleviated to some extent by use of exotic 
metals and metal alloys, primarily for the tubes. Metals and alloys which 
can withstand the chemical attack are not immune to the erosion by solid 
particulates in the gas. Ceramic materials, on the other hand, are 
effectively resistant to both corrosion by chemical impurities and also to 
erosion by particulate matter, and thus appear to be the most viable 
alternative. However, practical application of ceramic materials in a high 
temperature heat exchanger is complicated by the relatively low strength 
and low ductility of ceramic tubes, as well as the difficulty encountered 
in fabricating long ceramic tubes which are sufficiently straight. The 
application is further hampered by the differential thermal expansions 
between, for example, ceramic tubes and metals used in the construction of 
a heat exchanger pressure shell. Adequate solutions to these concerns have 
not appeared. 
It is thus desirable to provide a high temperature heat exchanger, 
particularly for coal gasification-gas turbine applications, which 
overcomes the discussed concerns. It is further desirable to provide a 
heat exchanger which effectively utilizes ceramic components, overcoming 
the strength, ductility, lack of straightness and differential thermal 
expansion characteristics heretofore detrimentally associated with ceramic 
components. 
SUMMARY OF THE INVENTION 
This invention provides a heat exchanger, particularly useful for combined 
coal gasification-gas turbine application, which allows high temperature 
heat exchange among fluid mediums, at least one of which is hostile as a 
result of its chemical nature and containment of chemical impurities and 
particulate matter. The invention further allows use of ceramic 
components, effectively alleviating previous limitations resulting from 
low ductility, low strength, lack of straightness and thermal expansion 
characteristics of ceramic components for heat exchanger utilization. 
In a preferred form the heat exchanger is a vertically oriented tube and 
shell type. The shell is metallic and the tubes are ceramic, laterally 
supported along their length by a plurality of tube sheet structures which 
divide the shell interior into three main chambers. Portions of the tube 
sheets, such as cylindrical inserts fitting about the ceramic tubes, are 
porous and flexible. The flexibility of the inserts allows axial expansion 
and contraction of the tubes without generating excessive stresses, and 
the porosity of the inserts provides controlled fluid communication 
between selected chambers. 
A contaminated hotter fluid medium, such as raw fuel gas, enters the bottom 
chamber and passes through the interior of the tubes. A second cooler 
fluid medium, such as clean fuel gas, passes into and through an upper 
chamber, across the tubes, absorbing heat from the raw fuel gas. A third 
intermediate chamber is interposed between the upper and lower chambers, 
and a clean gaseous medium, which may be the same as the medium in the 
upper chamber, is injected into the intermediate chamber at a higher 
pressure than the pressure in either of the other two chambers. This 
intermediate medium, because of its higher pressure, passes into the other 
two chambers through the porous inserts, thus forming a dynamic fluid seal 
between the lower and upper chambers. 
The ceramic tubes additionally have integral generally spherical flanges at 
their upper ends which seat in matingly configured hemispherical 
perforations in an upper tube sheet so as to form a ball joint type 
connection. The tubes are thus able to accommodate axial expansion and 
significant curvature of the ceramic tube without generating excessive 
stresses, particularly at the flange. The preferred tube material is dense 
silicon carbide (SiC), and the porous inserts are preferably fabricated 
from a dense mat of high alloy wire, textured similar to dense steel wool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 there is shown a high temperature tube and shell 
type heat exchanger 10. The shell assembly 12 includes a body 14, upper 
head 16 and lower head 18. The heads 16, 18 are joined to the body 14 
through flanges 20, to form a pressure bearing assembly. An upper 22 and 
lower 24 perforated tube sheet are preferably supported between the 
flanges 20, although the tube sheets 22, 24 can alternatively be supported 
from any of the three components making up the shell assembly 12. An 
intermediate tube sheet 26 is supported within the shell, preferably 
closer to the lower tube sheet 24 than to the upper tube sheet 22. 
A plurality of ceramic tubes 28 (one shown), preferably of dense silicon 
carbide (SiC), are supported vertically within the shell assembly 12. 
Tubes comprised of other ceramics such as beryllium oxide, dense alumina, 
zirconia and its oxides, as well as cermets, mixtures of sintered oxides 
and metals, can also be utilized. The upper end of each tube includes a 
generally spherical flange 30, which is integrally formed with the tube 28 
and which seats in a matingly configured generally hemispherical 
perforation 32 in the upper tube sheet 22, as shown in additional detail 
in FIG. 2. Hemispherical refers to the general shape of, for example, the 
perforation, although the bottom portion is flattened where the body of 
the tube 28 passes through. This arrangement provides a ball joint type 
seat which, coupled with the flexible lateral support of the lower portion 
of the tube, discussed further below, allows a degree of non-linearity of 
the ceramic tube 28 so as to alleviate detrimental stresses. A plate 34, 
preferably affixed to the tube sheet 22 by fasteners 23, is positioned 
atop the tubes 28 to provide a force which holds the flanges 30 in the 
perforations 32. The tubes 28 extend downwardly through the intermediate 
26 and lower 24 perforated tube sheets. 
The tube sheets divide the interior of the shell assembly 12 into a 
plurality of chambers. A hot first fluid medium, such as raw fuel gas 
including particulate matter, enters a first chamber 36 through an inlet 
nozzle 38, flows upwardly through the interior of the tubes 28, and is 
discharged from an outlet chamber 39 through outlet nozzle 40. A fluid 
medium to be heated, such as a clean fuel gas, enters a second chamber 42 
through an inlet nozzle 44, flows about the tubes, preferably in a 
serpentine pattern controlled by a plurality of flow baffles 46, absorbing 
heat energy from the raw gas within the tubes, and is discharged through 
an outlet nozzle 51. In a typical application, raw fuel gas from, for 
example, a coal gasification reactor, enters the tubes at approximately 
1750.degree. F. and at a pressure of fifteen to forty atmospheres, and is 
discharged at approximately 650.degree. F.; the clean fuel gas enters the 
shell assembly 12 at aproximately 230.degree. F. and is discharged, for 
example to a gas turbine, at 1430.degree. F. 
Interposed between the chambers 36 and 42 is an intermediate chamber 48 
bounded by the tube sheets 24, 26. A clean intermediate fluid medium is 
injected, through inlet nozzle 50, into the intermediate chamber 48 at a 
pressure which is higher than that in either chamber 36 or chamber 42, 
preferably at a differential of between two and ten psi. The intermediate 
medium can, for example, be clean fuel gas at a pressure higher than the 
fuel gas flowing through chamber 42, for example, comprising a clean cool 
fuel gas at 230.degree. F. passed through a compressor 52 and then inlet 
nozzle 50, as shown in FIG. 3. 
The primary structure of the tube sheets 24 and 26 is preferably 
impermeable to the contiguous fluid mediums, and selected portions about 
the ceramic tubes 28 are permeable to the high pressure medium injected 
into the chamber 48. The permeable portions preferably take the form of 
generally cylindrical inserts 54 disposed through perforations 56 in the 
tube sheets 24, 26. The inserts 54, which can comprise cylindrical 
components fabricated from wire made from such alloys as, for example, 
Stellite 6B or Haynes 188 (commercially available from the Cabot 
Corporation), or Thermalloy 63WC (commercially available from the Abex 
Corporation) are sized to flexibly receive the ceramic tubes 28 while 
allowing a degree of axial motion or eccentric position with respect to 
the tube sheet perforations 56 without developing excessive stresses in 
the tubes 28. The inserts thus not only provide for substantially 
unrestrained axial expansion and contraction of the ceramic tubes 28 
relative to the shell assembly 12 and affixed components, but also form a 
dynamic seal preventing direct communication between the chambers above 42 
and below 36, the intermediate chamber 48. The lower tube sheets also 
provide a radial restraint for the bottom portion of the tubes 28. For 
fabrication, the tubes 28 can be inserted through the perforations 56, and 
the porous flexible insert material subsequently packed about the tube. 
The inserts can also be positioned prior to tube insertion. Where high 
temperature fluids are utilized the outer portions of the insert material 
may self-weld to the metallic tube sheet perforation during operation. 
However, no similar reaction will occur between the metallic insert and 
the ceramic tube, maintaining the flexible relation. 
In order to maintain the preferably carbon steel sheel assembly 12 at 
acceptable operational temperatures, an insulating layer 58 is provided, 
for example, a monolytic refractory such as commercially available 
mixtures of Al.sub.2 O.sub.3, MgO and SiO.sub.2. To prevent contamination 
of the clean fuel gas by spalling of the refractory layer 58, the layer 58 
is preferably lined with a jacket 60 of high temperature alloy, for 
example 1N-657 (commercially available from the Huntington Alloys 
Corporation) or 310 stainless steel, which extends between the tube sheet 
22 and tube sheet 26. Other interior portions of the shell assembly 12 can 
also be lined, such as the intermediate chamber between the tube sheet 26 
and the tube sheet 24. The tube sheets, being exposed to high temperature 
mediums, should also be comprised of similar high temperature alloys. 
It will be apparent that the disclosed arrangements can benefically be 
applied to a variety of fluid mediums. For example, a dynamic fluid seal 
in combination with ceramic or metallic tubes can be utilized in processes 
involving heat exchange among corrosive liquids, such as acids. 
Numerous other applications and modifications may be made with the 
above-described apparatus without departing from the spirit and scope 
thereof. It therefore is intended that all matter contained in the 
foregoing description shall be interpreted as illustrative and not in a 
limiting sense.