Note: Descriptions are shown in the official language in which they were submitted.
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PRESSURE TANK CONSTRUCTIOM
FOR CORROSIVE MEDIUM
Field oE the Invention
This invention relates to improvements in hot water
steel tanks having corrosion inhibiting coatings on
interior surfaces.
Background of the Invention
Typically a ceramic or glass lined hot water tank
includes a cylindrical side wall with a cylindrical top
having a concave interior surface to define a "plus"
head for the tank. The bottom of the tank has a convex
interior surface to define a "minus" bottom for the
tank. This arrangement has been manufactured for a
considerable number of years where the technique is
normally to weld the top to the cylindrical side wall
rolled from a flat rectangular blank and to coat the
interior of the bottom open ended tank. A precoated
bottom having the convex interior surface is then welded
to the base of the tank.
Although such construction is acceptable for hot
water tanks, it has been found that tank failure is a
problem in the industry normally due to the hot water at
the top of the tank which increases the corrosion at the
joint between the tank top and the side wall. Such
failure is normally caused by interruption in the
glazing or coating applied to this area during the
coating process. An additional drawback of this
construction is that the "minus" bottom for the tank
requires a material thickness considerably thicker than
the side wall of the tank to ensure that under pressure
the tank bottom, ar least in its central area, does not
flex which could crack or in one way or another disrupt
the coating. An additional drawback with the "minus"
" head construction is that it subtracts considerably from
the assembled tank volume, hence the overall height of
the hot water tank must be greater. The hot water at
; the top of the tank increases any corrosive reaction
because released gases, which includes oxygen, are free
to chemically react with and corrode any bare metal
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exposed by poor ~lazing or coatings. It is also
appreciated that the speed of chemical reaction doubles
with every 10C increase in temperature. In addition
the abutting nature of the fit of the tank top with the
, 5 tank side is not necessaril~ perfect because of nicks or
other imperfections in the outer periphery of the
cylindrical portion of the tank side wall and the top.
Hence it is qui-te possible, even with the greatest of
care in applying the glazing material that some uncoated
areas exist at the joint between the tank top and the
tank side wall.
To increase the tank volume, it would be beneficial
to provide not only a "plus" head portion for the hot
water tank but also a "plus" bottom portion. This would
increase capacity of the tank for a given diameter with
possible reduction in 1-ank height depending on the shape
of the "plus" head. ~owever with the existing
technology, there is no known approach to connect the
"plus" bottom to the tank after the tank with welded
"plus" top has been coated with ceramic material. If a
"plus" bottom end for the tank as precoated were welded
to the tank side wall in the manner in which the top is
welded, the coating would be severely degraded or
damaged due to the heat of welding. Such degradation
would result in uncoated portions of steel which would
be corroded by the hot water.
Considering prior art tank constructions which
provide "plus" heads and bottoms for cylindrical tank
construction, U.S. Patent 3,199,711 discloses a fire
extinguishing tank which may contain pressurized
materials. The tank is constructed of materials which
are not corroded by the contQnts hence no protective or
corrosion inhibiting coating is applied to the tank
interior. For example in containing fire extinguishing
li~uids, the tank may be formed of copper or the like
where ~he components are soldered together. A similar
system is provided in U.S. Pa~tent 3,952,90~ wherein
again the material of construction is not corroded by
its contents. The plastic bottom of concave interior
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shape i5 sonic welded to the plastic side walls of the
barrier where a seal is provided at the connection to
prevent leakage. No protective coating is required in
the system nor is any welding used in making the
connection where the heat of the weld could in any way
affect an interior protective coating.
Other types of containers having "plus" end
portions on cylindrical tan]c side walls are clisclosed in
U.S. Patents 3,098,577 and 3,132,618. Neither patent
contemplates interior coating of the tank surfaces to
inhibit corrosion caused by its contents. The
considerations given above to the prior form of hot
water tank construction is not solved by the tank
construction disclosed in these patents.
Summary of the Invention
According to an aspect of the invention, a tank for
containing a corrosive medium has a corrosion inhibiting
coating on interior surfaces thereof. The tank
comprises a cylindrical steel side wall and first and
second ends for securement to and closure of the tank.
At least the first end is concave-shaped and projects
outwardly of the tank interior. At ]east the first end
is secured to the tank by a connecting means having an
annu]ar body portion for controlling effect of welding
heat on the corrosion inhibiting coating as previously
applie~ to the tank side wall interior surface and the
first end interior surface. A first portion of the
annular body portion is connected at a first connection
peripherally of the first end. A second portion of the
annular body portion is connected at a second connection
to a cylindrical end portion of the side wall. One of
the first and second portions and corresponding first or
second connection is coated with the corrosion
inhibiting coating. The other of the first and second
portions is a welded connection. The annular body
portion controls temperature to which the coating is
heated by spacing the welded connection a predetermined
extent from the corrosion inhibiting coating.
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According to another aspect of the invention, a
process is provided for connecting a steel end to a
corresponding end portion of a steel tank for contalning
a corrosive medium. The tank has a cylindrical side
wall with an interior surface, the end being circular
and having a concave shape e~tending outwardly of tank
interior and connecting means for connecting the end to
the tank side wall end portions. The process comprises
connecting the connecting means to either of the end or
the tank side wall. The interior sur~ace of the tank,
interior concave surface of the end and an exposed
portion of the connecting means are coated with a
corrosion inhibiting coating. The connecting means is
welded to the end or the tank side wall. To complete
connection of the end to the tank side wall, the coated
exposed portion of the connecting means abuts the
coating on the end interior surface of the tank side
wall interior surface to form a coating juncture.
Temperature to which the coating juncture is heated is
controlled by predetermining an extent to which the
- connecting means spaces the welding of the connecting
means from the coating juncture.
Brief Description of the Drawings
Preferred embodiments of the invention are ,shown in
the drawings wherein:
Figure 1 is a perspective view of a hot water tank
characteristic of the prior art devices;
Figure 2 is a section through the bottom and top
end portions of a tank of Figure l;
Figure 3 is a section through the hot water tank
bottom incorporating the method of connection of the
"plus" bottom to the tank in accordance with an
embodiment of this invention;
Figures 4 through 9 illustrate different
embodiments of the invention for connecting the "plus"
bottom of the hot water tank to the side wall of the
tank in accordance with this invention; and
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Figure 10 is a section through the hot water tank
base showing an alternative arrangement for supporting
the hot water tank with a "plus" bottom.
Detailed Description of the Preferred Embodiments
This invention is applicable to many ~orms of tank
construction which contain mediums corrosive to the tank
material; for example, steel tanks for containing water,
gas cylinders for acidic gases, fire extinguishers and
beer kegs. The preferred embodiments of this invention
will be demonstrated in a hot water steel tank
construction~ ~owever, it is appreciated that such
embodiments are equally applicable in other tank
constructions.
Hot water tanks are used in residential and
commercial establishments to heat and store water,
normally at temperatures in the range of 120F to 160F,
where the source of heat is normally by electrical
heating elements or gas fired heating devices. A
typical tank used in electrical type of water heater is
shown in Figures l and 2. The tank 10 consists of a
cylindrical side wall 12 with a first or bottom end 14
and a second or top end 16. The ends of the tank are
welded to the cylindrical side wall to seal the tank and
provide a compartment in which water is heated and
stored as hot water. It is understood that the water
inlet and apertures for electrical heating elements, as
provided in the side wall 12, are not shown. Usually
the hot water is taken from the top of the hot water
heater vi.a the outlet 18.
The assembly of the hot water heater of the prior
art device is normally accomplished by welaing the end
16 to the side wall 12. With reference to Figure 2, the
top end 16 has a peripheral end portion 20 which is
fitted within the side wall upper end portion 22. The
top end 16 is welded to the side wall end portion 22 by
a fillet weld ~4 which connects the upper edge 26 to the
outer side wall 28 of the end 16. A bottom open-ended
tank is therefore provided where the tank interior with
the end 16 in place is coated with a corrosion
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inhibitiny coating. This may be accomplished b~
cleaning the inside of the tank and end by use of
abrasives or acid pickling process to remove the oxides
and dirt from the steel interior surfaces and at the
same time etch the surface to provide for satisfactory
bond of the corrosion inhibiting coating to the interior
of the tank. After cleaning, the tank interior is
coated with a glazing slurry which is commonly used in
the art and consists of finely ground essentially
ceramic powder suspended in water. The coating is
applied to all internal surfaces including the interior
concave surface 30 of the end 16 and also about the
joint area 32 interior of the tank along with the tank
side wall interior surface 34. To facilitate welding of
the bottom end 14 to the tank side wall, the coating is
wiped off the bottom edge 36 as well as a marginal
interior portion of the tank side wall. The coated tank
is then passed through a drier to remove water from the
glazing slurry, and placed into a glazing furnace which
operates normally at a temperature of about 1600F.
This results in a smooth corrosion resistant coating on
the tank interior.
The bottom end 14 which has a convex interior
surface is cleaned and glazed in a similar manner to the
tank she]l assembly. The glazing coating is wiped off
in area 38 so as to be adjacent the cleaned end portion
36 of the side wall. The bottom 14 is inserted within
the tank. The coating on the interior surface 40 of the
tank bottom abuts the coating on the in-terior surface 3
of the side wall. A weld 42 is applied between surfaces
36 and 38. Usually the weld 42 is sufficiently remote
from the area generally designated 44, which is the
juncture of the coatings on interior surfaces 34 and 40,
that the heat from the welding process does not degrade
the coatings. Normally such welding is carried out by a
type of submerged-arc process.
This type of tank construction has been used for
many years in providing a "plus" head on the top of the
tank and a "minus" head on the bottom of the tank. The
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"minus" head considerahly reduces -the volume for the
tank considering its overall height. The bottom portion
with the convex interior surface must be formed of
material which is thicker than the side wall to
withstand the pressures within the hot water tank. If
the bottom were made of thinner material the bottom
could flex resulting in cracking of the coating and
opening areas to corrosion causing tank failure. The
material thickness for the bottom is usually in the
range of 1.6 times thicker than the material thickness
for the "plus" head portion providing the dished shapes
are similar, which in turn is usually thicker than the
material thickness for the side wall. This results in
considerable inventory problems and the need to carry at
least three different thicknesses of material depending
on the parts made. An additional requirement is the
extra tooling and machine set ups in manufacturing the
parts.
To increase volume of the tank for its diameter and
to permit use of thinner material thicknesses at least
on the bottom of the tank, the provision of "plus" end
at the bottom of the tank is a desired embodiment of the
invention. However, after assembly of the tank side
walls to the tank top and coating and subsequent welding
~5 of the tank "plus" bottom to the tank side wall, the
welding would be on the tank bottom portion in a area
which is coated. The heat from the weld would travel
directly through the tank bottom wall portion thereby
degrading the ceramic coating. This even becomes more
of a problem if non-ceramic coatings are used such as
polymeric coatings which include various suitable
polyamides; for example Nylon II ttrademark). Such
degradation of the coating can result in its destruction
and loss of corrosion inhibiting properties.
As already noted, another problem with existing
tank construction is that several of ~he tank failures
are due to a breakdown in the coating of the top seam
area of the tank top to the side wall~ Hence a
construction ~hich would avoid such breakdown is desired
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for the top construction of the hot water tank.
~ ccording to this invention a connection is
provided for either end of the tank cylindrical slde
wall which can constitute either the top or bottom of
the tank when in use. As shown in Figure 3, the
cylindrical tank side wall 12 has fitted to its bo-ttom
end portion 36 a "plus" bottom end 46. The end 46 has a
concave interior surface 48 which extends outwardly of
the interior of the tank. According to this embodiment
the concave interior surface 48 has a semi-ellipsoidal
shape. However, it is appreciated that the end may be
torispherical in shape as defined in ASME Boiler and
Pressure Vessel Code, Section VIII, ~ivision 1, 1977
Edition. Due to essentially constant material thickness
the exterior surface 52 of the end 46 approximates the
same shape. To provide for connection of the end 46 to
the interior of the side wall 12, a connecting device
generally designated 54 having an annular boby portion
is used. According to one aspect of the invention, the
connecting device 54 is secured to the peripheral edge
portion 56 of the exterior of the end 46. Such
connection may be by welding or o-ther suitable securing
means.
Prior to assembly of the end 46 to the cylindrical
side wall 12 of the tank, the interior surface 34 of the
tank is coated as well as the interior surface 48 and
the exterior portion of the connecting means 54. Any
excess applied coating in region 57, which is to be
subsequently welded -to the tank wall is wiped off. The
tank interior end is also wiped off to leave opposing
uncoated surfaces for welding. The end portion 46 is
then inserted into the end 36 of the cylindrical side
wall where the coatings on the connecting means 54 and
on the interior surface 34 of the tank overlap and
contact one another to seal the tank end. The
connecting means is then welded to the end 36 by a
fillet weld 58. The connecting device 54 is arranged to
control the temperature to which the juncture of the
overlapped coatings is heated by the process of welding
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the connecting means -to the end 36. Such control on
temperature is exercised by predetermining the extent to
which the connecting device 54 spaces the fillet weld 58
from the juncture of the overlapped coating and also
from the coating on the interior surface of the end and
tank side wall. The connecting device 54 conducts heat
from the welding region of weld 58 towards the juncture
- of the coatlngs. By trial and error, the extent to
which the connecting device spaces the weld from the
juncture of the coatings can provide a control on the
temperature to which the coatings are heated. It may be
desired to control the temperature to an extent which
minimizes or avoids any effect of welding heat on the
coatings at the juncture. Alternatively, lt may be
desirable to control temperature to the extent that the
coatings at least at the juncture are fused during the
welding process. It is appreciated that, when it is
desired to increase the temperature at the coating
juncture, a shorter height for the connecting device is
provided, or the mass of the connecting device is
reduced, or is made from material which conducts heat at
a faster rate to the juncture area. Further detail of
the overlap of and formation of a juncture of the
coatings and the welding technique will be discussed
with respect to the various embodiments of the invention
as shown in Figures 4 through 9.
To complete the assembly of the botto~ of the hot
water tank a cylindrical base or stand 60 is provided.
The stand 60 has a inwardly stepped portion 62 which is
secured to the interior surface 64 of the connecting
device. The stand has a peripheral bottom edge 66 which
extends below the lowermost portion 68 of the end 46 to
act as the support feet for the hot water tank when it
is stood upright for use.
The coating for the connecting device 54 for the
end 46 and also for the interior 34 of the tank side
wall 12 is shown in more detail in Figure 4. The
connecting device 54 comprises an annular skirt portion
having exterior surface 76 which is welded at 74 to the
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exterior surface 56 of the end portion. The skirt
extencls outwardly of the end portion and is stepped in
area 84 to facilitate insertion of the end 46 into the
tank. The coating 70 as applied to the interior sur-face
34 may be of a fired ceramic composition or of some
polymeric composition, such as "Nylon". As already
discussed in providing a ceramic or glass lined coating,
a slurry lncluding ceramic particles is coated onto the
interior surface 34 of the tank side wall. The lower
circumferential end portion of the shell interior 72
does not include coating material hence this area is
left uncoated after the tank side wall has been dried
and fired. The end 46 and the connecting device 54 also
have the slurried ceramic coating applied thereto over
the entire interior surface 48 of the end, and
overlapping the welded area 74 which includes the
connection of the connecting device 54 to the external
edge portion 56 of the end. The exterior surface 76 of
the connecting device is also coated with the slurried
; 20 material to provide thereby an essentially continuous
coating 78. The lower portion 80 is left uncoated to
~` provide an uncoated peripheral area 80 about the
perimeter of the connecting device 54.
The coatings, which have been applied to the tank
side wall interior surface and to the end ~6, are then
glazed, cured or fused. The end is then inserted into
the tank end until the coated surfaces 70 and 78 c~ntact
in the region designated 82 on the connecting device
exterior surface 76 and also on the lower portion of the
coating 70 of the tank side wall. Such contacting of
the coatings forms a sea~ about the periphery of the
tank bottom end to prevent water leakage and thereby
form effectively a continuous coating from the side wall
onto the end 46. In providing parallel coated surfaces
which form the juncture of coatings, a significant
circumferential band of contact area is provided for the
` contacting coatings. Such band area of contact ensures
; a seal about the peripheral juncture of coatings,
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because of the band height over which contact between
the coatings can be established. If there is difficulty
in providing a seal about the peripheral juncture of the
coatings, as discussed, it is possible by way of the
connecting device to control the temperature to which
the juncture of coatings is heated to fuse coatings in
that area without totally degrading the coatings and
losing any potential for a seal at this juncture. This
is particularly applicable when the coatings are of a
polymeric material, such as "Nylon" which can be fused
when heated to the proper temperatures. It is
appreciated that a sealant, such as an epoxy coating or
gasket, may be applied to the juncture before welding.
The heat of welding is then controlled by the connecting
device to ensure that the epoxy sealant or ~asket is not
degraded.
The end portion is now in a position to be secured
to the tank side wall 12. The connecting device 54
extends a sufficient distance outwardly of the juncture
82 of the coatings such that when the uncoated portion
80 is welded to the uncoated portion 72 of the tank side
wall, the heat of the welding does not in any way
degrade the non-corrosive characteristic of the coatings
at least in the area of juncture 82 and inwardly
thereof. Such connecting device 54 serves to space the
welded area away from the coating not only on the
connecting device 54 but also on the interior surface of
the end 46 such that when the weld is completed a
continuous coating remains on the bottom surface and
side wall area of the tank. The welding which may be
used to connect the connecting device 54 to the outer
peripheral edge portion 56 of the end is done before the
coating is applied to the end and connecting device
hence the heat of the weld of that connection has no
effect on the subsequent coating process. In actual
fact, the coating overlaps the seamed area and any weld
which may be present to coat and protect that area from
corrosion by the contained liquid which is normally
water.
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12
It ls appreciated that the principle of the
invention may be accomplishec1 in a variety of
configurations for connecting an end portion to the tank
side wall. With reference to Figure 5 through 9 such
alternative arrangements are shown where it is
understood that the surfaces as exposed to the contained
liquid are coated with the corrosion inhibiting
materials.
As shown in Figwre 5, the end 46 has a concave
interior surface 86 which may be of the ellipsoidal,
torispherical or hemispherical configuration as shown in
Figure 3. The connecting device 54 includes the annular
skirt portion 88 which extends outwardly of the interior
of the tank. According to this embodiment the skirt
portion 88 is a continuation of the wall portion 90 of
the end as provided for by a reverse bend ~2. The end
46 as fabricated from an essentially constant material
thickness of steel is coated prior to insertion within
the tank side wall 12 which has a coating on its
interior surface 34~ The coatings are such to form a
juncture in the region 94 which effectively forms a seal
about the bottom of the tank. Uncoated portions of the
tank side wall in region 72 and 96 of the skirt exterior
are provided so that a fillet weld 98 may be used to
connect the end 46 to the tank side wall 12. The skirt
of the connecting device 54 extends outwardly of the
tank interior a sufficient distance 50 as to locate the
fillet weld 98 at a region where the heat of the weld
does not degrade the non-corrosive properties of the
coating at least in the area of ~uncture 94 about the
perimeter of the connecting device 54. By providing
this reverse bend arrangement 92 for the end 46, the
entire end portion may be fabricated from a single blank
of steel sheet material without requiring any welding
35 operation in the reverse bend region 92.
Wlth reference to Figure 6, another embodiment is
shown for the end 46 of the hot water tank for
installation and connectlon to the tan~ side wall 12.
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With this type of assembly, the top and bottom ends can
be o~ the same diameter. This approach considerably
reduces inventory costs in keeping separate tank ends
for completing the tank construction. The end 46 has a
concave interior surface 100. The connecting device 54
comprises a skirt portion 102 having its upper end which
is angled -towards the bottom portion 104 of the end 46.
A fillet weld at 106 connects the angle portion 108 of
the skirt to the end 46. The assembly of the connector
device 54 and the end 46 are coated in the manner
similarly used in coating the other embodiments. The
interior surface 34 of the side wall is coated, such
that when the end 46 is inserted within the tank side
wall the coatings contact and form a seal in the
juncture region 110. With the end portion assembled
within the tank the uncoated portions of the tank end 72
and the connector skirt portion 112 are connected by
fillet weld 114, the heat of which does not affect the
coatings in the region 110.
The embodiment of Figure 7 is an alternative to
that of Figure 6, where an arrangement is provided which
permits the use of an end 46 which is the same as the
other end used in connecting to the top portion of the
tank side wall in accordance with the technique shown in
Figure 2. To accommodate the thickness of the
connecting device 54 the side wall 12 is stepped in
region 116. According to this embodiment the connecting
device 54 is a skirt portion 118 which is welded at 120
to the peripheral edge portion 122 of the end 46. The
end 46 with the connecting device 54 is coated
independentIy of the side wall interior surface 34. In
coating the end portion 46 the coating covers the seam
area which includes the weld 120. After the end 46 is
inserted in the tank end portion, the end is secured by
way of a weld 124. The skirt 118 extends sufficiently
outwardly of the tank so as to locate the weld 124 a
sufficient distance from the region 126 about which the
coatings on the interior surface 34 of the tank and on
:~ the connecting device 118 contact one another. The heat
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of weld from welding 124 does not deyrade coatings in
this area or interior thereof including the coating on
the interior 128 of the end 46.
Another arrangement is provided in Figure 8 for
connecting the end portion 46 to the tank side wall.
The connecting device 54 includes a skirt portion 130
which is welded at seam 132 to the peripheral edge 134
of the end 46. The welded assembly is then curved
inwardly to the extent shown in Figure 8 to provide for
a lead in portion in inserting the end 46 within the
tank side wall 12. The tank side wall 12 is provided
with the coating 70 in the manner discussed with respect
to Figure 4, the tank end 46 with connecting device 54
is coated with a coa~ting 136 which extends over the seam
area 132 and downwardly of the outer wall portion 138 of
the skirt 130. A fairly close tolerance fit is provided
between the uncoated portion of the outer surface 138 of
the skirt 130 and the corresponding uncoated portion 72
of the tank side wall. The weld 140 is located a
sufficient distance from the juncture 141 of the coating
so as to not degrade same. Similarly the skirt 130
spaces the weld 140 from the coating 136 on the interior
of the end 46.
It is also possible to provide for the connection
of the tank side wall to a "plus" end, bottom or top
which has an internal diameter at its circumferential
portion greater than the external diameter of the tank
shell. This type of connection is shown in Figure 9,
wherein the tank side wall 12 has the connecting device
54 secured at a first portion 142 to the e~terior 144 of
the tank side wall. The inner surface 34 of the tank is
coated with the eorrosion inhibiting eoating including
the eonneetion area 146 and remaining exposed external
surfaee 148 of the connecting deviee 54. The "plus"
end, whieh may be the bottom 46, has a eoating on its
interior surface 48 which overlaps the coating on
surface 148 o~ the connecting device in the region 150.
~; The seeond portion 152 of the eonneeting deviee is
fillet welded at 154 to the end 46 to eomplete the
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assembly of the end to the tank side wall. The
connectlng device 54 spaces the fillet weld 154 a
sufficient distance from the overlapped region 150 to
control the -temperature to which the overlapped region
of coatings is heated during the welding operation. By
this technique, a larger end can be assembled to a
smaller tank shell.
An alternatlve stand assembly for the hot water
tank is provided in Figure 10. The end 46 is assembled
to the tank side wall 12 in accordance with the
structure generally shown in Figure 8. For hot water
tanks which are insulated with a foaming material such
as urethane foam a prefabricated foamed base 156 is
provided which receives the shape of the end 46. With
the tank bottom on the foam base 156 a shell assembly
158 may be assembled around the base and the tank
leaving a cavity 160 between the tank exterior 15~ and
the tank side wall 12 and also about the top portion
(not shown). With the assembly complete, a foam
~0 material is in~roduced into the cavity 160 to fill the
cavity with an insulating foam material and solidify the
structure once the foam urethane composition has set.
This provides for a quick assembly technique for the hot
water tank construction where the shell 158 including a
platform 162 is now complete and ready for use where the
platform includes projecting feet portions 164.
~ s previously discussed with respect to Figure 2,
the prior art technique of assembling the top to the
tank end has resulted in failures. This can be avoided
in tanks having a "minus" bottom by using the same
technique for assembling the "plus" bottom end of the
tank, as discussed ~ith r~spect to Figures 3 through 10,
to similarly assemble the "plus" top to the tank end.
It is appreciated that in connecting a top end to the
tank a skirt extension or the like 60 as shown with
respect to Figure 3 i5 not required. In the case of
foam insulating the tank the tank upper portion may be
the mirror image of that shown in Figure 10 without the
use of feet or the like where appropriate openings and
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piping arrangement are provided for the outlet of the
hot water tank.
By this method of assembly in isolating the welding
for connecting a tank end to the tank side wall so that
the tank coatings are not degraded, one can provide
"plus" head configurations at each end of the tank to
increase the tank volume and in addition permit the use
of thinner material thicknesses for the tank ends since
the concave shape for the tank end can accommodate
considerably greater stresses without yielding. By
considerably reducing the material thickness of the end
portions which may be equal to or less than the
thickness of the tank side wall, appreciable savings in
tank construction are achieved. For example, with a
tank construction where the top is also used as the
bottom and assembled in the manner shown with respect to
Figure 7, there is a material saving of approximately
8%. With a tank end configuration having ends
approximating the ellipsoidal configuration of Figure 3
or a similar torispherical shape, there can be a
material saving of approximately 20~. When the tank
ends approach a hemispherical shape the material saving
can be approximately 31%. This is accomplished by the
fact that as the tank ends approach a hemispherical
shape considerably thinner material thickness for the
tank ends may be used while still accommodatin~ the tank
pressures.
The relationship of the material thickness of the
tank end compared to the thickness of the tank side wall
for resisting stresses created by pressures within the
tank are well understood in pressure tank design. This
permits the use of thinner materials in the tank ends to
considerably reduce the amount of material used in
manufacturing the tanks knowing that the tank end can be
designed to accommodate a predetermined stress.
Surprisingly with the prior art construction of Figure 2
even with the "plu5" head configuration for the tank
top, the material thicknesses for the tank top is
usually 1.~ times the tank side thickness and for the
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17
bottom "minus" head the thickness usually averages more
than 1.9 times the tank side wall thickness. In
accordance with this invention where the "plus" head
arrangement is used at both the tank top and bottom, it
has been found that both ends may be of a -thickness less
than the tank side wall thickness. This results in
significant material savings, to upwards oE 28%, and
considerably reduces failures of the tank in the region
of the joint of tank top to side wall. In addition the
design provides for a significant glazing energy saving
which can be realized in the range of 10 to 30%
depending upon the shape for the tank ends. Another
form of energy saving can be realized due to the overall
reduction in tank surface area for the volume of water
contained. Due to the reduced tank surface area, there
is less heat loss so that less energy is required to
maintain the water at the desired temperature.
The method, according to this invention, is
conducted in the manner described where the significant
advantage with one of the embodiments of the invention
is that the tank side wall may be coated independently
of the end portions. Hence the interior of the side
wall can be inspected for flaws before the ends are
connected to the tank side wall. In addition, the
threads of the outlet 18 can be independently inspected.
This is particularly important when coatings of ceramic
slurry are applied to the tank interior. In addition,
with coating the tank ends with ceramic slurry the
effectiveness of the coating can be inspected
30 particularly about the seaming area of the connecting
device.
~ lthough preferred embodiments of the invention
have been described herein in detail it will be
understood by those skilled in the art that variations
5 may be made thereto without departing from the spirit of
the invention or the scope of the appended claims.
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