Note: Descriptions are shown in the official language in which they were submitted.
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PYROMETALLURGICAL REACTOR COOLING ELEMENT AND ITS
MANUFACTURE
The present invention relates to a method of manufacturing a cooling
element for pyrometallurgical reactors, said element having at least one flow
channel, and where the manufacture of the element is made by continuous
casting, i.e. slip casting. In order to enhance the heat transfer capability
of
the element, the wall surface area of the cooling channel wall is increased
with respect to its round or oval shape on cross-section without increasing
the diameter or length of the flow channel. The invention also relates to the
element manufactured by this method.
The refractory of reactors in pyrometallurgical processes is protected by
water-cooled cooling elements so that, as a result of cooling, the heat
coming to the refractory surface is transferred via the cooling element to
water, whereby the wear of the lining is significantly reduced compared with
a reactor which is not cooled. Reduced wear is caused by the effect of
cooling, which brings about forming of so called autogenic lining, which fixes
to the surface of a heat resistant lining and which is formed from slag and
other substances precipitated from the molten phases.
Conventionally cooling elements are manufactured in two ways: primarily,
elements can be manufactured by sand casting, where cooling pipes made
of a highly thermal conductive material such as copper are set in a
sand-formed mould, and are cooled with air or water during the casting
around the pipes. The element cast around the pipes is also of highly
thermal conductive material, preferably copper. This kind of manufacturing
method is described in e.g. GB patent no. 1386645. One problem with this
method is the uneven attachment of the piping acting as cooling channel to
the cast material surrounding it because some of the pipes may be
completely free of the element cast around it and part of the pipe may be
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completely melted and thus damaged. If no metallic bond is formed between
the cooling pipe and the rest of the cast element around it, heat transfer
will
not be efficient. Again if the piping melts completely, that will prevent the
flow of cooling water. The casting properties of the cast material can be
improved, for example, by mixing phosphorus with the copper to improve the
metallic bond formed between the piping and the cast material, but in that
case, the heat transfer properties (thermal conductivity) of the copper are
significantly weakened by even a small addition. One advantage of this
method worth mentioning is the comparatively low manufacturing cost and
independence from dimensions.
Another method of manufacture is used, whereby glass tubing in the shape
of a channel is set into the cooling element mould, which is broken after
casting to form a channel inside the element.
US patent 4,382,585 describes another, much used method of
manufacturing cooling elements, according to which the element is
manufactured for example from rolled copper plate by machining the
necessary channels into it. The advantage of an element manufactured this
way, is its dense, strong structure and good heat transfer from the element
to a cooling medium such as water. Its disadvantages are dimensional
limitations (size) and high cost.
A well-known method in the prior art has been to manufacture a cooling
element for a pyrometallurgical reactor by casting a hollow profile as
continuous casting i.e. slip casting through a mandrel. The element is
manufactured of a highly thermal conductive metal such as copper. The
advantage of this method is a dense cast structure, good surface quality and
the cast cooling channel gives good heat transfer from the element to the
cooling medium, so that no effects impeding heat transfer occur, rather the
heat coming from the reactor to the cooling element is transferred without
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any excess heat transfer resistance directly to the surface of the channel
and onwards to the cooling water. The cross-section of the cooling channel
is generally round or oval and the mandrel has a smooth surface. This type
of cooling channel is mentioned in US patent 5,772,955.
In order to improve the heat transfer capability of a cooling element it is
however preferable to increase the heat transfer surface area of the
element. As demonstrated by the explanation below, according to the
present invention this occurs by increasing the wall surface area of the flow
channel without enlarging the diameter or adding length. The wall surface
area of the cooling element flow channel is increased by forming grooves in
the channel wall during casting or by machining grooves or threads in the
channel after casting so that the cross-section of the channel remains
essentially round or oval. As a result, with the same amount of heat, a
smaller difference in temperature is needed between the water and the flow
channel wall and an even lower cooling element temperature. The invention
also relates to cooling elements manufactured by this method. The essential
features of the invention will become apparent in the attached patent claims.
The ability of a cooling element to receive heat can be presented by means
of the following formula:
Q = ax A x dT, where
Q = amount of heat being transferred (W)
a heat transfer coefficient between flow channel wall and water [W/KmZ]
A heat transfer surface area [mz]
AT = difference in temperature between flow channel wall and water [K]
Heat transfer coefficient a can be determined theoretically from the formula
Nu =aD/.1
A = thermal conductivity of water [W/mK]
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D = hydraulic diameter [m]
Or Nu = 0.023 x Re~0.8Pr~0.4,
where
Re=wDplq
w = speed [m/s]
D = hydraulic diameter of channel [m]
p = density of water [kg/m3]
I = dynamic viscosity
Pr = Prandtl number [ ]
Thus, according to the above, it is possible to influence the amount of heat
transferred in a cooling element by influencing the difference in temperature,
the heat transfer coefficient or the heat transfer surface area.
The difference in temperature between the wall and the tube is limited by the
fact that water boils at 100 C, when the heat transfer properties at normal
pressure become significantly worse due to boiling. In practice, it is more
advantageous to operate at the lowest possible flow channel wall
temperature.
The heat transfer coefficient can be influenced largely by changing the flow
speed, i.e. by affecting the Reynolds number. This is limited however by the
increased loss in pressure in the tubing as the flow rate increases, which
raises the costs of pumping the cooling water and pump investment costs
also grow considerably after a certain limit is exceeded.
In a conventional solution, the heat transfer surface area can be influenced
either by increasing the diameter of the cooling channel and/or its length.
The cooling channel diameter cannot be increased unrestrictedly in such a
way as to be still economically viable, since an increase in channel diameter
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increases the amount of water required to achieve a certain flow rate and
furthermore, the energy requirement for pumping. On the other hand, the
channel diameter is limited by the physical size of the cooling element, which
for reasons of minimizing investment costs, is preferably made as small and
5 light as possible. Another limitation on length is the physical size of the
cooling element itself, i.e. the quantity of cooling channel that will fit in
a
given area.
When it is desired to increase the heat transfer surface of the cooling
element presented herein, it is done by changing the wall shape of the slip
cast cooling element flow channel to achieve a greater heat transfer surface
area, calculated per flow channel length unit, with the same flow
cross-section (same rate is achieved with the same amount of water). This
increase in surface area is achieved, for example, by the following means:
- At least one flow channel, essentially round in cross-section, is formed in
the slip-cast cooling element during casting, and threads are machined
into the flow channel after casting.
- At least one flow channel, essentially round in cross-section, is formed in
the cast cooling element during slip casting, and rifle-like grooves are
machined into the flow channel after casting. The grooves are
advantageously made by using a so-called expanding mandrel, which is
drawn through the flow channel. Grooving can be made to e.g. a hole
closed at one end, in which case the mandrel is drawn outwards. A hole
made in the channel, which is open at both ends, is made either by
pushing or drawing a purpose-designed tool through the channel.
- The most advantageous increase in surface area is obtained by forming,
during casting, one or several grooved, preferably straight-grooved, flow
channels in the cooling element, using a purpose-designed, grooved
casting mandrel. Despite the grooving, the shape of the flow channel is
still essentially round or oval in cross-section. Using this method will
avoid mechanical machining stages after casting.
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In all the methods described above, it is evident that,
should there be channel parts in the flow channel transverse
with regard to the casting direction, these parts are made
mechanically by machining, for instance by drilling, and the
openings not belonging to the chanriel are plugged.
According to a broad aspect of the present invention
there is provided a method to manufacture a plate-like
pyrometallurgical reactor cooling element. The element is
slip-cast manufactured of a highly thermal conductive metal
and has at least one cooling water flow channel, essentially
round or oval in cross-section, formed in the cooling
element during casting. The method is characterized in
that, in order to enhance the heat transfer capability of
the cooling element, the wall surface area of the flow
channel inside the cooling element is increased without
increasing the diameter or length of the flow channel by
forming one or several grooves'in the surface of the flow
channel by means of a grooved mandrel during casting or by
machining threads or rifle-like grooves after casting.
According to a still further broad aspect of the
present invention there is provided a plate-like
pyrometallurgical reactor cooling element, slip-cast
manufactured of highly thermal conductive metal and_ having
at least one cooling water flow channel, essentially round
or oval in cross-section, formed 'in the cooling element
during casting,. The plate-like pyrometallurgical reactor
cooling element is characterized in that the wall surface
area of the flow channel inside the cooling element is
increased, without enlarging the diameter of the flow
channel or adding to the length, by one or several grooves
formed in the surface of the flow channel by means of a
grooved mandrel during casting or by threads or rifle-like
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grooves machined in the surface of the flow channel after
casting.
The benefit of the method to increase heat transfer
surface area described in this invention was compared with a
method of the prior art with the aid of the example given
here. In connection with the example there are some diagrams
to illustrate the invention, in which
Figure 1 shows a principle drawing of the cooling
element used in the tests,
Figure 2 shows a cross-sectional profile of the test
cooling element,
Figures 3a-3d indicate the temperature inside the
element at different measuring points as a function of melt
temperature,
Figure 4 presents the heat transfer coefficient
calculated from the measurements taken as a function of the
melt, and
Figure 5 presents the differences in temperature of the
cooling water and the channel wall at different cooling
levels for normalized cooling elements.
Example
The cooling elements relating to the present invention
were tested in practical tests, where said elements A, B, C
and D were immersed in about lcm deep molten lead from the
bottom surface. Cooling element A had a conventional smooth-
surfaced channel, and this element was used for comparative
measurements. The amount of cooling water and the
temperatures both before feeding the water into the cooling
element and afterwards were carefully measured in the tests.
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The temperature of the molten lead and the temperatures
inside the cooling element itself were also carefully
measured at seven different measuring points.
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Figure 1 shows the cooling element 1 used in the tests, and the flow channel
2 inside it. The dimensions of the cooling element were as follows: height
300 mm, width 400 mm and thickness 75 mm. The cooling tube or flow
channel was situated inside the element as in Figure 1, so that the centre of
the horizontal part of the tube in the figure was 87 mm from the bottom of the
element and each vertical piece was 50 mm from the edge of the plate. The
horizontal part of the tube is made by drilling, and one end of the horizontal
opening is plugged (not shown in detail). Figure 1 also shows the location of
temperature measuring points T1 - T7. Figure 2 presents the surface shape
of the cooling channels and Table 1 contains the dimensions of the test
cooling element channels and the calculated heat transfer surfaces per
metre as well as the relative heat transfer surfaces.
Table 1
Diameter Flow Heat transfer Relative heat
cross-sectional surface / I m transfer surface
Mm area mz/1 m area
mm2
A 21.0 346 0.066 1.00
B 23.0 415 0.095 1.44
C 23.0 484 0.127 1.92
D 20.5 485 0.144 2.18
Figures 3a - 3d demonstrate that the temperatures of cooling elements B, C
and D were lower at all cooling water flow rates than the reference
measurements taken from cooling element A. However, since the flow
cross-sections of the said test pieces had to be made with different
dimensions for technical manufacturing reasons, the efficiency of the heat
transfer cannot be compared directly from the results in Figures 3a - 3d.
Therefore the test results were normalized as follows:
Stationary heat transfer between two points can be written:
Q = S xA x(T, - T2), where
Q = amount of heat transferred between the points [W]
S = shape factor (dependent on the geometry) [m]
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A= thermal conductivity of the medium [W/mK]
T, = temperature of point 1[K]
TZ = temperature of point 2 [K]
Applying the above equation to the test results, the following quantities are
obtained:
Q = measured thermal power transferred to cooling water
A= thermal conductivity of copper [W/mK]
T,= temperature at base of element as calculated from tests [K]
TZ = temperature of water channel wall as calculated from tests [K]
S = shape factor for a finite cylinder buried in a semi-infinite member
(length
L, diameter D) shape factor can be determined according to the equation
S = 2nUln(4z/D) when Z>1.5D,
z depth of immersion measured from the centre line of the cylinder [m].
The heat transfer coefficients determined in the above way are presented in
Figure 4. According to multivariate analysis a very good correlation is
obtained between the heat transfer coefficient and the water flow rate as well
as the amount of heat transferred to the water. The regression equation heat
transfer coefficients for each cooling element are presented in Table 2.
Thus a[W/m2K] = c + a x v[m/s] + b x Q[kW].
Table 2
C A b r-2
A 4078.6 1478.1 110.1 0.99
B 3865.8 1287.2 91.6 0.99
C 2448.9 1402.1 151.2 0.99
D 2056.5 2612.6 179.7 0.96
To make the results comparable, the cross-sectional areas of the flow
channels were normalized so that the amount of water flow corresponds to
the same flow rate. The flow channel dimensions and heat transfer surface
areas normalized according to the flow amount and rate are presented in
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Table 3. Using the dimensions given in Table 3 for cases A', B', C' and D'
and the heat transfer coefficients determined as above, the temperature
difference of the wall and water for normalized cases regarding the flow
amount were calculated as a function of water flow rate for 5, 10, 20 and 30
kW heat amounts with the equation
dT=QI(axA)
Table 3
Diameter Flow Heat transfer Relative heat
cross-sectionai surface / 1 m transfer surface
mm area m2/1 m area
mm2
A* 21.0 346 0.066 1.00
B* 21.0 346 0.087 1.32
C* 19.2 346 0.120 1.82
D* 15.7 346 0.129 1.95
The results are shown in Figure 5. The figure shows that all the cooling
elements manufactured according to this invention achieve a certain amount
of heat transfer with a smaller temperature difference between the water and
the cooling channel wall, which illustrates the effectiveness of the method.
For example, at a cooling power of 30kW and water flow rate of 3 m/s, the
temperature difference between the wall and water in different cases is:
Table 4
dT [K] Relative dT [%]
A' 38 100
B' 33 85
C' 22 58
D' 24 61
When the results are compared with the heat transfer surfaces, it is found
that the temperature difference between the wall and the water needed to
transfer the same amount of heat is inversely proportional to the relative
heat transfer surface. This means that the changes in surface area
described in this invention can significantly influence the efficiency of heat
transfer.
