EP1153254B1

EP1153254B1 - Pyrometallurgical reactor cooling element and its manufacture

Info

Publication number: EP1153254B1
Application number: EP99961080A
Authority: EP
Inventors: Eero Hugg; Ilkka Kojo; Pertti MÄKINEN; Raimo Koota
Original assignee: Outokumpu Oyj
Current assignee: Outokumpu Oyj
Priority date: 1998-12-22
Filing date: 1999-12-14
Publication date: 2004-08-25
Anticipated expiration: 2019-12-14
Also published as: RS49726B; FI982771A; BR9916469A; CN1331791A; DE69919745T2; PL193460B1; WO2000037870A1; AU1781800A; CN100449242C; ATE274683T1; AU768282B2; CA2356138C; YU44801A; EA200100690A1; FI982771A0; FI108751B; KR100640706B1; AR021961A1; ZA200104860B; KR20010099863A

Description

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 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 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 = α x A x ΔT, where
Q = amount of heat being transferred [W]
α = heat transfer coefficient between flow channel wall and water [W/Km²]
A = heat transfer surface area [m²]
ΔT = difference in temperature between flow channel wall and water [K]
Heat transfer coefficient a can be determined theoretically from the formula Nu = αD/λ λ = thermal conductivity of water [W/mK]
D = hydraulic diameter [m]
Or Nu = 0.023 x Re^0.8Pr^0.4, where
Re = wDρ/η
w = speed [m/s]
D = hydraulic diameter of channel [m]
ρ = density of water [kg/m³]
η = 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 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 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.

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 channel are plugged.
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 1cm 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. The temperature of the molten lead and the temperatures inside the cooling element itself were also carefully measured at seven different measuring points.

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.

	Diameter Mm	Flow cross-sectional area mm²	Heat transfer surface / 1m m²/1m	Relative heat transfer surface area
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 × λ × (T1 - T2), where
Q = amount of heat transferred between the points [W]
S = shape factor (dependent on the geometry) [m]
λ = thermal conductivity of the medium [W/mK]
T₁ = temperature of point 1 [K]
T₂ = 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
λ= thermal conductivity of copper [W/mK]
T₁= temperature at base of element as calculated from tests [K]
T₂ = 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 = 2πL/In(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 α [W/m2K] = c + a x v [m/s] + b x Q [kW].

C A b r²

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 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 ΔT = Q/(α x A)

	Diameter mm	Flow cross-sectional area mm²	Heat transfer surface / 1m m²/1 m	Relative heat transfer surface area
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:

ΔT [K] Relative ΔT [%]

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.

Claims

A method to manufacture a pyrometallurgical reactor cooling plate, said plate being slip-cast manufactured of highly thermal conductive coper and having at least one cooling water flow channel, substantially round or oval in cross-section, which cooling water flow channel is formed in the cooling plate during casting, characterized in that
in order to enhance the heat transfer capability of the cooling plate the wall surface area of the at least one flow channel inside the cooling plate is increased by forming one or several grooves inside the surface of the flow channel by means of a grooved mandrel during casting or by machining threads or rifle-like grooves after casting.
A method according to claim 1, characterized in that
the rifle-like grooves are made by means of an expanding mandrel.
A pyrometallurgical reactor cooling plate, slip-cast manufactured of highly thermal conductive coper and having at least one cooling water flow channel, substantially round or oval in cross-section, characterized in that it is manufactured according to claim 1 and
in order to enhance the heat transfer capability of the cooling element, the wall surface area of the at least one flow channel inside the cooling element is increased by means of grooves, threads or rifle-like grooves or the like.
A cooling plate according to claim 3, characterized in that
the rifle-like grooves are made by means of an expanding mandrel.