METALLURGICAL FURNACE WITH FLUID-COOLING SYSTEM FIELD OF THE INVENTION The invention relates to a metallurgical furnace. More particularly, the invention relates to a fluid-cooling system for a metallurgical furnace. BACKGROUND TO THE INVENTION It is known that a metallurgical furnace typically comprises a shell and a roof. The shell defines a taphole below a taphole lintel and the roof defines at least one opening for an electrode and at least one further opening for at least one chute for feeding raw material into the furnace. Electrode clamps are used to hold the electrode and feed the electrode through the at least one opening. The at least one chute may extend into a chamber defined by the shell and may terminate at a distal end in a chute tip. In order to extend the working life of modern metallurgical furnaces, including submerged-arc, brush-arc and DC-arc furnaces (i.e., non-tilting furnaces having a fixed taphole), high intensity cooling of the furnace and more particularly the shell of the furnace is required. In the art, effective cooling of a furnace shell is achieved by circulating cooling water through a conduit which is welded onto a metal cladding layer or jacket of the furnace shell. The cooling water is typically circulated through the conduit at a pressure of about 500kPa (above the prevailing atmospheric pressure). In this manner, heat is transferred by means of conduction, thermal radiation and convection from the hot furnace shell to the conduit, and to the cooling water. The heated cooling water is allowed to exit the conduit and to flow through a heat exchanger into a cooling water reservoir, before it is re-circulated through the conduit. It is known in the art that the portion of a furnace shell that locates below the taphole of the furnace is a high-risk area. Cracks may form in such a shell portion and metal, matte, or slag may burn through said shell portion. In this high-risk area of the furnace shell, a conduit which conveys pressurised cooling water cannot be used. Should a crack be formed or burn-through occur in this portion of the shell, there is a high likelihood that pressurised cooling water could be forced into a chamber of the furnace. Cooling water that is forced into the chamber of a furnace may come into contact with hot liquid metal, matte or slag and this can give rise to hazardous explosions. These explosions can result in damage to equipment, furnace down-time and also present a major hazard to the safety of furnace operators. In view of the above hazards, alternative shell cooling systems are typically used in the portion of a furnace shell that locates below the taphole of the furnace. Alternative shell cooling systems include air-cooling and water- spray cooling systems. An air-cooling system for the portion of the furnace shell that locates below the taphole of the furnace has a very low efficiency when compared to water-cooling. This is due to water having a thermal conductivity which is at least twenty times higher than that of air. Furthermore, air-cooling cowlings and ducts often get blocked by splashing metal, matte or slag during tapping operations. Spray-water cooling systems are prone to scale formation on the furnace shell, which in turn causes poor heat transfer and lowers the cooling efficiency significantly. During tapping, the splashing of metal, matte, or slag can also cause blockages of spray nozzles and water catchment throughs. From the above it is apparent that there remains a need in the art for a fluid- cooling system and method of cooling that can more effectively and safely cool certain critical components or parts of a furnace. Such critical components or parts include, but is not limited to: the portion of the shell which locates below the taphole of the furnace; the taphole lintel; a portion of the roof in a region of the at least one opening for an electrode; a seal for the electrode; an electrode clamp and a region towards a distal end of the at least one chute. OBJECT OF THE INVENTION It is an object of the present invention to provide a metallurgical furnace and a method of cooling a part of a metallurgical furnace with which the applicant believes the above disadvantages may at least partially be overcome, or which would provide a useful alternative to known metallurgical furnaces and methods of cooling. SUMMARY OF THE INVENTION According to the invention there is provided a metallurgical furnace comprising a shell and a roof, the shell defining a taphole and the roof defining at least one opening for an electrode and at least one further opening for at least one chute for feeding raw material into the furnace and - at least a first fluid-cooling system for a first part of the furnace; the at least first fluid-cooling system comprising: o a conduit that is in heat exchanging relationship with the first part of the vessel, the conduit having an inlet for receiving a cooling fluid and an outlet for discharging the cooling fluid from the conduit; o a pump for creating a negative pressure within the conduit, the pump having an inlet that is in fluid flow communication with the outlet of the conduit and an outlet for discharging pressurised cooling fluid from the pump; and o a cooling fluid reservoir having an inlet that is in fluid flow communication with the outlet of the pump and an outlet that is in fluid flow communication with the inlet of the conduit, wherein, the pump draws the cooling fluid from the cooling fluid reservoir through the inlet of the conduit and out the outlet of the conduit in order to transfer heat from the first part of the furnace to the cooling fluid that flows through the conduit. The first part of the furnace may comprise at least one of: a portion of the shell which locates below the taphole of the furnace; a taphole lintel; a portion of the roof in a region of the at least one opening; a seal for the electrode; an electrode clamp and a region towards a distal end of the at least one chute. The cooling fluid may be a waterless coolant fluid with an elevated boiling temperature of about 190°C. Alternatively, the cooling fluid may comprise one of water and a water-based coolant, with elevated boiling temperature at about 130°C to 140°C. The conduit may be arranged in one of a coiled and a serpentine configuration. The conduit may be manufactured from a heat conductive material. The heat conductive material may be a metal such as copper, stainless steel or any other metal with a suitable heat transfer coefficient. The pump may be a suction pump. One example of a suitable suction pump is a positive displacement pump. The metallurgical furnace may comprise a heat exchanger that is in fluid flow communication with the outlet of the pump and the inlet of the cooling fluid reservoir. The heat exchanger serves to lower the temperature of the cooling fluid that has passed through the conduit and the pump, prior to it being fed to the cooling fluid reservoir. The heat exchanger may comprise a plate heat exchanger that is cooled by means of a closed-circuit water-cooling system. The cooling fluid reservoir may be positioned relative to the conduit inlet such that a top surface of cooling fluid that locates in the cooling fluid reservoir is at the same elevation as the inlet of the conduit or at an elevation lower than the inlet of the conduit. In some preferred embodiments, the first part of the furnace comprises the portion of the shell which locates below the taphole of the furnace. The shell may comprise a metal cladding layer and the conduit may be attached to the metal cladding layer by means of a welded joint. In other embodiments, the conduit may be one of: embedded in the shell; and integrally formed within the shell of the furnace. Further according to the invention, the metallurgical furnace may also comprise at least a second fluid-cooling system for a second part of the furnace, the second part of the furnace comprising another of: the portion of the shell which locates below the taphole of the furnace; the taphole lintel; the portion of the roof in a region of the at least one opening; the seal for the electrode; the electrode clamp and the region towards a distal end of the at least one chute, the at least second fluid-cooling system comprising: - a conduit that is in heat exchanging relationship with the second part, the conduit having an inlet for receiving a cooling fluid and an outlet for discharging the cooling fluid from the conduit; - a pump for creating a negative pressure within the conduit, the pump having an inlet that is in fluid flow communication with the outlet of the conduit and an outlet for discharging pressurised cooling fluid from the pump; and - a cooling fluid reservoir having an inlet that is in fluid flow communication with the outlet of the pump and an outlet that is in fluid flow communication with the inlet of the conduit, wherein, the pump draws the cooling fluid from the cooling fluid reservoir through the inlet of the conduit and out the outlet of the conduit in order to transfer heat from the second part of the vessel to the cooling fluid that flows through the conduit. Hence, it is envisaged that a plurality of fluid-cooling systems may be used to cool respective parts of the furnace. Each of these fluid-cooling systems may operate independently from one another. Hence, in such an embodiment, each fluid-cooling system may have its own pump, heat exchanger and cooling fluid reservoir. A plurality of such cooling systems may increase the safety of operating the furnace. Another part or balance of the shell may be cooled by a conventional cooling system utilizing a cooling fluid under positive pressure. Also included within the scope of the invention is a method of cooling a first part of a metallurgical furnace, the first part having a conduit in heat exchanging relationship with the first part, the conduit having an inlet for receiving a cooling fluid from a cooling fluid reservoir and an outlet for discharging the cooling fluid, the method comprising: − creating a negative pressure within the conduit such that the cooling fluid is drawn from the cooling fluid reservoir through the inlet of the conduit and discharged at the outlet of the conduit; − allowing a temperature of the discharged cooling fluid to decrease; and − supplying the discharged cooling fluid to the cooling fluid reservoir. BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAM figure 1 is a schematic diagram of a metallurgical furnace that includes a fluid-cooling system; and figure 2 is a relevant Moody diagram. DETAILED DESCRIPTION OF THE INVENTION An example embodiment of a metallurgical furnace is generally designated by the reference numeral 10 in figure 1. The furnace 10 comprises a shell 12 and a roof 14. The shell defines a taphole 16 below a taphole lintel 18 and the roof 14 defines at least one opening 20 for an electrode 22 and at least one further opening 24 for at least one chute 26 for feeding raw material into the furnace. Electrode clamps 28 are used, in known manner, to hold the electrode 22 and feed the electrode through the at least one opening 20. The at least one chute 26 extends into a chamber 30 defined by the shell and terminates at a distal end thereof in a chute tip 32. The shell may comprise a metal cladding layer 33. The furnace 10 further comprises at least a first fluid-cooling system 40 for a first part 42 of the furnace. The at least first fluid-cooling system comprises a conduit 44 that is in heat exchanging relationship with the first part 42 of the vessel. The conduit 44 has an inlet 46 for receiving a cooling fluid 47 and an outlet 48 for discharging the cooling fluid from the conduit. A pump, typically a suction pump 50, creates a negative pressure within the conduit 44. The pump has an inlet 52 that is in fluid flow communication with the outlet 48 of the conduit and an outlet 54 for discharging pressurised cooling fluid from the pump. A cooling fluid reservoir 56 has an inlet 58 that is in fluid flow communication with the outlet 54 of the pump and an outlet 60 that is in fluid flow communication with the inlet 46 of the conduit 44. The pump 50 draws the cooling fluid 47 from the cooling fluid reservoir 56 through the inlet 46 of the conduit 44 and out the outlet 48 of the conduit in order to transfer heat from the first part 42 of the furnace to the cooling fluid that flows through the conduit. The first part 42 of the furnace 10 may comprise any suitable part of the furnace, but typically a critical part as referred to in the introduction of this specification, namely, but not limited to, at least one of: the portion 42 of the shell 12 which locates below the taphole 16 of the furnace; the taphole lintel 18; a portion 62 of the roof 14 in a region of the at least one opening 20; a seal 64 for the electrode 22; the electrode clamp 28 and the region 32 towards the distal end of the at least one chute 26. Surprisingly, it has been found that the first fluid cooling system 40 utilizing negative pressure as referred to above, is effective and efficient to cool critical parts of the furnace which ordinarily have relatively smaller surface areas, for example the part 42 below the taphole 16 having a surface area of in the order of 5m ² . The first fluid cooling system 40 may further comprise a heat exchanger 70. The heat exchanger 70 is typically a plate heat exchanger. The heat exchanger has an inlet 72 which is connected to the outlet 54 of pump 50 by a conduit 73. An outlet 74 of the heat exchanger is connected to the inlet 58 of reservoir 56 by a conduit 75. Heated cooling fluid from outlet 48 of the conduit 44 enters the heat exchanger at inlet 72 and is cooled by means of cooling water (not shown) from a closed-circuit water-cooling system 76. In some embodiments, the cooling fluid 47 is a waterless coolant fluid with elevated boiling point of about 190°C. Hence, such waterless coolant fluids typically boil at higher temperatures than water, provide sufficient heat transfer and are non-corrosive. Alternatively, the cooling fluid may comprise one of water and a water-based coolant with elevated boiling point of about 130°C to 140°C. In a case where the first part of the furnace is the portion 42 below the taphole, the conduit 44 is made of a heat conductive material and is attached to the metal cladding layer 33 of the part 42 such that the conduit 44 is in thermal communication with the part 42, typically by welding the conduit 44 onto the cladding layer 33. The conduit may be arranged in at least one of coiled and serpentine configuration. In this manner, a single conduit 44 could cover the part 42. The conduit 44 is welded to the cladding 33 such that the inlet 46 and the outlet 48 are located spaced from the taphole 16. A supply line conduit 80 is connected between the outlet 60 of the cooling fluid reservoir 56 and the inlet 46 of the conduit 44. A return line conduit 82 is connected between the outlet 48 of the conduit 44 and the inlet 52 of the pump 50. The location of the inlet 46 and outlet 48 allows the supply line conduit 80 and the return line conduit 82, respectively, to be located in spaced relation from the taphole 16. The cooling fluid reservoir 56 is positioned relative to the inlet 46 of the conduit 44 such as to ensure that a top surface or level 84 of the cooling fluid 57 that locates in the cooling fluid reservoir 56 is at the same elevation or level as the inlet 46 of the conduit 44 or lower, as shown in figure 1. Hence, in use, the portion 42 is cooled by circulating the cooling fluid 47 from the cooling fluid reservoir 56, through the conduit 44 and back to the reservoir 56. When the pump 50 is activated, a negative pressure (i.e., relative to the prevailing atmospheric pressure) is created in the conduit 44. The pressure at the inlet 46 of the conduit 44 is typically minus 10kPa. The pressure at the outlet 48 of the conduit 44 is typically minus 40kPa. In this manner, the pump 50 creates suction, which causes the cooling fluid 47 to flow from the cooling fluid reservoir 56 via supply line conduit 80 and inlet 46 through the conduit 44. Whilst the cooling fluid 47 flows through the conduit 44, heat is transferred by means of conduction, thermal radiation and convection from the part 42 via conduit 44 to the cooling fluid 47 in the conduit 44. The heated cooling fluid that exits the outlet 48 of the conduit 44 is sucked by the pump 50 through the return line conduit 82 towards the inlet 52 of the pump 50. Under the action of the pump 50, the heated cooling fluid is pressurised and forced out of outlet 54 of the pump 50, through conduit 73 and towards the inlet 72 of heat exchanger 70. The cooling fluid exits the heat exchanger 70 under the pressure of the pump 50, and flows through the outlet 74, conduit 75 and the inlet 58 of the cooling fluid reservoir 56 into the reservoir 56. Various process monitors and control logic may be employed to control the operating power of the pump 50. By controlling the operating power of the pump 50, a flow rate of the cooling fluid 47 through the conduit 44 can be controlled. In use, that is when the part 42 is heated due to furnace use, the flow rate of the cooling liquid 47 correlates with a rate at which heat is transferred from the part 42 to the cooling fluid 47. Therefore, by controlling the operating power of the pump 50, the rate at which heat is transferred from the hot portion 42 of the furnace shell to the conduit 44 and hence to the cooling fluid 47 can be controlled. Process parameters that are typically used to control the operating power of the pump 50 include: − the temperature of the portion 42 of the furnace shell 12; − the temperature of the cooling fluid 47 in the cooling fluid reservoir 56; − the temperature of the cooling fluid 47 in any one or more of conduits 44, 80 and 82; and − the volumetric flow rate of the cooling fluid 47 through any one or more of conduits 44, 80 and 82. A major advantage of the above-described fluid-cooling system 40 is that, owing to the negative pressure in the conduit 44, cooling fluid 47 that circulates through the conduit 44 will not be forced into the furnace chamber 30 if a crack is formed or burn-through occurs in part 42 of the furnace shell 12. Rather, in such an eventuality, at least some of the cooling fluid within the conduit 44 would still be sucked through the conduit 44 and out of outlet 48. The cooling fluid (which is not sucked through the conduit) would not be forced into a chamber 30 of the furnace 10. Thus, the cooling fluid that circulates through the conduit 44 should not come into contact with hot liquid metal, matte or slag (not shown) that locates in the chamber of the furnace 10. The furnace 10 may also comprise at least a second and similar fluid- cooling system (not shown) comprising a conduit, such as conduit 44, for cooperating with and cooling a second part of the furnace 10, such as, but not limited to, the taphole lintel 18; the portion 62 of the roof 14 in a region of the at least one opening 20; the seal 64 for the electrode 22; the electrode clamp 28 and the region 32 towards the distal end of the at least one chute 26. In the case of some critical parts, such as the taphole lintel 18, the chute 24 and the clamps 28, the conduit may be embedded in or formed integrally with a body or wall of the part. In some embodiments the conduit 44 for cooperating with the first furnace part and the conduit cooperating with the second furnace part may be connected in series, as part of a single fluid-cooling system 40 as described above. As shown in figure 1, another and conventional fluid-cooling system 200 having a conduit 202 may be attached to a balance of the furnace shell 12 in a serpentine fashion. Another cooling fluid (not shown) is forced at positive pressure of about 500kPa through the conduit 202 in order to transfer heat from the balance of the furnace shell 12 to the other cooling fluid that flows through the conduit 202. The other fluid-cooling system 200 would be independent from the first fluid-cooling system 40. The following are design calculations which are applicable to the system as disclosed. To determine a total pressure drop across the system, pressure losses across bends as well as straight sections of the system need to be calculated. For these calculations, relative roughness and Reynold's Number is needed to read a friction coefficient from the Moody Diagram depicted in Figure 2. For water With cooling channels running across a cooling area of 1.2m by 3m, the cooling area is 3.6m ² of shell surface area below the tap hole. Total cooling channel length is 6 x 3m=18 m. Cooling channels dimensions are b≔125 mm and h≔50 mm, which are used for hydraulic diameter calculations. Relative roughness Absolute roughness for carbon steel is ε = 0.025 ∙ 10 ⁻³ m.

Reynold’s Number LLD is a characteristic of linear dimension. μ is the dynamic viscosity of the fluid. Friction coefficient: To determine a friction coefficient, the above calculated Reynolds number and Relative Roughness are used to read the friction coefficient “f” from the Moody Diagram in figure 2. In this case f=0.0225. Pressure drop across bends

Temperature rise calculations: The heat transfer taken into consideration is heat transfer between the furnace shell cold face and the cooling water only. Heat transfer between the fluid-cooling channels -atmosphere will be neglected. This can be exothermic or endothermic depending on the ambient temperature. For these calculations, heat transfer between the furnace shell and the cooling system will be by assumed as convective heat transfer only. Assume heat flux removed by cooling system is Thus, to cool he cooling water temperature will increase with ∆ T _W = 3,824∆℃ between the inlet and outlet of the conduit. It will be appreciated by those skilled in the art that the invention is not limited to the precise details as described herein and that many variations are possible without departing from the scope of the invention.