GB2105018A - Vertical shaft furnaces - Google Patents

Abstract

A vertical shaft furnace installation includes a cupola (C) and gas- cleaning apparatus at the outlet of the cupola (C). In order to avoid heat damage to the gas cleaning equipment, diluent air may be added to the hot gases emerging from the cupola (C) in order to reduce their temperature, in particular at the end of the blow-down period. Should the temperature of the gases entering the gas cleaning equipment rise inordinately, a reduction in the blast of the furnace is arranged to occur; the reduction may be a progressive one with temperature increase or may be achieved in a single cut in the air- weight of the blast. Control of the blast reduction may be achieved by mechanical means or by a microprocessor or other electronically-controlled system. <IMAGE>

Description

SPECIFICATiON Vertical shaft furnaces When metal is melted in a vertical shaft furnace fuelled with coke and through which air is blown, a great deal of hot gas emerges from the top of the furnace. This gas contains grit and dust which is removed by passing the gas through suitable gas-cleaning equipment such as a filter plant or cycione. Before this can be done, however, the gas must be cooled so as not to damage the gas-cleaning equipment. Cooling may, for example, be achieved by diluting the gas with cold air, which may be drawn in either entirely through the charge hole or through specially provided air intakes as well as the charge hole.

Metal is melted by charging the necessary metallic raw materials into the top of the furnace to form layers interspersed with layers of coke. The furnace holds a number of such layers within the shaft while it is operating.

When it is desired to stop melting, further charging of these layers ceases and the successive charges of metal and coke already in the furnace descend as the coke is burned away and the metal melts. Descending charges absorb heat from the gas produced by burning the coke at the bottom of the furnace where air is blown in. As the shaft empties, less heat can be absorbed and the temperature of the gas begins to rise. In order to avoid the risk of damage to the gascleaning equipment, cooling of the gas has to be sufficient. In the case of air-dilution cooling this is normaily achieved by designing the plant dilution airflow to give sufficient cooling of the gases even under the conditions prevailing as the shaft empties. The result is that a larger flow of gas/air mixture has to be accommodated by the gas-cleaning equipment than is required during normal running when the shaft is filled.This equipment must therefore be large enough to cope with the increased flow. Gas-cleaning equipment is very expensive and considerable capital and running cost savings could be made by making it as small as possible. Where other means of cooling the effluent gas are employed, for example, by heat exchangers or water evaporation, the capacity of the cooling system must be large enough to cope with the increased cooling demand.

According to the invention there is provided a vertical shaft furnace installation in which hot gases emerging from a furnace are fed to gas cleaning equipment and in which the combustion air blast to the furnace is automatically reduced in response to an increase in the heat content of the emerging gases. It has been found that with such an arrangement, in furnaces employing air-dilution cooling the temperature of the gases entering the gas-cleaning equipment may be maintained at a substantially constant temperature as the shaft empties. Where other means of cooling are employed the maximum design capacity of the cooling system can be reduced.

The temperature of the emerging gases is preferably sensed at the input to the gascleaning equipment.

Whilst the blast could be reduced progressively with increase in the sensed temperature so as to maintain the temperature of the gas entering the gas-cleaning equipment at a constant value, it is alternatively possible to make a single reduction as soon as the sensed temperature exceeds a predetermined level. A reduction of about 30% in the combustion air weight at this stage will ensure that gascleaning equipment designed for normal operating conditions will not be overloaded during the blow-down period (the period after charging ceases until the end of a melt). The only penalty compared with progressive reduction is a slightly lower melting rate and longer melting time during the last stages of the melt, as compared with a conventional system.

A progressive reduction in the blast with increase in the sensed temperature could be achieved by employment of a microprocessor or similar electronic equipment or modified standard flow control device deriving its input from temperature and flow sensors and programmed or pre-set to activate blast air flow control valves of the installation according to the required system response.

The invention will now be described, by way of example only using a modified standard air/mechanical air control device, with reference to the accompanying drawings, of which: Figure 1 is a schematic layout of a vertical furnace installation according to the invention; Figure 2 is a schematic layout of an airflow control system for the vertical shaft furnace installation of Fig. 1; Figure 3 is a view of the drive mechanism and friction clutch assembly of the system of Fig. 1; Figure 4 is an end view of the friction clutch assembiy, showing the mechanism for setting the blow-down level; and Figure 5 is a diagram of the blow-down control circuit.

Fig. 1 shows a vertical shaft furnace installation comprising a cupola (vertical shaft furnace) C into the lower end of which air is blown, and from the top of which hot gases emerge which are cleaned prior to their leaving the installation. The gases would typically emerge from the furnace at temperatures of around 400"C although this could rise to 1 100"C at the end of blow down. The emerging gases must be cooled to avoid damage to the gas cleaning equipment in this case by diluting the gas with cold air as shown typically in the proportion of seven parts by volume of air to one part by volume of gas.

Referring now to Fig. 2, the combustion air supply duct 1 to the cupola contains an orifice plate 2. A sensor 3 senses the difference in air pressure created across the plate 2 and produces a pneumatic signal proportional to that pressure difference. (The pressure difference is related to the air flow through the duct.) The sensor 3 may also incorporate a temperature sensor which produces another pneumatic signal proportional to the temperature of the air in the duct. The pressure and temperature signals pass along tubes 4 to an air-flow control unit 5. The control unit produces control signals which are in turn fed along further tubes 6 to the actuator 7 of a damper 8 which regulates the air flow through the duct.The control unit operates the damper s > . as to produce a pressure drop across the orifice plate that corresponds to a desired air flow as set by a manually operated control 9 on the control unit. The control may be varied continuously between a maximum figure for the particular cupola and zero.

Under normal running conditions the air flow to the cupola will remain substantially constant at the preset figure regardless of changes in temperature and pressure of the air supply, but in the event that a filter plant or other gas-cleaning equipment receiving the effluent gas from the cupola begins to overheat, the air flow to the cupola is automatically reduced. The means by which this is achieved will be described in more detail below.

A temperature sensor 1 0 in the form of a thermo-couple is mounted in the inlet of the filter plant or other suitable position to monitor its operating temperature. The electrical output of the sensor 10 is fed via a lead 11 to a blow-down control circuit 1 2. When the temperature exceeds a predetermined danger level, the circuit 1 2 sends a signal along a cabie 1 3 to the air-flow control unit 5 causing the pre-set air flow to be reduced to a predetermined safe level, indicated at 14. The mechanism for effecting this reduction in flow is illustrated in Figs. 2 to 4.

Turning to Fig. 3, the shaft 1 5 is the control shaft of a conventional air-flow control unit. The right hand end of this shaft (when viewed as shown) is connected to a knob or similar control (not shown) by which the shaft can be turned by hand to set the air flow through the duct 1 to the desired figure. A pinion 1 6 is mounted cn the shaft to drive the shaft through a conventional friction clutch assembly. The pinion 1 6 is driven by a similar pinion 1 7 mounted on the output shaft 1 8 of a suitably geared electric motor (not shown).

The use of a friction clutch allows the shaft to be manually rotated without hindrance by the electric motor and its gear mechanism.

The left hand end of the shaft i 5 (viewed as in Fig. 2) is coupled to one end of a lever 30 (Fig. 4) of which the other end is pivotally connected to a clevis 31 on one end of a linkage rod 32. The linkage 32 is bent such that the axes of its end portions are parallel but spaced apart from one another. The end remote from the clevis 31 has a head 33 which is pivotally connected to a point at th6 periphery of a first of two cam discs 34, 35.

The cam discs are in face contact with each other and are mounted for rotation about a common axis A passing through the centre ol both discs. The discs both have a part-circumferential portion of reduced radius, subtending an angle of about 105 , 34a and 35a. The first disc 34, to which the linkage 32 is attached, has an arcuate slot and the second disc has a tapped hole (not shown) which is positioned such that a screw 37 can be inserted through the slot in the first disc and screwed into the tapped hole to limit the angle through which the second disc can be rotated relative to the first.When the bolt 37 is at the clockwise end of the slot (when viewed as shown) the portions of reduced radius coincide, but as the second disc is rotated anti-clockwise from that position (relative to the first disc) the degree of coincidence is reduced until, at the fully anti-clockwise position, there is no overlap between the portions. The discs are located together at a desired angular position by tightening the bolt.

A microswitch 38 is mounted with its operating arm 39 touching the edges of the discs in the portions of reduced diameter.

Before we describe how the above mechanism operates it will be convenient to consider the blow-down control circuit 12, shown in Fig. 5. The output of the temperature sensor 10 is connected to terminals 40 and when the temperature exceeds the danger level, a relay coil RLA, connected across the terminals, is energised and a warning lamp L1 lights. The contacts RLA1 of the relay close permitting current to flow through a further relay RLB, via the closed contacts 38' of microswitch 38, from an AC supply L-N. Relay RLB is thus energised causing its contacts RLB1 and RLB2 to close. Closure of contacts RLB2 energises the electric motor M of the air flow reduction mechanism. The motor drives the shaft 1 5 through the slipping clutch, turning down the air-flow setting. Rotation of the shaft also causes the discs 34, 35, to rotate anti-clockwise under the action of the ever 30 and linkage 32.

On the initial reduction in air flow the temperature of the filter plant may well fall below the danger level, de-energising relay RLA, but since the contacts RLB1 are connected across contacts RLA1 the motor will continue to run, causing a further reduction in air flow.

Eventually the arm 39 of the microswitch reaches the end of the portion of reduced diameter on the second disc 35 and the arm is then moved radially outwards, breaking switch contacts 38' and making contacts 38". Thus, relay RLB is de-energised and the motor is switched off. Lamp L2 lights, indicating that the system is operating under blowdown conditions (reduced air flow).

The blow-down setting can be adjusted by altering the angular relationship between the discs 34 and 35.

As long as the temperature at the point of measurement is above the danger level any attempt to turn up the air flow manually will operate switch 38 to re-start the motor, which returns the setting to the blow-down level.

The arrangement described above is intended particularly for cupolas in which dilution air is used for cooling the effluent gas from the furnace. In such cases it is possible to: a. Reduce the capacity of the gas-cleaning plant by about 30%.

b. Reduce the power requirement of the gas-cleaning plant by over 30%.

c. Reduce the cost of the gas-cleaning plant by over 10%.

Addtional advantages may be obtained in furnaces using heat exchangers or water evaporation to cool the effluent gas prior to cleaning, because costs and power consumption of the cooling system can be reduced.

Further advantages may be obtained in furnace installations utilising a microprocessor or other electronic control system rather than a mechanical arrangement such as that described above. Any desired response to an increase in the temperature of the emerging gases could be programmed or pre-set in such a system; in particular it is simpler to achieve a progressive reduction in the blast in response to an increase in the sensed temperature in this manner than it is in using a mechanical arrangement. In such an arrangement electronic sensors would replace sensor 3 and provide the input to, for instance, a microprocessor programmed to operate the damper 8 as required and which could, if required, also perform the function of the blow-down control circuit 1 2 in response to an input from temperature sensor 10, the blow-down settings being stored in the program and thereby eliminating the disc 34, 35 and their associated apparatus.

Claims (8)

1. A vertical shaft furnace installation in which hot gases emerging from a furnace are fed to gas cleaning equipment and in which the combustion air blast to the furnace is automatically reduced in response to an increase in the heat content of the emerging gases.

2. A furnace installation according to Claim 1 in which the temperature of the emerging gases is sensed at the input to the gas cleaning equipment or other suitable position.

3. A furnace installation according to Claim 1 or Claim 2 in which the temperature of the emerging gases is secured by a thermocouple.

4. A furnace installation according to any preceding claim in which the hot gases emerging from the furnace are cooled by dilution of the gas with cold air prior to entering the gas cleaning equipment.

5. A furnace installation according to any preceding claim in which hot gases emerging from the furnace are cooled by heat exchanger or water evaporation prior to entering the gas cleaning equipment.

6. A furnace installation according to any of Claims 2 to 4 in which the blast is reduced progressively with increase in the sensed temperature so as to maintain the temperature of the gas entering the gas cleaning equipment at a substantially constant value.

7. A furnace installation according to any of Claims 2 to 4 in which a single reduction in the blast is made as soon as the sensed temperature exceeds a pre-determined level.

8. A vertical shaft furnace installation substantially as described with reference to and as illustrated in the accompanying drawings.