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February, 1932

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It has been our privilege to follow the development of Lord Londonderry's Vane-Tempest Colliery from cutting the first sod. In June, 1925, we referred to the difficulty of sinking through the water-bearing Permian rocks and described the method of drilling the boreholes in readiness for freezing. By February, 1926, the boreholes were completed, and in April, 1926, we described the refrigerating plant and explained the method of freezing. In 1930 we paid a third visit to the colliery. By this time the shafts were completed, the surface equipment erected, and coal was being won, although only in small quantity. Our account was published in April, 1930.

Through the courtesy of Mr. Malcolm Dillon, the managing director of Londonderry Collieries Limited, we have visited the colliery once again, this time to examine the developing workings underground and to inspect additional boiler plant necessitated thereby. Three seams are now being developed, viz., the Five Quarter (239 fths.), the Low Main (287 fths.), and the Hutton (307 fths.). Coal is won on the bord and pillar system, the pillars being at 44-yard centres, and in the Hutton seam the winning has advanced to 1,000 yards north of the pit bottom.

Water from the three seams is collected in a standage in the Hutton, and is delivered to the surface by a three-throw Mackley pump rated at 300 g.p.m. against 2,100 ft. head and driven by a 220-H.P. 2,750-volt 40-cycle Metrovick slipring motor. Two small air-driven Evans pumps in the Hutton seam deliver water to the standage from previous workings, and a 4-in, air-driven Evans pump performs a similar duty in the Five Quarter seam.

The main haulage duty in the Hutton seam is undertaken by an M. B. Wild 200-H.P. endless haulage driven by a British Electric Plant motor geared from 580 to 8 r.p.m. Rope tension is maintained by a tension wheel on a trolley behind the haulage and weighted by 2 tons of cheese-weights. At the time of our visit a similar 200-H.P. M. B. Wild haulage was being erected in the Five Quarter seam, the motor in this case being by J. H. Holmes & Co. Ltd. Subsidiary to these haulages over 50 Markham vertical compressed-air haulages are in use in the three seams.

Mavor & Coulson and British Jeffrey-Diamond chain arcwallers are at work in the Five Quarter seam and an Anderson-Boyes machine in the Low Main seam. In addition about two dozen Hardy punching machines are in use for heading out.

Steel girders mounted on brickwork are used to support most of the main roads, but arch girders are in use for a length of 500 yards of roadway.

Boiler Plant.

As the underground workings have developed, the load on the power plant has naturally increased, and it has become necessary to supplement the existing 20,000-lb. Clarke-Chapman Woodeson boilers. Babcock & Wilcox Limited were accordingly commissioned to enlarge the boiler house and to install two 30,000/36,000-lb. boilers complete with pulverised-fuel apparatus and coal and ash-handling plant. Drawings of the new plant are reproduced in Figs. 1 and 2, and Fig. 3 gives a view of the boiler fronts. The boilers are unusually tall, being 37 ft. high to the centre-line of the drum, a fact that is accounted for by the large combustion chambers, which are some 26 ft. high. As will be seen from Fig. 2, the combustion chambers are plain refractory lined. These boilers are somewhat too small to justify financially a water-cooled furnace, having regard to the particular fuel used, and it has accordingly been necessary to provide a large furnace volume to keep down the rate of heat liberation per cubic foot to a figure suitable for the refractory material. Each boiler has a heating surface of 5,142 sq. ft., and supplies steam at 200 lb. per sq. in. and 488° F. Behind each boiler is an economiser, of 1,750 sq. ft. heating surface, of Babcock & Wilcox forged-steel return-bend type. This design replaces expanded joints by oval-flanged joints, allows all joints to be inspected while in service, and enables individual tubes to be cleaned internally after removing the appropriate return bends.

Pulverised-fuel firing is on the unit system. Each boiler is served by a Fuller-Bonnot mill capable of handling 4,500 lb. of coal per hour and driven by a 50-H.P. motor. A photograph of one of the mills with its attendant fan and burner is reproduced in Fig. 5, and a section through the mill is given in Fig. 8. It is a horizontal ball mill, comprising a cylinder lined with chilled-iron liners and containing a charge of mixed-size balls, the raw coal being fed in at one end of the cylinder. It is, however, of special design as regards the method of withdrawing and classifying the pulverised coal. The usual ball mill in which the pulverised coal is withdrawn to the burner through an orifice at the discharge end of the mill suffers from two disadvantages, viz., the power required may be excessive and the product may not be uniform. Regarding the first point, pulverised coal commences to be produced at the entry end of the mill; the bulk of this product, however, is not withdrawn immediately, but has to traverse the length of the ball charge, because the current of air bypasses most of the charge and exerts most of its effect in the vicinity of the orifice at the discharge end of the mill. This pulverised coal clogs the balls, retards the grinding process and necessitates extra power. To reduce this trouble there is a temptation to make the mill unduly short; but this gives rise to the second trouble, that the coal is not completely pulverised by the time it reaches the discharge end, so that coarse particles find their way to the burner.

In the Fuller-Bonnot mill the withdrawal of the pulverised coal from the mill and its final grading are separated into two processes, so that each may be performed efficiently. Within the mill is an internal classifier through which the pulverised coal is exhausted. It extends about three-quarters of the length of the mill and consists of a perforated sheet-steel cylinder with a blank end. These perforations distribute the suction over the whole length of the bed and direct the draught at right-angles to the direction of flow of the coal. The bed is thus kept clear of pulverised coal as soon as it is formed, and a preliminary grading is effected. Final grading is performed by an external classifier, which allows the fully pulverised particles to be withdrawn to the burner, but returns the coarser particles to the mill. A reciprocating feeder controls the coal supply to the mill, and the throughput is controlled by varying the stroke of this feeder. Preheating of the primary air (i.e., that passing through the mill) is achieved in the present instance by withdrawing hot gas from the furnace. The arrangement Is best seen from the plan and the longitudinal elevation given in Figs. 1 and 2. Furnace gas is withdrawn from near the base of each furnace and led through ducting beneath the firing floor to the two pulverisers. To the right of Fig. 7 is seen the regulator for one of the boilers. The preheat temperature is held at about 400° F. by adjusting a damper in this regulator, which allows a greater or less proportion of cold air to be drawn into the ducting, together with the furnace gas. By means of this preheat coal can be handled with a moisture content up to 15 per cent. or over. When we inspected the plant the mills were running very quietly and smoothly — a characteristic we have noticed previously with Babcock mills.

In the foreground in Fig. 5 is shown the exhauster fan which withdraws the pulverised coal from the mill and delivers it to the burner. It is rated at 5,000 cu. ft. per min. and is driven by an 18-H.P. variable-speed slipring motor controllable from 600 to 1,200 r.p.m. The burner is also seen in this illustration, but is better shown in Fig. 7. It is a 14-in. Lodi turbulent burner, and, as will be seen from Fig. 9, is of simple design. Coal and primary air are fed in through the central pipe and split up by the vanes in front of the pipe mouth. Secondary air is introduced into the outer chamber, and enters the furnace through a sharp-edged orifice which gives rise to a vena contracta effect with a high degree of turbulence in the zone of restoration, at which point approximately the primary air and coal streams mix with the secondary air. The secondary-air fan and ducting are shown in Fig. 7; the fan is rated at 11,500 cu. ft. per min. and is driven by a 22-H.P., 760-r.p.m. motor. Figs. 5 and 7, of course, show the pulverised-fuel plant for only one boiler; the layout for the second boiler is shown on the plan, Fig. 1. An induced-draught fan is located behind each boiler, and is driven by a 22-H.P., 575-r.p.m. motor. All the fans are by Davidson & Co. Ltd., and the motors by the English Electric Co. Ltd. Visco air-filters are fitted to all the motors.

Instruments include one Electroflo CO2 recorder available for either boiler, Hopkinson pressure gauges, and a Cambridge multi-point pyrometer.

Coal and Ash-Handling Plant.

Coal-handling plant, also by Babcock & Wilcox Limited, has been supplied to handle alternatively raw coal or dust from the dry-cleaning plant. Raw coal is received in a concrete hopper placed under the railway siding and fitted with a 3-in, steel grid. A rotary feed at the outlet of this hopper supplies coal to the boot of a vertical chain-and-bucket elevator. This has 78-ft. centres and delivers 10 tons of coal per hour. It is fitted with 10-in, mild-steel buckets at 18-in, pitch on a single-strand bushed chain running at 100 ft. per min. This elevator discharges into a small hopper feeding a 15-in, diameter screw conveyor 42 ft. long running at 30 r.p.m. and supplying two bunkers through four outlets. As will be seen from the longitudinal elevation, Fig. 2, the bunkers are arranged against the wall inside the boiler house. They are of 40 tons capacity each, and have coal-level indicators operating pointers which read on scales fixed to two of the boiler-house stanchions.

The dust from a Birtley dry-cleaning plant is carried to the boiler house by a 334-ft. enclosed belt conveyor, having an 18-in, belt running at 100 ft. per mm. and delivering 10 tons of dust per hour. Fig. 6 shows the commencement of this conveyor leading from the dust cyclone of the dry-cleaning plant, and its termination at the boiler house is shown to the right of Fig. 4. This conveyor discharges into the same hopper as the elevator, so that the screw conveyor distributes the dust as well as the coal. A common 5-H.P. motor drives either the raw-coal elevator or the dust conveyor through clutches.

Ash is discharged into an Usco water immersed scraper conveyor, which has a capacity of 4 tons per hour. It is arranged at present to handle the ashes from the two Babcock & Wilcox boilers, but is arranged for future extension to serve the four Woodeson boilers. A 3-H.P. motor is fitted, and a variable-speed gear provides for the conveyor to be driven at 4, 7 or 10 ft. per min. An inclined skip hoist receives the ash from the scraper conveyor and elevates it to a 30-ton overhead bunker. The skip is hoisted by a 2-ton winch which is completely automatic, performing the actions of filling, hoisting, emptying, and lowering at predetermined intervals. The skip hoist, winch, and ash bunker are shown in Fig. 4, and in the drawings, Fig. 2.

A 6,000-g.p.h. water softener has been supplied by United Water Softeners Limited. It employs the hot process, and is supplemented by a continuous blow-down heat interchanger. A 10,000-g.p.h. Harland feed pump supplies both boilers, delivery being controlled by Copes regulators. Fig. 10 shows the new water softener on the right; that for the existing boilers being on the left. Hopkinson valves are fitted throughout the new boiler plant.

Drawings and Photographs accompanying the article