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South African Class 25 4-8-4
Owing to the difficulties experienced to obtain adequate supplies of suitable water in arid regions like the Great Karoo between Touws River and Kimberley and from De Aar into South West Africa (SWA), the South African Railways (SAR) began to give serious consideration to the possibility of introducing condensing locomotives as far back as the late 1930s. At one time it was considered to convert Class 12A 4-8-2 locomotives to condensing engines, but the idea was not put into practice.
Condensing locomotives were a rarity, but no novelty to South Africa, since the first condensing steam locomotives had already entered service in the Cape of Good Hope in the late nineteenth century. Between 1886 and 1888, three well-tank condensing locomotives with a 0-4-0 wheel arrangement were placed in service by the Cape Copper Mining Company on its Namaqualand Railway, a 2 ft 6 in (762 mm) gauge line between Port Nolloth and O'okiep
On the SAR, it was only after World War II, that extensive condensing tests were carried out with the modified Class 20 locomotive. The approximately 90% water and 10% coal savings which were achieved during the tests with the Class 20 in the Eastern Transvaal and the Karoo in 1950 and 1951 led to the decision to proceed with the design of a new condensing locomotive.
The result, the Class 25 4-8-4 Northern type condensing locomotive, can be considered as the ultimate in SAR non-articulated steam locomotive design. It was designed under the direction of L.C. Grubb, Chief Mechanical Engineer of the SAR from 1949 to 1954.
The design work on the locomotive's condensing apparatus and the condensing tender was carried out by Henschel & Son, who built one locomotive complete with tender, no. 3451 with works number 28730. After being trialled in Kassel, it was then dispatched to the North British Locomotive Company (NBL) in Glasgow who built the rest of the Class 25 locomotives, numbered in the range from 3452 to 3540. They were delivered between 1953 and 1955.
Apart from the complete engine and tender no. 3451, Henschel built sixty more of the condensing tenders to which they held the patent, with works numbers in the range from 28780 to 28839, as well as four spare boilers for the Classes 25 and 25NC with works numbers in the range from 28770 to 28773. The last 29 condensing tenders were built by NBL.
In 1963, one more condensing tender, no. 3541, was built by the Salt River shops of the SAR on a spare cast frame which had been delivered as part of the original order which called for three spare frames, one for an engine and one each for a condenser and non-condenser tender.
The boiler was similar to the Watson Standard no. 3B boiler of the Class 15F, but with the distance between tubeplates reduced from 22 feet 6 inches (6,858 millimetres) to 19 feet (5,791 millimetres) by the addition of a combustion chamber. While this resulted in a slight decrease in the total evaporative heating surface, there was a 40% increase in the firebox heating surface and a better ratio of firebox to grate area. The boiler was lagged with Cape Asbestos mattresses, manufactured and fitted in South Africa.
On the condensing locomotive, spent steam was recycled and condensed back to water for repeated use. Since the steam wasn't expelled up the chimney, the Class 25's smokebox contained a steam turbine-driven fan beneath the chimney to keep the draught going, with deflector plates that were supposed to prevent char from causing excessive wear on the fan blades.
The smokebox of the Class 25 was similar to that of the Class 25NC, but with the front extended to accommodate the centrifugal blower which was driven by an exhaust steam turbine mounted under the front of the smokebox. Apart from the usual round smokebox door in the centre of the smokebox front plate, the complete front smokebox plate on both classes was hinged to the wrapper plate on the right hand side of the smokebox in addition to being bolted to the angle ring at the smokebox front, to facilitate less cramped access to the blower equipment, superheater header and tubes.
The draught turbine gave the locomotive its characteristic whining sound while running. As delivered, the Class 25 had the usual round smokebox front, but this was later modified by adding a banjo-shaped extension to also cover the exhaust steam turbine. Spent steam was fed through a thick pipe on the engine's left hand side back to the condensing tender.
The banjo-shaped extension modification to the front of the smokebox was found necessary to cope with the copious amounts of char generated by brittle Witbank coal and mechanical stoking, since the original Henschel design proved to have too little capacity to keep accumulated char away from the exhaust turbine. Most of the char was collected in the trough of the banjo at the bottom of the smokebox front from where it was periodically ejected through a vertical steam ejector pipe inside the banjo door which exhausted immediately in front of the chimney.
In service, the turbine-induced draught of the Class 25 actually performed better than the regular draught of the Class 25NC. One advantage of the condenser was that while getting up steam, the blower could be opened at a boiler pressure of about 15 to 20 pounds per square inch (103 to 138 kilopascals) to start the fan spinning, which drew the fire and accelerated getting up steam pressure. On a non-condensing locomotive the blower would be weak while steam pressure was still that low.[
Timken roller bearings were used throughout, including on the three-axle tender bogies, the coupling and connecting rods as well as the crosshead gudgeon pins, while the locomotive's leading bogies and coupled wheels had Cannon-type axle boxes. Compared to earlier SAR practice, a novelty was the adoption of mechanical lubrication. A sixteen-feed lubricator was driven off the reversing link trunnion. Since the Class 25 was entirely mounted on roller bearings, very little effort was required to move these huge locomotives.
The cylinders and frames were cast in one piece while the steel cylinders and steam chests were fitted with cast iron liners. The tender frame was also a one-piece steel casting. The Alligator type crossheads were split on the vertical centre line and clamped on to the end of the piston rods, which had three coned rings engaging in grooves in the crossheads. The original coupling rods differed from the usual in the provision of three independent rods, thereby doing away with four knuckle joints and pins.
The locomotive was able to negotiate curves with a radius of 275 feet (84 metres), with 1⁄4 inch (6 millimetres) gauge widening and 4
1⁄2 inches (114 millimetres) superelevation. The wheels, axles and axleboxes of the leading bogie were designed to be interchangeable with those of the Class 15F, while the wheels and axles of the trailing bogie were interchangeable with those of the Class 24's trailing bogie.
The locomotive's brakes were operated by two 24 inches (610 millimetres) diameter cylinders on the engine and four 21 inches (533 millimetres) diameter cylinders on the tender. The brake riggings on the tender bogies were independent of each other and the front tender bogie was equipped with a hand brake. The engine and tender brake pipelines were fitted with delaying valves which delayed brake application on the locomotive until a predetermined degree of braking had been established throughout the train. A separate driver's brake valve permitted the continued application of locomotive brakes while the train brakes were being released or vacuum was being built up.
Almost one third of the total length of the Type CZ condensing tender was taken up by the coal bunker, which included the oil separator equipment to remove oil from spent steam and the mechanical stoker equipment which had a maximum delivery rate of 12,000 pounds (5,443 kilograms) of coal per hour. The rear two-thirds was taken up by eight large radiators on each side, cooled by five steam-driven roof-mounted fans. The 5,000 imperial gallons (22,700 litres) water capacity consisted of two tanks, a 4,400 imperial gallons (20,000 litres) fresh water tank in the centre of the tender between the radiators and a 600 imperial gallons (2,730 litres) condensate tank under the tender belly between the bogies. Feedwater was taken directly from the condensate tank's hot contents rather than from the main tank's cold contents.
Since the temperature of the condensed feedwater was too high for the use of ordinary injectors, the boiler was fed by two turbo-pumps located under the cab. Each pump had a capacity of approximately 88 imperial gallons (400 litres) per minute.
The system proved to be extremely efficient and reduced water consumption by as much as 90% by using the same water up to eight times over, giving the Class 25 locomotive a range of 800 kilometres (500 miles) between water refills. In addition, the hot condensate feedwater resulted in a 7% reduction in coal consumption.
The condensing tenders were rather appropriately classified as Type CZ, since CZ is also the motor vehicle registration letters of Beaufort West, the capital town of the Karoo where the Class 25 was to serve. Since spent steam was not expelled through the chimney, the condensers sounded unlike any other steam locomotive on South African rails. Their non-condensing and free exhausting Class 25NC sister locomotives had the usual sharp bark of a steam locomotive, especially under load, while the condensing Class 25 had more of a hoarse hollow chuff sound in addition to its turbine whine.
SAR Class 25 Condenser Original Factory Design
- Brand: Mini World Models (South Africa)
- Product Code: SAR Class 25 Condenser Original Factory Design
- Availability: 2-3 Days