All About Engines

298 All About Engines twenty-eight fans. Fresh air reaches the stokeholds through the ventilators on the upper deck, and as it gets warmed and vitiated it is forced by the fans through tubes which are heated by waste gases, so that the furnaces are fed with hot air. Some- thing like 700 h.-p. is needed to drive the fans. The steam pipes for each boiler are about 8 inches in diameter, but as they collect the steam from all the boilers they gradually increase in size up to 21 inches and 25 inches in diameter. The main steam pipes in the boiler and turbine rooms vary from 16 inches to 32 inches in diameter. The valves admit- ting steam to the various turbines are operated by seven engines of a special type which is made by Messrs. Brown Bros., of Edinburgh. They are a combination of steam and hydraulic, and the largest, which works the 53-inch change-over valve, has a cylinder 16 inches bore and 15J inches stroke ! The turbines are arranged for triple expansion when going ahead, the outer screws being driven by high and intermediate pressure, and the two inner screws by low-pressure turbines; but valves are provided which enable them to be used in almost any combination. The sizes of the turbines are given in the table on the following page. The only way to realise what these figures mean is to measure up some of the sizes and then attempt to form a mental picture of the thing itself by the aid of what you have read in Chapter VI. The general arrangement will be understood from