How Compound Steam Engine Works?

Compound Steam Engine:

A steam engine, in which the expansion of steam takes place in two or more stages, is known as a “compound steam engine”.

Compound engine is one in which the steam from the boiler expands to a certain extent in one cylinder and then exhausts into a larger cylinder, where the expansion may be completed.A compound steam engine with two cylinders is called duplex steam engine.

And the cylinder, which receives the high pressure steam, is known as “high pressure(H.P.)” cylinder. The steam after expanding in the high pressure cylinder, exhausts into a larger cylinder known as “low pressure(L.P.)” cylinder. In this cylinder, the last stage of expansion is performed. The L.P. cylinder generally exhausts into a condenser. That is why, the compound steam engines are generally condensing type. But they may be non-condensing as well. If expansion of steam takes place in three cylinders, the engine is called “triple expansion engine”. Similarly, if the expansion is carried out in four cylinders, it is known “quadruple expansion engine”.

In case of triple expansion engines, the first stage of expansion is performed in H.P.cylinder, intermediate expansion takes place in intermediate pressure(I.P.) cylinder, and the last expansion is completed in L.P. cylinder. In quadruple expansion engine, the intermediate expansion is carried out in two I.P.cylinders.Compound steam engines are generally condensing engines. 

Few advantages of compounding the expansion of steam in two or more cylinders: 

1. There is considerable economy in steam for high pressure operation.
2. The temperature range per cylinder is reduced, with corresponding reduction in the condensation.The temperature range is the difference between the highest and lowest temperatures of steam within the cylinder 

3. The ratio of expansion is reduced, thus reducing the length of stroke. 

4. The leakage past the valves and piston is reduced, because of the reduced pressure difference in each cylinder. 

5. The steam can be reheated after expansion in one cylinder, and before entering the next. 

6. Because of lighter parts used in this type of engine.The mechanical balance can be made more nearly perfect, and therefore high speed are possible.

7. In case of a breakdown, the engine can be modified to continue working on reduced load.This feature gives great advantages especially in marine propulsion. 

8. More uniform turning moment is exerted on the crank shaft, by spacing the cranks at 900 in the case of a two cylinder engines or at 1200 in triple expansion engines. Thus, a lighter flywheel is required. 

9. There are less forces in the working parts are reduced, as the forces are distributed over more parts. 

10. The cost of the engine, for the same power and economy, is less than of a simple steam engine.

11. The compound steam engine may start in any position which is an advantage for locomotive and marine engines.

Classification of Compound Steam Engine : 

The compound steam engines may be classified according to the arrangement of cranks, and the angles between them. Two-cylinder compound engines are generally classified as : 
1. Tandem type compound engines,
2. Woolf type compound engines, and 
3. Receiver type compound engines.

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