Compound Steam Engine - Advantages Of Compound Steam Engine

What is compound Steam Engine?

Compound steam engine is nothing but a steam expansion arrangements where one or more than one cylinders are available to divide the expansion of steam into different stages.

Generally, in compound steam engine two cylinders are available.1.high-pressure cylinder (H.P.) 2. low-pressure cylinder (L.P.). The cylinder which receives high-pressure steam is called high-pressure cylinder and after expansion of steam in high-pressure cylinder chamber, it exhausts into a large cylinder known as low-pressure cylinder. High-pressure cylinder is generally smaller size in low-pressure cylinder. In compound steam engine high-pressure steam enters into engine cylinder where the steam expands and divide into different stages. Each stage of steam is performing different cylinder of the engine. At first high-pressure steam partly expand in high-pressure cylinder and then steam exhausted into the low-pressure cylinder where the expansion is completed. This method reduces expansion ratio of steam in the engine cylinder in which the length of stroke is reduced. It reduces the temperature range in each cylinder. In low pressure cylinder chamber, exhaust steam is condensed. that's why compound steam engine is condensing type. According to the number of engine cylinder, if the expansion of steam takes place in three cylinders the engine is called triple expansion engine and expansion is carried out in four cylinders is known as quadruple expansion engine. this type of expansion engine, steam pressure expend in between high-pressure and a low-pressure cylinder called intermediate pressure cylinder(I.P.). For triple and quadruple expansion engine, steam expands one and two times in IP cylinder.


Working principle of a steam engine with high steam pressure faces some problems into the engine cylinder. To minimise this problem,compound steam engine is developed.The difficulties are follows

1.High-pressure steam enters comparatively cold cylinder walls which cause initial condensation.

2.Steam expansion falls down to the condenser pressure causes a large temperature range in the engine cylinder.

3.The ratio of expansion is the large due to a greater range of pressure difference.

4.The stroke of the piston is large due to the large ratio of expansion.

5.Two cylinder compound steam engine is generally classified according to the arrangements of cranks and the angles between them, discussed in next page.


Steam engine with more nubber of cylinders have following advantages:

1.The cost of the compound steam engine is less than simple steam engine for same power and economy.

2.There is considerable economy in steam for high pressure operations.

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

4.High speed is possible due to its perfect balancing.

5.In case of breakdown, the engine can be modified to continue working on reducing load.

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

7.The temperature range per cylinder is reduced for reducing the condensation.

8.The force in the working parts are reduced.


In the previous page, we have seen the power developed by simple steam engine.In the compound steam engine, there is some minor modification.Generally two following guiding factors are consider for developing power by the compound steam engine.

1.Total work done by a compound steam engine is shared equally by both the cylinders.Or in other words, both the H.P. na L.P. cylinders are same.

The initial thrust or load on the piston of both the H.P. and L.P. cylinders is the same.

Let, pa = Actual mean effective pressure referred to L.P. cylinder in bar,

L = Stroke length in meters,

A = Area of the L.P. cylinder or piston in m2,

N = Engine speed in r.p.m.

Then, power developed (I.P.) by compound steam engine is given by following relation:-

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