For greater efficiency and less space per horse power, high pressure steam is used in steam engine. Now with the use of this high pressure steam the range of expansion is correspondingly increased. If full expansion is takes place in single cylinder the walls of the cylinder to be made very thick to withstand the high pressure and also required a heavy flywheel. All this made a simple steam engine very heavy in construction, large variations of temperature and more leakage past the piston.
To overcome these difficulties, the expansion of steam is made in two or more cylinder. This called as “Compounding the expansion of steam”. There are three different methods of compounding of steam engines as below,
01) Tandem Compounding:-
In this type, the cylinders are arranged on common axis and two pistons have a common piston rod, connecting rod and crankshaft. The exhaust steam from high pressure cylinder passes directly into low pressure cylinder where it further expands.
The turning moment in this type of this type of engine is not uniform due to common crank and piston rod where all the torque acts at the same time. This is the basic disadvantage of such type of engines and requires a large flywheel. Tandem arrangement gives constructional economy because of less components of the engine.
02) Woolf Compounding:-
In this type, two cylinders are arranged side by side while the two cranks are placed at 180 degree to each other. The pistons of high pressure and low pressure cylinders begin and end their strokes together respectively.
The exhaust of steam from high pressure cylinder coincides with suction of steam in low pressure cylinder. In this case also exhaust steam from high pressure cylinder passes directly into low pressure cylinder where it further expands.
Since the two cranks are at 180 degree to each other, the two cycles are in phase and this causes large variation in the turning moment of crankshaft and necessitating the use of heavy flywheel.
03) Receiver Type Compounding:-
In this type, two cranks are mounted at 90 degree to each other that is why steam from high pressure cylinder can not exhaust directly into low pressure cylinder.
A container, known as receiver is used to collect steam from high pressure cylinder while low pressure cylinder draws steam from receiver for further expansion.
The turning moment in receiver type compounding steam is more uniform therefore a lighter flywheel can be used. With three cylinder engine, the cranks are arranged at 120 degree to each other but still small turning moment variation can be achieved.
You may also interest in following...
Advantages of Steam Turbine over Reciprocating Engine.
How To Cool Exhaust Valve Of Internal Combustion