Classification of the Compound Steam Engine

Compound steam engines are usually classified into two types:

  1. Tandem compound steam engine
  2. Cross compound steam engine

Tandem Compound Steam Engine

In this type of engine, the two cylinders usually have a common piston rod and are fixed in tandem, working on the same crank shaft. These cylinders may be observed as having a crank at zero degrees to each other.

Tandem Engine Type Compound Steam Engine

Steam generated in the boiler is supplied to one side of the piston of the high pressure (H.P) cylinder. On the other hand, the high pressure piston exhaust takes place simultaneously, and this exhaust steam now acts on the piston of the low pressure (L.P) cylinder. The valves should be operated in such a way that there should be continuous admission of steam in the high pressure as well as low pressure cylinders simultaneously.

After the steam supply is cut off in the high pressure cylinder, the steam expands, and after the high pressure exhaust steam is admittedto the  low pressure cylinder up to the cut-off point, it further expands to the condensing pressure if the engine is considered or to the atmosphere for non-condensing engines.

Since the steam engines are double acting, the steam flow takes place during the return stroke, as shown by the thin arrow heads.

As the cycle of H.P and L.P cylinders is in phase, the maximum turning moment on the crank-shaft, due to each and every cylinder, will certainly act at the same instant. This is the disadvantage of this type of compound engine, since a large flywheel is consequently required.

Cross-composite steam engine

  1. Woolfe type compound steam engine
  2. Receiver-type compound steam engine

Woolfe type compound steam engine

Woolfe Type Compound Steam Engine

In this type of two cylinder compound engine, the crankshaft of the cylinders is at an angle of 180 degrees to each other. The cylinders are arranged in a side-by-side manner, and the exhaust steam from the H.P cylinder passes directly into the L.P cylinder. The expansion is, therefore, continuous during the stroke. As the crank is at an angle of 180° the two cycles are in phase and cause a large variation in the turning moment on the crankshaft; this is the same disadvantage as in the tandem type of compound steam engine.

Receiver type Compound Steam Engine

In this type, the steam from one cylinder is not directly discharged into the next cylinder but it is discharged into a chamber known as the “receiver”. So the receiver is nothing but a reservoir of steam from where the steam is admitted into the L.P cylinder during its admission stroke. In this arrangement, the crank angle will be less than the angle of 180°. In a two cylinder compound engine, the angle is 90°. This type of engine can start in any position. It can also be run at reduced loads, with one cylinder in operation.

Receiver Type Compound Steam Engine

There is always an unavoidable pressure drop in the receiver due to condensation of steam, but it can be reduced by steam jacketing the receiver. The reservoir should be large enough to keep the pressure in it fairly constant; its volume should be about 1.5 times the H.P cylinder volume.

According to the number of expansion stages, the compound steam engine may also be classified as follows:

  1. Double expansion
  2. Triple expansion

Double expansion

In a double expansion engine, the expansion of steam takes place in two cylinders. First the steam expands in the high pressure cylinder, and then it is discharged into the low-pressure cylinders. Finally, it is exhausted into the condenser.

Triple expansion

In this type of engine, the expansion of steam is completed in three cylinders. The steam from the high pressure cylinder is exhausted into intermediate pressure cylinder and then the steam is discharged into the low pressure cylinder. The steam from the low pressure cylinder is discharged into the condenser.

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