Compound Steam Engine: Meaning, Advantages & Classification | Thermal Engineering

In this article we will discuss about the compound steam engine:- 1. Meaning of Compound Steam Engines 2. Classification of Compound Engines 3. Governing 4. Advantages.

Meaning of Compound Steam Engines:

Improvement of the thermal performance of a steam engine is possible by increasing the ranges of pressure and temperature through which the steam may be expanded within the cylinder of a steam engine. In such cases, the ratio of expansion will be very large if we take full advantage of high boiler pressures and the high condenser vacuum available in modern practice.

The large expansion ratio in one cylinder results in much condensation of steam, large stroke of the piston and greater temperature range. In order to avoid these difficulties and to effect the thermal improvement, engine are built with two or more cylinders for the successive expansion of steam.

The cylinder in which expansion first occurs is known as high pressure (H.P.) cylinder; the exhaust from the high pressure cylinder is passed on to do the work in the second cylinder, which is known as low pressure (L.P.) cylinder before being finally exhausted into a condenser.

In triple expansion engine the expansion of steam is carried out in three Cylinders. Steam enters the high pressure cylinder which exhausts the steam into the intermediate pressure (I.P.) cylinder from which the steam is exhausted into the low pressure cylinder.

It is not uncommon to find triple expansion engine with five cylinders: one for the partial expansion of high pressure steam, two low pressure cylinders. A quadruple expansion engine has four cylinders, a high pressure cylinder, a high intermediate pressure cylinder, a low intermediate pressure cylinder and a low pressure cylinder. Now-a-days quadruple expansion engines are generally not used.

Compound engines range in capacity from 40 to 3000 kW. They are operated condensing at 10 to 14 bar gauge boiler pressure and sometimes, if the valves are properly designed, on superheated steam. The initial pressure in a quadruple expansion engine may be from 14 to 20 bar gauge.

Classification of Compound Engines:

Compound engines are classified with regard to:

(a) The locations of the cylinders in relation to each other and the crankshaft of the engine and

(b) The method of transfer of steam from one cylinder to another.

In tandem compound engines the pistons of H.P. and L.P. cylinders have a common piston rod, and the axes of both the cylinders lie along the same straight line. It has only one crosshead and one connecting rod. In cross compound engines two cylinders are parallel and are on the same side of the crankshaft.

In a duplex compound engine, the cylinders are parallel and adjacent to each other. The piston rods are connected to the same crosshead. The L.P. cylinder is mounted on the H.P. cylinder. This type of engine occupies the same floor space as does a simple steam engine but has the advantages of a compound engine.

In angle compound engines, the cylinders are placed at right angles to each other, one horizontal and the other vertical. The connecting rods are connected to the same crankshaft and to the same crank pin.

In Woolf compound engines the steam from H.P. cylinder exhausts directly into the L.P. cylinder, while in receiver compound engines the steam is delivered from the H.P. cylinder to a receiver and thence to the L.P. cylinder.

In two cylinders compound engines the cranks may be at 0°, 90° and 180° to one another. When the cranks are either 0° or 180° to one another, the two pistons are at the ends of their respective strokes at the same time so the steam from H.P. cylinder can be exhausted directly into the L.P. cylinder.

Two cylinders are connected throughout the stroke. In this case there will be no independent cut­off in the L.P. cylinder. These arrangements have one disadvantage of a large variation in turning moment. As the steam pipes connecting the cylinders offer a considerable temporary storage capacity, it is not necessary that the L.P. cylinder should be ready to receive steam immediately as it is released from the H.P. cylinder and the cranks for the cylinders may be placed at the position which will give the greatest uniformity in turning moment.

In order to bring uniformity in turning moment, in cross compound arrangement the L.P. cylinder crank is 90° ahead of H.P. cylinder crank in the direction of rotation. In this arrangement we require a temporary storage known as the receiver in which the steam from H.P. cylinder will be exhausted and will be held there until the L.P. cylinder is ready to draw from it. The pressure in the receiver will vary throughout the stroke and it will depend on the supply of steam from H.P. cylinder and on demand of L.P. cylinder.

The receiver between H.P. cylinder and L.P. cylinder is necessary in following cases:

(i) When the exhaust from H.P. cylinder cannot be directly exhausted into the L.P. cylinder.

(ii) When the engines have L.P. crank ahead of H.P. crank.

(iii) When the engines require admission of steam to L.P. cylinder when the H.P. cylinder is not exhausting.

(iv) In triple expansion engines with cranks at 120°.

The volume of a receiver should be atleast 1 to 1.5 times the H.P. cylinder volume. Receivers should be provided with pop safety valves to prevent damage in case the L.P. cylinder admission valves do not function properly.

It is fitted with a combination type pressure gauge, a drain cock and a by-pass valve for admitting live steam to the L.P. cylinder of the compound engine. The by-pass permits warming up the receiver and L.P. cylinder before starting the engine and it also facilitates starting the engine when H.P. crank is at dead centre position.

In modern practice we do not require separate receiver but ample space is provided by the connecting pipe and L.P. cylinder steam chest.

Governing of Compound Engines:

The power output of a compound engine can be controlled in one of the two ways. One possibility is the use of a special gear which will permit the cut-off to be varied and this method is called cut-off governing. In throttle governing the steam is throttled before admission to steam chest, thereby reducing the inlet pressure and effective heat drop available from the expansion.

In cut-off governed engine the reduction in work is obtained by a reduction in the mass of steam admitted and the ideal heat drop per unit mass of steam remains unchanged. In the throttle governed steam engine the work is reduced by a reduction of steam chest pressure, which results in an appreciable reduction in available heat per kilogramme of steam. The mass of admitted steam is slightly reduced because of increase in specific volume of steam due to throttling. The cut-off governing is more efficient.

The effects of various methods of governing of compound engines are shown with the help of theoretical indicator diagrams where the effect of clearance volume is neglected.

(I) Varying Cut-Off in H.P. Cylinder (Fig. 9-23):

In fig. 9-23 abcga and gcdefg represent the H.P. and L.P. diagrams on the assumption of equal distribution of work. By making the cut-off in H.P. cylinder later or earlier, the expansion curve will be changed from bed to b’c’d’ or b^”c”d^”.

The areas under these curves will represent the total work done. The maximum volume of the H.P. cylinder remains the same and hence the cut-off ratio in the L.P. cylinder remains unchanged. A later cut-off in the H.P.. cylinder will raise the pressure in the receiver and throw an increasing proportion of total load onto the L.P. cylinder.

An earlier cut-off will lower the pressure in the receiver and throw an increasing proportion of total load onto the H.P. cylinder. When the engine runs on light load, the work done in the L.P. cylinder may become almost negligible.

(II) Varying Cut-Off in L.P. Cylinder (Fig. 9-24):

In fig. 9-24 abcga and gcdefg represent the H.P. and L.P. diagrams on the assumption of equal distribution of work. As the cut-off in H.P. remains the same, the total amount of steam passing through the engine remains the same and hence the variation of the cut-off in L.P.. will not affect the total work done.

The earlier cut-off in the L.P. cylinder will raise the receiver pressure and the amount of work represented by g’c’cgg’ will be transferred from the H.P. cylinder to the L.P. cylinder. The later cut-off in the L.P. cylinder will lower the receiver pressure and also decrease the load on the L.P. cylinder.

(III) Throttling of Steam Supply to H.P. Cylinder (Fig. 9-25):

In fig. 9-25 abcga and gcdefg represent the H.P. and L.P. diagrams on the assumption of an equal distribution of work. In this method of governing the cut-off in each cylinder remains the same.

It will be evident from the diagrams that the effect of throttling the steam supply to the H.P. cylinder is to reduce the power developed in it while power developed in the L.P. cylinder will remain more or less constant. When the engine runs on light load, the power developed in the H.P. cylinder may be very small. This is the reverse of what happens with cut-off governing on H.P. cylinder.

Advantages of Compounding of Steam Engine:

The following are the advantages gained by compounding:

(i) The high pressure steam is admitted to only H.P. cylinder which is of small size and can be made of adequate strength without undue weight. The large L.P. cylinder and piston are never subjected to the boiler pressure and can, therefore, be of much lighter construction than would be the case, were the whole expansion carried out in one cylinder.

(ii) The cylinder condensation is reduced because of the lesser temperature range in each cylinder.

(iii) The action of high pressure on entering the cylinder is largely affected by the pressure in the clearance space. In H.P. cylinder there is less clearance space so undesirable effects can be controlled.

(iv) As the net pressure on each piston is reduced by compounding, the leakage past valves and pistons is reduced.

The pressure difference is not much less in H.P. cylinder than it is in the simple steam engine, but the H.P. cylinder is much smaller for the same power output and the volume of the leakage is, therefore, very small. Also, the steam which leaks past H.P. piston is effective in doing work in L.P. cylinder.

(v) The steam condensed on H.P. cylinder walls is evaporated during the exhaust stroke and augments the supply of live steam to L.P. cylinders i.e. the steam expanding in L.P. cylinder appears to be drier at a given pressure than would be the case if the expansion occurred in a single cylinder.

(vi) The engine may start in any position.

(vii) Improvement in steam economy is effected.

In general, compounding increases, the steam economy at rated load, 10 to 25% for non-condensing and 15 to 40% for condensing operation. At fractional’ loads the saving in steam due to compounding is smaller.

Sometimes, compound engine may use more steam, at light load, than a simple engine would use at the same load. The saving shown by the compound engine would use at the same load. The saving shown by the compound engine over the simple engine is greater at higher boiler pressures.

(viii) Improvement is made with regard to engine balance and turning moment.

(ix) Owing to the permissible lightening of the reciprocating parts, engine vibrations and friction are reduced.

(x) In event of breakdown the engine may be modified to continue working.

(xi) Forces in working parts are reduced as the forces are distributed over more components.

The main disadvantages of the compound engine are its greater first cost, its greater complexity and the larger floor space.