Compound Steam Engines: Classification, Advantages and Disadvantages | Thermodynamics

In this article we will discuss about:- 1. Introduction to Compound Steam Engines 2. Classification of Compound Steam Engines 3. Intermediate Pressure 4. Advantages and Disadvantages.

Introduction to Compound Steam Engines:

Nowadays high pressure steam from the high pressure boilers is generally used for power generation purpose because-

1. It gives higher cycle efficiency

2. Plant requires less space/kW of power generation.

If the high pressure steam is used with a single cylinder engine, then it will have the following limitations:

1. A huge and robust cylinder is required to with stand and accommodates high pressure steam.

2. Hence reciprocating parts are very large.

3. So, large variation, in torque.

4. To even out this variation, a larger flywheel is required.

5. More balancing problems.

6. More condensation losses—Due to large pressure difference between the inlet and exhaust conditions, by the properties of steam, there will be large temperature difference and which results in more condensation losses.

To overcome these difficulties a compound steam engine is used whenever pressure ranges are high. A compound steam engine consists of more than one cylinder. If two cylinders are used, then it is called as Double expansion compound steam engine.

Here high pressure steam from high pressure boiler will be first admitted in the HP cylinder, where partial expansion of steam takes place and the remaining expansion will be completed in LP cylinder.

If three cylinders are used, then it is called as Triple expansion compound steam engine. In this case an Intermediate Pressure (IP) cylinder is provided in between the HP and LP cylinders.

Classification of Compound Steam Engines:

I. Tandem Compounding

II. Cross compounding

(a) Woolfe type

(b) Receiver type.

I. Tandem Compound Steam Engine:

In this case there is only one piston rod, one cross head, one connecting rod and one crank shaft. In this case piston for the HP cylinder and LP cylinder are mounted on the same piston rod.

Let the high pressure steam from the high pressure boiler be admitted to one side of the piston of HP cylinders as shown by solid arrow heads. Steam from the other side of the HP piston, which is shown by solid arrow heads will be simultaneously exhausted/admitted into the LP cylinder. The valve setting should be such that admission of steam continues simultaneously in both the cylinders. The steam from the LP cylinder will be exhausted either to the condenser or to the atmosphere.

Since normally the steam engines are double acting, the steam flow in the return stroke is shown by dotted arrow heads.

Since the pistons are mounted on the common piston rod, so that when the piston of HP cylinder is at IDC (Inner Dead Centre), the piston of LF cylinder will be at IDC, (Inner Dead Centre) and similarly when the piston of HP cylinder is at ODC (Outer Dead Centre) then the piston of LP cylinder will be at ODC, so the operation in both the cylinders are in phase. So, the maximum and minimum torques will occur approximately at the same crank angle.

As shown in Fig. 18.13 there is large variation in turning moment, so a large fly wheel is required to even out this variation.

(II) Cross Compounding Steam Engine:

(a) Woolfe Type:

In Woolfe type two-cylinder CSE, the cranks of the two cylinders are 180° out of phase, i.e.,

(i) When the crank of HP cylinder is at TDC, the crank of LP cylinder will be at BDC and

(ii) When the piston of HP cylinder is at TDC, the piston of LP cylinder will be at BDC and

(iii) When the piston of HP cylinder is moving downwards, the piston of LP cylinder will be moving upwards.

Cylinders are arranged side by side. The pistons are connected to separate piston rods, which are in turn connected to crank through separate connecting rods.

In this case the HP cylinder directly exhausts into LP cylinder so no receiver is required. The turning moment variation is exactly similar to that of Tandem type. So this arrangement also requires a heavy Flywheel to even out the variation.

(b) Receiver Type:

In this type of engine the cranks of the cylinders are placed at 90° to each other, the strokes of the two cylinders are thus out of phase. The HP cylinder cannot exhaust directly to the LP cylinder. In order to overcome this, a receiver is provided in between HP and LP cylinder. The HP cylinder exhausts in to the receiver while the LP cylinder draws from the receiver. Therefore to minimise fluctua­tions in pressure, the volume capacity of the receiver must be sufficiently large.

Intermediate Pressure for the Compound Steam Engine:

There are two possibilities:

1. Equal initial piston loads

2. Equal work per cylinder

1. Equal Initial Piston Loads:

In a double-acting steam engine, the net pressure on the piston will be the difference between the pressures on either side of the piston.

For the high-pressure cylinder.

Net pressure = P₁ – P₇

For the low-pressure cylinder,

Net pressure = P₇ – P₅

Now

Force on piston = Net pressure on piston x piston area

Thus, for equal initial piston loads.

(P₁ – P₇)A_HP – (P₇ – P₅) A_LP …(1)

Where A_HP = area of high-pressure piston.

A_LP = area of low-pressure piston.

Now, the strokes of the pistons of a compound engine are usually made the same. Thus, if the stroke = L, then from Eq. (1).

(P₁ – P₇)V_HP – P = (P₇ – P₅) V_LPL …(2)

But neglecting clearance.

A_HPL = V_HP = volume of high-pressure cylinder also

A_LP L = V_LP = volume of low-pressure cylinder hence, Eq. (2) becomes,

(P₁ – P₇)V_HP – P = (P₇ – P₅) V_LP …(3)

2. Equal Work per Cylinder:

For this to be the case, the area of the high-pressure and low-pressure diagrams must be the same.

For the high-pressure cylinder.

Work done = Area 1 2 6 7

(Area under 1 – 2) + area (under area 2-6) – (area under 6-7)

Note that the high-pressure cylinder volume is the same as the cut-off volume, 7-6, of the low-pressure cylinder. Note also that the total work done is given by the total area 1 2 3 4 5. This would also be the work done if the steam were admitted and expanded in the low-pressure cylinder only. This is a point which is made use of in the solution of some steam engine problems.

Advantages and Disadvantages of Compound Steam Engines:

Advantages of Compound Steam Engines:

1. By compound pressure range/cylinder is reduced and in turn the temperature range/cylinder is reduced—so less stress and less condensation losses.

2. Only HP cylinder is subjected to high pressure steam and therefore can be made of stronger material. But cheaper materials can be used for LPG cylinder because of low pressure range.

3. Since pressure range/cylinder is reduced, leakage of steam will be less.

4. Steam can be reheated in a separate heater after expansion in each cylinder, which increases the efficiency of the engine.

5. More uniform, torque can be obtained in receiver type engines and a smaller flywheel can be used.

6. Engine vibrations are reduced because of lighter reciprocating parts. Therefore Balancing will be simpler.

7. Engines can start in any position.

8. Cost of CSE/kW of power produced is less than single cylinder engines.

9. During breakdown, the engine may be modified to continue to operate on reduced loads.

Disadvantages of Compound Steam Engines:

1. The arrangement is complex, needs greater care and maintenance.

2. Increased wear and tear.

3. More lubrication problems.