MODERN HIGH PRESSURE BOILERS

Modern trend is to build supercritical boilers which produce steam at pressures above critical value. [Critical pressure (221.2 bar) is that pressure at which specific enthalpy (latent heat) is zero and water evaporates at 374°C]. They improve the thermal efficiency of the power plants. Several thermodynamic principles are utilised in order to obtain higher efficiencies. The following are the chief characteristics and salient features of modern high pressure boilers.

(i) Method of Water Circulation : This may be natural circulation due to static head and convective current; for pressures above 160 bar, forced circulation using pumps is advisable. In this case water flows with high velocity. Hence, for a given output, boilers may have small size tubes.

(ii) Arrangement of Drums and Tubing : The high pressure boilers are characterised by the use of very small steam drums or by complete elimination of drums. Water is evaporated and converted into steam in a single continuous tube. Parallel set of tubes may be arranged to avoid or to minimise large resistance to flow of water.

(iii) Improved Methods of Heating : These methods include improved arrangements of heating devices such as heating the feed water by superheated-steam; saving of latent heat by evaporating water at pressures above critical pressure (225 bar); and increasing the heat flow through the tubes by using hot gases travelling with supersonic speed.

La Mont Boiler Working Principle:

La Mont Boiler (Figure) is a forced circulation, high pressure water tube boiler. The principle involved is, by employing a high velocity water higher rate of heat transmission can be obtained. It deals with smaller quantity of water and operates at higher pressures.

Water is supplied through an economiser to a steam separating drum which is set practically outside the boiler. A circulating pump draws water from drum and delivers it to the evaporator. Water circulated is about ten times the steam evaporated. Hence overheating of the tubes is avoided. Water is rapidly evaporated in the tubes. The mixture of water and steam from these tubes passes into the drum. Here vapour is separated and is flown into superheater. Superheater which is placed in the path of hot gases is of convective type. Steam gets superheated in the superheater and leaves to produce work in a primemover. The air preheated by the flue gases may be used for economic combustion.

The level of water in the drum is kept constant by supplying feed water equivalent to the quantity of steam drawn out.

The main parts of Lamont boiler are

1. Hotwell

2. Feed pump

3. Economiser

4. Steam Drum

5. Circulating pump

6. Evaporator

7. Superheater

8. Air preheater

9. Chimney

10. Combustion Chamber

Advantages :

• Lighter in weight for a given output.

• Reduction in number of drums.

• High evaporation rate (45,000 to 70,000 kg/hr)

• Absence from scaling trouble in the pipes.

• Quick starting even in cold conditions.

• High pressure steam (200 bar) can be generated.