Water tube boilers have to a large extent superseded the smoke tube or fire tube boilers for the supply of steam to main and auxiliary machinery. Even donkey (i.e. auxiliary) boilers are frequently found to be water tube and certainly all modern turbine plants use them for main steam supply.

The advantages of water tube boilers are:

1. High efficiency (generally greater than 85 %) hence reduced fuel consumption.
2. Flexibility of design – important space consideration.
3. Capable of high output (i.e. high evaporation rate).
4. High pressures and temperatures improve turbine plant efficiency.
5. Flexible in operation to meet fluctuating demands of the plant – superheat control rapidly responsive to changing demands.
6. Generally all surfaces are circular hence no supporting stays are required.
7. Steam can be raised rapidly from cold if the occasion demands (3 to 4 hours compared to 24 hours for smoke tube or fire tube boilers) because of the positive circulation.
8. Consider a fire tube or smoke tube boiler and a water tube boiler with similar evaporation rates the water tube boiler would be compact and relatively light by comparison and its water content would be about 7 tonnes or less compared with the smoke tube or fire tube boiler’s 30 tonnes.
9. With double casing radiation loss can be cut to 1 % or less.

 

References

“Steam Engineering Knowledge for Engineers” by Thomas D. Morton