From the options given below, which type of boiler operates at a higher pressure and is generally at a relatively greater risk of bursting if safety systems fail?

A fusible plug for a steam boiler typically consists of a plug body filled with an alloy of:

The air pre-heater increases the temperature of ___________ before it enters the furnace.

In a steam boiler, the actual evaporation is 8 kg of steam per kg of fuel burnt. The heat supplied per kg of steam is 2500 kJ/kg and the higher calorific value of the fuel is 25,000 kJ/kg. What is the boiler efficiency?