Complex sentence: A boiler incorporates a firebox or furnace in order to burn the fuel and generate heat; the heat is initially transferred to water to make steam; this produces saturated steam at ebullition temperature saturated steam which can vary according to the pressure above the boiling water.
Split into simpler sentences: A boiler incorporates a firebox or furnace in order to burn the fuel and generate heat. This produces saturated steam at a rate which can vary according to the pressure above the boiling water.