**Water tube boilers are externally fired**

Water tube boilers are a type of boiler in which water circulates in tubes heated externally by the fire. The combustion gases flow around the outside of the tubes, transferring heat to the water inside the tubes. This design allows for a higher heat transfer efficiency compared to internally fired boilers.

**Explanation:**

1. **Definition of Water Tube Boilers:**
- Water tube boilers are a type of boiler where the water is contained in tubes and heated externally by combustion gases.
- The hot combustion gases flow around the outside of the tubes, transferring heat to the water inside the tubes.
- This design allows for efficient heat transfer and higher pressure operation.

2. **Internally Fired Boilers:**
- Internally fired boilers are a type of boiler where the combustion of fuel takes place within the boiler shell itself.
- The fire and hot gases are located inside the boiler, surrounded by water.
- The heat from the combustion process is transferred directly to the water surrounding the firebox.
- Examples of internally fired boilers include Scotch marine boilers and firebox boilers.

3. **Externally Fired Boilers:**
- Externally fired boilers are a type of boiler where the combustion of fuel takes place outside the boiler shell.
- The fire and hot gases are located outside the boiler, and the heat is transferred to the water inside the tubes.
- The combustion gases flow around the tubes, transferring heat to the water inside.
- Examples of externally fired boilers include water tube boilers and locomotive boilers.

4. **Water Tube Boilers:**
- Water tube boilers are a type of externally fired boiler.
- In water tube boilers, the water circulates through the tubes, which are heated externally by the combustion gases.
- The combustion gases flow around the outside of the tubes, transferring heat to the water inside.
- This design allows for a large heating surface area and efficient heat transfer.
- Water tube boilers are commonly used in power plants, industrial facilities, and marine applications.

5. **Conclusion:**
- In conclusion, water tube boilers are externally fired boilers.
- The combustion gases flow around the outside of the tubes, heating the water inside.
- This design allows for efficient heat transfer and higher pressure operation.
- Internally fired boilers, on the other hand, have the combustion process located inside the boiler shell.