Water tube boilers and fire tube boilers are two common boiler types, the main difference between them is the way the hot gas and water flow:


1. Flow of hot gas: In a water tube boiler, hot gas flows inside the tube and water flows outside the tube. In a fire tube boiler, hot gases flow through tubes and water surrounds them.


2. Heat transfer mechanism: In a water tube boiler, heat is transferred from hot gas to water through the tube wall. In a fire tube boiler, heat is transferred from the hot gas to the water by direct contact and convection.


3. Thermal efficiency: Since the hot gas in the water tube boiler is in direct contact with the water and the heat transfer efficiency is high, the thermal efficiency of the water tube boiler is usually higher than that of the fire tube boiler.


4. Structural complexity: The structure of water tube boilers is relatively complex. Due to the need to handle high-pressure water flow, more pipes and equipment are required. The structure of a fire tube boiler is relatively simple and usually consists of a set of tubes arranged in parallel.


5. Operational safety: Since the structure of water tube boilers is complex and requires more pipes and equipment, more monitoring and maintenance are required during operation to ensure safe operation. The structure of fire tube boilers is relatively simple and maintenance is relatively easy.


6. Response to load fluctuations: Due to their design, water tube boilers respond better to load fluctuations, allowing them to produce steam faster. Fire tube boilers have a slower response time.


In summary, the main differences between water tube boilers and fire tube boilers are the flow of hot gas and water, heat transfer mechanisms, thermal efficiency, structural complexity, operational safety and response to load fluctuations. Choosing a suitable boiler type depends on specific requirements and conditions and should be determined based on specific usage needs.