In a low-pressure serpentine watertube boiler, the tubes are attached by either _______ or by using ferrules.

In a low-pressure serpentine watertube boiler, the correct method for attaching the tubes is through methods such as seamless fittings or ferrules. Seamless fittings allow for a smooth transition and connection between the tubes, ensuring that there are no potential weak points that could lead to leaks or failures. Ferrules provide a secure grip around the tube, creating a solid joint that can withstand pressure changes and temperature fluctuations within the boiler system.

While threading is commonly used in various piping systems, it is not typically associated with watertube boilers where a more robust form of connection is necessary to handle the demands of high-temperature and high-pressure applications. This is why other methods are favored in this context. Using methods like welding or expanding can also be effective but are not the primary focus here concerning the given options. The need for secure and reliable connections in boiler systems underscores the importance of choosing the right attachment method to maintain safety and efficiency in operation.