Question 59 AEL01 - Assistant Engineer - Limited

The Correct Answer is A. ### Explanation of Correct Option (A) **A) the steam-water mixture that exits the coils is separated in a flash chamber** Coil-type water-tube boilers (often referred to as 'once-through' boilers or 'forced circulation' boilers) are designed to minimize water volume and maximize heat transfer efficiency, which allows them to produce steam quickly. They operate by pumping feedwater through a continuous, long coil exposed to hot gases. The water gradually heats up, vaporizes, and often leaves the coil as a high-velocity, high-pressure mixture of steam and unevaporated water (a two-phase mixture). A large steam drum is traditionally needed in conventional boilers to provide a large surface area where the density difference between steam bubbles and liquid water allows for natural gravitational separation. Because coil boilers rely on forced circulation and the water residence time is very short, the steam/water mixture exiting the coil must be mechanically separated. This separation is accomplished in a smaller, external device known as a **steam separator** or **flash chamber**. This device efficiently separates the dry steam for use and directs the remaining liquid water back into the feedwater system or blows it down. Because this external, high-efficiency mechanical separator is used, the large internal volume and complexity of a traditional steam drum are unnecessary. ### Explanation of Incorrect Options **B) the heat of combustion is sufficient to remove all moisture from the steam** This is incorrect. While the heat is very intense, it is impractical and inefficient to design the coil to achieve 100% steam dryness (superheating) by the time it exits the coil bundle, particularly under varying load conditions. Coil boilers are intentionally designed to leave some water content (a wet steam mixture) to ensure the tubes do not overheat and fail, relying on the flash chamber for final separation. **C) the volume of steam is small at low pressures** This is incorrect on two counts. First, auxiliary boilers are often run at relatively high pressures where steam volume is actually smaller (denser) than at low pressures. Second, regardless of the boiler's operating pressure, the need for a mechanism to separate liquid water from saturated steam remains critical to prevent water carryover and maintain safety/efficiency. **D) automatic burner cycling controls steam volume and quality** This is incorrect. Automatic burner cycling controls the rate of heat input to match the steam demand (volume/pressure), but it does not physically separate liquid water from steam. Separation is a physical process requiring a dedicated component (the steam drum or flash chamber), independent of the operational controls of the burner.