What is true regarding steam and water in a closed feedwater heater?

In a closed feedwater heater, steam and water do not mix due to the design of the system. The function of a closed feedwater heater is to preheat the water before it enters the boiler, improving thermal efficiency. The steam enters the heater and transfers its heat to the water without physically mixing with it, allowing the water to increase in temperature while remaining in a separate phase. This separation ensures that the steam can condense and be reused in the system, maintaining the efficiency of the overall steam cycle.

The other options do not accurately describe the operation of a closed feedwater heater. For instance, the idea that they mix thoroughly contradicts the fundamental principle of heat exchange in closed systems, where phase separation is key. The notion that water is converted to steam is incorrect in this context, as the primary role of the feedwater heater is to raise the temperature of the water, not convert it to steam. Lastly, suggesting that steam is wasted does not align with the principle that steam is effectively used for heat transfer and continues in the system to maximize efficiency.