Boiler feedwater is directed through the economizer before entering the steam drum for heating and separation. This design is a cornerstone of modern boiler engineering, resulting from extensive optimization for energy efficiency, equipment protection, and process integration. The rationale for this sequence is explored in detail below.

Principle and Structure of the Economizer

The economizer is a critical heat exchange surface. Its core function is to harness waste heat from low-temperature flue gas in the boiler's tail end to preheat the feedwater, forming an essential part of the regenerative cycle.

Structurally, it consists of multiple rows of serpentine steel or finned tubes densely arranged in the tail-end flue duct. Here, the flue gas—having passed through the superheater and reheater—has cooled to approximately 300-400°C but retains significant thermal energy. Feedwater from the deaerator (typically at 100-150°C) flows through these tubes, absorbing heat from the gas. This raises its temperature to closely approach the drum's saturation temperature (e.g., 250-300°C in high-pressure boilers).

This configuration offers key advantages: the serpentine tubes maximize the heat transfer area, and the manageable temperature difference (100-200°C) between the cool water and warm gas allows for highly efficient heat exchange without risking overheating of the economizer's tubes.

Economic Benefits: Enhancing Efficiency and Reducing Cost

Bypassing the economizer would lead to substantial energy waste. Its inclusion delivers major economic advantages:

1. Lower Exhaust Gas Heat Loss

Exhaust gas heat loss is the largest source of energy loss in a boiler, typically accounting for 5-10% of total heat. The economizer captures this waste heat, reducing the flue gas temperature by 50-100°C (e.g., from 350°C to 250°C). Thermodynamically, a 10°C reduction in exhaust temperature can improve boiler efficiency by 0.5-1%, leading to direct fuel savings and lower operational costs.

2. Reduced Heating Load on the Steam Drum

Preheated feedwater enters the steam drum near its saturation temperature. This drastically reduces the energy required within the drum to bring the water to a boil. Consequently, the water walls can focus more energy on converting saturated water into steam, improving the targeted utilization of combustion heat. Introducing cold water directly would absorb saturated heat, forcing the system to consume extra fuel to maintain pressure and temperature.

Safety: Mitigating Thermal Stress and Equipment Damage

This design is crucial for the operational safety and longevity of key components.

1. Protecting the Steam Drum

The steam drum is a thick-walled pressure vessel. Introducing cold water (e.g., 150°C) directly into it while it contains saturated water (e.g., 311°C at 10 MPa) creates a severe temperature differential. This causes intense thermal stress as the inner wall contracts while the outer wall remains expanded, potentially leading to fatigue cracking over time. Preheating the water minimizes this temperature difference to a safe 20-50°C, virtually eliminating this risk.

2. Self-Protection via Recirculation

The economizer protects itself during low-flow periods (startup/shutdown) via a recirculation pipe connected to the steam drum. If feedwater flow is insufficient, saturated water from the drum flows back to the economizer. The evaporation of this water absorbs heat, preventing the tubes from overheating and being damaged by "dry firing," a safeguard that would be absent without the economizer.

Process Rationality: Energy Cascade Utilization

The entire boiler system is designed on the principle of temperature matching and energy cascade use:

• High-Temp Gas (1000°C+): Heats the highest-temperature medium (superheated steam) via the superheater.

• Medium-Temp Gas (500-600°C): Heats medium-temperature mediums (reheat steam).

• Low-Temp Gas (300-400°C): Heats low-temperature feedwater in the economizer.

• Final Exhaust (200-300°C): Preheats combustion air in the air preheater.

Introducing cold water directly into the steam drum would disrupt this cascade, wasting high-grade heat on a low-temperature task and failing to recover low-grade energy from the exhaust, thus violating fundamental thermodynamic optimization principles.

In summary, routing feedwater through the economizer first is a non-negotiable design element that maximizes economic efficiency, ensures operational safety, and adheres to sound thermodynamic practice for overall system optimization.