Water-tube heat recovery boiler

The choice depends mainly on the source and quality of the gases.

A HRSG is usually best suited when the gases come from gas turbines, engines or systems with relatively clean gases and stable operating conditions. It is common in cogeneration, combined cycle and power generation.

A WHRB is usually most suitable when the gases come from industrial processes with particulates, corrosive compounds, variable loads or risk of fouling, such as refining, petrochemical, process furnaces, FCC units, cement or glass.

At AITESA we do not start from the name of the equipment. We analyze the gas profile, steam requirements, plant integration, allowable pressure drop and maintenance conditions to define the most appropriate solution.

The main difference is in the quality of the gases and the expected level of fouling.

An HRSG operates with cleaner gases, for example from gas turbines or engines. In these cases, maintenance focuses on the inspection of tube bundles, control of the water-steam circuit, instrumentation, valves, safety elements and availability and operational reliability of the system.

A WHRB is exposed to more complex process gases, with particulates, corrosive compounds, deposits or varying conditions. Therefore, the design must pay special attention to accessibility, tube bundle cleanliness, material selection, corrosion, fouling control and ease of inspection.

In both cases, maintenance should not be treated as an afterthought, but as a design criterion from the initial phase of the project.

It is possible in some cases, but it depends on the scope of the project and the level of prior preparation.

In retrofit or revamping projects, the shutdown window conditions the engineering, manufacturing, prefabrication, assembly and plant integration strategy. To reduce risks, it is important to define in advance the connection points, available space, logistics, access, interference with other equipment and the necessary tests before start-up.

When the shutdown window is small, the value lies in anticipating as much as possible in the workshop and arriving on site with a clear technical plan. However, each case must be analyzed according to the existing installation and the operational constraints of the plant.

Fouling is managed by design, not just maintenance.

First, the gas composition, the presence of particles, the temperature, the operating regime and the type of deposits to be expected are analyzed. This information is used to define aspects such as tube bundle layout, flow velocities, materials, cleaning access, inspection systems and, where appropriate, specific solutions to facilitate operation and reduce downtime.

In gases with high particulate content or corrosive components, it may also be necessary to adjust the design to reduce accumulations, provide accessible areas and facilitate periodic cleaning.

The objective is to maintain heat transfer, limit pressure drop and preserve equipment availability throughout its service life.

A recovery boiler harnesses thermal energy that is already available in the exhaust or process gases. By transforming that energy into useful steam, it can significantly reduce the need to generate that steam using additional fuel.

The actual impact depends on each installation: gas temperature and flow rate, operating hours, steam demand, required pressure and temperature, performance of the existing system and integration with the rest of the plant.

Therefore, the equipment alone should not be evaluated in isolation. Heat recovery must be analyzed within the overall energy balance of the installation, considering efficiency, OPEX, availability, return on investment and operational constraints.

For a first technical evaluation, the basic data of the gas and steam required are usually needed.

The most relevant data are:

• Source of gases.
• Gas flow and temperature.
• Gas composition, presence of particles and possible corrosivity.
• Steam requirements: flow rate, pressure, temperature and whether saturated or superheated steam is required.
• Operating regime: operating hours, load variability, starts and stops.
• Installation constraints: available space, allowable pressure drop, access, maintenance and existing connections.
• Regulations or technical specifications applicable to the project.

With this information, the type of solution, preliminary scope and additional data needed to advance the technical definition can be assessed.

Before defining a recovery boiler it is necessary to review the actual plant integration conditions.

The most common issues are available space, connection points to the gas train, allowable pressure drop, impact on upstream equipment, interfaces with the water-steam system, instrumentation and control, maintenance access and available shutdown windows.

In retrofit or revamping projects, these factors can be as decisive as the thermal dimensioning itself. It is therefore advisable to analyze them from the initial phase, before closing the technical scope of the equipment.

Not always, although in demanding industrial applications it is common to resort to water-tube designs.

The water-tube configuration allows working with higher pressures and temperatures, better adapting to high gas flow rates and responding to more complex operating conditions. It is therefore a frequent solution in heat recovery projects for refining, petrochemical, cogeneration, power generation and other industrial processes.

The final choice depends on gas conditions, required steam parameters, available space, allowable pressure drop, regulatory requirements, and operation and maintenance needs.