Bare Tube Vertical Boiler Economizer

Bare Tube Vertical Boiler Economizer

A boiler economizer is a heat exchanger installed in the flue gas path of a boiler (typically downstream of the boiler tubes or furnace). Its primary role is to preheat the boiler feedwater using the residual heat from flue gases (which would otherwise be wasted). This reduces the energy required to heat water to steam in the boiler, thereby lowering fuel consumption and emissions.

A "bare tube vertical" design specifies two key features:

Bare tubes: Tubes without fins or extended surfaces (unlike finned-tube economizers, which use fins to increase heat transfer area).

Vertical orientation: Tubes are arranged vertically (rather than horizontally), with flue gases and feedwater flowing through/around them in a vertical or counter-vertical path.

Structure & Working Principle

Key Components

Vertical bare tubes: Typically made of carbon steel or alloy steel (resistant to corrosion from flue gases), these tubes form a bundle. Feedwater flows inside the tubes.

Headers: Upper and lower headers that distribute feedwater into the tube bundle (lower header) and collect preheated water (upper header) before it enters the boiler.

Flue gas path: Flue gases (hot, from boiler combustion) flow outside the vertical tubes, transferring heat to the feedwater inside via conduction and convection.

Heat Transfer Process

Hot flue gases (150–350°C, depending on the boiler) exit the boiler furnace and pass over the external surface of the vertical bare tubes.

Heat from the flue gases transfers through the tube walls to the cooler feedwater (entering the economizer at 50–100°C, depending on the system) flowing inside the tubes.

Preheated feedwater (now at a higher temperature, e.g., 100–200°C) exits the economizer and enters the boiler, reducing the energy needed to convert it to steam.

Cooled flue gases (30–100°C cooler than their inlet temperature) exit the economizer and are released through the stack.

Advantages of Bare Tube Vertical Design

Resistance to fouling and corrosion: Bare tubes (without fins) have a smooth surface, making them less prone to accumulating ash, soot, or particulate matter from flue gases. This is critical for boilers burning solid fuels (e.g., coal, biomass) or high-sulfur fuels, where flue gases carry heavy dust or corrosive components (e.g., sulfur oxides).

Ease of maintenance: The smooth, vertical tubes are easier to clean (e.g., via sootblowers, water washing) compared to finned tubes, which trap debris between fins.

Durability in high-ash environments: Vertical orientation allows ash and particulates to fall freely under gravity, reducing buildup on tube surfaces—ideal for boilers with high flue gas dust loads.

Simplicity and cost-effectiveness: Bare tubes are cheaper to manufacture than finned tubes, and the vertical design simplifies installation (e.g., alignment with flue gas ducts).

Bare tube vertical economizers are most commonly used in:

Solid fuel boilers: Coal-fired, biomass-fired, or waste-to-energy boilers, where flue gases contain high levels of ash or particulates.

Industrial boilers with harsh flue gas conditions: Boilers burning high-sulfur fuels (e.g., heavy oil) or processes with corrosive flue gas components, where finned tubes would foul or corrode rapidly.

Small to medium-sized boilers: Where simplicity, low maintenance, and cost are prioritized over compactness.