Recuperative Tubular Air Preheater

A tubular air preheater is a type of heat exchanger that is commonly used in thermal power plants to improve the efficiency of the boiler by recovering waste heat from the flue gases. It preheats the combustion air before it enters the boiler, which helps to reduce the energy required to heat the air inside the boiler. This leads to improved thermal efficiency and reduced fuel consumption.

In this article, we will provide an in-depth explanation of the tubular air preheater, including its construction, components, working principle, and advantages.

Construction and Components of a Tubular Air Preheater

A tubular air preheater consists of a series of parallel tubes that are arranged in a rectangular or circular pattern. The tubes are made of materials that are resistant to high temperatures, such as stainless steel or carbon steel. The tubes are welded together to form a tube bundle, which is housed in a casing. The casing is made of materials that are also resistant to high temperatures, such as cast iron or steel.

The combustion air flows through the tubes, while the hot flue gases flow around the tubes. The tubes are placed in the path of the flue gases, which transfer heat to the air flowing inside the tubes. The heat transfer occurs through a process called convection. The hot gases transfer heat to the walls of the tubes, and the walls of the tubes transfer heat to the combustion air flowing inside the tubes. The preheated air is then supplied to the boiler.

The tubular air preheater is equipped with several components that help to improve its performance and ensure efficient operation. These include:

1. Inlet and Outlet Ducts: The inlet and outlet ducts are used to connect the air preheater to the combustion air duct and the boiler. The ducts are designed to minimize pressure loss and ensure a smooth flow of air through the preheater.
2. Bypass Dampers: The bypass dampers are used to control the flow of air through the preheater. They can be opened or closed to regulate the amount of air passing through the preheater, depending on the operating conditions of the power plant.
3. Cleaning System: The tubular air preheater is equipped with a cleaning system that is used to remove ash and other debris that may accumulate on the surface of the tubes. The cleaning system can be manual or automated, depending on the design of the preheater.

Working of a Tubular Air Preheater

The working of a tubular air preheater is based on the principle of heat transfer through convection. The hot flue gases leaving the boiler enter the air preheater through the inlet duct. The gases flow around the tubes, transferring heat to the walls of the tubes. The tubes absorb the heat and transfer it to the air flowing inside the tubes.

The air flowing inside the tubes is preheated before it enters the boiler. The preheated air reduces the amount of heat required to raise the temperature of the air to the desired level inside the boiler. This results in improved thermal efficiency and reduced fuel consumption.

The preheated air is supplied to the boiler through the outlet duct. The bypass dampers are used to regulate the flow of air through the preheater. When the power plant is operating at low loads, the bypass dampers are opened to allow some of the air to bypass the preheater. This prevents excessive heating of the air, which can lead to a decrease in thermal efficiency.

Recuperative Tubular Air Preheaters:-

A recuperative tubular air preheater is a type of heat exchanger that transfers heat from the hot flue gases leaving the boiler to the combustion air entering the boiler. The heat transfer takes place through the walls of the tubes that separate the hot flue gases and the combustion air. In this type of air preheater, the hot flue gases flow through the outside of the tubes, while the combustion air flows through the inside of the tubes.

Recuperative tubular air preheaters can be further classified into two types:

• Bare Tube Air Preheater: This type of preheater consists of a bundle of plain tubes that are welded together to form a tube bank. The combustion air flows inside the tubes, while the flue gases flow around the tubes. This type of preheater is not very efficient, as the heat transfer rate is relatively low due to the absence of any fins or extended surfaces on the tubes.
• Finned Tube Air Preheater: This type of preheater consists of a bundle of tubes that have fins or extended surfaces on their outer surface. The fins increase the surface area of the tubes, which improves the heat transfer rate between the hot flue gases and the combustion air. Finned tube air preheaters are more efficient than bare tube preheaters, as they can achieve a higher heat transfer rate with a smaller surface area.