Radiant section is a part of a fired heater, which is a device used to heat fluids in various industries such as refining, petrochemical, chemical, etc. The main function of the radiant section is to transfer heat from the combustion of fuels to the process fluid flowing inside the tubes. The heat transfer is mainly by radiation, which means the emission of electromagnetic waves from the hot surfaces of the flame and the refractory walls.
Components of Radiant Section
The main components of the radiant section are:
- Burners: These are devices that mix fuel and air and ignite them to produce a flame. The burners are usually located at the bottom or the side of the radiant section. The number, type, and arrangement of the burners depend on the design and capacity of the fired heater.
- Tubes: These are metal pipes that carry the process fluid through the radiant section. The tubes are arranged in rows or coils along the walls and the roof of the radiant section. The tubes are usually made of carbon steel or alloy steel, depending on the temperature and pressure of the process fluid. The tubes have different diameters, thicknesses, and lengths, depending on the heat transfer requirements and the fluid properties.
- Tube supports: These are structures that hold the tubes in place and prevent them from sagging or buckling due to thermal expansion and contraction. The tube supports are usually made of refractory materials or metal bars. The tube supports can be fixed or sliding, depending on the design and operation of the fired heater.
- Refractory: This is a material that lines the walls and the roof of the radiant section. The refractory acts as an insulation and a protection for the metal casing of the fired heater. The refractory also reflects some of the heat back to the tubes, enhancing the heat transfer by radiation. The refractory can be made of bricks, castables, or ceramic fibers, depending on the temperature and the service life of the fired heater.
Heat Transfer in Radiant Section
The heat transfer in the radiant section can be divided into three steps:
- Combustion: This is the process of burning the fuel and air mixture in the burners, producing heat and flue gas. The combustion is controlled by the fuel flow rate, the air flow rate, and the air-fuel ratio. The combustion efficiency is the ratio of the actual heat release to the theoretical heat release of the fuel. The combustion efficiency depends on the type and quality of the fuel, the burner design, and the operating conditions. The combustion efficiency can be improved by optimizing the fuel and air distribution, minimizing the excess air, and reducing the heat losses.
- Radiation: This is the process of emitting electromagnetic waves from the hot surfaces of the flame and the refractory. The radiation intensity is proportional to the fourth power of the absolute temperature of the surface. The radiation intensity also depends on the emissivity of the surface, which is a measure of how well the surface emits or absorbs radiation. The emissivity ranges from 0 to 1, where 0 means no emission or absorption, and 1 means perfect emission or absorption. The emissivity of the flame and the refractory are usually close to 1, while the emissivity of the tubes are lower, depending on the material and the surface condition. The radiation heat transfer is calculated by using the Stefan-Boltzmann law, which states that the net heat transfer between two surfaces is proportional to the difference of the fourth power of their absolute temperatures and their emissivities. The radiation heat transfer can be enhanced by increasing the temperature and the emissivity of the flame and the refractory, and decreasing the temperature and the emissivity of the tubes.
- Convection: This is the process of transferring heat by the movement of the flue gas and the process fluid. The convection heat transfer is calculated by using the Newton’s law of cooling, which states that the heat transfer between a surface and a fluid is proportional to the difference of their temperatures and the convection heat transfer coefficient. The convection heat transfer coefficient depends on the fluid properties, the fluid velocity, and the surface geometry. The convection heat transfer can be enhanced by increasing the fluid velocity and the convection heat transfer coefficient.
Example of Radiant Section
The following table shows an example of the radiant section of a fired heater that heats crude oil from 100°C to 350°C. The fired heater has 12 burners, each with a heat release of 10 MW. The radiant section has 120 tubes, each with a diameter of 150 mm, a thickness of 5 mm, and a length of 10 m. The tubes are arranged in 10 rows, with 12 tubes per row. The refractory has a thickness of 200 mm and an emissivity of 0.8. The crude oil has a mass flow rate of 100 kg/s, a specific heat of 2.5 kJ/kg.K, and a density of 800 kg/m3. The flue gas has a temperature of 1200°C, a specific heat of 1.2 kJ/kg.K, and a density of 1.2 kg/m3. The convection heat transfer coefficient of the flue gas is 50 W/m2.K, and the convection heat transfer coefficient of the crude oil is 100 W/m2.K. The emissivity of the tubes is 0.6, and the emissivity of the flame is 1.
| Parameter | Value | Unit |
|---|---|---|
| Heat release of burners | 120 | MW |
| Heat transfer by radiation | 108 | MW |
| Heat transfer by convection | 12 | MW |
| Heat absorbed by tubes | 108 | MW |
| Heat absorbed by crude oil | 100 | MW |
| Heat loss by flue gas | 8 | MW |
| Combustion efficiency | 83.3 | % |
| Radiation efficiency | 90 | % |
| Convection efficiency | 10 | % |
| Overall efficiency | 75 | % |
| Flame temperature | 1200 | °C |
| Effective gas temperature | 1000 | °C |
| Tube-wall temperature | 500 | °C |
| Refractory temperature | 800 | °C |
