Introduction

furnace (fired heater) is a device used to heat up chemicals or chemical mixtures. Fired heaters transfer heat generated by the combustion of natural gas, ethane, propane, or fuel oil. Furnaces consist essentially of a battery of pipes or tubes that pass through a firebox. These tubes run along the inside walls and roof of a furnace. The heat released by the burners is transferred through the tubes and into the process fluid. The fluid remains in the furnace just long enough to reach operating conditions before exiting and being pumped to the processing unit.

Furnaces are used in crude processing, cracking, olefins production, and many other processes. Furnaces heat up raw materials so that they can produce products such as gasoline, oil, kerosene, chemicals, plastic, and rubber. The chemical-processing industry uses a variety of fired heater designs. These elaborate furnace systems can be complicated and equipped with the latest technology.

Heat Transfer
The primary means of heat transfer in a fired heater are radiant heat transfer and convection; however, heat must pass through the walls by conduction to be absorbed by the flowing fluid. In the fired furnace, the flame on the burner is the radiant heat source. Radiant heat transfer takes place primarily in the firebox. Tubes located in the firebox are referred to as radiant coils or tubes. The tubes transfer heat to the fluid by conduction. In a fired furnace, radiant heat is emitted from the combustion of natural gas or light oil. As the radiant heat travels from the bottom of the furnace, contacting the tubes or passing in the furnace, and then continues to the top, heat is transferred to the surrounding air. This process initiates the convective heat transfer process that causes the lighter air and hot combustion gases to rise above the radiant heat source. The top of the furnace is referred to as the convection section because most of the heat it receives is by convection.

Combustion
Combustion is a rapid chemical reaction that occurs when the proper amounts of fuel and oxygen (O2) come into contact with an ignition source and release heat and light. Furnaces use this principle to provide heat. Complete combustion occurs when reactants are ignited in the correct proportions. Incomplete combustion occurs in a fired furnace when not enough oxygen exists to completely convert all of the fuel to water and carbon dioxide. Many furnaces use natural gas or methane (CH4) as fuel for the burners. Methane (CH4) reacts with O2 to form carbon dioxide (CO2) and water (H2O):

CH4 + 2O2 → CO2 + 2H2O

Incomplete combustion may result in the production of carbon monoxide. The chemical processing industry also uses ethane, propane, and light oils for fuel.

Fuel Heat Value
Different fuels release different amounts of heat energy as they are burned. The heat energy released, referred to as the heat value, is measured in British thermal units per cubic foot. The British thermal unit (Btu) is a measurement of heat energy. One Btu is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.Hydrogen has the lowest fuel heat value (274 Btu/foot3), whereas natural gas, or methane, has a heat value of 909 Btu/foot3. Charts are available that list the heating values of fuels used in furnaces. It is important to realize that the more Btus a fuel gives off, the more oxygen is required for
combustion.

References

1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas