A Fired Heater is used for heat generation by combusting of oilFired Heater or gas or a mixture of them. Fluids coming in the heater will be heated without changing of state (input/output at liquid state or gaseous state) or with vaporization already started at heater entrance (mixed phase) and which will increase during the process inside the heater. Heaters used in industry for steam generation and petroleum refining are known as fired heater, process heaters, furnaces or process furnaces. Fired heaters are used throughout hydrocarbon and chemical processing industries such as refineries, gas plants, petrochemicals, chemicals and synthetics, olefins, ammonia and fertilizer plants. There are some special heaters where chemical reactions occurs, for example steam reforming heaters or cracking furnaces.
The common type of fired heaters are the following:
- cylindrical vertical. The cylindrical vertical type are usually used up to about 20-25 million of Kcal/h, cabin type or box type for higher power.
BOX TYPE: This type of heater are usually composed of one or more chambers with rectangular section, the tubes are usually in horizontal position arranged on one on more rows along the wall of the section. Burners can be located on the floor or on the wall of the radiant section. Flue gases run through the convection section and exit from the stack in the atmosphere.
CABIN TYPE: These type of heaters have usually a radiant section in the lower part and a convection section in the upper one. Fluid to be heated run through the convection section and then through the radiant one, to have an higher thermal head. Coil are composed of many tubes in parallel and connected with manifold at the end. Burners can be located on the floor with vertical axis in one or more rows or they can be located on the walls with horizontal axis. Tubes in convection section have triangular pitch.
CYLINDRICAL VERTICAL: These type of heaters are usually used for quite low power. In this case they can be cheaper then other solutions. These heaters can be “all radiant” type or they can have both radiant and convection section. In the first case the coil is composed of vertical tubes located around a circle, at the base of the heater there are the burners. The heater with radiant and convection section can reach higher power and allow higher efficiency. The convection section, which has horizontal axis, is located above the radiant section. In the convection section, the first rows of tubes are smooth and they are called “shield”. Process fluid run through the convection section and then in the radiant one by the “crossover”.
Conventional fired heaters designs include both radiant and convection sections. In certain fired heater designs known as the “all radiant” type, there is no separate convection section. Where heaters employ both radiant and convection sections, it is conventional to mount the convection coil above the radiant coil and in the path of the flue gas exhausting from the combustion chamber.
The radiant tubes, either horizontal or vertical, are located along the walls in the radiant section of the heater and receive radiant heat directly from the burners or target wall. Flue gas heat is transferred by radiance and conduction and in minor quantity by convection through the tube wall. The radiant zone with its refractory lining is the costliest part of the heater and most of the heat is gained there. This is also called the firebox.
This section is usually located in the upper part of the heater. Flue gas heat is transferred on the tubes wall by conduction. The tubes are usually finned type or studded type. Flue gases which transfer radiant heat, should go out the heater very hot. But in this way the efficiency will be reduced. To recover this part of heat, the tubes are put in contact with the flue gas before they exiting from the stack. In this way the heat in the flue gas are removed by convection. Too much heat picked up in the convection section is a sign of too much draft. Tube temperature is taken in both convection and radiant sections.
Just below the convection section is the shield (or shocktube) section, containing rows of tubing which shield the convection tubes from the direct radiant heat. Several important measurements are normally made just below the shield section. The bridgewall or breakwall temperature is the temperature of the flue gas after the radiant heat is removed by the radiant tubes and before it hits the convection section. Measurement of the draft at this point is also very important since this determines how well the heater is set up. This is also the ideal place for flue gas oxygen and ppm (parts per million) combustibles measurement.
Stack and Breeching
The transition from the convection section to the stack is called the breeching. By the time the flue gas exits the stack, most of the heat should be recovered and the temperature is much less. From a measurement point of view, this location places fewer demands on the analyzer but is much less desirable for the ability to control the process. Measurement of stack emissions for compliance purposes is normally made here.