Direct-fired combustion equipment is that in which the flame and/or products of combustion are used to achieve the desired result by radiation and convection. Common examples include rotary kilns and open-hearth furnaces. Indirect-fired combustion equipment is that in which the flame and products of combustion are separated from any contact with the principal material in the process by metallic or refractory walls. Examples are steam boilers, vaporizers, heat exchangers, and melting pots.
The heat exchangers previously discussed rely on convection and conduction for heat transfer. In a fired heater, such as shown in Figure 3-19. radiation plays a major role in heat transfer. The process fluid flows through tubes around a flame. These tubes receive most of the heat directly by radiation from the flame. A small amount of heat is also received by convection from the air between the tubes and flame. Heaters are not common in most field installations but are much more commonly used in plant situations in which competent operators routinely main tain and inspect the equipment.
For safety reasons, heaters are most often used to heat a heat medium system (water, steam, or heat transfer fluid) rather than to heat the gas or oil stream directly. If the fluid to be heated contains hydrocarbons, the heater can be located safely away from other equipment. If it catches on fire, damage can be limited.
The tubes that are around the flame get most of their heat energy from radiation. The tubes in the top of the chamber get their heat from convection as the hot exhaust gases rise up through the heater and heat the process fluid in the tubes. The principal classification of fired heaters relates to the orientation of the heating coil in the radiant section. The tube coils of vertical fired heaters are placed vertically along the walls of the combustion chamber. Firing also occurs vertically from the floor of the heater. All the tubes are subjected to radiant energy.
minimum of plot area. Typical duties run from 0.5 to 200 MMBtu/hr. Six types of vertical-tube-fired heaters are shown in Figure 3-20.
The radiant section tube coils of horizontal fired heaters are arranged horizontally so as to line the sidewalls and the roof of the combustion chamber. In addition, there is a convection section of tube coils, which are positioned as a horizontal bank of tubes above the combustion chamber. Normally the tubes are fired vertically from the floor, but they can also be fired horizontally by side wall mounted burners located below the tube coil. This economical, high efficiency design currently represents the majority of new horizontal-tube-fired heater installations. Duties run from 5 to 250 MMBtu/hr. Six types of horizontal-tube-fired heaters are shown in Figure 3-21.