Chapter 10 - Furnaces
Including
Feed composition
Indirect fired
Fire-tube heaters are indirect fired, containing the combustion gases in tubes that occupy a small percentage of the overall volume of the heater.
Basic Components of a Furnace
Fired heaters come in a variety of shapes and sizes.
Flow Control
Flow is controlled upstream of the furnace.
Forced-Draft Process Heater
Forced-draft furnaces utilize a centrifugal blower to push preheated air to the burner for combustion.
Draft
Furnace draft can be natural, forced, induced, or balanced.
Direct Fired and Indirect Fired
Furnaces are classified as direct fired or indirect fired
Types of soot blowers:
Wall blowers Finned tube blowers
Direct fired
the combustion gases typically fill the interior. Ex. Cabin, cylindrical, box, A-frame furnaces
Fired heaters
transfer heat generated by the combustion of natural gas, ethane, propane, or fuel oil.
Forced-draft furnaces
use a fan to push fresh air to burners for combustion.
Box Furnace
A box furnace takes its name from the square or rectangular design of heating unit.
Cylindrical Furnace
A direct-fired heater design used by industry.
Cabin Furnace
A very popular direct-fired heated used in the chemical-processing industry for large commercial operations.
Burners
Burners can be arranged on the floor or the lower walls of the firebox
Stack Damper
Combustion gases leave the furnace through the stack and are dispersed into the atmosphere at a height to ensure against any effect such as carbon monoxide poisoning.
Combustion
Combustion is a rapid chemical reaction that occurs when proper amounts of fuel and oxygen come into contact with an ignition source and release heat and light.
Fuel Heat Value
Different fuels release different amounts of heat energy
Preheating the Charge
During cabin furnace operation, the initial charge is pumped through a shell-and-tube heat exchanger to heat the fluid before it is sent to the furnace.
Referred as:
East West North South pass
Applications
Furnaces heat up raw materials so products can be produced such as gasoline, oil, kerosene, chemicals, plastics, and rubber.
Furnaces
Heat released by the burners is transferred through the tubes and into the process fluid.
Balanced-draft furnaces requires two fans:
One inducing flow out the stack; One providing positive pressure to the burners
Soot Blower
Soot blowers are devices found in the convection section of process heaters
Bridgewall Section
The bridgewall section is the sloping section of the upper furnace that connects the radiant section to the convection section.
Number of Passes
The charge or flow entering a furnace is often split into two or more flows called passes.
Fuel System
The fuel is stored in a tank located a safe distance from the furnace
Heat Transfer
The primary means of heat transfer in a fired heater are radiant heat transfer (via burners) and convection,
Firebox and Refractory Layer
The section in a furnace that contains the burners and open flames is called the firebox which is lined with a refractory layer (bricks) that reflect heat back into the furnace.
Radiant And Convection Tubes
The tubes located along the walls of the firebox are called the radiant tubes or coils.
Furnaces can be classified by several features:
Type of draft Number of fireboxes Number of passes Volume occupied by combustion gases Shape
Applications
Used in crude processing, cracking, olefins production, and chemical processing
A furnace or fired heater
a device used to heat up chemicals or chemical mixtures.
Induced-draft furnace
a fan located below the stack pulls air up through the firebox and out the stack.
Natural-draft furnace,
buoyancy forces induce draft as the hot air rises through the stack and creates a negative pressure inside the firebox.
Furnaces
consist essentially of a battery of pipes or tubes that run along the sides and roof of the firebox.