Introduction

Industrial furnace is very important to achieve better processing temperatures than the open-air system. It also deals with temperatures higher than 4000C. It can be either used to deliver heat for processes or can be a reactor for reactions to happen. It generates heat either by mixing fuel with air or from electrical energy. The excess heat from the furnace will exit as flue gases. Furnaces are used in the hydrocarbon and chemical industries such as refiners, gas plants, petrochemicals, and fertilizer plants. In refinery, furnaces are used in some units such as: distillation, fluidized catalytic cracking (FCC) and catalytic reforming.


Categories of furnaces

1- Direct fired heaters: Combustion gases take most of furnace volume and heat the stream in pipes.
2- Fire tube heaters: There is a fire tube surrounds by a liquid that fills the heater shell and heats the stream in the coils.

Different types of direct fired heater

1- Cylindrical: It requires small area for a given duty.
2- Cabin: it can be side-firing or end-firing burners instead of vertical firing and thus, the floor of the heater will be close to the ground.
Coil arrangements: it can be horizontal, vertical, or helical.

Furnace mechanical components



1- Radiant section: heat is transferred to the tubes by radiation.
2- Arch: flat or slopped portion of the radiant section opposite to the floor.
3- Convection section: heat is transferred to the tubes by convection.
4- Crossover: connecting piping between coil sections.
5- Breeching: it is a section where flue gases are collected after the last convection coil for transmission to the stack.
6- Stack: vertical conduit used to discharge flue gas to the atmosphere.
7- Damper: a device in the stack to introduce resistance to the flow of the flue gas.
8- Burner: device that introduces fuel and air into a heater for ignition and combustion.


9- Pilot: small burner provides ignition energy to the main burner.
10- Terminal: flanged or welded connection to or from the coil providing for inlet and outlet of fluids.
11- Pass/stream: flow circuit of one or more tubes in series.

Two of the most important parts of the furnace are:

Convection section:



1- Shield section/shock section: tubes that shield the remaining convection section tubes from direct radiation.
2- Extended surface: fins attached to the heat-absorbing surface.
3- Tube support/tube sheet: device used to support tubes.
4- Header (return bend): cast fitting used to connect two or more tubes.
5- Header box: insulated structure separated from the flue-gas stream which is used to enclose several headers.

Radiation section:

The radiant tubes are either horizontal or vertical along the walls to receive radiant heat from burners. It is the costliest part of the furnace and 85% of the heat is gained. It is called a fire box.


References:

- TUSLA HEATERS MIDSTREAM, (Beginner Guide to Fired Heaters).
- Kolmetz, Karl. (REFINERY SELECTION, SIZING AND TROUBLE SHOOTING: Kolmetz Handbook of Process Equipment Design). KLM Technology Group: West Malaysia, 2017.
- Journal of physics: Conference series, (ANALYSIS AND REDUCTION OF HEAT TRANSFER IN A FURNACE OF REFINERY), (Gunasekaran N, Shanmugaraja M, Bharathwaaj R, Shilpha Dharshini J, Vivishnu Kumar M and Yogesh Kumar P S),2020.
- AMEIEK (PROCESS INSTRUMENTS), (Process Heaters, Furnaces and Fired Heaters), USA:2006.
- Geoff Barker, The Engineer’s Guide to Plant Layout and Piping Design for the Oil and Gas Industries, 2018.