3.2.2 Fired Heat
Fired heaters are used for process heating duties above the highest temperatures that can be reached
using high pressure steam, typically about 250 ºC (482 ºF). Process streams may be heated directly
in the furnace tubes, or indirectly using a hot oil circuit or heat transfer fluid, as described in
Section 3.2.4. The design of fired heaters is described in Section 19.17. The cost of fired heat can
be calculated from the price of the fuel fired. Most fired process heaters use natural gas as fuel, as
it is cleaner burning than fuel oil and therefore easier to fit NOx control systems and obtain permits.
Natural gas also requires less maintenance of burners and fuel lines and natural gas burners can
often co-fire process waste streams such as hydrogen, light organic compounds, or air saturated
with organic compounds.
Natural gas and heating oil are traded as commodities and prices can be found at any online trading
site or business news site (e.g., www.cnn.money.com). Historic prices for forecasting can be
found in the Oil and Gas Journal or from the U.S. Energy Information Adminstration (www.eia.gov).
The fuel consumed in a fired heater can be estimated from the fired heater duty divided by the
furnace efficiency. The furnace efficiency will typically be about 0.85 if both the radiant and convective
sections are used (see Chapter 19) and about 0.6 if the process heating is in the radiant section only