Gas or Oil Heater ?
Process heaters may burn gas, oil, or both. The choice depends on several factors, including the source reliability, operator attention, and response time.
Process gas (fuel gas) has always been preferred because it is easier to handle, burns cleaner and is less expensive. Many locations use some natural gas. However, most natural gas contracts provide that industrial gas can be cut off on short notice whenever the demand by domestic users exceeds the supply. Consequently, those locations have some provisions for backup oil burning in selectedgas is curtailed.
Steam boilers are often chosen for supplemental oil firing when the local facilities are subject to possible natural gas curtailment because they are normally attended, generally use larger quantities of fuel, and operate more nearly at constant load. If more units have to be switched to oil, the process heaters usually selected are large users in less severe services with steady loads, like atmospheric crude heaters.
Design of oil fired heaters is more complicated than that for gas fired heaters:
• The oil requires separate oil guns and steam for atomization, along with separate controls.
• The flame for a given heat release is longer and of a less predictable shape than the flame in a gas fired heater, which affects firebox geometry. For example, when heating a temperature-sensitive process fluid with gas, we often specify a clearance of 5 feet between burner centerline and nearest tube surface. With oil the figure is 6 feet.
• Because oil burning produces ash and soot, design of the heater’s convection section should include provision for tube cleaning by onstream water washing or soot blowing.
• Choice of refractory material may be affected due to chemical attack from components in the oil. See a specialist for more information.
• Because of the soot produced with oil, the maximum number of fins per inch of convection tubes is only three (maximum is five with gas). This affects thermal efficiency and draft.
• Based on operating experience, the design excess air with gas fuel is 10%; with oil, 15%. This also affects the draft requirement.
• Fuel oil has a higher sulfur content than gas. This may affect the minimum temperature allowed for steel surfaces in direct contact with flue gas. The concern is sulfuric acid condensation on metal surfaces, which may present corrosion and air pollution problems. The table below tells you the minimum required steel surface temperature (not flue gas temperature) which is necessary to prevent acid condensation. The temperature varies depending on the sulfur content of fuel (measured in percent of weight).
<1% 300°F <0.1% 275°F • The vanadium often present in fuel oil may cause severe corrosion of hot metal parts like tube supports. • Oil guns foul quickly, requiring frequent attention. • Oil firing requires much more maintenance time and operator attention.