When crude is extracted from the earth, it needs to be heated to make it suitable for handling. In addition to petroleum, crude contains sand, water and other impurities that need to be removed. Heavy crude is too thick or viscous to be pumped easily. Special heaters are needed to heat the material and reduce its viscosity.
Special-purpose heaters are designed to handle the job of heating heavy crude. For instance, for a major oil company, one heater maker built a convection heater designed for heating substances with high sensitivity to overheating. Designed to heat fluids by means of convection, the convection heater has a natural gas burner that has an output of 36 million BTU/hr. Combustion gases heat the crude as it flows through heating coils in the heater, raising its temperature from 61 to 122°F (16 to 50°C). Crude flows through the heater at a rate of approximately 3,000 gal/min.
The heater has special features to avoid overheating the boundary layer — the portion of crude in contact with walls of the heating coils. (Temperatures at the boundary layer are known as film temperatures while temperatures in the central zone are known as bulk temperatures.) Without these features, the boundary layer would be heated much higher than the portions that flow through the central zone of the coils. Overheating occurs when film temperatures significantly exceed bulk temperatures. Overheating can rapidly carbonize the crude and clog the heating coils.
Two key features prevent overheating:
• The coils are not exposed to radiant energy produced by the burner. Instead, they are heated only by the energy from combustion gases.
• Some of the convection gases are recirculated back to the combustion chamber, where they reduce combustion gas temperatures but increase the volume of gases flowing around the coils for greater heat transfer. Recirculating the gases also increases efficiency of the heater and significantly reduces the amount of NOX that exits the exhaust stack. Thermal efficiencies up to 90 percent (LVH) can be achieved.
As a result, film temperatures of crude at the walls of the heating coils are precisely controlled and do not exceed bulk temperatures more than 5°F (2.7°C). Moreover, the coils are finless, which helps reduce hot spots.
Convection heaters like this crude heater also are used in a number of other industries: chemical, food, wood, automotive and aeronautical. Available in a range of sizes with outputs ranging from 2 to 125 million BTU/hr, they can be used to heat gases, thermal fluids and cryogenic liquids such as liquefied natural gas, liquid ammonia and nitrogen.