Series TRO, TSO and TSOL use a dielectric material to support the coiled heating elements, and the heater tube is pressed to increase thermal conductivity. Used for oil preheating, oil baths, wax and other heat transfer fluids, oil immersion heaters provide years of trouble-free service when the tubes are completely covered with fluid, used on correct voltage and kept free of carbonization. With watt densities of 16 to 22 W/in2, they are designed for use with heat transfer oil and have a maximum operating temperature of 550°F (288°C). Heaters with lower watt densities are designed for use with Bunker C fuel oil preheaters and line and suction heaters with a maximum operating temperature of 250°F (121°C). All are constructed with steel heads and tubing.