Also known as hot oil (or sometimes Dowtherm) systems, a thermal fluid heater is an industrial heating system wherein a special heat transfer liquid is recirculated by a pump through a fired (or electric) heat exchanger, where the temperature is raised for use in heating various processes. Usually used in the range from 200 to 750 oF (93 to 399oC), thermal fluid heaters typically are used in applications such as:
The main advantage of a thermal fluid heating system is that it runs at a low pressure -- typically just the pressure required to pump the liquid around the system. When designed correctly, they reliably and efficiently produce high temperature heat. They are simple to maintain with only the burner and pump needing regular maintenance.
Disadvantages include the fact that the heat transfer fluids used are flammable and have a low viscosity, so they are prone to leakage. The fluids also can break down (thermally and by oxidation) in poorly designed systems, causing deposits and sludges that hurt the system performance.
How Does it Work?The heater has cylindrical coils through which the recirculating fluid passes. A gas- or oil-fired burner fires into the center of the coil system, heating up the fluid. Typically, the fluid temperature rises 20 to 100oF (11 and 55oC) between the heater inlet and outlet. Some designs can have as many as three-passes in the flue gas arrangement to maximize efficiency. Typically, the burner model and fuel type are selected and the application engineered to the exact needs of the particular heater.
For applications up to 600oF (316oC), air-cooled mechanical seal pumps may be used. For higher temperatures, water-cooled or magnetically coupled pumps can be used. A pump strainer, or coarse mesh screen, is used to stop solids larger than 0.125" from entering and possibly damaging the pump. A sidestream filter with a much finer screen is sometimes used for systems that have small solids resulting from fluid breakdown in circulation.
A degasser, or a specially designed tank located at the pump suction, helps to separate any air or vapors from the circulating fluid and vents them up to the expansion tank.
Although there are no special requirements for isolation valves, valves with a leakproof and maintenance-free metal bellows sealing arrangement may be recommended because the initially higher cost is offset by the reduced maintenance requirements.
All thermal fluid manufacturers publish data for the volume expansion of their heat transfer fluid from ambient to the system operating temperature. Typically, the heater manufacturer calculates the initial system volume and applies this expansion factor, then adds an allowance for minimum and maximum tank fill and selects the expansion tank size accordingly. PH
SIDEBAR: Troubleshooting TipsTo ensure safe, effective operation of your thermal fluid heater, it should be maintained by well-trained personnel. Provide them with access to the supplier's equipment manuals and post the supplier's contact information on the equipment so they know who to contact with questions. In addition, a preventive maintenance plan that includes normal readings of temperature, pressure, flow and other process variables as well as comparison of those readings with regular operating conditions should be in place. Beyond that, remember to:
Clive Stone is president and T.J. Morris is national sales manager at Thermal Fluid Systems Inc., Kennesaw, Ga., a manufacturer of thermal fluid heating systems. For more information about TFS's heaters, call (770) 425-5556; e-mail firstname.lastname@example.org; or visit www.tfsheat.com.