Thermal fluid heaters, or hot oil heaters as they commonly are called, are typically electric, gas or oil fired. The operation of the system is straightforward: The heat transfer fluid flowing through a direct-fired heater's coils is circulated through a closed-loop system to all heating equipment in the process. Heat energy then is transferred from the fluid to the various heat consumers before returning to the heater and continuing the cycle.
Among the advantages of thermal fluid heating vs. steam or hot water systems are the elimination of such maintenance concerns as water treatment, steam traps and blowdown systems. Thermal fluid heaters also do not need the added expense of full-time, licensed boiler operators. Thermal fluid is commonly used in liquid phase at atmospheric pressure up to 650oF (343oC) and also in vapor phase, to transfer heat at a uniform temperature, up to 750oF (399oC). A steam system would need to be 1,525 psig to reach just 600oF (316oC). For these reasons and others, the cost of maintaining and operating a thermal fluid heater is significantly less than a steam or hot water system.
Once you have decided that a thermal fluid heating system is the best option for your process heating needs, what should you consider next? You must choose the correct thermal fluid for your specific operating conditions, and your process heat load requirement must be determined (remember to add at least 15 percent as a safety factor to make certain your heater is not undersized). Still, there are many other factors to consider before purchasing any equipment. These tips will help you choose the best heater for your application and ensure a long and trouble-free life for your process heating equipment.
TIP 1: Look for a Compact Design, But Not Too CompactReal estate is at a premium in most plants, and choosing a compact heater design can save room for future expansion of your plant's production capacity. A compact design also saves on shipping and installation costs. Nevertheless, the heater manufacturer needs to be certain the dimensions are not too compact. A heater's combustion chamber must be sized according to the burner's flame dimensions. The length of the heater should be adequate to allow the flame to completely extinguish before reaching the end of the chamber, and the width of the chamber should be sufficiently sized so the flame never flame impinges on the coils. Before purchasing the equipment, you should ask the heater manufacturer if they have considered the flame dimensions in the heater design.
TIP 2: Select an Appropriate Coil ConfigurationMost efficiently designed, direct-fired thermal fluid heaters employ a dual helical coil configuration. Two concentric coils are spaced so that hot gases make three passes over the coils before exiting the heater to the exhaust stack. This configuration allows the thermal fluid to gain ample heat while reducing fuel consumption. It also allows for the best use of the coils in that the useable heating surface area of the coils is large enough to provide a low heat flux into the thermal fluid.
TIP 3: Keep Heat Flux LowThe heat flux is a measure of how much heat energy is transferred through the process coils and thus into the thermal fluid flowing within them. It is calculated by dividing the required energy (heat load) by the surface area of the heater coils. This number should be kept to a minimum because excessive heat flux can damage the fluid, shortening its service life. Although they improve the heater's overall efficiency, combustion air preheaters should not be included in the heat flux equation because they have no impact on the intensity of heat dispensed into the coils.
TIP 4: Maximize System EfficiencyThe firing rate of a burner must be greater than the heat load of the process it is serving. A certain amount of heat will not be transferred to the fluid but instead exhausted through the stack. System efficiency is a measure of how much of the energy supplied through combustion actually is absorbed into the thermal fluid and delivered to the heat consumers in the process.
An efficient heating system can pay for itself quickly. For example, a company can save roughly $30,000 per year simply by employing a 10 million BTU/hr heater with an efficiency of 88 percent rather than an equally sized heater with an efficiency of 80 percent. Throughout the course of a highly efficient heater's service life, it can pay for itself many times over.
TIP 5: Minimize Film Temperature To Extend Fluid LifeDefined as the temperature of the thermal fluid at the inside surface of the coil, film temperature always is higher than the bulk fluid temperature. This is important because thermal degradation of the fluid will be quickest at its highest temperature. To minimize film temperature, fluid velocity through the coils must be great enough to create a highly turbulent flow. Especially in the radiant section of the heater, the Reynolds number -- a measure of a fluid's turbulence -- divided by the coil diameter should never fall below 100,000/in.
The customer must choose the correct thermal fluid based on the maximum film temperature to which the fluid will be exposed. The film temperature should be calculated by the heater manufacturer, who also can recommend a fluid based on this and other parameters. It is not uncommon for a well-designed heater with low film temperatures to operate for ten years without changing the thermal fluid.
TIP 6: Insulate Your Heater to Protect Personnel and EquipmentFor personnel protection, OSHA guidelines should be followed to ensure a heater surface temperature below 130oF (54oC). Usually, the best means of achieving this is via ceramic fiber blanket insulation. The thickness of the ceramic fiber should be based on the specific conditions of your process.
Refractory insulation, which once was widely used in thermal fluid heaters, should be kept to a minimum because it can crack and break off during shipment. Also, it has a dangerous tendency to retain heat. If your plant has a power outage, even after the hot flue gases have evacuated the heater via natural draft from an adequately sized stack, refractory will continue radiating the heat it has absorbed back to the stagnant fluid in the coils. Thermal degradation of the fluid can occur quickly given this situation, significantly lessening the life of the fluid and impairing its heat transfer properties. This will also reduce the overall system efficiency and can damage the heater's coils.
TIP 7: Employ Controls for Safe and Effective Heater OperationThe leading cause of thermal fluid heater failure is insufficient flow through the coils. It is vital to the heating system that the flow rate through the heater coils is proven and measured via an orifice and flow meter. It is not enough to electrically interlock the pump to the control system -- the mechanical portion of a pump could fail without triggering any alarms. If a low flow condition arises, a shutdown of the system must be automatically activated to protect the fluid and heater.
Reliable thermostats must be employed to provide a system shutdown upon high temperature in the coils or the exhaust stack. If the temperature rises above a predetermined level, then something is awry and the system must be shut down and inspected.
A low level switch installed in the expansion tank will detect fluid losses in the system. This level switch should be tied into the control panel so that the system will shut down on its signal.
A dependable burner management system -- consisting of a burner firing rate controller, flame scanner and flame safeguard -- is crucial to a safe and effective heater operation. The burner management system should display a history of any faults that have occurred, and the heater manufacturer should be called in to fix any recurring problems.
TIP 8: Specify Heater To Ensure Code ComplianceThird-party approval is an important aspect in any manufacturing process. It ensures that the highest quality equipment will be installed in your facility. A typical thermal fluid heater should have "U" stamped, A106B coils, and the heater should bear an ASME Section VIII stamp. All electrical work should be UL listed at a minimum. You also will want to remember to tell the heater manufacturer if you require Factory Mutual (FM), Industrial Risk Insurers (IRI), National Electrical Manufacturers Association (NEMA) ratings, American Petroleum Institute (API) or any other third-party guidelines or approvals for your system.
TIP 9: Perform Period Maintenance to Extend Heater LifeA properly designed and installed thermal fluid heating system should give between 20 to 30 years of reliable service, but periodic maintenance still is necessary for safe and effective operation. The thermal fluid should be checked regularly to verify that it has retained its heat transfer properties. Burners and pumps need periodic service to maintain optimum functioning, and other equipment will need to be inspected occasionally as well. The user may want to consider entering into a service contract with the manufacturer or a reliable service agent to perform a heater checkout on a yearly basis.
TIP 10: Look at Heater Purchase as a Long-Term Relationship and Select the Supplier with an Eye Toward Customer ServiceBefore purchasing any equipment, you should check the heater manufacturer's references thoroughly. The manufacturer should have an extensive history of being highly responsive to customers' needs and a track record of solving problems or answering concerns their customers may have.
Taking the time to consider these tips before purchasing your thermal fluid heating equipment will save your company a great deal in operation and maintenance costs in the long run and help you choose the best overall heater for your production.