Many processes today require an indirect method of heating, which implies the use of a heat transfer medium instead of circulating the process directly into a heater. Thermal oil, water, glycol mixtures and steam are the most commonly used heat mediums for indirect heating. Thermal oil often is preferred over steam for high temperature applications because there is less maintenance such as water conditioning and blowdown systems. Steam processes require higher pressures when compared to oil systems at comparable temperatures as well as more ongoing regulatory oversight.
Thermal oil heaters typically are used in a non-pressurized, closed-loop system. Primary components include a pump, heater, piping and an expansion tank. Many factors govern the size and selection of the heater and associated equipment. The following guidelines will help navigate sizing and selection, which can be a challenging and time-consuming task.
1. Know Your Industrial Process Heating Application
Knowing your overall process will help in determining many factors regarding the thermal fluid heating system. Jacketed tanks and reactors typically have lower pressure drops than extruders or presses. Shell-and-tube or plate-type heat exchanger manufacturers might have already assumed a certain pressure drop for the unit you purchased. This pressure drop is instrumental in selecting the proper pump and motor.
Knowing how much thermal fluid is in the process and interconnecting piping will help determine the size of the expansion tank. If you run a batch process that sees regular temperature cycling like an injection-molding machine, multi-opening press or jacketed tank or reactor, the head load of the wetted parts within your machinery will need to be considered in addition to the product within when making the overall heat transfer calculation. The initial heatup of the process may dictate the heater size rather than the actual steady-state process. For continuous applications for which you need to reach temperature quickly and maintain that temperature, a margin should be applied to the steady-state heat load to make sure you can reach that operating temperature quickly.
2. Know Your Heat Transfer Fluid
Several types of heat transfer fluids are available, and each has its own particular set of advantages and disadvantages. If you consider only the heat transfer properties, water is an ideal heat transfer medium, but it will obviously build pressure as the temperature increases beyond the boiling point. Thermal oils offer high temperature capability at low pressures. They include synthetics or aromatics, petroleum-based fluids and silicones. Molten salt has become popular as an ultra-high temperature heat transfer fluid, offering operation up to 1,000°F (538°C). Not all thermal fluids will be suited to your process. Different fluids will have different boiling points, flashpoints and operating temperatures. It is important to know the vapor pressure of the fluid because this will determine whether there is a need for a pressurized expansion tank. Maximum film temperatures must be considered because the fluid flows over the heating elements. The fluid you select will be a significant investment. The time spent selecting that fluid will be good insurance to protect that investment.
3. Know Your Piping
Your piping layout is critical to making sure your system works properly. It is important in determining the amount of head to overcome for pump sizing. Smaller piping will decrease cost but it will cause more pressure drop, thus increasing the motor horsepower and pump size. Knowing the highest point of piping is important because the expansion tank will need to be elevated above this point or pressurized. If this is not considered, this location will form air pockets at shutdown and possibly overflow the expansion tank. It also is important that drain valves be located at piping low points and vents at piping high points. When installing and testing the piping for a hot oil system, it is important that water not be used or, if used, completely removed to avoid excessive cook-out times -
4. Selecting the Pump for Thermal Fluid Heating
Once the process, fluid and piping are known, pump selection can begin. The pump must overcome the friction losses in the piping, the pressure drop through the process equipment and the pressure drop through the heater. Other factors to be considered are restriction orifices, valves or other instrumentation.
Flow rate is the next important decision to make. Fluid velocity across the heating elements is critical to ensure the thermal fluid does not degrade due to overheating and cracking or, in the case of water, possibly flash. To ensure that you have the optimal velocity inside the heater vessel, you should let your heater manufacturer determine the required flow rate for the system. The flow rate to the user can be adjusted and varied using a control or bypass valve.
The two typical types of pumps are positive displacement (sometimes called gear pumps) and centrifugal. Gear pumps are good for low flow (i.e., below 50 gal/min) but have slippage around the gears in high temperature oil applications due to reduced surface tension. This causes lower flow rates. The spacing between the gears and the pump housing is small, and they are typically different materials. Due to the difference in thermal expansion, gear pumps should be heated gradually to prevent seizing of the pump.
Centrifugal pumps are more flexible and are recommended for thermal fluid heating systems. Many types of centrifugal pumps on the market are suitable for hot oil, water or water-glycol systems. These include air-cooled mechanically sealed pumps, sealless pumps such as canned and mag-drive pumps, and packed pumps. Packed pumps are not recommended for thermal fluid heating systems due to leaking. It should be noted that only a small number of application-specific pumps can be used for molten salt, and only a vendor that sells a pump specifically for this service should be chosen.
5. Choosing the Heater
Heater selection is one of the most important steps in the thermal fluid heating system. Things to consider are heater watt density, velocity through the heater, heater orientation and control method. The watt density of the electric heater is critical to protect the oil from degradation. Typical watt densities of heaters are 30 W/in2 for water/glycols; 20 W/in2 for lower temperature hot oil systems; and 12 to 15 W/in2 for higher temperature hot oil units and heat transfer salts. Smaller diameter heaters are preferred due to the increased velocity over the heating elements. Typically, a 1:1 ratio between flow measured in gallons per minute and kilowatts will result in approximately 15°F (8.3°C) temperature rise through the heater.
Horizontal heaters tend to be self-venting while vertical heaters require an additional method to vent. Always vent a liquid heater from the top to ensure the heater is flooded at all times. Always include a temperature sensor on the top element of a thermal fluid system for overtemperature protection. Ensure that you have allowed space for pulling and inspecting the heater bundle periodically. When cooling is required, do not bypass the heater immediately. Always ensure there is adequate flow through the heater to remove residual heat from the internal elements before attempting to bypass flow. Care must be taken to protect the heater during an accidental blocked-in condition. A pressure safety valve should be included to prevent overpressure of the heater caused by thermal expansion of the thermal fluid. Heater pressure safety valves typically are sized for thermal relief.
6. Specifying Heater Controls
Heater controls are just as important as heater selection in a thermal fluid heating system. Thyristor control is preferred over on/off control to allow the heater to run at lower temperatures and maintain tight control. Always keep the overtemperature safety system separate from the control system. Use dedicated safety contactors to provide positive shutoff of the power to the heater. This ensures a failsafe method of protection.
Typical controls should be capable of receiving a remote setpoint from the user control system, and they also should be able to transmit the process variable to the user control system. The control panel typically is mounted on the heater and pump skid and prewired to them. For hazardous areas, the panel can be mounted in a remote, safe location. Ensure the pump is interlocked to the heater so that the heater will not energize if the pump is not running.
7. Know the Types of Insulation
Many types of insulation are available on the market but not all of them are suitable for thermal fluid system service. Fiberglass insulation is easy and inexpensive to install for water and water-glycol applications. For thermal oil applications, fiberglass should be avoided as it creates a fire hazard. Instead, closed-cell insulation such as foam glass should always be used. Ensure there is enough insulation on the heater and piping to prevent injuries to personnel. OSHA typically recommends a surface temperature limit of 140°F (60°C) although you should verify with your local authority or facility to be certain. Insulation also reduces heat losses to the atmosphere and preserves system efficiency. Even if personnel safety is not in question, your equipment and piping should still be insulated.
8. Account for Liquid Expansion in Thermal Fluid Heating System Design
As the thermal fluid is heated, it will expand or possibly even contract. Water and water-glycol have minimal expansion — typically 10 percent or less. Thermal oil can expand as much as 30 to 40 percent; this will vary widely based on the oil type and manufacturer. Molten salt actually contracts as it melts and experiences little expansion as it continues to heat.
An expansion tank must be provided in thermal oil, water and water-glycol systems to store the expanded fluid and prevent overpressure of the system. Ensure there is a liquid-level switch installed in the tank and interlocked to the pump and heater for protection. A level gauge is recommended to provide visual inspection of the fluid level. Many expansion tanks are vented to the atmosphere. At higher temperatures, it is recommended that a nitrogen blanket be installed on the tank to prevent oxidation of the thermal oils. It also is necessary to pressurize the system with nitrogen when the bottom of the expansion tank is not at the highest point of elevation in the system piping. It is recommended that the expansion tank be designed and built to ASME Section VIII, Division 1, but the ultimate decision is at the discretion of the facility owner.
9. Know Your Area Classification
Electric thermal fluid heating systems may be installed in hazardous and non-hazardous areas. It is the facility owner’s responsibility to determine the area classification based on the plant safety design considerations. NEMA 4 or IP56 heater terminal housings are well suited for Class I, Division 2 or Zone 2 locations. Temperature controls are required to be purged with dry instrument air or NEMA 7. For IEC applications, the panel must be manufactured per Ex “p” or Ex “d.” Ensure there is a third-party certification by a nationally recognized testing laboratory (NRTL).
10. Know Your Startup Options
The commissioning and startup of your new thermal fluid heating system should be taken seriously. The system cannot simply be turned on and operated without proper commissioning. Care must be taken to remove air from water and water-glycol systems to prevent pump cavitation and potential damage. Air, steam and contaminants must be cooked out of hot oil systems in a systematic and cautious way to prevent system overpressure and potential spills of hot oil. Molten salt is in solid form during startup, and these systems are even more complex to commission. Insist that your vendor provides this service, and be sure they are providing a service technician that has significant experience in commissioning thermal fluid heating systems.