Thermal Fluid Systems May Be Simple and Straightforward in Their Design

Thermal fluid systems provide an efficient source of heat when temperatures as high as 750°F (399°C) are required. Depending on the application's requirements, thermal fluid systems may be simple and straightforward in their design — a single heater providing heat to one user at a constant flow rate. Or, they may be complex and flexible — a hot oil system may be designed to heat multiple users at various temperatures, for both heating and cooling. To design the most efficient system for an application, heating and cooling requirements must be determined. Then, it is critical to establish whether thermal fluid is the only medium required or if the processes also require hot water or steam. Provide Temperature Control The most basic system includes a thermal fluid heater providing heat to a single user at a constant flow rate. While simple in design, this arrangement provides minimal temperature control. Modulating thermal fluid control valves may be added to the system to control the flow of fluid to the user. But, remember that when flow to the user is controlled, all users will be subjected to the same supply temperature. Either two-way or three-way modulating control valves may be employed. It is important to remember that unless they are manually isolated, three-way valves require thermal fluid flow at all times. This must be considered when selecting the pump and sizing the piping. Using two-way control valves offers flexibility if all of the potential users are not operating simultaneously. However, when using two-way control valves in a system, it is critical to design the system such that the minimum flow through the heater is achieved at all times, regardless of the position of the control valves. Backpressure relief valves (mechanical or pneumatic) may be used for this purpose. Within the modulation range of the burner, most thermal fluid heaters can control temperature to ±5°F (±2.7°C). If the heater cycles off, the system could lose up to 50°F (28°C), depending on the system size, quality of the insulation etc. If tighter temperature control is required, a primary/secondary loop system may be employed. With the primary loop operating 25 to 50°F (13 to 28°C) above the required secondary loop temperatures, even if the heater cycles off, temperature control of ±2°F (± 1.1°C) may be achieved. Also, the use of primary/secondary loop systems also allows multiple users to operate simultaneously at different hot oil temperatures. A temperature control unit (TCU) is a modified version of a primary/secondary loop system. TCUs provide aqueous or non-aqueous heating and, if needed, cooling for batch or continuous processes. TCUs may utilize central hot (thermal fluid or steam) or cold loops (cooling towers or glycol), or they may be self contained. Heat sources include thermal fluid, electricity and steam while cooling media includes water, glycol or nitrogen. Temperature control units provide tight temperature control to ±2°F (± 1.1°C) with repeatable results. They often are used in chemical and pharmaceutical processing. TCUs also can be utilized as part of a larger, more complex hot oil system. Achieve Heating and Cooling Hot oil systems typically are considered for heating-only applications. However, when properly designed, a hot oil system can be used for both heating and cooling. Cooling media may be city water, cooling tower water, glycol or another thermal fluid. Shell-and-tube or plate-and-frame heat exchangers may be used to cool the hot oil. Heat exchangers must be designed appropriately for the application, taking into consideration the material, temperature and pressure compatibility with the fluids used. The simplest heat/cool system includes a single user, three-way control valve and the heat exchanger (cooler). This approach is best for small systems as the entire system volume will be either heated or cooled. For systems with many users operating at a single temperature, a heat/cool system with a dedicated heating loop and dedicated cooling loop may be utilized. This system allows for simultaneous heating/cooling; however, all the hot and cold supply temperatures are the same for each user. For multiple users with different heat/cool profiles, coolers may be incorporated into a primary/secondary loop system. This system provides the most flexibility as users may be controlled at different temperatures, and heating may occur for some users while others are cooled. While offering the most flexible design, this system will be the most costly. Add in Hot Water and Steam Generation In addition to hot oil requirements, many applications include requirements for steam or hot water as part of the process. Examples of processes requiring multiple media include food processing, waste treatment and industrial laundries. Thermal fluid may be used to generate steam and hot water through the use of temperature control valves and heat exchangers. Shell-and-tube heat exchangers are the most common for this application; however, properly selected plate-and-frame units also may be used. Heat exchangers must be designed considering the media used, pressure, temperature, material requirements and all applicable American Society of Mechanical Engineers (ASME) calculations and testing. Using thermal fluid to generate steam or hot water eliminates the need for a separate boiler, with its associated fuel lines and stack requirements. If the steam or hot water usage is not simultaneously required with the hot oil users, a larger thermal fluid heater may not be necessary. Steam and hot water generators can be added to existing hot oil systems. Final Considerations Hot oil systems may be either liquid phase or vapor phase. In a liquid-phase system, the hot oil remains in the liquid phase throughout the entire system. In a vapor-phase system, the thermal fluid is liquid while in the heater. At the heater outlet, the fluid flows into a flash tank and is flashed to vapor. The vapor then is used as the heat transfer media to heat the users. While giving up its heat to the process, the vapor condenses to liquid, and the condensate then is returned to the main loop to begin the cycle again. Liquid-phase systems are common in food processing, chemical processing, asphalt heating, tank heating and industrial laundries. Vapor-phase systems are most common in chemical processing applications. A vapor-phase system can provide uniform temperature control as the heat is transferred at the saturation temperature, whereas a liquid phase system has a temperature difference between the fluid inlet and outlet. Before designing a thermal fluid system, it is critical to consider all process requirements. When these requirements are taken into consideration and the system is properly designed, a single hot oil system may be capable of meeting heating, cooling, steam and hot water design needs. Source: process-heating.com