Thermal liquid heating is a specialized form of process heating that uses the forced circulation of a special liquid heating medium called thermal fluid. In many types of process heating, high-temperature thermal liquid is preferred over high-pressure steam or hot water. Generally, thermal liquid systems can be used at high temperatures without a corresponding high pressure.
In the late 1800s, European processors started using packaged thermal fluid systems, but it was not until the early 1950s that this technology became popular in the United States. Because of the many advantages of these systems, they are becoming even more popular today. In addition to achieving high temperature heat without high pressure, there are many cases where thermal liquid systems are not subject to local inspection, permitting or attendance requirements as would be typical with a high-pressure steam or hot water boiler. In addition, thermal liquid does not require the daily maintenance of water softening, water treatment or blowoffs.
System ComponentsIt is important to understand the system and all its components in order to take advantage of the benefits that thermal fluid systems have to offer.
The Heater. Users will want to select a heater designed only for thermal liquids. An all-welded, bent steel tube or helical coil design allows for the continuous expansion and contraction to which the heater can be subjected to without damage. In some models, double-welded construction eliminates the problems of rolled or poor joining of tubes. This welded type of heater may be preferred by fluid manufacturers because thermal liquids at advanced temperatures are so thin that only the finest welding can contain the fluid without leakage. Because thermal liquid fluid will burn in the presence of a gas flame, leakage could lead to fire.
The heater design also must account for potential thermal liquid shock, proper velocity and flow control to keep heat flux, fluid film temperature and fluid bulk temperature within the proper ranges. Generally, a velocity of 8 to 12 ft/sec is required across the internal tube surface. Due to the higher anticipated heater temperatures, a high-temperature insulated cabinet should be incorporated to improve overall thermal efficiency. Additionally, the tube design must extract an optimum amount of heat from the burners.
Numerous special controls must be employed that are not standard to other boilers. Higher range temperature operating and high limit controls should be provided. As a safety check for proper flow, a pressure control is provided to monitor the pressure differential across the heater. In addition, a current sensor or relay is interlocked to the heater pump motor. These controls are tied to the burner circuit to ensure that the pump is running prior to burner operation. Factory Mutual (FM) trim or other control trim upgrades also are a good idea. Units should be available to burn natural gas, propane, oil and bio gas and be available with low-NOX burners, if required. Full modulation is a preferred firing method for these burners and processes.
Pumps. In closed recirculation hot water systems, pump selection with regard to sizing (for flow rate in gal/min) is based on the desired temperature rise through the heater and the system head pressure. Pump selection in a thermal liquid system is based on velocity or flow across the tube surface and the total system head. Some fluids reduce viscosity by 100 times from 50 centistokes at 100°F (38°C) to 0.5 centistokes at 600°F (316°C). Velocity flow should not be less than 8 ft/sec, which, with 1" pipe or equal as boiler tubing, represents approximately 20 gal/min per tube. Due to the change in viscosity with change in temperature, pump motor horsepower must be selected by considering the cold point and hot point of the system. Air-cooled mechanical seal type pumps are the most common, but fluid-cooled and sealless pumps also are available.
The tank should be mounted at the highest point in the system if it is to be vented to atmospheric to meet certain local code requirements or exemptions. It is always better to have a small amount of an inert gas blanketing the vapor section of this tank: This prolongs the life of fluid and can help with NPSH of pumps in certain situations. It must have this inert gas blanket if it is not the highest point of the system. Provisions should always be made for degassing and venting any high points of the piping system.
Flow Control. Because velocity is so important, continuous flow through the heater is required at all times. This can be as simple as heater and pump arrangement in series with the entire system. In most cases, a three-way diverting valve is installed to ensure flow through the heater even when heat from the loop is not required. A simpler way to ensure consistent flow is a primary/secondary loop. This arrangement normally uses a central heater/system, which is a distribution tank with special baffling that serves as a buffer. This separates the heater pump from the process or system pumping requirements and is recommended when required flow through the heater cannot be guaranteed with a one pump system.
All system piping larger than 1" should be welded. When flanges are required, a high-grade, high-temperature spiral-wound gasket should be used. No petroleum-type pipe dope should be used. All long circulating lines should be protected with expansion joints to prevent damage due to thermal expansion.