Gas Technology: Thermal fluid could be a better way
Taking the pressure off
Thermal fluid heaters use a liquid-phase heat transfer medium to deliver heat energy for industrial processes. Thermal oil, glycol, or even water are all possible heat mediums. Fluid is heated and circulated to process points using a closed loop system. Thermal oil systems are especially attractive because of their ability to deliver fluid at temperatures up to 750° F without the need for a high-pressure design such as is required with a steam or hot water boiler.
With pumped fluid heat transfer using oil, risks of scale formation, corrosion and frost are avoidable. Thermal fluid heaters are typically easier to operate and maintain. Also, the inherent simplicity and a wide range of heat recovery options make it possible to operate such systems at peak energy efficiency. Because these systems are generally not highly pressurized, it is usually not necessary to have a licensed operator on duty. Heaters are built with reliable automatic safety devices that allow unattended operation. Because the volume of liquid is relatively small, they can be brought up to temperature quickly after an overnight shutdown period.
Thermal fluid heaters use an energy source to heat the fluid in a horizontal or vertical tank. Natural gas is often the fuel of choice because of its clean burning characteristics and attractive price. Units are available in a wide range of sizes, from 500,000 Btu/hour to 40 MMBtu/hour. Even larger process outputs are possible with multiple units. The working fluid is usually a high-temperature oil with a low oxidation characteristic and high heat transfer capability. In the unit design, it is important to keep the heated oil moving to avoid oxidation at the heat transfer point.
The heater itself is typically a gas-fired heat exchanger, with the circulating fluid passing through enclosed helical or serpentine coils in the combustion area. The heater is equipped with redundant temperature and pressure relays to prevent over-heating and to assure accurate delivery of the working fluid at the set temperature.
Oils for Specific Temperatures and Applications
Thermal oils of both mineral and synthetic origin are available with a range of thermal and physical characteristics from a variety of suppliers. The fluid used must be free of suspended solids, non-toxic in the case of leaks, and stable at temperatures at least 100 degrees above the planned operating temperatures. Manufacturers of thermal fluid heaters recommend periodic analysis of the oil to assure that it is not oxidizing or picking up any contaminants. Most offer analysis services.
Because of pumping, the oil is pressurized and piping must be specified to meet this operating pressure. Heaters are equipped with systems to remove both air and moisture from the circulating fluid. Most units are completely factory-assembled and need only to be connected to a fuel supply (for example, a gas train), to exhaust venting, and to the thermal oil distribution system.
Wide Range of Markets
Thermal fluid heaters have found applications in a variety of markets, from asphalt processing to plastics manufacturing, from food and paper processing to the pharmaceutical industry. Ideal applications are where there is a need for controlled thermal input from a sealed system. One leader in fluid heaters is Fulton Thermal Corporation, headquartered in Pulaski, New York. Fulton offers both horizontal and vertical tank units in a wide range of sizes.
Mike Roberts from Fulton indicates that thermal fluid heaters have thermal and combustion efficiencies about the same as similar-sized boilers. But he adds, “Where you see a gain is in the system efficiencies. With a thermal system, you don’t lose efficiency by blowing down hot water or wasting heat through steam traps like you do with a steam system.”
Heat recovery can add to the efficiency of systems. Typically this is achieved by use of an economizer on the heater exhaust train, with captured heat used to pre-heat returning fluid or combustion air. Fluid pre-heating can also be accomplished with other industrial energy sources such as boiler exhausts or product cooling processes.
Roberts also points to another advantage, “With a thermal system, maintenance is much less.” For owners this can translate to reduced downtime, longer production runs, reduced labor costs, and lower process startup costs. Roberts points out, “Some maintenance is required, just like any other fuel-fired piece of equipment. You have to maintain the combustion system. But you don’t have to maintain water treatment equipment, or any steam-side components. You still have required maintenance to keep the system going.” Regarding system life, Roberts indicate that well-maintained systems can have a life of 30+ years.
Whether yours is a new industrial facility, or you are replacing an older steam, hot water or thermal system, it is important to correctly specify your needed thermal range, and the total process heat requirement. Manufacturers offer factory assistance in selecting the right size and type of system. Thermal fluid systems offer simplicity, reduced maintenance, and the ability to avoid the regulatory requirements of a steam plant. Now might be the time to consider this option.
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