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Table: Sample Data Sheet for an EPH circulation Heater (This table is not available electronically. Please see the February, 1999 issue.)
Illustration: FIGURE 1. Left, a typical circulation electric process heater
Illustration: FIGURE 2. The heater's temperature increases exponentially
as the process flowrate decreases
Illustration: FIGURE 3. Silicon-controlled rectifier (SCR) control schemes can be used, as well as Stepper SCR schemes. Both are sketched here
Illustration: FIGURE 4. This flowchart breaks EPH sizing down into iterative steps
Illustration: FIGURE 5. Data required for optimum sizing are summarized on this datasheet
Use of electric process heaters (EPHs) is growing, not only in the chemical process industries (CPI), but in areas such as offshore oil production and electric power generation. The devices are smaller and weigh less than traditional heat- transfer equipment such as direct and indirect fired heaters and shell-and-tube heat exchangers. In many cases, they offer better leak and corrosion resistance, and enhance safety and temperature control.
Nevertheless, the heaters can be difficult to size and specify, as they combine aspects of pressure-vessel, heat-exchanger, and rotating-equipment design. In fact, the heater's closest relative is the pump, which also uses electrical energy to change a system's thermodynamics.
Immersion heaters are electric process heaters that are immersed into a process. In these devices, heat transfer is accomplished mainly by conduction and natural convection. Such units can be used, for instance, to keep water from freezing in a tank.
A circulation heater (Figure 1) is one type of immersion heater. With a circulation heater, process fluid, or any combination of gases, liquids and solids, is forced across a heating element. A circulation heater can be used, for example, to heat 10 million ft3/d of fuel gas from 70 to 100 degrees F at an operating pressure of 300 psig.
Circulation heaters convert electrical energy into heat. A properly sized EPH will use forced convection to transfer all the heat generated into the process stream. For high-temperature gas applications, heat can also be transferred by radiation. In this case, heat radiates directly to the gas, but also to the heater vessel. The vessel gets hotter than the gas and transfers heat to the gas via forced convection. EPHs generally are sized assuming...