Decarbonisation Technology August 2022 issue

Figure 2 Heater bundle

housing protects the electrical connections. Temperature monitoring of one or several hairpin elements is typically done with a resistance temperature detector (RTD) sensor. Other arrangements exist, such as duct heaters, indirect heaters, and various special designs. For example, an air or gas heater installed within a process duct in low-pressure high-flow applications could also be considered an EPH. The heater bundle can be contained in a fitted pressure vessel for maximum fluid velocity across the elements, or inserted into a tank to keep the process warm or to vaporise liquids. Sizes range from a few kilowatts to a few megawatts. Most engineers are familiar with shell-and-tube exchangers, but the heat transfer of an EPH is different. A heat exchanger’s maximum temperature is determined by the hot inlet fluid temperature, whereas the maximum temperature of an EPH is determined by the heat transfer coefficient. EPH applications also can be used for high- temperature heating as the heater elements can easily operate above 1200°F/650°C, and higher, for gas applications. At bundle temperatures over 1000°F/540°C, the internal wall of the pressure vessel will rise above the process gas temperature and essentially double the heat transfer area. There are also similarities, such as higher pressure drop gives better heat transfer. A good viewpoint of EPH heat transfer is that the process cools the heater elements, so higher flow allows for higher watt density. Optimising the EPH bundle into smaller diameter and using full silicon-controlled rectifier (SCR) control allows for higher heat flux. However, if on/

off control is used, the bundle temperature will increase with a decrease in flow. The advantage of full SCR control is that the sheath temperature of the EPH will drop with less power applied (see Figure 3 ). Three basic EPH calculations determine power, film temperature, and pressure drop: • Power is a function of mass flow, specific heat, and delta temperature • Sheath temperature is a function of four physical properties (density, specific heat, thermal conductivity, and viscosity), the Reynolds number, and the heat flux (watt density) • Pressure drop is a function of density, velocity, pressure vessel, and heater bundle geometry. In the 1990s, the largest EPHs were 100’s of kilowatts (KW), while by the 2000s, the largest EPHs had increased to 1000’s of KW, and now we are seeing 10’s of megawatts per EPH. These sizes are required for decarbonisation processes, such as keeping

Flow rate 10 to 100 percent Sheath temp - On/o Sheath temp - Full SCR Pressure drop Power

Figure 3 EPH bundle temperature vs flow rate