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Heat Transfer Fundamentals
 
  INDIRECT HEATING OF TANKS

  FLUIDE & LIQUID HEATING

The heating of tanks is a common industrial practice in many applications. Heating may be required to maintain pumping viscosity of heavy oil or resins, to prevent crystalline precipitation (sodium hydroxide), to facilitate production processes, for freeze protection, and hundreds of other applications. MATRUSREE offers literally hundreds of ways of heating tanks of all sizes. Included in this guide is an overview of the types of tank heating approaches. The manual is not meant to be a complete guide to each of these heating methods, rather an overview of what is available. Once a specific method is selected, further investigation should be made into the suitability of the heater for the application. Finally, the manual is a detailed guide on sizing heaters for large tanks. This guide includes sizing details that are beyond the scope of the sizing guide presented in the general catalog.
  INDIRECT HEATING OF TANKS
Indirect heating uses a heat transfer medium to transfer the heat to the tank. Indirect methods can vary from external heating of the tank using the tank wall as the heating medium to utilizing a heat transfer medium to carry the heat to the tank. In addition, pipe insert heaters have been included in this category because they use an air space between the element and the process to convey the heat. There are various advantages to indirect heating. The biggest advantage is that the heater can typically be serviced without draining the tank. Second, indirect heating often allows watt density exposed to the process fluid to be lowered by spreading the heat over a larger surface. Finally, overheat conditions can be limited in many instances by simply limiting the temperature of the heat transfer medium. There are a few minor disadvantages to indirect heating that may be critical to your process. The primary disadvantage is the thermal lag caused by using a heat transfer medium to carry the heat. The delay is caused by the fact that the heater must first heat the heat transfer medium before the heat transfer medium can heat the process. If there is a large mass of heat transfer medium, larger heating capacities will be required to raise temperatures.
  INDIRECT HEATING USING HEAT TRANSFER SYSTEMS
Heat transfer systems are utilized in indirect heating by using a heat exchanger near or in the tank. The heat Transfer fluid is circulated to the tank by the heat transfer system where the heat exchanger transfers the heat from the fluid to the process. The heat exchanger may consist of a jacketed vessel or a heat exchanger immersed directly into the process fluid. The main advantage to this heating approach is the process fluid is never exposed to temperatures higher than the heat transfer fluid temperature. This is important in fluids that could be damaged by high temperatures. A common jacketed vessel application is the heating of chocolate. Chocolate, when melted, is very viscous and susceptible to damage from even moderately high temperatures. Melting and heating chocolate indirectly with a water bath or jacketed vessel is normally recommended. The water bath allows for the use of a higher watt density heater to heat the water and subsequently a smaller heater without subjecting the chocolate to the relatively high element temperatures normally associated with high watt densities in a viscous liquid. Therefore, by using a double boiler or jacketed vessel approach the process is protected from high temperatures. Finally, this approach is useful when the process may not be compatible with a standard heater. Instead of designing a custom heater that will withstand the harsh process, a heat transfer fluid is used to transfer the heat from a standard heat transfer system.
  INDIRECT HEATING USING BOILERS
Boilers are used as heat transfer systems for indirect heating. Instead of a liquid, steam is used to transfer the energy to the process. Again, a heat exchanger is used in the process to transfer the energy from the steam to the process. This may be in the form of a jacketed vessel or a heat exchanger directly in the tank (see Figures 14-16). The advantage of steam is it can transfer large amounts of energy to the process efficiently. The Disadvantage of steam is the higher cost of piping materials required by the high pressure associated with higher temperatures. Operation above 200C can become costly due to the associated piping materials required to operate at high pressure. However, when operating below these temperatures the boiler may be the most cost effective approach. The steam heat exchanger can typically be sized smaller than an equivalent liquid heat exchanger due to the higher efficiency of condensing steam to transfer energy to the process.
Temperature vs. pressure. Note the high pressure required to achieve 200oC
Typical boiler installation showing required accessories for a complete boiler heat transfer system
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