Control Systems
Fluide & Liquid Heating
Heater Selection
Thermal System Basics
Heat Transfer Fundamentals

Direct heating places the heater in direct contact with the heated medium. The heating element is immersed in the process fluid utilizing various mounting styles. The advantage of heating directly is that the heaters very efficient with this method. This is because all heat that is generated is absorbed directly by the process. This helps to speed heat-up and eliminate thermal lag. There is no intermediate heat transfer medium that could result in heat losses. The disadvantages of direct heating include the element surface limitation of the heater to deliver the energy. Large surface areas require more space for the heater. If the tank is small, there may not be room for a properly sized heater. Additionally, consideration of the heater material must be made to insure that the element is compatible and will not degrade due to corrosion or pitting from the process. Because of the efficient heat transfer with directly immersed heaters, the relative watt density is typically high in these applications. Therefore, the heater must be designed so that it is not exposed to air while operating which could lead to heater failure due to high element temperatures. Finally, the element must be protected from sludge build up in the tank that could limit the elements ability to transfer the heat. Figure 2 is an example of immersion heater mounting and demonstrates installation mounting position above sludge line and below fluid line. A review of direct heating methods with the application pictures are shown throughout the next sections.

Screw plug immersion heaters are typically applied in small tanks or reservoirs requiring relatively small amounts of heat. Many of the tanks are open top style and are used in the finishing industry or industrial process tanks. Many screw plug heaters include a built-in mechanical thermostat, which can often control the heater without any additional equipment. MATRUSREE, however, typically recommends the use of an over temperature cut out and/or level control.

Typical screw plug immersion heater installations
  • Locate Heater as low as possible for maximum heated liquid storage capacity Heat does not move downwards
  • Mounting a screw plug heater to the tank
Because screw plugs have limited element space flanged heaters are used for larger wattage applications. Flanged immersion heaters are typically applied in large tanks or where process requirements dictate high wattage. Flange heaters provide a high amount of wattage in a relatively small space because of the large amount of heating elements that can fit into the flange. Flanged heaters require an appropriately sized nozzle in the tank and typically cannot be removed without draining the tank. The size of the nozzle will limit how many elements can be put in the tank due to the flange size. A smaller nozzle will require a longer heated length or multiple heaters if the watt density is to be held constant. Nozzle size must always be considered when sizing a flanged immersion heater application.
       Mounting a flanged Immersion Heater
 Flange heater mounted
on each endof a hot
water storage tank for
an Efficient shower
system Heater
Typical flanged immersion heater installations
Over-The-Side immersion heaters provide heating solutions for tanks without openings in the side of the tank for insertion of a heater. They also are beneficial in small tanks or where heater portability is required. They are typically applied as an after market modification to the tank. Another advantage to the Over-The-Side approach is the ability to remove or install the heater without draining the tank. Finally, large tanks where there is only a manhole cover at the top available for heater access are ideal for Over-The-Side style heaters with manhole construction like the deep tank heaters.
Circulation heaters are utilized in direct heating applications. The process fluid is circulated directly through the heater. The reason you would choose a circulation heater over an immersion heater directly installed in the tank is either due to space limitations or due to watt density limitations requiring high fluid velocity over the elements to increase heat transfer. Furthermore, if the circulation heater is piped and valved properly, the heater may be serviced without draining the tank. The circulation of the process fluid is provided by a pump or natural convection. In a pumping design, the process fluid is pumped from the bottom of the tank, through a strainer, the pump discharges through the heater, and the fluid is returned to the top of the tank. In convection tank heating applications, the natural convection of fluid is used to circulate the fluid through the heater. This approach is often referred to as side-arm heating. This design requires careful consideration of viscosity and watt density to prevent damage to heater or fluid due to low-flow conditions. Another type of tank heating that uses a similar approach to side-arm heating is vaporization. This is applied on low boiling point applications such as ammonia. The heater is mounted side arm style low enough on the tank to remain constantly flooded and the natural convection draws the fluid through the heater. The heater adds the energy consumed by the vaporization of the fluid. Again, careful consideration of watt density must be considered. Additionally, level control should be installed to cut the heater out on fluid low-level conditions. Finally, a circulation heater may be used to heat the fluid on demand as it is drawn from the tank. This method allows the tank to be maintained at lower temperatures or even remain at ambient temperature.
There are many approaches to pipe insert heaters utilizing various heaters. A pipe insert heater uses an element inserted into a sealed pipe. The advantage to the pipe insert heater is that the element is isolated from the process. This allows the element to be removed without draining the tank, isolates hazardous or corrosive materials from deteriorating the element, and if sized properly may allow the heat to be distributed over a larger surface area reducing the watt density exposed to the process. Consideration of element expansion both in length and in the element supports on the inner diameter must be considered when designing a pipe insert heater. The internal heaters of pipe insert heaters consist of screw plugs, flanged heaters, or open coil elements (OCE) inserted into a pipe. Pipe insert heaters must have a separate temperature controller mounted outside of the pipe to regulate the process temperature. An over temperature device should be installed and attached to the top of the pipe at the highest point. Packaged Systems offer a control panel with the necessary switchgear and temperature controls along with the heaters.
Heat transfer systems can be designed to heat tanks directly. The best example ofthis application is freeze protection or temperature maintenance of large tanks. In this example, the fluid is drawn directly from the bottom of the tank, through the heat transfer system and pumped back into the top of the tank. Consideration must be given to the fluid being heated and the process conditions so that the pump and heater may be properly specified for the application. In this approach, the process tank must be maintained at levels high enough to provide sufficient static head. Additionally, the depth of the tank must be maintained high enough over the outlet nozzle to prevent vortexing of the fluid and drawing air into the heat transfer system. Two feet of depth in the tank is recommended for every foot per second of velocity in the suction pipe exiting the tank. Finally, the tank should be vented to prevent high pressure from damaging the heat transfer system.
Some tank heating applications can benefit from the use of suction heating assemblies. They consist of a flanged immersion heating unit mounted in a heating chamber with one end open so the liquid may be heated as it is pumped from the tank. This eliminates the need to maintain the total storage tank at process or pumping temperature. This simple design results in less piping required and lower installation costs over a circulation heater design. Suction heaters are generally employed for heavy fuel oil and other viscous mediums. Capacity sizing is based on the pumping rate and temperature rise required to obtain proper pumping viscosity. Suction assemblies have the added advantage, using proper valving and valve accessories, of allowing the removal of the flanged immersion heater for maintenance without draining the tank. If the material is highly viscous, the piping between the heater and the pump may need to be heat traced to maintain the temperature of the Figure 12: suction heater piping and installation configuration process fluid at pumping temperatures in periods of downtime.
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