Important Information When Designing a Circulation Heater

Indeeco electric circulation heaters offer an efficient, economical means of heating many processes liquids like water heating and oil heating or air heating or gas heating. Careful attention to heating requirements is essential to ensure that the heater we design will provide optimal performance for your specific application and offer years of efficient operation.

Selecting the proper watt density, or rate of heat output per unit surface area of heating element, is critical to both the life of the heater and the integrity of the fluid being heated. Watt densities that are too high, may carbonize or break down chemically, or the heater elements may burn out prematurely from not being able to dissipate heat properly. 

Standard heaters are designed to meet the needs of a myriad of generalized applications and have their place in certain situations. However, the nature of most circulation heater applications requires a customized solution.  Indeeco circulation heaters can range from heating chocolate to high temperature gases to cryogenic fluids, which require more than simple search for an acceptable heater watt density from tables of values for generalized applications.  Or to fit the proper material needs for the process fluid. A custom heater design and specification take into consideration your application’s unique combination of parameters that are critical in determining a cost-effective and safe heater design. 

APPLICATION ANALYSIS

The specification process begins with a well-defined picture of the heater’s intended use. At a minimum, our sales and design engineers would need the design parameters below.

• Process Media
• Flow rate – minimum & maximum 
• Operating pressure of a gas
• Temperature in and out of the circulation heater
• NEMA Rating
• Voltage
• Area classification, if needed
• Materials
• Controls
  • Hi-limit thermocouple
  • Process thermocouple
  • SCR or Contactor
• Any space constraints

Each application has its own unique combination of process parameters that should be used to estimate the operating temperature of the electric heating element sheaths during normal operation. Using the proper element sheath temperature will maximize the life of the heating elements and help prevent possible degradation of the fluid being heated. Other parameters include thermal fluid properties such as maximum allowable heater element sheath temperature, specific heat, thermal conductivity, viscosity and coefficient of thermal expansion are all critical pieces of information that should be accounted for when designing a new circulation heater.

HEATER DESIGN

Knowing the heater design specifications, we can also consider the physical orientation of the circulation heater as well as the fluid flow direction and fluid flow type during its evaluation. Together, these characteristics dictate the interworking relationship of forced and natural convection, which is the processes by which heat is transferred via the movement of the heated fluid. Fluid flow type can be laminar, turbulent or transition. With laminar flow, the fluid travels smoothly or in regular paths, and fluid velocity, pressure and other flow properties remain constant. The preferred or optimal flow path is turbulent flow.  This occurs when a fluid undergoes irregular fluctuations or mixing. The fluid’s speed at any point is continuously undergoing changes in both magnitude and direction. Transition flow is the largely indeterminate region between the turbulent and laminar flow patterns. As the design analysis moves between laminar and turbulent flow, the more prominent mode of heat transfer transitions from natural convection to forced convection. Natural convection occurs as a fluid is heated and its mass per unit volume (density) decreases. Thus, the hotter, lighter fluid rises while the colder, heavier fluid sinks. Forced convection is achieved by putting the fluid between a differential pressure; thus, forcing motion to occur according to the laws of fluid mechanics. These two modes of heat transfer can work either in unison or opposition. Of course, the most effective heat transfer occurs when they work together such as in a vertically oriented vessel with vertical upward flow (figure 2). Because laminar flow requires considerable length to become fully developed and typical electric heat exchangers are relatively short with element bundle-support spacers that interrupt the flow, pure laminar flow is rare. Extensive empirical data exists to support fully turbulent and fully developed laminar flow regimes as well as pure natural convection. However, in most real-world applications, one mode of heat transfer will dominate or the resultant mode of heat transfer will be a combination of the types. Properly understanding and applying these heat transfer phenomena can dramatically affect which watt density you select for your circulation heater application. For example, in higher temperature gas applications, the acceptable watt density can be greatly improved by designing the heater’s geometry (i.e., shell diameter and baffling types) to ensure that the flow regime within the heater vessel is in the turbulent region.

HEATER OPTIMIZATION

With the widely accepted use of computer-aided design tools in today’s engineering, the complicated iterations involved in evaluating criteria for circulation heater selection have been greatly facilitated. Individual applications can be quickly and accurately assessed using multiple scenarios to determine the optimal design options based upon your current and future heating needs. For example, we can simulate a seamless transition from no-flow to high-flow conditions that accounts for forced convection in laminar, transition and turbulent flow regimes in addition to natural convection. 

Geometry also plays an important role in heat transfer, as Vertical and horizontal installations have different natural convection heat transfer characteristics. Computer-aided design tools can account for vessel orientation and flow direction. Computer-aided design programs also assist in the calculation of pressure drop, which generally acts inversely to heat transfer, for a given geometry. In gas applications, heater vessel shell temperature calculations take into account the additional heat radiating from the heating elements, which can be extremely dangerous if overlooked and have high-pressure effects on gas properties (heat capacity, thermal conductivity and viscosity). 

Employing computer aided design programs to manipulate the heater’s physical design and optimize these critical design parameters will provide the best engineered and most economical circulation heater for your specific application.

Contact any of Thermal Products sales engineers to discuss your application today!