Challenges in Design and Engineering of Electric Heaters
Ritabrata Pramanik
Mechanical Engineering
Fluor Daniel India Pvt. Ltd.
Srinath NR
Mechanical Engineering
Fluor Daniel India Pvt. Ltd.

This article discusses the basic design and engineering issues of Electric heaters which, if addressed properly in early stages can minimize the changes during detailed engineering stage.

Electric heaters are used in the Process Industries as an alternative to fired heaters or process exchangers for some specific applications. When specified and used properly, electric heaters will last for many years without problems. However, unlike shell and tube heat exchangers, the design and engineering of Electric Heaters are carried out by the suppliers. This fact and the lack of international standards and specifications pose challenges to detailed engineering contractors in selection and detailed design of these heaters. More over Electric heater is just not a piece of heat transfer equipment with pressure components; they are equipment with electric terminal box, electrical control panel and transformer in some cases. Hence other than supplier and Process engineers interface with Electrical and Instrument engineers are also required to design and engineer these heaters.

Abbreviations:
1. ASME: American Society of Mechanical Engineers
2. DEC: Detailed Engineering Contractor
3. NEMA: National Electrical Manufacturers Association
4. IEC: International Electrotechnical Commission
5. MAP: Maximum Allowable Pressure
6. MAWP: Maximum Allowable Working Pressure
7. FEA: Finite Element Analysis

Basic Heater Configuration:

Electric heaters are used in applications where the process duties are low but fluids are heated to high temperature. Configuration is generally specified by Process Licensors mainly based on one of the following two factors:
  • The heat required to bring the process up to the desired temperature
  • The heat required to maintain the process at a particular operating temperature.
In both these cases, the heater bundle is inserted inside a shell, more like a shell and tube exchanger with the fluid being heated placed in the shell side and heating element acting like tubes.. In former case the fluid will flow through the shell with certain velocity to attain the required process temperature, whereas in latter case the fluid is just stored in the shell to maintain the process operating temperature. The mechanical design aspect of the heater in the former case is more critical and will be discussed in detail in this article.

Electric Heater Construction:

For the given duty the number of heater bundles is selected based on allowable watt density of element, which in turn is dependent on the properties of the fluid being heated. The heater element material is selected based on factors like maximum fluid temperature, fluid properties etc. Both the material of heating element and the allowable watt density are generally specified by the Process licensor. In Hydrocarbon Industries watt density varies between 4650 W/m2 to 34000 W/ m2 where various forms of hydrocarbons and hydrogen are the most commonly heated fluids. The heaters are oriented either in horizontal (Fig.1) or in vertical (Fig.2) position. Vertical orientation is preferred in services with very high operating process temperature where the heaters are required to be supported on spring supports to avoid transfer of high loads on heater supports and connected piping system due to higher heater shell expansion.

Components of Electric Heaters:

Heater bundle: This is a non pressure part and comprises of seamless tubular electric heating elements, rod baffles, heat shield baffles, cooling baffle and electric terminal box.
• Heating element: The material of construction for heating element varies from steel to nickel based alloys. In hydrocarbon industries, generally with high temperature application, heating elements are made of 80 Ni - 20 Cr alloy heating wire packed with magnesium oxide insulation enclosed in a tubular element sheath. Some portion of the heating element will be ineffective, known as unheated element length, which will be occupying the space from the heater flange face to the nearest tangent line of the welding tee opening, as shown in Fig.1 and Fig.2.











Figure 1: XElectric heater oriented horizontally



























Figure 2: Electric heater oriented vertically

  • Rod baffle: Unlike a conventional shell and tube exchangers, where the flow of fluid inside the shell is a combination of co-current/ counter current and cross flow, the flow inside the electric heater shell is longitudinal. Hence the tubular elements inside the heater shell are supported on rod baffles.
  • Heat shield baffle: A minimum two full solid heat shield baffles (as shown in Fig 1 & 2) will be provided in unheated section to reduce the heat loss through the heater flange.
  • Cooling baffle: A cooling baffle outside the heater flange is provided to avoid heat transfer to heater terminal box, thereby preventing the terminal box from getting heated.
  • Electric terminal box: An electric terminal box is provided on one end of heater bundle and inbuilt duplex type thermocouples will be welded on element sheath to measure and control the temperature of heater bundle.
Heater tube sheet and Shell: These are the pressure components of Electric heaters. All the heating elements are welded to the heater tube sheet. Heater bundle along with the tube sheet is inserted into the heater shell. Heater shell has a body flange on one end where the tube sheet of the heater bundle is bolted and another end is connected to piping system. The process fluid flows longitudinally over the heater elements inside the shell from cold end to the hot end of the shell. Tee connections are required for interconnecting the heater vessels in case of multiple bundles.

Power Control Panel: This is required to provide control power supply to the heater. Tripping of heater during any abnormal condition such as ground fault, over temperature, etc is controlled by power control panel.

Transformer: Step down transformer is required for the high duty, low voltage heaters. In case of high duty heaters the required current ampere will be higher. Hence medium voltage supply will be provided to avoid bigger cable size and loss of current. However, since there are very few Suppliers who supply medium voltage heaters, in many cases we may have to go for low voltage heaters with the step down transformer.

Challenges in design and engineering of heater components:

The basic configuration and orientation is generally specified by Process Licensor much before the start of detailed engineering and is finalized by the heater supplier when the detailed engineering is at its peak. In order to avoid changes in heater configuration at late stage of engineering, it is advisable to have a detailed review of the following aspects of design and downstream engineering by the detailed engineering contractor prior to design freeze.

Selection of watt density: Since the selection of heater configuration is largely dependent on watt density, this needs to be reviewed when specified in process datasheet. If not specified, the DEC should ask the Process Licensor to specify the same.

Orientation of heater orientation: The heater orientation should be finalized after discussion with Process Licensor and Piping before issuing enquiry to bidders to avoid unnecessary changes at a later stage with adverse impact on cost and schedule.

Optimum voltage level of the heating element: The higher is the voltage level, lower is the size of the electric heater. However, not all electric heater suppliers have capability to manufacture heater with high voltage level. Hence DEC needs to carry out a detailed study to check the commercial viability of heater with high voltage, with limit suppliers, vis-à-vis low voltage heater with additional step down transformer but with more number of suppliers.

Proper specification of Power Control Panel: Generally heater power control panels are located away from the hazardous area, either indoor or outdoor. However sometimes the control panel needs to be located outside, under a hazardous area, due to lack of space inside Electrical & Instruments room. Since the price of power control panel, suitable for hazardous area, will be much higher due to requirement of additional safety protection when compared to panel suitable for non hazardous area, a prior study in consultation with Electrical group regarding the pros and cons before specifying the requirement in the enquiry document to avoid changes during later stage. For example, if the hazardous area classification is class 1, zone 2, then the panel shall be pressurized by air purging and panel enclosure require closed loop cooling system or outside heat sink to put out the heat produced inside the panel, making the panel bigger and costlier. The enquiry document shall also specify the following in order to avoid unnecessary changes at a later stage with adverse impact on cost and schedule
  • The minimum and maximum design temperatures of the panel. In many cases the minimum temperature will become critical design due to issue in digital display at temperature below 0°C.
  • Control panel enclosure type in accordance with NEMA or IEC standards, suitable for the installation location.
  • The width and swing angle of enclosure access doors based on available layout.
  • Required spare quantities and
  • spare component mounting space inside the panel.
Selection of Design Code: Since the purpose of the Electric Heater is to heat up the process fluid and not merely to transport the process fluid, Pressure Vessel design code is a more appropriate code for mechanical design of the heater. In addition, ASME Code Section VIII Div.1 contains mandatory appendix 41 that is specific for the design of Electric Heaters tube sheet. DEC needs to check whether any mechanical design code has been specified by the Process Licensor and resolve the design code issue with the Process Licensor in case piping design code is specified instead as per Pressure Vessel design code.

Calculation of MAP and MAWP: Electric Heater suppliers tend to consider design pressure as the MAP and MAWP in order to avoid engineering effort to calculate MAP & MAWP as per design code. DEC shall clearly specify the requirement to calculate MAP & MAWP as per applicable design code, in case the Project Design Basis calls for it.

Tube sheet design: Electric heater tube sheet is generally designed as per Appendix 41 of ASME Sec VIII Div.1.[a] DEC shall specify the complete applicable design conditions for tube sheet design, including that of full vacuum, if any. Generally heater supplier will adopt tube sheet thickness based on standard blind flange and perform strength calculation per appendix 41 for internal pressure at design temperature to check the adequacy of adopted thickness. Hence in scenario where full vacuum design condition is applicable then same shall be specifically included for tube sheet design also to avoid changes during later stage.

Shell design: Equipment Vendors generally design electric heater shells as per ASME Sec VIII Div.1. Structural strength of shell shall be adequate for system pressure, for supporting heater(s) and for applicable piping loads. The shell strength check for heater support reaction loads and piping loads are performed by FEA method. Stress analysis of shell shall be carried out for following types of piping loads,
1. Sustained load- Operating: Weight (operating) + thermal load (operating)
2. Sustained load- Design: Weight (operating) + thermal load (design)
3. Occasional load: Thermal (operating) + weight (operating) + wind loads.

With the above load combinations, in the calculated stresses may exceed the allowable stress of the heater shell with standard wall thickness. Hence to ensure an optimum design, only the following combinations sustained and occasional load cases of may be adopted:
  • Sustained load-Operating: Weight (operating) + thermal load (operating), since the nozzles will "see" only the operating temperatures.
  • Occasional load: Thermal (operating) + weight (operating) + wind loads.
Electrical and thermocouple terminal box: In hydrocarbon processing industries heaters area are classified as hazardous area. Hence these terminal boxes shall be designed for hazardous area. Example terminal box are rated for NEMA 4X and IP6 as per NEMA and IEC standards respectively are suitable for class 1, zone 2 area. Since there will be a minimum of two duplex thermocouples for each bundle, and in general each thermocouple requires separate junction box, the purchase order shall specify the minimum required junction box properly.

Step down transformer: As explained earlier this is required only in application where the high voltage supply is provided to low voltage heater. Since transformers are located nearer or within the heater skid, the hazardous area classification will be applicable. Generally NEMA 3R rated transformers are suitable for hazardous area - class1, zone 2. An additional weather proof kit may be necessary for Dry type transformers for protection against blowing snow in cold countries.

Heater Hydro test: If designed as per ASME Sec VIII Div 1, hydro test pressure for heater will be as per UG-99 of ASME Sec VIII Div.1, which requires hydro test pressure to be 1.3 x MAWP with temperature correction.[a] However, in specific application, where the heater is connected to piping system by butt welded joints (Fig.2), and no “golden joint” is allowed as per local code, the applicable site hydro test pressure need to be as per ASME B31.3, which requires the hydro test pressure for the butt joints to be 1.5 x Design Pressure with temperature correction.[b] In this case, the heater pressure parts needs to withstand this hydro test pressure without exceeding 90% of yield strength.[b] This needs to be specified in the requisition document to avoid unnecessary changes at a later stage with adverse impact on cost and schedule. It is good practice to hydro test piping system to 1.5 x Flange rating pressure (after ensuring that heater and connected piping can take the stress) to account for possible re rating of piping system in the future.

Conclusion:

These days since the end users are expecting more cost competitive and schedule driven projects, detailed engineering contractors are expected to correctly and completely specify all the technical requirements in the requisition document to avoid changes at a later stage which may affect the project cost and schedule. A close coordination among various engineering disciplines, Process Licensor, Client and Suppliers can help the DEC for early finalization of the selection and design of the electric heater and minimize changes during the detailed engineering stage.

References:

1. ASME Boiler and Pressure Vessel Code (Section VIII, Division 1)
2. Process Piping - ASME Code for Pressure Piping (ASME B31.3)