Hazardous Area Electric Heater Selection Guide

INTRODUCTION

“Explosion-proof electric heater” is a common commercial term. In technical sections, this guide uses “Ex heater” or “equipment for explosive atmospheres” where more precise wording is helpful.

An Ex heater should never be selected only from power and voltage.

A complete selection starts with two groups of information:

1. Process duty — medium, flow, temperature, pressure, viscosity or gas properties, heating power and allowable pressure drop.
2. Hazardous-area duty — area classification, gas or vapor group, temperature class, ambient temperature, required protection concept, certification scheme and installation requirements.

Both groups must be reviewed together because heater surface temperature, terminal enclosure, sensors, cable entries, control architecture and shutdown logic can all affect the final design.

1. Define the Hazardous Area

The project should provide the site hazardous-area classification according to the applicable code or standard.

Typical project information may include:

• Zone or other area classification
• required Equipment Protection Level (EPL), where applicable
• gas / vapor group
• temperature class
• ambient-temperature range
• gas or vapor identity
• installation location
• indoor / outdoor condition
• dust classification if combustible dust is also present

Area classification should normally be determined by the plant owner, EPC, qualified hazardous-area engineer or authority responsible for the installation.

2. Confirm the Heating Medium

The heater may be used for:

• clean air
• process gas
• nitrogen
• compressed air
• hydrocarbon gas
• water
• thermal oil
• crude oil
• chemical liquid
• tank heating
• duct heating
• circulation heating

The heating medium affects element sheath material, watt density, flow protection, pressure design, temperature control and fouling risk.

3. Confirm Process Conditions

Provide:

• normal and maximum flow
• inlet temperature
• required outlet temperature
• working pressure
• design pressure
• allowable pressure drop
• medium composition
• viscosity where relevant
• corrosive or fouling constituents
• required heating power or process heat duty

For gas and liquid heaters, the process design must be completed together with the hazardous-area design.

4. Heater Type

Common arrangements include:

Inline pressure-bearing heater:

For closed gas or liquid pipelines.

Duct heater:

For process air or ventilation ducts where pressure is relatively low.

Flanged immersion heater:

For tanks and vessels.

Circulation heater:

For recirculating liquids or thermal fluids.

Skid-mounted package:

For projects requiring heater, pump, piping, valves, instruments and controls as one integrated unit.

5. Protection Concept and Ex Construction

The final protection concept depends on the required hazardous-area standard and equipment architecture.

Examples in IEC 60079-based systems can include flameproof enclosure protection “d”, increased safety “e”, intrinsic safety “i”, pressurization “p” and other permitted concepts.

For electric heaters, the heater terminal box, sensors, junction boxes, cable entry devices / cable glands and control equipment must all be reviewed according to their location and function.

Do not assume that one explosion-protection technique applies automatically to every component.

6. Temperature Class Is a Design Constraint

Temperature class is not the process outlet-medium temperature and is not the internal resistance-wire temperature.

The relevant maximum surface temperature of Ex equipment must remain within the permitted limit for the specified temperature class under the required operating and fault conditions defined by the applicable standard and certification.

The thermal design therefore needs to consider:

• element sheath temperature
• heater body or exposed surface temperature
• terminal-box temperature
• local hot spots
• ambient temperature
• flow loss
• temperature-control failure
• insulation and heat retention

7. Control and Safety Functions

Depending on the application, protection can include:

• independent high-temperature cutout
• outlet-temperature control
• element or sheath temperature monitoring
• low-flow trip
• low-liquid-level trip
• pump interlock
• pressure monitoring
• differential-pressure monitoring
• fan permissive
• emergency stop
• manual reset after safety trip where required
• PLC / DCS interlock

The safety philosophy should be agreed before final design.

8. Control Cabinet Location

A practical design question is whether the main control cabinet is located:

• inside the hazardous area
• in a safe area
• in a pressurized enclosure
• in a remote electrical room

Placing standard power-control equipment in a safe area can simplify certification and maintenance, but the final architecture depends on the project.

Field devices installed in the hazardous area must match the applicable certification and protection requirements.

9. Certification Route

The customer should identify the required certification scheme before order.

Examples may include:

• China-market explosion-protection certification requirements, including CCC where applicable
• IECEx certification
• ATEX conformity for products placed on the EU market
• project-specific third-party approval
• marine or offshore requirements where applicable

IECEx and ATEX are related to equipment for explosive atmospheres but are different conformity frameworks. IECEx is an international conformity-assessment system, while ATEX Directive 2014/34/EU is EU product legislation for equipment and protective systems intended for use in potentially explosive atmospheres. One should not be represented as automatically replacing the other.

10. Reference Standards

Projects based on IEC 60079 may refer to standards such as:

• IEC 60079-0 — General requirements
• IEC 60079-1 — Flameproof enclosures “d”
• IEC 60079-10-1 — Classification of areas — explosive gas atmospheres
• IEC 60079-14 — Electrical installations design, selection and erection

The applicable edition, national adoption and project specification should be confirmed for each project. The list above is illustrative rather than a claim that every project requires every part.

11. RFQ Checklist

Provide:

1. Heating medium and composition
2. Flow
3. Inlet and outlet temperature
4. Working and design pressure
5. Allowable pressure drop
6. Required heating power
7. Voltage, phase and frequency
8. Hazardous-area classification
9. Gas group
10. Temperature class
11. Ambient-temperature range
12. Required certification scheme
13. Indoor / outdoor installation
14. Control-cabinet location
15. Required alarms and trips
16. Customer PLC / DCS interface
17. Required documentation
18. Supply boundary

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Engineering Note

This series is for technical explanation and preliminary engineering reference. Hazardous-area classification, protection concept, certification route, temperature class, installation requirements and final equipment selection must be confirmed from the actual project, applicable code, authority having jurisdiction and agreed certification scope. A certificate issued under one national or regional scheme should not be assumed to satisfy another scheme automatically.