Explosion-Proof Heater Gas Group, Temperature Class and Surface Temperature Guide

INTRODUCTION

Gas group and temperature class are two separate parts of hazardous-area equipment selection.

Gas group relates to the ignition characteristics and severity of the explosive gas or vapor atmosphere.

Temperature class limits the maximum permitted surface temperature of equipment exposed to the hazardous atmosphere.

Both must be matched to the site requirement.

1. Gas Groups IIA, IIB and IIC

In IEC 60079-based gas-atmosphere classification, Group II equipment is used for non-mining explosive gas atmospheres and may be subdivided into IIA, IIB and IIC.

The required gas group should be provided by the project hazardous-area classification.

IIC is generally the more demanding gas subgroup. Equipment certified for IIC may be suitable for IIB or IIA applications only when the complete Ex marking, protection concept, certificate, ambient range and any special conditions of use support that application. Do not infer suitability from the subgroup letters alone.

Do not select only from the letters IIA/IIB/IIC without reviewing the full Ex marking.

2. Temperature Classes

Common gas/vapor temperature classes are:

T1 — maximum equipment surface temperature 450°C

T2 — 300°C

T3 — 200°C

T4 — 135°C

T5 — 100°C

T6 — 85°C

A lower numerical surface-temperature limit represents a more restrictive temperature class.

The site classification should specify the required temperature class based on the ignition characteristics of the hazardous material and applicable rules.

3. Temperature Class Is Not Heater Outlet Temperature

A process heater can heat a gas or liquid to a particular outlet temperature while the hazardous-area design separately controls the temperature of relevant exposed Ex surfaces.

These are different engineering quantities.

The design may need to consider:

  • heater body surface
  • terminal enclosure
  • cable gland area
  • sensor body
  • nearby external surfaces
  • abnormal flow conditions
  • insulation
  • ambient temperature

4. Element Sheath Temperature

Heating-element sheath temperature can be substantially higher than the bulk process-medium temperature.

It depends on:

  • surface watt density
  • flow velocity
  • heat-transfer coefficient
  • fouling
  • element spacing
  • medium properties
  • loss of flow
  • control response

Therefore element-sheath temperature should be reviewed separately from both outlet-medium temperature and external Ex surface-temperature limits.

5. Ambient Temperature

Ex certification and equipment marking can depend on the permitted ambient-temperature range.

A project in a desert, arctic, offshore or enclosed hot environment may require a different thermal review than a standard indoor installation.

Always provide the expected minimum and maximum ambient temperature.

6. Insulation and External Surface Temperature

Thermal insulation reduces heat loss but can also change equipment surface temperatures.

The insulation design should consider:

  • personnel protection
  • process heat loss
  • Ex surface-temperature requirements
  • terminal-box heat exposure
  • inspection access
  • outdoor weather protection

Do not add or change insulation around certified Ex components without checking the approved design.

7. Fault Conditions and Interlocks

The temperature-class design may depend on protective functions such as:

  • high-limit cutout
  • low-flow trip
  • fan interlock
  • pump interlock
  • low-level trip
  • independent temperature sensor
  • manual reset
  • redundant shutdown logic where required

The required protective devices and their safety function should be documented.

8. Full Marking Matters

A complete Ex marking can include more than gas group and temperature class, such as:

  • protection concept
  • equipment group
  • EPL
  • ambient range
  • certificate number
  • special conditions of use
  • ingress protection
  • other project-specific information

Selection should therefore be based on the full certificate and marking.

9. Data Required

Provide:

  • hazardous material
  • area classification
  • gas group
  • temperature class
  • EPL where applicable
  • ambient range
  • process temperature
  • heater location
  • insulation requirement
  • required certification scheme
  • required protection concept
  • project standard

Related Xiangyuan Products

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FAQ

Is IIC “better” than IIB?
IIC represents a more demanding gas-group capability in IEC-based gas classification, but equipment suitability must still be checked from the complete marking and certificate.
Is T6 a higher allowed surface temperature than T4?
No. T6 is more restrictive: its maximum permitted surface temperature is 85°C, while T4 is 135°C.
Can an outlet gas temperature exceed the T-class number?
The T-class limit applies to relevant equipment surface temperature, not directly to process outlet temperature. The complete certified design and process arrangement must be reviewed.
Can insulation be added after the heater is certified?
Not automatically. Insulation can affect surface temperature and should match the approved design or be reviewed before modification.

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.

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