Determining temperature class for electric process heaters

Electric process heaters that interface with flammable materials in explosive environments need to be properly evaluated to protect the equipment against catastrophic failures.

Johann Lainer

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Article Summary

There are several possible protection methods that may be employed to protect from a potential explosion. A temperature class requirement will be provided by the user based on the explosive gases that may be present in the installation area. The maximum surface temperature of the process heater must then be evaluated to assure compliance.

Temperature classes that are based on the maximum operating temperature for heaters prescribe the level of safeguards. Determining the temperature class for a specific electric process heater is unique compared to other electrical components installed in classified locations. The temperature code, or T-code, is highly dependent on the process conditions of the heating application. Global Temperature Classes/Codes are broken down into the following various levels.

System details
Some end users may believe and request that the temperature classification be based solely on the temperature of the terminal enclosure. However, current standards for the general requirements of equipment to be installed in explosive atmospheres state that an external source of heating must be considered in the product evaluation. If end users do not factor in temperatures from the flange to the electrical enclosure, the wrong T-code could be used, which could mean a heater may not be properly certified and fail inspection thereby increasing delays and costs for end users.

Definitions and envelope boundary
The first step is to understand the primary definitions and envelope boundary for the temperatures of interest.

Per EN/IEC 60079-0, Section 3, Terms and Definitions
Maximum Surface Temperature: The highest temperature attained during service under the most adverse conditions (but within the specified operating tolerances) by any part or surface of the heater bundle that is exposed to the atmosphere. NOTE: For electrical equipment in an explosive gas atmosphere, this temperature may occur on an internal component (internal to the enclosure) or on the external surface of the enclosure/heater flange/standoff, etc., depending upon the type of protection employed.

Enclosure Service Temperature: The maximum or minimum temperature reached at specific points of the equipment when the equipment is operating at rated conditions, including ambient temperature and any external sources of heating or cooling.

Enclosure Internal Temperature: The maximum air temperature reached inside the terminal enclosure when the equipment is operating at rated conditions, including ambient temperature and any external sources of heating or cooling.

Certification requirements
Next, we need to understand the requirements of the applicable codes, which typically are determined by the country where installation happens. North American regulations (NEC Article 500 and CSA C22.2) are different than European regulations (EN 60079-0) as well as international regulations (IEC 60079-0). However, each of these standards specify that influence from an external source of heating must be included in the determination of the maximum surface temperature. For an electric process heater assembly, this means that the heat generated inside the terminal enclosure needs to be considered as well as the temperature of the flange and other exposed surfaces. The temperature impact of this entire area must be included in the evaluation of maximum surface temperature.
Per EN/IEC 60079-0 section 5 references below:

1. The first step in calculating a preliminary T-code is to determine the maximum surface temperature and internal enclosure air temperature based on the heater design operating in the most demanding conditions while remaining within design specifications. These conditions include: maximum process temperature influence, full power/amperage operation, maximum ambient temperature for installation location and solar load if applicable.

As Watlow engineers have proven, the internal enclosure air temperature is impacted by the pins that attach heating elements to the electrical termination. With enough amperage flowing through them, the pins and other electrical supply connections can act as individual “small” heaters and could dramatically increase the temperature within the enclosure. (See Watlow’s “Better Predicting Terminal Enclosure Temperature to Improve Heater Reliability” white paper)

2. Once the preliminary T-code is determined, electric process heater designers must answer an important question: Is the preliminary T-code cooler, same or hotter than the customer specified T-code? The answer to that question is an important guide in the development of the heater.
If it is cooler than specifications, then proceed with the cooler T-code. If it is the same, then proceed with the customer specified T-code. If the preliminary T-code is hotter, then the proposed design does not meet the customer requirement. At that point, design changes must be considered to meet the customer specified T-code.

3. The last step is to verify that the enclosure service temperature rating will not be exceeded. The maximum surface temperature is evaluated such that even if it were to reach the assigned T-code temperature limit, the enclosure service temperature rating will not be exceeded. This will ensure that all of the internal enclosure components are suitable for the expected worst-case temperatures.

An enclosure operating with a service temperature in excess of the maximum could cause two problems. First there is a potential safety risk as the heater will be operating at service temperatures beyond its rated temperature. If ignition happens at that elevated temperature, the integrity of the enclosure may fail and not contain an explosive event. Additionally, if the temperature exceeds internal component ratings, there is a higher chance of component degradation and an increased chance of early equipment failure.

It is very important to comply with the current standards that define the evaluation scope to be from the flange (heat source) to the electrical enclosure, as well as fully accounting for all the heat generated inside and outside the enclosure. Failing to do so could mean an unsafe situation or could result in the authority having jurisdiction (AHJ) not approving the equipment installation, causing potential schedule delays and/or cost overruns.

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