Ex d switchgear for ventilation and heating systems in a refinery – problem of excessive internal temperature rise solved
High dissipated power and high ambient temperatures are typical challenges for Ex equipment. In this Ex d switchgear project for a refinery, we used solutions that enabled the system, which generates more than 500 W of thermal loss, to operate safely – without exceeding permissible temperatures and in full ATEX compliance.
Too high a temperature inside the switchboard
Our task was to develop an explosion safety concept and supply a complete switchgear for an ATEX II 2G (Ex d IIB T3 Gb) hazardous area. The unit was to be a key component of the control system for the ventilation and cooling system at the refinery site. In a nutshell: it was responsible for powering and controlling the fan, compressor, condenser and electric heater.
The system was to operate in two distinct modes:
- Winter mode: fan control inverter + electric heater.
- Summer mode: fan control inverter + compressor + condenser.
Already at the analysis stage of the technical enquiry, we determined that the entire system would be capable of generating more than 500 W of distributed power. In a closed Ex d enclosure, operating at ambient temperatures reaching 40°C in summer, this amount of thermal energy would act as a heater.
This risked exceeding the permissible operating temperature of the internal components (55 °C ) and thus losing the stability and durability of the entire system. It was the need to contain the heat in a confined space that became a key design challenge for us.
Why was an Ex d enclosure necessary
High temperature was not the only design consideration. A number of components in industrial design, which do not have ATEX certification, were used in the design.
This type of situation is typical for individually designed solutions. Two approaches are then possible:
- Certification of the entire finished unit (e.g. switchgear) as a complete Ex product – a lengthy and costly process, justified only for series production.
- The use of an Ex d flameproof enclosure, allowing components of normal industrial design to be safely installed inside it.
After appropriate analysis and consultation with the client, we chose the second option.
The Ex d enclosure admits the presence of ignition sources (spark, arc, hot surface) and is designed to withstand a possible internal explosion and, thanks to the extinguishing slots, to contain the flame and hot gases inside. As a result, the switchgear can safely operate in atmospheres with explosive mixtures, even if ignition occurs inside.
Dissipated power limitation and design separation
We wanted to ensure a safe temperature level, so the power dissipated in a single enclosure could not exceed 140 W. Exceeding this value would lead to an internal temperature rise above the limit of 55°C.
We therefore decided to split the unit into three Ex d enclosures with Ex e enclosures interconnected by cable feed-throughs. Each performed a separate function:
- Module 1 – power and main apparatus
- Module 2 – System control and logic
- Module 3 – Inverters
In the allocation process, we also took into account the dependence of the system operation on the outside temperature. Some of the components responsible for powering the heating circuits were only activated when the ambient temperature fell below +21°C. In this way, we were able to concentrate more distributed power in a single housing, without the risk of overheating during summer operation. This is because the 140 W value referred to the most stressful scenario, i.e. when the ambient temperature reaches 40°C.
Connections between enclosures and cable entries
Another key element of the design was the insufficient number of feedthroughs, which would have prevented the proper connection of the individual switchgear sections. Therefore, the flow of power and control cables between enclosures was carefully calculated by us during the design phase. This allowed us to select the correct number and diameter of certified resin-sealed cable grommets to ensure the correct level of explosion safety.
An additional challenge was the introduction of special cables, such as optical fibres and Ethernet cables, which require special barrier glands. In the case of fibre optic cables, it was also necessary to change the original type of cable proposed by the customer, as it was not suitable for resin sealing.
Component layout and thermal management
When designing the interior of the enclosures, we performed an analysis of the vertical temperature distribution. The results showed a difference of up to several degrees between the lower and upper zones of the enclosure. Consequently, we placed the most temperature-sensitive components, including the inverters, in the lowest possible part of the enclosure, where the temperature is lower. Higher up were the passive and more thermally resistant components. This arrangement ensured that heat was evenly distributed and that a safe temperature margin was maintained even when operating at maximum heat loads.
Effect and investor requirements
The design, developed by our experts, enabled the safe use of industrial components in an Ex zone with a dissipation power of more than 500 W. The three-module Ex d switchgear with Ex e enclosures allows a stable operating temperature to be maintained regardless of atmospheric conditions, eliminating the risk of overheating and ensuring full compliance with ATEX requirements and IEC 60079 standards. The investor positively evaluated our design and commissioned us to carry out this solution.