Revamps 2026 Issue

section. TWTs at the cold end are therefore significantly higher compared to coun- ter-current operation under the same ambient conditions. The trade-off is real; co-current flow reduces the LMTD, which means a larger surface area is needed to achieve the same heat transfer duty. However, in services where the alter- native is periodic blockage and the associated down- time, the additional surface area is a worthwhile invest-

Counter-current arrangement

Co-current arrangement

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Inlet

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Exit

Inlet

Figure 1 Recovering from a congealing event is never as easy as it sounds, which is why co-current flow is the preferred arrangement for air coolers in congealing service

preheat was uncomfortably high (i.e., high enough to raise serious concerns about operating costs and steam system loading). The design was technically meeting its thermal objective. However, it was doing so in a way that would be expensive to run and potentially difficult to sustain during periods of peak steam demand elsewhere in the plant. External air recirculation was considered as an alternative. The concept involves taking a portion of the hot air leaving the bundle and routing it back to the inlet, essentially blend- ing it with the cold ambient air to raise the mixed inlet tem- perature. It works, and refineries have used it successfully. However, in this particular case, the plot space simply was not available for the additional ducting and fan infra- structure that recirculation requires, and the capital cost did not help the business case either. So, the team went back to the drawing board and looked at what else could be done within the existing equipment envelope. Troubleshooting measures Revisiting the flow arrangement One of the more important and often overlooked design decisions for air coolers in congealing service is the flow direction between the process fluid and the air stream. Standard practice defaults to counter-current flow, and for good reason: it maximises the log mean temperature differ- ence (LMTD), which means you need less surface area for the same duty. From a pure heat transfer efficiency stand - point, counter-current is almost always the right answer. However, in congealing service, counter-current flow cre - ates a specific problem at the cold end of the bundle. This is where the process fluid has already given up most of its heat and is at its lowest temperature, and where it meets the coldest incoming air. The combination of cold process fluid and cold air means TWTs at the outlet end can drop uncomfortably close to, or even below, the pour point. This is exactly where solidification initiates. Switching to co-current flow addresses this directly. In a co-current arrangement, the hottest process fluid enters at the same end as the coldest ambient air (see Figure 1 ). As the process fluid cools along the bundle length, the air is simultaneously warming up, so by the time the process fluid reaches its lowest temperature at the outlet end, the air it meets has already picked up heat from the hotter inlet

ment. In this case, the thermal penalty was evaluated care- fully against the operational risk, and co-current flow was selected as the preferred arrangement. Moving to 100% variable speed drive fans The original design had incorporated variable speed drives (VSDs) on 50% of the fans, which is a fairly common con- figuration. It offers some degree of flexibility while keeping capital costs down. However, during cold weather opera- tion, the limitation of this approach becomes apparent: the fans running at fixed speed continue to pull ambient air across the bundles at a rate that the steam coils may strug- gle to adequately preheat, particularly during the coldest hours of the night. The recommendation was to extend VSD coverage to all fans. This gives operators genuine control over the air- side flow rate during cold ambient conditions. By slowing down fan speed or, in very cold conditions, shutting down selected fans entirely, the air mass flow across the bun - dles is reduced, the air-to-process temperature difference decreases, and TWTs can be maintained above the pour point without having to lean as heavily on steam preheat- ing. It also provides flexibility during normal operation to trim fan speed based on actual process outlet temper- ature, which has energy savings benefits beyond just winterisation. The capital cost difference between 50% and 100% VSD coverage is relatively modest compared to the opera- tional value it provides, particularly in a service where cold weather upsets can be costly. Steam coil distribution and control The steam coil system itself was reviewed not just for capacity, but for how evenly it distributed heat across the air inlet face. Uneven steam distribution is more common than people expect when considering that partially blocked headers, poorly functioning traps, or inadequate conden- sate drainage can all result in sections of the coil operating below design temperature. This creates cold lanes in the air stream entering the bundle, and those cold lanes find their way to the tubes. Improved monitoring was put in place to give operators bet- ter visibility into coil performance, and the control philosophy

Revamps 2026