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Exploration • Drilling • Field Services


Ultra compact air-cooled coolers on platforms


Johan van der Kamp and Hans van Essen explain how to design ultra compact air- cooled coolers that fulfil the stringent space requirements on offshore platforms.


A


ir-cooled cooling has advantages compared to cooling with water. More and more freshwater cooling (rivers, lakes and below ground) is disallowed because of its potential


thermal and bio-pollution. Sea and ocean water requires the use of expensive materials because of its high corrosion potential. Air-cooled cooling also has one main disadvantage: it requires a lot of plot space so that on offshore rigs space might become a particularly big problem.


Te Litoral-A field is a big oil and gas field in the


Gulf of Mexico. Since the accident in the deep part of the Gulf of Mexico several years ago, Petroleos Mexicanos (Pemex) has been developing fields in the shallow part of the Gulf. Te Litoral-A field is in the Bay of Campeche


where the local water depth is 26 metres. In order to boost the current oil and gas production, the platform design is aimed at improving the flexibility of the Litoral Processing Centre and reduce flare gas using high pressure compression. Te platform is to be capable of increasing the gas pressure to 85bar. Te compressors for this platform were designed and supplied by a Houston-based compressor supplier, in the USA. Air-cooled coolers were to be used for cooling. Bronswerk and this supplier have supplied exciting combinations of high pressure compressors and coolers in various offshore projects. Te Litoral-A Project required a design combining safe and reliable cooling in a limited space. Te supplier approached Bronswerk at an early stage with the challenge of the project to find a solution for this situation and produce a design which would permit safe and reliable cooling in a tight space.


Each of the compressors needs gas coolers for a 1st stage and a 2nd stage, a recycle gas cooler and an oil cooler. For this project Bronswerk gave the supplier an initial indication of the size of the coolers based on the gas composition, flow, temperature, pressure. It soon became evident that space, weight and absorbed fan power were critical points in the overall module designs. Te disadvantage of conventional air-cooled cooling now


● Increase the number of rows of finned tubes in combination with a conventional fan1


Fig. 1. A conventional design. ). Using


more rows while maintaining the external pressure drop of the finned tubes within an acceptable level will reduce the flow of cooling air. Leading to higher outlet temperatures of the air and a lower temperature difference between process flow and air. It became apparent that it would be impossible to meet the space requirements and on top of that the absorbed power was over 30 per cent higher than permissible. Furthermore, the noise level would be unacceptably high. Various tube diameter sizes were considered, but none suited the specifications. To overcome the high airside pressure drop, one fan below the bundle (forced draft), and one fan above (induced draft) was looked at. No improvement in the absorbed power and noise level was obtained. And, most importantly, this produced an unreliable design due to the unpredictability of the conventional fan performance. On the process side no limitations or problems


were encountered. Within the specified maximum tube side pressure drop, several trial designs resulted in acceptable heat transfer. Te problem was to ensure that the required flow


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became apparent: there was not enough space for common air-cooled coolers. Conventional designs:


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