Ultra compact air-cooled coolers on platforms

Paul Boughton

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.

Air-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.

The 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.

The 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. The platform is to be capable of increasing the gas pressure to 85bar.

The 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. The Litoral-A Project required a design combining safe and reliable cooling in a limited space.

The 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. The disadvantage of conventional air-cooled cooling now became apparent: there was not enough space for common air-cooled coolers. Conventional designs:

- Increase the number of rows of finned tubes in combination with a conventional fan1). 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.

The problem was to ensure that the required flow of cooling air could be 'pressed' through the finned tube bundle. Smaller bundle space means higher air velocity and whence a higher pressure drop for a given air flow. An increased number of layers (to bring the required cooling surface area within the limited plot area) also increases the pressure drop.

The resulting pressure drop on the airside was far higher than is common in air-cooled cooler applications and higher pressure drop means higher driving power and a higher noise level. The only feasible solution would be to employ a fan which delivered an optimal volume flow and static head within the constraint of maximum permissible power (in other words: highest efficiency). Also, the noise produced by that fan should stay below the maximum permissible noise level.

The ultra-high efficiency, ultra-low noise fan, the Whizz-Wheel, was considered. This fan is capable of achieving:

- A 50 per cent reduction of absorbed motor power or

- For the maximum allowable motor power produce a higher airflow in combination with a higher static pressure head.

The selection of this Whizz-Wheel fan in combination with the higher number of tube rows lead to the result that all of the suppliers strict requirements regarding space, absorbed motor power and noise level could be fulfilled. This placed the supplier in 'pole position' to win this job.

Compared with the best (smallest) design of the coolers when fitted with conventional fans1), the final Whizz-Wheel based design requires 40 per cent less space. In spite of this smaller space, the consumed motor power of the coolers is 25 per cent below the maximum allowable level.

With conventional fans1 (even with the larger space) the permissible noise level was exceeded by 5dB(A). In the final Bronswerk design, the noise level is within the permissible limit.

Although the coolers have more rows of finned tubes, the total height of the coolers is reduced by 0.6 metres. Even the weight of the coolers is 15,000kg less than with the conventional fans1.

The othe main benefits are not only for the smaller cooler but also for the overall project:

- The ability to provide a suitable module on an existing platform without modifying the platform structure;

- Maintaining the weight of the module below the maximum crane lift capacity to avoid the need for a heavier crane;

- Fewer fans, so fewer E-power connection points and cables, and fewer components requiring maintenance;

- No self-induced vibrations from the fans to the structure.

The Whizz-Wheel design has led to several applications in which the space occupied by the coolers could be brought to allowable ultra compact dimensions. Together with the two other advantages of these fans, ie strongly reduced power consumption and strongly reduced noise generation, the result is that for many air-cooled coolers or condensers, specifications can be met that were unimaginable before the Whizz-Wheel.

More information at www.engineerlive.com/iog

1. 'Conventional fan' should be understood as the best available low noise, high efficiency axial flow fans before the Whizz-Wheel was available.

Johan van der Kamp and Hans van Essen are with Bronswerk Heat Transfer BV, Nijkerk, The Netherlands. www.bronswerk.com