DIESTA: an High-Efficiency Tube for Enhanced Air Coolers
Developed with support from Total and the French Environment and Energy Management Agency (ADEME), DIESTA improves the efficiency of the air coolers that are widely used for natural gas liquefaction. With its next-generation finned tubes, specially designed to boost heat exchange efficiency, DIESTA improves both the environmental and cost performance of LNG plants, which are critical to expanding the use of natural gas and meet the global warming challenges.
Liquefaction Train Energy Efficiency: A Major Challenge
The liquefied natural gas (LNG) sector is a backbone of Total’s climate strategy for meeting the challenges outlined in the International Energy Agency’s 2°C Scenario. As an essential driver to boost natural gas’s share of the overall fossil fuel mix, LNG represents one of the most promising paths to improve carbon intensity of our production mix.
But the current natural gas liquefaction process is energy inefficient. Up to 10% of the gas supplied to an LNG plant is consumed in the process of being pretreated and cooled to a liquefaction temperature of -163°C. The new technology known as DIESTA (Dual Internally and Externally Structured Tube for Air Coolers) is designed to tackle the major challenge of improving that energy performance. Developed jointly by WIELAND, KELVION and TECHNIPFMC, with support from Total through the Energy Efficiency Program launched in 2008 with the French Environment and Energy Management Agency (ADEME) in 2008, DIESTA is specially designed for use in the cooling cycles required to liquefy natural gas.
DIESTA : An High-Efficiency Tube for Enhanced Air Coolers
Air coolers are used to chill, condense and subcool the propane used as a coolant in liquefaction trains. Their finned tubes are essential components for heat transfer between the fluid to be chilled or condensed (the propane) and the cold source (the air).
The next-generation DIESTA finned tube was specifically designed to enhance that air cooling process. Its innovative design, which includes modifications to the interior surface of both the tube and the outside fins, improves the rate of heat transfer between the propane in the tube and the airside fins by some 15% over a conventional finned tube.
- The aluminum alloy sleeve that covers DIESTA’s exterior surface holds fins that are dimpled and grooved, rather than smooth as they would be on a standard tube. That allows for better air distribution and increased turbulence, two factors that help to improve the heat transfer coefficient.
- Earlier finned tubes had smooth interior surfaces, but the DIESTA tube has a textured surface. Grooves on the surface increase turbulence, which improves the rate of heat transfer to the fluid in the tube.
Air cooler compressor gas lift aboard the FPU Alima. Moho-Bilondo deep offshore field.
Air compressor gas lift aboard the FPU Alima, Moho-Bilondo deep offshore field.
Gladstone LNG refrigerant and air conditioners trains
New technology known as DIESTA (Dual Internally and Externally Structured Tube for Air Coolers) is designed to tackle the major challenge of improving that energy performance.
Substantial Benefits in Three Areas: Cost, Footprint and Process Safety
This innovation fulfills all of its designers’ objectives. Compatible with existing air cooling technology and compliant with industry standards that include API 661 and ISO 13706, DIESTA offers crucial advantages.
By shrinking the footprint of each liquefaction train, DIESTA reduces construction costs. The length of an LNG train depends on the number of air cooler units that must be lined up to obtain the necessary cooling power. Since DIESTA increases the amount of heat transferred to each unit, the total number of units — and therefore the LNG train’s length — can be decreased by up to 20%. LNG trains that process millions of tons of LNG annually can be several hundred meters long. A substantial reduction in each train’s footprint yields cost savings on pipes, structures, civil works, wiring and more, above and beyond the smaller number of air coolers.
When less equipment is required, the volume of gas stored at the facility can be reduced — and that improves safety.
With more efficient air coolers, LNG production capacity can be increased with no change to the cooling cycle infrastructure. Higher-efficiency air coolers improve operating conditions, with the result that compressors can operate more efficiently at lower power — and compressors are the most energy-intensive components of a liquefaction train. Thanks to that lower energy consumption during the cooling cycle, operators can choose, based on the design they use, whether to:
- Produce the same quantity of LNG using up to 3% less energy.
- Increase LNG production by up to 3%  through improved cooling cycle efficiency. This debottlenecking requires only minor modifications, without changing the compressor drive unit configuration. That makes it highly attractive from a financial standpoint.
This innovative technology for building more cost-effective and environmentally efficient LNG plants will now be routinely considered for all greenfield and revamping projects.
 Combined with a lower temperature approach on the propane evaporators.