Benefits of Plate & Shell technology in LNG applications

The use of natural gas around the world is growing and in energy production, the traditional oil and coal are expected to be eclipsed by natural gas use in the coming years.

With the introduction of the European Sulphur directive and other regulations over the world, LNG has recently gained an increased relevance as a fuel source for ships due to its relative environmental friendliness. With energy density somewhat comparable to oil, essentially nonexistent particle- and sulfur emissions, and very low NOx emissions, liquefied natural gas is seen by many as a competitive modern fuel. Many cruise liners and passenger ferries have already been converted and many new ships are built with LNG compatibility in mind. One example is the conversion of the container ship Wes Amelie to use LNG as a fuel source, where Vahterus in conjunction with TGE Marine Gas Engineering Gmbh provided the necessary skid and fuel system.

The transportation of natural gas has therefore become an increasingly important issue. While natural gas can be pumped through pipelines, these pipes can reach only specific locations and require everything from environmental planning to land use permissions and licenses throughout the entire pipeline. A common alternative is liquefaction. LNG, which is over 400 times denser than the gaseous form, allows it to be stored and transported much like other liquid products. This can be both in bulk in large carrier vessels between terminals, and in smaller quantities by trucks from factories or refueling stations.

However, before LNG can be used it must be regasified and heated back to ambient temperatures. Unlike bulky Shell & Tube units or ambient air LNG vaporizer towers, Vahterus Plate & Shell technology allows for the vaporization and superheating of LNG from its storage temperature directly to ambient and above in a single, small unit. The compact size of the Vahterus Plate & Shell is a major advantage for any vessel conversion, as it leaves more space in the engine room for other necessities, such as instrumentation and piping. As the materials required for cryogenic operations tend to be stainless steel, in comparison to Shell & Tube heat exchangers the small size of Vahterus products usually translates to a cost advantage as well.

The same applies for heating or cooling boil-off gas, a problem any cryogenic liquid storage tank, no matter how well insulated, has to account for due to relatively warmer ambient conditions. Vahterus has extensive experience in both cargo condensing and with heaters to make use of naturally evaporated gases. Vahterus PSHE units are at this point well-proven technology on board marine and floating systems, and with cryogenics.

Due to the nature of the round plate being highly resistant to thermal stresses and able to handle large temperature differences between the plates, cold recovery for HVAC or freezer systems becomes possible to use glycol and similar fluids as a secondary circuit.

The Plate & Shell heat exchanger is well suited to these applications and can greatly increase the overall efficiency of a small- scale LNG system.

Another alternative is to use an indirect system, where an intermediate thermosiphon loop is created. Driven by gravity and thus requiring no pumps, this loop contains some media that is evaporated and condensed in a continuous cycle. With such an indirect system, it is possible to use a wide range of heat sources for LNG vaporization: seawater, engine coolant water and many other possible alternatives exist. A custom manufactured indirect system is designed to fit a customers’ needs, and is usually no larger than a meter wide and two or three meters tall.

The overall duty of these systems is no more limited than any Vahterus heat exchanger. Range of delivered units is wide, beginning with small evaporators onboard tugboats, where LNG use can be only 100 kg/h or even less. Others are helping to fuel large ferries or power systems, providing vaporization of thousands of kilograms per hour of LNG. On the high end of the scale, Vahterus can manufacture even larger units, which are in service in LNG terminals where major regasification operations of over ten thousand kilograms per hour can take place, which have boil-off gas handling corresponding to their largescale storage.