46 CFR §151.40-1

1. (a)Boiloff. Cargo pressure and temperatures are maintained by permitting the cargo to boil naturally and the cargo vapor thus generated removed from the tank by venting.
2. (b)External cargo cooling—(1) Cargo vapor compression. A refrigeration system in which the cargo vapors generated within the tank are withdrawn, compressed, and the lower energy vapor or its condensate returned to the tank.
   1. (2)External heat exchange. A refrigeration system in which the cargo vapor or liquid is cooled outside the cargo tanks by being passed through a heat exchanger. Refrigeration is not accomplished by direct compression of the cargo.
3. (c)Internal heat exchange. A refrigeration system in which a cooling fluid is passed through heat transfer coils immersed in the cargo tank liquid or vapor phases.
4. (d)Tank refrigeration. A refrigeration system in which the cooling fluid is passed around the cargo tank exterior in order to remove heat from the tank or its surroundings.
5. (e)No refrigeration. A system that allows the liquefied gas to warm up and increase in pressure. The insulation and tank design pressure shall be adequate to provide for a suitable margin for the operating time and temperatures involved.
6. (f)Tank heating.
   1. (1)A system in which the cargo is heated by means of steam or other heat transfer fluid running through coils within or around the tank. The cargo itself does not leave the tank.
   2. (2)A recirculating system in which the cargo leaves the tank, is pumped through a heater and then returned to the tank.