Air-heated vaporiser capacity across different media

The principle behind an ambient-air vaporiser is straightforward. Ambient air is the heat source; a finned-tube exchanger passes that heat to a cryogenic liquid, which absorbs it and boils. A liquid such as LNG at −162 °C flows through the fins while air outside the tube stays far warmer. Heat travels through the tube wall, the liquid turns to gas and expands roughly 600 times in volume, and the gas leaves at near ambient temperature.

The exchanger obeys basic thermodynamics: every joule needed for vaporisation comes from the surrounding air. Capacity therefore depends on two variables, heat-transfer area and the temperature difference between air and liquid. More area and a larger ΔT mean more heat per hour and a higher capacity. The governing formula in engineering practice is Q = m × r, where Q is the total heat required, m the mass vaporised per hour and r the latent heat of vaporisation of the liquid.

Run the same ambient-air unit on different media and the capacity gap is large, because the physical properties differ. Latent heat comes first: liquid oxygen about 213 kJ/kg, liquid nitrogen about 199 kJ/kg, LNG about 510 kJ/kg. Vaporising LNG therefore takes roughly 2.4 times the heat needed for liquid oxygen, and the same unit running on LNG delivers only about 40 % of its liquid-oxygen output.

Density comes second: liquid oxygen about 1140 kg/m³, liquid nitrogen about 808 kg/m³, LNG about 450 kg/m³. Measured as volumetric flow, a given volume of LNG carries far less mass and therefore needs much less heat than the same volume of liquid oxygen. Boiling point comes third: liquid oxygen −183 °C, liquid nitrogen −196 °C, LNG about −162 °C. Boiling point sets the driving ΔT against ambient air — the colder the liquid, the larger the temperature difference and the stronger the heat-transfer driving force.

Taken together, experience ranks capacity under identical ambient conditions and identical hardware as liquid nitrogen highest, liquid oxygen next, LNG lowest. In selection practice an LNG vaporiser is typically sized two to three times larger than a nitrogen unit of the same duty.

Ambient temperature is the single most important external factor. On an LNG vaporiser, capacity at 35 °C in summer can reach 120 % of the rating, while at −20 °C in winter it may fall to roughly 60 %. This is exactly why northern sites buy a larger unit or add water-bath topping heat. Humidity matters too: at high humidity the fins frost over, the frost layer insulates, heat transfer drops and so does capacity.