Ambient Vaporizers

Natural draft units rely on free convection of ambient air across finned aluminium extrusions, with no fans, no power supply and no moving parts. Forced draft units add fans to raise the air-side heat transfer coefficient, so the same duty is met from a smaller footprint or held at a lower ambient temperature. Natural draft generally suits intermittent duty on sites where plot area is available, while forced draft suits high continuous flow, restricted footprint or cold climates. Selection is reviewed against flow rate, ambient design temperature and duty cycle.

Sizing starts from the required gas flow in Nm3/h, the medium (LIN, LOX, LAR, CO2, LNG or LPG), inlet pressure, required outlet temperature and the ambient design temperature at the site. Duty cycle matters as much as peak flow: during continuous operation ice builds on the fin surface and reduces effective heat transfer, so a derating factor is applied to the clean-surface rating. Relative humidity, wind exposure and spacing between units also affect achievable duty. Vaporizers are sized from these project values rather than from a nominal catalogue figure.

The heat transfer principle is the same, but a unit is configured for one defined medium. Design pressure, material selection, cleanliness requirements and connection details differ between services, and oxygen duty in particular requires cleaning for oxygen service and material compatibility that is not applied to nitrogen or LPG units. Where a site handles several mediums, separate dedicated units are the normal arrangement.

Capacity falls as ambient temperature drops, because duty depends on the temperature difference between the air and the process fluid. Frost and ice on the fins add thermal resistance and reduce output further, and high relative humidity accelerates that build-up. Cold climate applications are addressed by selecting a lower ambient design temperature, increasing heat transfer surface, adding forced draft, or arranging duty and standby banks with switchover and defrost periods.

A manifolded bank is two or more vaporizers connected through a changeover arrangement so that one set is in service while the other defrosts. It is needed where the duty cycle is continuous, or long enough that icing would otherwise reduce output below the required flow before demand ends. Switchover can be time-based or triggered by outlet gas temperature. Number of units and switchover interval follow from flow rate, ambient design temperature and humidity.

Ambient vaporizers can serve continuous demand, but not as a single unit running indefinitely, since icing sets a practical limit on run time. Continuous duty is normally covered by duty and standby banks with defrost cycles, or by forced draft units where plot area is limited. Where flow is very high or ambient conditions are unfavourable, water bath or steam-heated vaporization may be the more appropriate technology, and that comparison is made case by case.

The vaporizer is connected to the liquid withdrawal line of the storage tank, normally through vacuum-insulated or otherwise insulated piping to limit heat gain and two-phase flow. Line sizing accounts for the design flow rate and the allowable pressure drop, with isolation and relief provisions arranged per the applicable piping code. Downstream, a pressure regulator or pressure build circuit and outlet temperature monitoring are typical. The exact arrangement follows the P&ID agreed for the installation.