Please help solve this problem: A supply tank requires a vaporizer to generate sufficient pressure to pump stored fluid up into a vehicle or tank. The available head is limited as the tank level falls and it is important to minimize the system pressure drop to maintain the desired flow rate.
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CSA Archive 
Designing pressure build-up heat exchangers can be tricky if the available pressure drop is limited. If we assume parallel vertical heat exchanger tubes, the lower inlet fluid is liquid and the upper exit fluid is partially or mostly vapor. (For nitrogen at low pressure the ratio of liquid to vapor density is about 175.) The volume of liquid and vapor increases greatly as liquid is boiled in the heat exchanger. This causes the flow velocity to accelerate which uses up some of the available pressure drop. Most of this pressure drop can be recovered by installing a diffuser at the exit end of each tube. Ideally, the diffuser exit diameter should be sized so that the exit velocity is the same as that at the inlet. To make the diffuser operate efficiently, its expansion angle should not exceed 7 degrees away from the straight heat exchanger inner tube. This rule sets the length of the diffuser.