Ambient air vaporizers are used to vaporize liquid cryogens for many applications in a range of industries. Because they have no moving parts and rely on the “heat” contained in the atmosphere to provide the energy vaporization, they are reliable.
Their principle of operation is simple. Liquid cryogens are passed through a number of connected parallel tube paths. The tubes are part of an aluminum extrusion that has several fins emanating from the center. These fins provide a large surface area on which the “ambient weather conditions” impinge and provide heat energy for vaporization. Because most cryogens are extremely cold (about -300°F [-184°C]), even the coldest ambient condition provides a sufficient temperature difference for adequate heat transfer.
The key to achieving enough heat transfer even in arduous ambient conditions is to have sufficient surface area per tube and per vaporizer. The optimum vaporizer construction is achieved by balancing the flow conditions; meeting the application requirements; and having enough surface area on the fin elements to provide sufficient heat energy to vaporize the flowing cryogen. Combinations of series and parallel paths through which the liquid cryogen flows solve the pressure drop problems associated with liquid flowing through pipes. As a result, extremely low pressure drops can be achieved. All series and all parallel paths are used, depending on the application and design of the vaporizer.
Not all vaporizer designs are the same, and the common practice of comparing vaporizer size and performance by counting the number of fin elements can be misleading. Units that have an identical number of fin elements, and even the same element length, can give widely differing performance in terms of continuous cryogen vaporization. Fin element design includes not only the external fin surface area but also the spacing between fins, the surface finish of the fins, and the design of the internal fluid passage. All of these variables affect the overall ability of the vaporizer to perform its intended function.
Spacing.The absolute number of fins is only one of several parameters that must be considered when evaluating an ambient vaporizer. Fin elements are held in place and spaced apart by clips or brackets. The resulting space between the fins is crucial to vaporizer performance. As the vaporizer performs the function of vaporizing cryogens, the moisture in the air freezes and forms frost on the fins, which slowly builds up on the surface. If the frost fills the space between the fins, preventing airflow through the vaporizer, performance will be impaired, and the system will no longer vaporize the cryogen efficiently. A widely spaced design helps avoid these problems.
It is important to note that ice buildup is not necessarily a negative. As the ice density increases with time, so does the thermal conductivity, providing an adequate mechanism for heat transfer to supply the necessary energy for vaporization. Provided the ice accumulation does not bridge the fin elements, the vaporizer will continue to function adequately. Some systems are designed with four fin elements in the vaporizer boiling zone, where the frost buildup is the highest, so that ice has nowhere to cling and cannot cause bridging.
Vaporizer fin elements all look similar, and their size is rarely mentioned in specifications. However, size is crucial to performance. Fins can be as large as 8" (203 mm) or as small as 5" (127 mm), tip to tip. The surface area difference between 8" and 5" dia. fins is a factor of nearly two, which can be a substantial variance when comparing two vaporizers with the same number of fin elements (figure 1).
Surface Finish.Surface finish also can affect operating costs. Systems in which the fin element is extruded to a high-quality, smooth surface-finish promote rapid defrosting and ice shedding. These benefits allow the use of smaller units to meet flow requirements, which reduces system cost and improves efficiency.
Cryogen Fluid Passage.Another variable of fin design that often is overlooked is the inner cryogen fluid passage. In many fin elements, the passage is a smooth, round orifice. Due to the temperature difference between the cryogen and the fin element, the liquid can flow unpredictably through this type of pipe. Temperature gradients are established quickly, and two-phase flow occurs readily. On short-length units, this design can lead to slugs of liquid finding their way through the vaporizer without vaporizing, which can cause gas exit temperature inconsistencies and low temperature problems.
Finned internal fluid passages help promote turbulence and cryogen boiling as well as provide increased internal surface area to enhance heat transfer into the cryogen as it flows through the vaporizer. The internal fins prevent the formation of liquid slugs that become entrained in the gas flow, thereby improving the overall efficiency of the vaporizer and eliminating the problems with gas exit temperature inconsistencies.
When evaluating vaporizers, process cooling engineers should go beyond simply comparing the number of fins. Variables such as spacing, surface area, surface finish and the design of the cryogen fluid passage affect the performance and cost of the system.
Sidebar: When Defrost is Needed: Manual vs. Automatic SwitchingExcept in regions with temperate climates year-round, all vaporizers will require defrosting at some point during operation to provide continuous gas flow. The length of time that elapses between the vaporizer being online and providing gas flow and being off-line defrosting is known as the “switching time” or “cycle time.” Computer simulations suggest that an optimum switching time lies between four and eight hours. Defrost operation is affected by the ambient conditions; less ice buildup occurs during the winter than during the summer, simply because of the lack of humidity in the winter months, while faster defrost and ice shedding occurs in the summer.
Switching can be performed manually or automatically, rapidly (less than four hours) or more slowly. Manual switching requires no power, uses simple cryogenic globe valves, is simple to install, requires little maintenance and is reliable. Conversely, automatic switching requires power (which to a certain extent defeats one of the primary advantages of an ambient vaporizer) and skilled installation. The more frequently the switching system operates, the higher the maintenance and the less reliable the switching system becomes, leading to the likelihood of gas flow interruptions.
Automatic switching also raises the question of adequate time for defrost. The quicker the switching time, the less time there is for defrosting. In the case of a backup system, which typically comes online during a power failure, automatic switching would be inappropriate.
Rapid switching has few advantages and several drawbacks. Rapid switching necessitates an automatic system, and there is no tolerance for error or variable ambient conditions, or accommodation of future increased gas flow rate requirements. In cases where the ambient conditions provide quick defrosting and facilitate rapid switching, a single, high-efficiency heat transfer unit can meet the process requirements with little risk and minimal maintenance requirements while providing reliability. For applications that require high-volume flow rates required on a continuous basis, switching between units is unavoidable. However, rapid switching is particularly undesirable in these cases.
Over the long term, manual switching offers a flexible extension of the concept of the simple ambient vaporizer. For convenience, automatic switching can be considered, but the switching time should be between four and eight hours to ensure maximum system efficiency.