How to Maintain Liquid Cryogen Quality
In my last column, I looked at how usage pattern affects insulation choice. In this third part, I will focus on how the required liquid quality affects piping system design.
Liquid cryogens normally exist as a two-phase fluid - a mix of liquid and gas. Liquid cryogen quality refers to the ratio of liquid cryogen within the two-phase cryogenic fluid. A higher percentage of liquid results in a higher quality cryogenic fluid as well as higher cooling capacity per pound of fluid. Consequently, cryogenic liquid quality is an important factor when designing a cryogenic piping system for process cooling.
One way to maintain high quality is to minimize heat leak into the piping system. This is best accomplished with vacuum-insulated, multilayer insulation. However, certain applications require a higher quality liquid than a vacuum-insulated piping system alone is able to deliver.
In my last column, I showed that it can be more cost effective to keep a vacuum-insulated piping system cold and filled with cryogen than to allow it to warm up during idle time. While the piping system is off, cryogenic liquid is vaporizing due to heat leak - as much as 181 BTU/hr for a 200' long, 1" dia. process line. This amount of heat vaporizes approximately 2 lb of liquid per hour, forming about 4 ft3 of cold gas. The cold gas volume is almost 20 times larger than the same weight of liquid. This increase in volume causes a pressure buildup in the pipe if the main valves are closed. If the main valves are kept open, the cold gas volume will tend to displace the liquid, forcing it back toward the storage tank. In either case, the liquid's quality at the use point is diminished and the fluid at the use point may be only cold gas.
In many applications, a few moments of cold gas at startup is not a problem. However, certain applications are sensitive to fluctuations in liquid quality. Environ-mental test chambers used to test sophisticated circuit boards and high intensity magnets used in magnetic resonance imaging can be adversely affected by changes in liquid quality. Two products can be added to a cryogenic piping system to maintain liquid quality at the use point: keep-fuls and phase separators.
Using Keep-FulsA keep-ful device bleeds cold gas that is produced in an idle liquid-full pipe. The device senses when a gas pocket has developed and opens a vent valve to relieve gas pressure out of the pipe. When gas is released, more liquid flows into the pipe to keep it full. Generally, the keep-ful is located near the end of a piping run at the piping system's high point so cold gas will gravitate upward along the pipe to the keep-ful. The cold gas is vented while the liquid drains away from the keep-ful. A keep-ful ensures high quality liquid whenever the application requires cryogen. The vent line must be properly sized to ensure that pressure drop is low (less than 5 psig). High pressure drops create high backpressure, which can interfere with operation and cause the unit to vent both cold gas and liquid.
Choosing Phase SeparatorsIn some cases, an application is so demanding that liquid quality must be maintained within a tighter tolerance than is possible with a keep-ful device. In these applications a phase separator is used. A phase separator is an insulated vessel with a liquid cryogen level controller and vent gas pressure control system. Cryogenic liquid flows into the phase separator, where its velocity is reduced to almost zero. In this quiescent state, gas bubbles in the fluid rise to the top of the liquid bath and exit through the vent system. The resulting liquid bath is nearly 100% liquid and has an extremely repeatable cooling capacity per pound of cryogen. This repeatability is particularly important in electronic testing applications.
If required, phase separators can be installed on each cooling operation, or one large phase separator may be sufficient to supply high quality liquid to multiple units. In all cases, piping from the phase separator to the use point must be kept short. It should be vacuum insulated and as straight as possible to minimize heat leak and pressure drop, which produce gas bubbles and reduce liquid quality.