Zero Gravity Phase Separator Technologies - Past, Present and Future 921160
Spacecraft life support equipment is often challenged with two phase flow, where liquid and gas exist together. In the zero gravity environment of an orbiting spacecraft, the behavior of a liquid/gas interface is dominated by forces not usually observed in one “G” due to the overwhelming effects of gravity. The normal perceptions no longer apply. Water does not run down hill and bubbles do not rise to the surface. Surface energy, capillary forces, wetting characteristics and momentum effects predominate. Techniques and equipment have been developed to separate the liquid/gas mixture into its constituent parts with various levels of efficiency and power consumption.
The solution to the classic zero gravity problem of removing condensate from a temperature and humidity control heat exchanger is followed through its evolution from the elbow and wick separator used on the Biosatellite program, through the air turbine driven rotary pitot tube separator used on the Apollo program's Lunar Module, to the motor driven rotary pitot tube separator and heat exchanger “slurper” configuration used on the Shuttle Orbiter. An elbow separator uses the momentum of the liquid droplets to force the water to the outside wall of an elbow in an air duct downstream of a condensing heat exchanger. A wick is used to trap the water by capillary force and move it to the surface of a high bubble point membrane where the water passes cut of the duct to a low pressure sump. The simplicity of no moving parts is attractive, but carryover of droplets in the air stream, and the need for a low pressure sump are often not acceptable. Separator development continued with the evolution of the air turbine and motor driven rotary separators. In these separators the momentum of liquid droplets in a centrifugal force field produced in a spinning drum is used to move water to the outer wall for collection in a spinning trough attached to the drum. A pitot tube removes the water using recovered energy from the spinning motion. The increase in pressure is used to deliver water through a check valve to a storage tank at a pressure above the ambient cabin pressure. The simplicity (one moving part) and automatic control (it stops pumping as the trough water level gets low) of the rotary pitot type separators have made them the separators of choice on all manned space flight life support systems to date. However, power consumption, carryover, and sensitivity to contamination may limit their use in future long duration space flight application such as Space Station Freedom. These problems that are currently being investigated are reviewed.