Mitigating Frost for Cooling 100+ kg of Air/sec to Subzero Temps

[TheFerruccio]

The SABRE engine, currently being developed by a company in Oxfordshire, England, is a single stage to orbit, experimental hybrid air-breathing/LOX rocket engine with a rather fascinating heat exchanger that pre-cools the air before entering the combustor. What fascinated me the most about this heat exchanger is that it will employ something absurd like 1400 kilometers of millimeter diameter inconel tubing that's with a wall thickness on the order of tens of microns.

This precooler apparently cools well over a hundred kilograms of air every second up to Mach 5. The papers I have read are deliberately vague on exactly how they were able to mitigate the frost accumulation during this cooling. I'm sure it's a company secret. But, like much proprietary information, the answer is not necessarily something that hasn't been speculated before.

For more information, go to http://www.reactionengines.co.uk/pdf_documents.html and specifically read the article titled "Heat exchanger development at REL." I have a general idea of how this engine is put together, and what the different heat exchangers do. However, I do not have nearly as firm of a grasp on thermodynamics, in general, as some other people on these forums might have.

If you were to try come up with a way of mitigating frost for cooling 100 kilograms of air to subzero temperatures every second, what methods would you try to keep the frost from forming?

 

[AlephZero]

I can't answer the question, but there this might be interesting. Includes what is claimed to be an exclusive video of the cooler test program, but not much technical detail. http://www.bbc.co.uk/news/science-environment-17864782

Even if it works technically, making it failsafe could be an "interesting" engineering challenge.

 

[Mike_In_Plano]

I noticed their return temperature on the He was fairly high, so I looks to be a counter-flow heat exchanger.
It gets physically larger as the gas gets cooler. I wonder what that's about? Slower air velocity / greater heat exchanger efficiency?
As to moisture, I can't see it not being somewhat of an issue at lower altitudes. Upper altitudes, it will fall below the dew point...
I suspect most of the ice would form in micro-crystalline solids in the center of each tube's airstream leaving the residual moisture on the walls to be dealt with. Perhaps this is handled with surface conditioning and the shear forces of the passing gas?

- Mike

 

[Enthalpy]

Neither did I find details about their heat exchanger, which obviously is a company's secret, since this is the difficult part of the program that prevented it run previously elsewhere.

I vaguely imagined the heat exchanger would let the tubes run nearly parallel to the air flow so frost would skid away with pipe in- and outlets put on the same unexposed face of the tubes, and a big hole at the rear would let air flow out with the frost.

Once air is cooled to 120K for instance, it's free of moisture and CO2, so they can carry on to liquefaction.

As for the tubes, I suggested for other uses to make some of nickel, not inconel, since electrolytic nickel is commonly produced with 8µm thickness:
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051#p23419
This bizarre idea can have puzzled some people. In the US, a lab made a light material by using light beams to define the mould in PMMA to be covered with nickel.